In radiology we dont use the term x-ray and radiograph synonymously. When talking about x-rays, he is actually talking about the member of the electromagnetic spectrum which is used to make the radiograph. This is the actual film or image. The X in x-ray is not capitalized. x is the algebraic unknown. The proper spelling if used as a noun is x ray. It is hyphenated as an adjective or verb. (deep thoughts by zack dow)
We start with a standard 110 volt outlet and must increase the voltage to 70,000 to 90,000 volts to produce x rays. We must have an AC source and transformers to achieve the desired voltage. If you have a current passing through a wire, you will have a magnetic field around that wire. The magnetic field does have a direction.
Left Hand Rule: Put your hand around the wire. The current will be passing in the direction that your thumb is pointing. The magnetic field will be around that wire in the direction of your fingers. It is very important to understand there is directionality.
Left Hand Dynamo Rule: Take your left hand and take the natural spread of you thumb and first two fingers. Pass the wire through the direction of your thumb, and you have a magnetic field in the direction of your index finger. Then the current will be flowing in the direction of your middle finger. It is important to remember that a direction exists, which is the basis for the simple alternating current generators. Generate a current first in one direction and then another. This generally happens 60 times a second. You are inducing a current. It starts at zero and flows in one direction. It then it goes back to zero and flows in the other direction. The reason for the flow is voltage, an electromagnetic pull. It pulls the current through the wire.
One complete cycle occurs every 1/60th of a second, hence sixty times a second (60 Hz). This is the standard current of a wall outlet. It goes in each direction for 1/120th of a second to complete a cycle. This is unlike a battery in which you dont get an alternating current. It is unidirectional.
110 volts starts the system. In x ray tubes we will require 70 kV peaks. We need to step up the voltage coming from the outlet. In order to do this we need a step up transformer. It is located between the primary and secondary sides. In a transformer, we have a series of coils on the primary side and another series on the secondary side. As you start to build the voltage in one direction, you will have a magnetic field around the wire. It cuts across the secondary wire. As we have seen before if we pass a wire through a magnetic filed, we will induce a current and a voltage. There is a ratio of the numbers of windings in each transformer. For, example, one has twice the windings as the other, effectively increasing the voltage by two times in the side with more windings. This is the same for 1000 times as many windings, which would produce 1000 times the voltage.
Windings go around a soft iron core to concentrate the effects of the magnetic filed. Generally the primary windings and secondary windings wrap around each other, but are insulated from each other to get maximum effect of inducing a magnetic field from one set of windings to another. We also have an alternating current in the secondary part as well.
Autotransformer-in primary side. Primary circuit had two subsets, a primary and secondary part. An autotransformer is located between the two. It carries this name because there is only one set of windings. Here you can select how many windings form part of the secondary. By adjusting windings, one can adjust the voltage in the secondary circuit. This adjusts the power coming out of the outlet. It allows multiple kV reading to happen. We are looking at kV meter, and adjust it to the desire setting.
We activate the whole system by pressing the exposure switch, which completes the circuit and allows current to flow through the whole system. We want enough current to pull electrons across the x-ray tube. We need an accurate timer for accurate exposure time. The x-ray tube itself is in the circuit on the secondary side of the major circuits. We produce the x rays in the vacuum tube. In order to produce x rays, we are going to have to accelerate electrons across the gap in the tube. Electrons are passed through a filament, a thin wire, within the tube. A rheostat will determine the current through the side. We then go through a step down transformer, which lowers the voltage and raises the current. We have a high current and low voltage passing through the filament. The high current heats the filament, which glows with a cloud of electrons around it. This is known as thermoionic emission. We have a cloud of electrons around the filament. Some electrons pass on and react with the Tungsten target producing x radiation.
Self Rectification: As the voltage is pulled in one direction from the filament to the target, you will have a cloud of electrons passing to the target. The pull cycles in each direction. We dont want to burn out the filament. Current flows in one direction half of the time and them stops the other half, hence self rectifying. It is allowing the current to flow in only one direction. We have a discontinuous stream of x rays. For example, if we set the time for one second, we will only be producing x rays for ½ second.
We must be careful not to overload the tube. They are very expensive to replace. Electrons can be pulled the opposite direction. Rectifiers are also placed in the system to help prevent all of the strain on the x-ray tube. This now becomes a half-rectifying system. We use these in the clinic. We them need longer exposure times.
Fully rectified circuit- flows in the same direction.
Electron volt- voltage is a difference in potential of electromotive forces. Its what moves electrons. Like moving the electrons through a wire. The electromagnetic force (pull) is measured in voltage and the energy it would take to move an electron across a difference in potential, of one volt, is one electron volt.
Bohr Model- We deal with atoms using the Bohr Model of the atom. This is the one that resembles a small planetary piece of crap, where electrons occupy discreet positions in specific orbits. The orbits are give specific labels such as K, L, M orbits, and each orbits electrons have specific binding energies. The K shell electron is bound at an energy level of a magnitude around 70,000 eV (70 ke V). The electrons are tightly bound to the nucleus. There will be different levels of attraction force depending on what element you are dealing with. We deal with tungsten. Tungsten has an electron holding energy near 70,000 eV. It is actually 69.4 ke V but 70 is close enough. You then go to the L shell. The binding energy drops off considerably so you are around 2000 eV (2keV). The L shell (rhymes) electrons are bound by the nucleus to a lesser degree than the K shell electrons. In the M shell the electrons are relatively loosely bound. Electrons are close to being free electrons because of the difference in the binding energy (the attraction energy the nucleus has to them).
Electrons can be moved to other shells. To move an electron inward there must be a space in the inner shell, but to move an electron out it can be easily ejected from the system. In order to move an inner electron (which is tightly bound) outward, there must be energy brought into the system. If an electron drops in from an outer shell to an inner shell it gives up energy. Energy is brought into the system to move an electron out. Energy is given up to move it in.
Electromagnetic spectrum: X-rays, gamma rays, visible light, ultra violet, infrared, radio waves, and microwaves are all members of the electromagnetic spectrum. The difference between all these is the energy level they have photons related to their wavelength. All parts of the electromagnetic spectrum vary by wavelength and energy. Analogy: Picture taking skipping rope and tying it to a tree and taking the other end and walking away from the tree. Holding the rope moderately tight, move your are slowly up and down, you would have a series of shorter waves moving along the rope. A shorter wavelength= higher energy; if you move your hands up and down rapidly you are putting more energy into the system. If you keep that in mind youll remember which is which. All members of the electromagnetic spectrum move at the speed of light C=ml. C=speed of light, in a vacuum, m=frequency (# of waves per unit time), and l=actual wavelength. If you were standing along side the skipping rope, as the wavelengths go up, fewer of these waves would pass by you in any given time.
History of radiology-review: In 1895, Renken was working with a Crooks evacuated tube. He attached the tube to a circuit; in this case a direct circuit. These were called vacuum tubes, but at the turn of the century these vacuum tubes were not as good as we have now. These tubes still had some air in the tube. In effect, if you tried to pass a current through the tube, you used electrons from the residual gas that remained in the tube to complete the circuit. The idea was that the electrons would pass from one electrode to the other. However, not all electrons always directly hit the other electrode. Some bypass the other electrode and strike the glass envelope. As Renken worked in his lab (in the dark) he had a plate of platinum-baring cyanide nearby and he noticed that when he completed the circuit, this plate glowed. Thus producing a flow of radiation, x rays. (You can look up the rest of the history).
The stationary x ray tube: Inside are located the molybdenum-focusing cup in which is the filament, in the front. When you start out you wind up also charging the molybdenum negatively and that has the effect of focusing the electrons to get a greater vacuum in the tube. Also, what we have here is a tungsten target imbedded in a big block of copper, and that block of copper is referred to as a heat seal. The dental x ray tube is a stationary target, or stationary anode tube. This is not technically correct because it is an alternating current so it is an anode only half of the time you are making x rays. It changes from the anode to cathode every 120th of a second. There is a lot of heat being generated when the electrons are going across the tube producing x rays. 99.8% of the interaction between the electrons and the tungsten target will generate heat. Only 0.2% will produce x rays.
The goal is to get rid of this heat. Tungsten has a very high melting point, 3000 C, but if you dont get rid of the heat you will melt a hole in the target. The block of copper acts to draw the heat out of the target. Also various things around the outside block, allow radiation to come out only the place that we want. The target is set up at an angle, usually 17 degrees in most diagnostic tubes. When you look at textbooks you will see a picture like this, but it is not a picture of reflection. This is a common misconception. The two rays interact in the tungsten. The beam going downward represents the x ray beams we want. It doesnt represent all of the x rays because they are going off in all directions. We are only interested in the ones going off in a specific downward direction.
Rotating anode (target) x ray tube: If you are using an exposure protocol that requires a larger exposure over a shorter period of time you are going to create more heat and the stationary target tube will not be able to handle it. Instead use a rotating anode x ray tube, normally referred to as a medical tube, but we use them in dentistry as well. On the filament side we have the same components. One the other side instead of having a stationary target embedded in copper, you have a tungsten target that has the same 17 degree bevel, but is shaped like a saucer looking from the side. It rotates throughout the entire procedure, constantly presenting a different part to the incoming stream of electrons. It distributes the heat that it produces over a wider area giving it off by convection. I
Ideally we would like to have a point source of radiation. We want all of the x rays produced from the same spot because in effect it is like shadow casting- if you have a small source of light and you are casting a shadow of something you are going to get a very sharp image, if you have a large source of light you are going to get a very poor image. So the idea is to try and keep your source of radiation to a very small apparent source. If the point source is too small you will shorten the like on your x ray tube because you will bore a hole right thru your target. So the idea is to try and get a compromise between the two extremes. You use a relatively small focal point and you get a relatively small edge gradient that is below the ability of the eye to discern. So you get a good image while having some control over the heat that is being produced at the target. There are two interactions that take place in the target in the x ray tube that produce X radiation: Braking Radiation or Bremsstrahlung= breaking radiation because you are slowing down the electrons. Electrons cross from the filament to the target. Some may pass close to a nucleus and be slowed down, thus deflected in their path by the nucleus. When they slow down they give up some of their energy. Depending on the amount they are slowed down, they will give off varying %s of their energy. A large range of energy will be given off, most as heat, but some as X Radiation. If it plows into the nucleus it will give up all of its energy. There is a spectrum of energy levels with a range of x rays and a lot of hear being produced. Characteristic Radiation: The electron comes across from the filament. For the interaction to take place the electron must have more energy than the binding energy of the K shell electron in Tungsten. The electron comes in and knocks the electron out so it must have more than 69.4 ke V, which is the binding energy of tungsten. We use basically 70 ke V. We have no characteristic radiation being produced in the x ray tubes that we use. By knocking out an inner shell electron (K) we allow an outer shell electron to drop in- giving up energy. This energy is the difference between the binding energy of the two electrons. All of this occurs at the same wavelength, not a spectrum as in breaking.
A great deal of the electrons will just pass through and not interact with the tungsten atoms because there is really a lot of space for electrons to pass. Again, different types of X radiation are given off- most as heat and some as X radiation.
Tube heads: these are filled with oil or Freon. The oil acts as an insulator and as a heat sink that pulls the heat out of the Copper sink, so that it can transfer it out of the unit.
Duty cycle: the time between exposures can produce enough heat to burn out the tube. Be careful because you dont want to exceed this cycle once you get out into private practice.
Heel Effect: The beam that comes out of the unit is not uniform from side to side in intensity. Lets look at a representation of the tungsten target from the side with a 17 degree angle. Say we are producing a bunch of x rays at a particular location and we are going to see what happens to say 1010 photons of x radiation. Different beams of radiation A,B, and C are going to interact with the tungsten at different levels. You find that the beam does not consist of a homogeneous distribution of energy levels. The greater the distance that the photons of x radiation are passing thru the tungsten target- the greater the likelihood they will interact with the tungsten and thus not get out of the tube. B acts as the middle of our beam and is 100% intensity. So at C the rays have a longer distance to get thru the target (C is on the anode side of the tube). At A, the beam has a shorter distance to pass thru the target (it is on the cathode side of the tube). It will have the higher intensity-more x radiation on that part of the beam.
When making exposures across the skull for orthodontics or oral surgery purposes. The big problem is that we have the soft tissue of the face and brain and the bone behind it. This can cause a balance problem. There is not a uniform distribution of what is blocking the x radiation. What you want to do is set the unit up with the cathode side (filament) projected towards the back of the skull and the anode (target side) towards the front of the skull. This is the part that has the least radiation. You try to equalize sides of the beam so that you dont under or over-expose your radiographs.
That sucked!
Fundamentals of Oral Radiology
April 17, 2001
Chris Granillo
Attenuation of X-Rays
We were talking about the attenuation of X-ray before, but specifically the attenuation of the X-ray beam. You always have to think that your not talking about a single photon, although the interactions that take place at the single photon level, but your talking about the whole beam and how much of it is going to get to where your going. So here people talk about absorption of X-rays, thats not technically a correct term but it is a widely used term, the proper term is attenuation and actually it is the attenuation of the X-ray beam.
Attenuation of the X-ray beam:
What were really talking about is this "if you have a beam that is aiming at a measuring point and you measure what the intensity is or how much energy, thats what were really talking about with X-ray, how much energy is there and then you put something in the path of the beam that causes some of the photons not to reach this point thats attenuation". It doesnt necessarily mean that the photons are actually taken up by the attenuator and that the energy stays there, that may be indeed what happens or maybe that it as a result of some interaction that some of the X-rays wind up leaving in a different direction usually as a result of being taken up and then given off again. As far as what were concerned is what we measure here after placing an attenuator in the path of the beam, which is "if there is a decrease in the amount of energy in the beam at the measuring point, then were going to call that attenuation".
Now what that means is that you could also get some what appears to be anomalies happening:
If you have a narrow beam, remember you can restrict the opening in the X-ray machine through which the X-rays leave, and if you have a smaller opening then you will have a narrower beam. Usually the entity that produces the size of the beam is called the culinator is made out of lead; it does not have to be but usually its lead. So you have what you can think about it as having like a big lead washer in there then if you happen to have a large hole in it then youll have a large beam and if you have one that has small hole then youll have a small beam. There are also moveable culinators that might be rectangular or square, but you can restrict the opening of the unit.
All these notes are verbatim but this part is particularly vague unless you follow along with his power point slide presentation, which I assume shows what the hell hes talking about, anyhow heres the web site for his power point presentation. www.anatomy.uiowa.edu/ruprecht/lectures.html
"You then have a narrow beam that comes out, have an attenuator in the path of the beam and measure here youll get a certain additional intensity. If you then have a wide beam then all you do is open up the opening in the machine you have otherwise the same geometry-you have the attenuator sitting in exactly the same position-you have your dosimeter (your measuring instrument) sitting in the same position-you may actually find that you get a greater amount of energy or x-radiation reaching the measuring point. Now by right that shouldnt be because youre still only going to be measuring whats coming in this part of the beam. You still got the attenuator there whether your measuring it this way or that way, that should still be exactly same whats out here reaching this point, which is the smaller area were you are measuring. What happens though is in the same way that you put an attenuator in the path of the beam, some of the X-rays will change their direction and wind up going up here a chance of giving your attenuation of that central part of the beam. Some of the x-rays that are out in this outer part of the beam will also have their direction change and of those some of them will wind up actually coming to the measuring point and so what will seem to happen is that your getting more X-radiation in this area because the beam is wider even though theoretically that shouldnt be happening. So you do get some effect of beam width upon attenuation as well, but that is something that we use under a different guide that will come up again in scatter radiation".
General Rule:
Equal thicknesses of a given attenuator will remove equal fractions of the beams energy.
-So whatever you are sticking in the path of the beam whatever the material is, its a pure material, you will remove a certain fraction or percentage of the energy of the beam, if you then put exactly the same material in this path again it will then remove another equal fraction.
Example:
24 packets of photons:
-1/2 -1/2 -1/2 =3
24 =12 =6
What you have is an asotonic reduction based on increasing the amount of attenautor. Now for practical purposes you will have a thickness for everything you remove but in theory it just keeps dropping off and dropping off until it gets to the point where you cant measure it.
Half Value Layer (HVL):
Is a concept that you will run across. It comes up somewhat in radiation protection but frankly it comes up more in the general area of just radiation physics. It will be of minimal interest to you per say in using diagnostic radiology but may be of more interest to you when you are doing board exam because it tends to show up.
The half value layer is defined as the thickness of an attenuator that reduces the intensity of the beam to one half. What this means is that it is "a measurement of the beam quality itself". It tells us what the quality of the beam is, the beam energy level (the bKev of the beam). It is a way of measuring the actual inherent energy of the beam and usually copper is used as the attenuator.
6 methods of Attenuation:
1) Geometry based (inverse square law). If you have a beam of light (could be square or round beam whatever) as it moves from its point source it spreads out. So if you check at a given distance the beam, whether its a foot or a mile it would have spread out over a certain area.
Ex.
If xx=x, then the area = x2 since the
Distance ratio is 2d/d then yy=2xx, or 2x
d x Thus area @ 2d is (2x)2 or 4x2. Each of the
2d x2 areas that are x 2 can only be receiving 1/4
x the amount of light or x-radiation of that
4x2 from the outside. So you double the
distance ie. 2, take that 2 and square it and
y y put it in the denominator, so you invert it
becomes 1/4, so basically what its telling
us is "that the further away you get from
the source, light beam or x-radiation, by a
matter of squaring inversely your going
to get a drop off of radiation. So at
3x the distance, youll get a 9th, at 4x the
distance a 16th drop off etc., etc.
For straight attenuation, most states have codes that state at which distance you have to be away from the X-ray machine if you dont have a barrier, between yourself and the operator in 49 states its 6 ft. away, but in Iowa its 12 ft. away.
Other 5 methods are interactive: This has to deal with different energy levels of photons, that is different wavelengths of the photons. We can think of them as sometimes as waves and sometimes as particles of energy.
Within an X-ray beam we have a spectrum of wavelengths, a spectrum of energy levels. Because we have a spectrum of different wavelengths of light we call this polycromatic.
1) Coherent Scatter: Two Subsets
a) Thomson Effect
Dealing with relatively low energy level X-ray, below 10Kev, with relatively long wavelengths. One of two things happens: along comes a wave and encounters an atom and it gives all of its energy to one electron within that atom, but the energy that is given is not sufficient to eject that electron, so it gets rid of it again but not necessarily in the same direction. So the actual photon is reemmitted unchanged but in a different direction. When this interaction is assumed to take place at the specific electron level this is called the Thompson effect.
b) Rayleigh Effect (same principle)
Assume again that the photon comes in but instead of giving its energy to one specific electron it gives its energy to the entire atom, the atom still has less energy it can do anything with, but it makes it unstable so it gives all the energy off, still has the same wavelength but in a different direction.
2) Photoelectric Effect - based on diagnostic radiology
This is what occurs in the X-ray unit (most of it), but some also occurs in the patient. The photoelectric effect is what allows us to create diagnostic images. This is what gives us the differential information in the X-ray beam that we then translate into some form of image we can see. In this particle case, the incoming photon has slightly an excess of the binding energy in the K shell electron of the atom in which its going to interact. It will come off; it has a smaller wavelength, probably around 20-30 Kev. It gives its energy to the K shell electron and ejects the electron as a result you have an ion, which is why X-radiation is a form of ionizing radiation because it can interact due to ionization at the atomic level. With the photoelectric effect you have no energy left.
*As I mentioned before the photoelectric effect is the basis for diagnostic radiology and the reason why is because the likelihood of this interaction taking place is based on the cubed of the atomic number of the element.
Ex. Calcium has an atomic # of 20 so 203 = 8000, which is the likelihood of this interaction taking place, in comparison to say Oxygen which has an atomic # of 8, so 83=512. What this means is that if there is something that has calcium in the path of the beam, each one of those atoms is 8000x more likely to interact with the beam than say hydrogen and somewhere around 16x more likely to react than something that has Oxygen. So what that means is that those tissues, which have a high calcium level, like bone, enamel, dentin, and cementum, will block the transmission of X-radiation because they will attenuate the beam at that part much more so than an other part of the body. There is a differential based on the components of the various parts of the body during the path of the beam. This differential translates into a different exposure of various parts of the film that are beyond that part of the body that the beam is passing through, which is what gives us the image.
2) Incoherent Scatter: Compton Effect
Occurs at the same energy levels that are used for diagnostic radiology that is at the same energy levels where photoelectric effect occurs. With the Compton effect you have a photon thats coming in that has a great deal in excess of the binding energy of the electron of which its going to interact. It gives off some of its energy to the electron and ejects it, but it keeps some of that energy, this will no longer have the same wavelength (it will be longer, remember wavelength is inversely related to energy), as a result incoherent and scattered. It will be at the exactly same angle but in the opposite side of the main path. There is a form of conservation of momentum that is going on here as well.
*The likelihood of the Compton effect occurring is based on the atomic number not the cubed of the atomic number. So if we take the three elements mentioned previously, (Ca-20, O-8, H-1), this is not much a differential and it does not add to our ability to be able to see and in fact it degrades our image because of all the scatter that is going on.
When you hear the term characteristic radiation this means that the energy level of that particular photon is characteristic of the element in which it was produced.
Soft radiation means low energy level it does not have a great deal of ability to penetrate
3) Pair production and Annihilation Reaction
Now youre dealing with the photons that are coming from different sources. Now were talking about 1.02 Mev. What is happening in this case is photons come in and passes close to the nucleus of an element that it is interacting with and you get a conversion of energy to matter. Now what you have is two electrons, a standard negatively charged electron or a negatron and a positively charged electron called a positron. This positron will interact with the first negative electron it encounters and then you get a reconversion of matter to energy, but instead of being converted into one photon at 1.02 Mev, it will be two photons each of which will have half that energy (.51 Mev) and each of which will go in exactly the opposite direction.
4) The 24 Gev= Photonuclear Disentigration
Photon falls into the nucleus, which may eject a proton, neutron, or an alpha particle.
Recap:
A. Secondary and Scattered Radiation consists of:
1) Secondary electrons The atoms may bounce into an electron
Photoelectron (photoelectric effect) in another shell and knock it out as well
Compton of recoil electron this is what is called an auger electron.
Pair production electron
-Get a positron and a negatron
B. Secondary and Scattered X-rays consist of:
Coherent (unmodified) scattered
Characteristic
Compton
Annihilation
C. Nuclear Particles:
Protons
Neutrons
Alpha particles
Other particles
**Special Transcript Version**
Fundamentals of Oral Radiology
May 21, 2001
Mark Naisbit (via the wife of Dan "Mercenary Man" Norris)
Biology of X rays, and Regulations
Ok lets get started. Did everyone get the syllabus and lab manuals? Just as a brief orientation , so youll be happy, upstairs there is a list of what group youre in. The lab exercises will take place in the clinic that is on the first floor. Ill be there and so will the staff, one of whom, Jenny, is here this morning. In the lab exercises you will be asked to carry out experiments and they will, or should, help to reinforce some of the material thats going to be presented when we come to the production of the x-rays and also some of the stuff about the production of the radiographs. You will also be able to demonstrate some of the stuff that Ive been telling you about, and you can see that it actually occurs. Also at the same time, in the early part of it, what you will be doing is becoming familiar with the radiology equipment so by the time you actually get around to seeing patients you will be familiar with the equipment. You will know where everything is and you will be able to concentrate on that particular task at hand, not worrying about how to make radiographical pictures at the same time as you are trying to figure out how to operate the equipment. There will be several things going on simultaneously, some of which will be in the background, that is to say, youll be learning without having to concentrate about what you are doing.
Some of you have left me either voice mail or email messages concerning conflict you have. I trust that you have all gone and spoken to Rosemary Stanley about them and have either been cleared up or are in the process of clearing up. Certainly if you havent done that yet, see her as soon as you can this week so that in the event that we have to make accommodations for you we have as much time as possible to handle that in the easiest fashion.
(Student question- Where upstairs would those assignments be posted?) Its in room S351. Do you know where the radiology clinic is? Do you know where the interpretation room is? Thats room S351 and its on the bulletin board. That actually brings up something else, apparently the sophomore identification codes are being compromised. It seems to happen every year, that at least twice during the year someone or other matches the poster in such a way that people know what they are. So currently as far as I know, youre codes are not valid. Which brings up the next exciting world of computer technology, because I just came back from the entitle course where the instructor tries to teach us everything that ever occurred in the world of computers. One of the things we delt with is web CTs. Are all of you already going onto web CT and looking at your marks somewhere? If everything worked out, you currently should have your marks posted on web CT. Now, the only one you have posted is that one quiz that you had, but thats where Im going to be putting the marks because so far, they havent reissued any IDs and I dont know whether theyre going to. It seems to totally escape when some of these courses are given. Weve also, at the end of the year, had the suggestion that instead of doing that, we can send the marks out to the students by mail, which would be twice a week, at 75 students- I dont think so. We are going to try Web CT. I would appreciate it when you have some time during the day, go on to Web CT and just see whether or not you can find your marks of 86-120, because if not, well have to see what we can do and I wont be here because I have a funeral to go to after the lecture. Are there any questions then about your activities?
When you show up in the clinic, although you probably will not be required to wear those blue clinic outfits that make you look like a smurf, please dont show up in clothes that make you look like youve actually come to change the oil in your car. Because you are in a patient area and patients get all sorts of ideas about what goes on in the building and whether or not theyd actually like to come here. Its important that patients come here or youre not going to get finished with your studies here, so dress in an appropriate manner.
What were going to be talking about today basically deals in some sense with radiation protection. The first thing were going to do is talk about radiation biology and then well talk about radiation protection per se. Most of what were going to be talking about in this first section will probably not be new information for you. You will have had some of it in one form or other in various assigned courses and its hardly going to be very in depth coverage in the field of radiation biology. People get masters and PhDs in this area and that takes anywhere from 3-7 years, so I dont think were going to be able to scale all of that into somewhat less than one hour. So, understand that this is just and overview and it really is just a reminder of things youve probably got stored away somewhere up in your brains and just havent thought about for a while. Now, radiation biology, that branch of biology is the science of life, it deals with the effects, adverse and beneficial, of radiation and the organisms. I think thats something you want to keep in mind. That is to say that not all effects of radiation on organisms are necessarily adverse. Some of them may be beneficial, we can assume that life as it exists on planet earth is at least partially due to the effects of radiation on organisms over a long period of time and we can only assume that what it has done is improve those organisms as far as being able to live in the environment that theyre in. Having said that, when we get into the clinic, that does not give us licence to experiment with our patients to create better Americans just because we happen to have a radiation source with us. But you do want to think about the fact that not all radiation effects are necessarily adverse. We do, however, tend to deal with them as if they were. In the sense that we try to keep any effects to an absolute minimum, consistent with the past that we havent had. Now, we talk about radiation, there are, if you will, two broad categories of radiation. There are those that are particulate and those that are non-particulate or wave like. And of the particulate, one of them is made up of alpha particles, and again this would be relatively a review for you. An alpha particle is a positively charged particle given off by certain radiographical substances consisting of tow protons and two neutrons. In affect, what you have is a helium nucleus with alpha particles. When you have a whole collection of these and they are moving, that is to say you have a stream of alpha particles, we would refer to that as an alpha ray. Now, alpha particles have a short range in tissues and they hide linear energy transfer, Ill talk about that in a moment. So, in affect, if you had tissue here, or anything else, since were talking about radiation biology were interested in tissues, specifically related to mammalian tissues and you have a stream of those alpha particles coming they will travel into that tissue and its now a matter of how they interact with the tissue- how far they go. If you were a nucleus of a helium atom, an alpha particle, and you were coming at a tissue, you would see most of that as being space. You would have nuclei here, here, here, here, but a lot of that would be space. So, as youre travelling there you would have the ability to travel a certain distance into that, but interact with some of those tissues there by giving your energy to those tissues. Now, linear energy transfer (LET), refers to the amount of energy that is given per linear path of travel in the tissues. So, if you have large particles coming in here, they will have a reasonably good chance that theyre going to interact with some of the components of the tissue and for our purposes think of it as running into the tissue and thereby giving up all of their energy. Because they are relatively large, as you will see in a comparison when were talking about electrons for example, these alpha particles will not be able to travel that far into the tissue before they give up all of their energy. So, they have what is referred to as a high linear energy transfer, which means they will give up at a great rate, their energy as they travel on a linear path for a relatively short distance. So they dont travel that far before theyve given up virtually all of their energy. What that means is that the likelihood is that a series of alpha particles, that is alpha rays, will give up all of its energy over a very short period of time so it will give it all pretty well in the same tissue. The tissue that is most likely to be affected is either going to be the skin, thats clear, or mucosa. Because, of course, these things may be given off internally as well as food that is ingested, or it can be breathed in. What that means is that if theyve given up their energy over a relatively short period and have therefore given up all, mor or less, in the same tissue, any potential damage that can occur from this energy transfer through ionization can be one and excitation can be the other will all occur more or less in the same tissue and therefore there is a greater likelihood that there will actually be laparoscopic damage done as a result. So, alpha particles are actually relatively destructive as fas as radiation effects are concerned, they are relatively dangerous.
Now, the next thing we have is also a form of particulate radiation and that is beta particles. A beta particle is an electron, either a negatively or positively charged electron that would either be an electron called a positron or a negative and positive tron. And unless we say something different, when we say electron, we mean the connective negative ones and positron. And so, a beta particle, that is and electron or positron ingested at high velocity from a nucleus undergoing beta decay. That is our most common source of a beta particle and then, just like the alpha particles, if you have a whole stream of them then we refer to them as a beta ray. Now, there are distinctive delta rays which are also electrons as well, but were not going to go into that all at great depth. Now, beta particles have a variable range in tissues. Theyre smaller, so if you think about it, an analogy might be that if you had a net full of basketballs and you were shooting a golf ball at it, it has got a chance that its going to make it between somewhere and going to go a certain distance, that would be like an alpha particle, but its not going to go very far. If you took a BB gun and were shooting it into there, it has got a greater chance that its going to get in there further because theyve got more spaces they can go. Thats a relatively loose analogy, but that may be the way you want to think of it. So if we have a variable range and its usually greater than alpha particles, we will go further into tissue. And thus we have a variable linear energy transfer (LET) which is used less as an alpha particle. So, if you have this series of electrons coming, in the same tissue, theyre also going to travel in and through, they will also give up some of their energy by interacting with these, but the chances are that some of them may actually make it through various tissues to theyll go further, so they have a lower linear exit transfer. It is still higher than non-particulate radiation, that is x-radiation and gamma radiation, that well get to in just a moment. But, it tends to be generally less than alpha particles. So, although beta rays are still relatively more likely to cause damage, they were used less than alpha particles. Which brings us into gamma rays. Now, gamma rays are electromagnetic radiation at a great penetration power, so they belong to the same spectrum of visible light that x-radiation belongs to and they are admitted by nucleus of radioactive substances. Again, thats as far as terrestrial sources are concerned, the most likely substances youre going to get. A gamma ray is similar to an x-ray and it may have the same wavelength and hence the same energy, or it can have a smaller wavelength and greater energy. So, generally speaking, in the electromagnetic spectrum, x-rays and gamma rays will occupy the same part of the spectrum, except that gamma rays will go further out, that is to say towards higher energy, lower or smaller wavelengths. If you were to take a gamma ray and an x-ray, if you could physically pick them up and look at them, which you obviously cant, but if you could, if you had a gamma ray or an x-ray that had the same energy level, that had the same wavelength, they are identical. The difference is their source. Generally speaking, if you talk about gamma rays coming from nuclear decay and x-rays being produced in a vacuum tube, but actually physically they are identical when they have the same energy level. But, you can get gamma radiation that will appear at much higher energy levels and hence lower wavelengths. Remember the inverse ratio of energy and wavelength. You remember the skipping rope moving up and down slowly, long wavelength, up and down rapidly with short wavelength, moving up and down rapidly, thus more energy so short wavelength bigger energy. X-rays are also then electromagnetic radiation of great penetration power and they are usually produced, as youve already seen, by a bombardment of a substance, usually a heavy metal, in diagnostic tubes that heavy metal being tungsten, by a stream high velocity of electrons in a vacuum tube, the wavelength is usually less than two angstroms and the ones that we use for diagnostic purposes are .1 to .3 angstroms. An angstrom, as you may remember, is 10 to the minus 8 centimetres. They will have a much lower linear energy transfer because they dont have, for all intents and purposes, any physical being, they can penetrate further, they will pass through tissues which, of course, is why we can produce radiographs by having the film on the far side. And so relative to beta rays and alpha rays, x-radiation and gamma radiation do not cause a great deal of tissue damage. If they caused a great deal of tissue damage it wouldnt be used for diagnostic purposes.
Now, another term Im sure youre quite familiar with is and ion. Which is and electrically charged atom or group of atoms resulting when a neutral atom or group of atoms loses or gains one or more electrons. In other words, you take an atom and you strip away an electron and youve got an ion. If you give it an electron youve got an ion, or if you strip away an electron and its wandering around by itself, thats also and ion. Like I said, most of what Im going to tell you here should not be new materials. And ionization, that is the production of ions due to the ejection of one or more electrons of and atom, very simple terms. Now youll hear the term ionizing radiation being used and the reason we refer to various other forms of radiation, including x-rays, as ionizing radiation is that they interact to produce ions. One of the ways in which x-radiation interacts with materials is through the ejection of electrons. Now we see that when we have the lecture on attenuation of x-radiations. But, when youre looking at that from the biological point of view, whats happening is you are producing ionization, hence x-radiation, alpha radiation, beta radiation, gamma radiation are all referred to as ionizing radiation because of the fact that when they interact with materials, not just biological, it will be at least partially through he production of ions. Now, one of the other things that does happen in the interactions is excitation and that the transfer of energy to an electron, so it has an excess, but it can not eject an electron from the atom. Weve seen that as well, we saw that in the Thompson effect. Youre not getting any sort of ionization, but you are transferring energy to an atom and usually what the atom does is it tries to get rid of it as quickly as possible so it can go back to its steady state. During the time that it has that excess energy, it has been excited and at that point it does have potential to undergo interactions with other atoms, or supposed these atoms are in molecules, the molecules which contain that atom can undergo interactions with other molecules and so at that point it has the potential also to have things happen that wouldnt normally happen. We think that excitation may actually be a more important occurrence than ionization. To explain some of what happens as a result of the interaction of radiational efficiency. And we know that there will be about 3 to 5 times as many excitation interaction take place as ionization interactions taking place, on average. So, it does tend to be a relatively important interaction. Nonetheless, everybody tends to think about ionization as being what occurs because we refer to all of these radiations as ionizing radiations. Now, you are all probably familiar with the term radical, or free radical. A free radical here is an atom that has an electron with respective spin. As you remember, when you look at the bohr model of an atom, which is the one that looks like a planetary system, youve got electrons going around the nucleus like little planets. And like little planets, they spin on their axis and what any of the atoms will try to do, and why you get combinations in the molecules, is to have an even number of electrons in their shells, so that for every electron that is spinning in a clockwise manner, there will be one spinning in a counter clockwise manner and that makes it balanced. If it has an odd number of electrons, or if an electron gets flipped so that you dont have a pair of electrons, one spinning one way or another and thats sort of what happens with an odd number of electrons, then you have whats referred to as a radical, and that is a highly reactive entity. We assume a lot of what happens as a result of the effects of radiation on a tissue is as a result of radical production, so a lot of the research in radiation biology is to try and develop ways to get radical scavengers to pick up radicals. Because radical, we think, are one of the major causes of tissue damage. Now, Ive used the term photon already, but Ill just remind you again that when were talking about x-radiation, we talk in terms of photons, the term photon comes from the Greek word Photos- meaning light. A photon is a quantum of electromagnetic energy having both particle and wave behaviors. X-rays and gamma rays will act at times as if theyre particles and they will act at times as if they were waves. We tend to think of the energy in the beam of x-rays as being made up of discreet quanta of electromagnetic energy and thats how well talk about it. Each one of those quanta packets are whats called a photon. And so the photon energy is the energy in a quanta of electromagentic energy. Just think of a beam of x-rays as being made up of a series of discreet packets of energy and each one of those packets is a photon and the amount of energy in a given photon is photon energy. Now, weve seen this in one form or another, this is an equation youll run up against either as an equation or as proportional formula. That is E the energy in a photon of x-radiation or in x-radiation is equal to HC where H is plancks constant, C is the speed of light over lambda, meaning that as a wavelength goes up, the energy goes down. Now, radiation is something that we are constantly subjected to. Radiation is something that is constantly part of our environment. Trees are out in the environment taking various things out of the soil and out of the atmosphere and amongst the things that they took in were radioactive materials. So when you then take your wood and you make it into furniture and you sit on it, you are sitting on radioactive material. Virtually everything has radioactivity, the levels of radioactivity may be relatively low in some material and relatively high with others. But from terrestrial sources we are constantly being bombarded by some form of radiation, we refer to that as background radiation. We also have extra-terrestrial sources of radiation coming from space. Now, we are protected somewhat from the atmosphere that we have around the earth, depending on where you are on the earth, you will be getting more or fewer levels of radiation. It also has to do with whether youre in a direct or oblique line with secondary radiation coming from the sun. So, if you think about it, there will be extra-terrestrial sources of radiation coming from a higher elevation, depending on how much atmosphere theyre going through, there will be a decreasing likelihood it will reach you, but it will reach you. So its not something that you only receive when you come in to have a radiograph made. It is constantly something that surrounds us. We live in a world of radiation. Radiation has effects on us and we talk about those in two broad categories. We talk about them as direct effect and indirect effect. Most people when they think about what radiation will do, think in terms of a direct effect, although it is not the major one of the two. The direct effect is the effect of radiation on a target molecule and usually when were talking of a target molecule were talking about DNA. But those are certainly not the only target molecules. There is also a direct hit of the radiation on the molecule, it strike it, it splits it, it does something to it. Indirect effect occurs when the target molecule is not struck directly by the x-radiation, rather the results are hit on a surrounding atom or molecule. In our case, thats usually once because our bodies are made up of over 70% water. So most of the effects of radiation on mammalian tissues are not as a result of a direct hit on the target molecule, but as a result of striking the water, and on the water molecule there was ionization perhaps, or excitation and then that interacted with the target molecule. So its an indirect effect and thats a much more important situation because if you think about it the number of target molecules that are here versus the number of water molecules that are there, make it a much more likely situation that the water is going to be affected and that, as a result of one or several intermediate interaction, the effects of radiation are going to be transferred to the target molecule and there are a larger number of effects as a result of water mediated indirect effects. For example, here youve got a water molecule and theres radiation and what can happen is you can get ionization. When the ionizing of H2O happens, they may disassociate into things referred to then as free radicals and a free radical is generally designated by an O , although they might be changing that to a slightly different symbol. So what happens is you get H2O plus disassociating into a hydrogen ion and an OH radical. Hydroxl radical, the way you get the H2O negative, associating into a hydrogen radical and a hydroxyl ion. Now, those two radicals are highly reactive and then they interact with each other and if they do so they will just return to water so a lot of what will happen in anything with respect to radiation is that in a very short time it just returns right back to where it was. So, in the case of water, you produce these products and some of them will interact in
order to return back to water. However, you could get things like this happening, that have 2 hydrogen radicals combining to give you hydrogen. If there were enough of it being produces the cell would be compromised. If it had 2 hydroxyl radicals interacting to give you hydrogen peroxide, hydrogen peroxide is highly toxic to cells. The same free radicals can also interact with surrounding water molecules. After they interact with each other, theyve still got lots of water around there, so you could have this type of thing happening where you have a hydrogen radical interacting with water to give you hydrogen gas and a hydroxyl radical. Or you could have this interacting in any number of other interactions with themselves, with products theyre producing by interacting with water which now interacts with some of the other radicals ions that are being produced. So, you can produce water or hydrogen radical, HO2 radicals, water, hydrogen peroxide, oxygen, water, oxygen- the point here is that there are a lot of interactions that can take place. Now if you happen to have a high oxygen tension in the system then you can introduce a whole other series of interactions and certainly oxygenation. Selective oxygenation is something that gets tried periodically when we use radiation. For therapeutic purposes, if you could introduce the oxygen into the tissues that were interested in, then we can increase the effects of radiation and do so selectively. If we can keep the oxygen out of either tissues, but having high oxygen tension in there does result in a greater likelihood of radiation effects. This is where it becomes important, that all of these interactions may not just be occurring amongst the components of water, which may be very interesting but I dont think by itself its going to cause much problem, but you can also get an interaction with constituent molecules of the cell. Now, some of those molecules may not be very important molecules and they may be very redundant molecules and so if you knocked off a few enzymes here and there it may not have any effect at all on the cell. On the other hand, if what youre interacting with are important molecules, DNA tends to be the one we think of. DNA and RNA are highly important and you could start to disrupt the ability of the cell to actually function and what happens is, again with effects on DNA, is you can actually start getting breaks and changes in there. If you have a couple of different kinds of DNA sitting there you get a break in one and a break in the other, first of all the most likely thing that will happen is they will just restitute and heal the break itself. But its possible to get cross over so this winds up going over there, and this winds up going over there. Now, that may or may not have effect on the cell if you exchange identical information, thats probably not going to have any effect at all. But, if you have a break through something that is important and you transfer it over to the other chromosome you may have actually screwed up the message that was in there. If the information that is needed is still here and is still functioning, it may still be transmitting information out, but if you start going into cell division, this is going to go a long way and this isnt coming with it. So youre going to wind up with a cell thats missing information. You can have deletions and inversions, but now you have a chromosome that has a great deal of its information reversed . Cross over occurs on a regular basis between chromosomes. You may wind up having some of the information wind up in one chromosome duplicated and wind up in the other chromosome deleted, again, that may or may not have any problems in that call at that point. But when that cell divides, it sends off that type of information to its target cell and now you may suddenly have one chromosome that has an excess of one bit of information and a deficiency in another and vice-versa, so the cells may or may not be able to function properly. So if you go through a cyclical mitosis gap, one synthesis gap two and cells are far more sensitive during various parts of their cell cycle. Sensitive radiation occurs in the latter part of the G1 phase, the first third of S, so thats an area where you have sensitivity and also in the G3 and M phases. So, you understand that cells are not throughout their entire life cycle, equally sensitive or equally resistant to radiation on top of that, when cells are in mitosis, they will generally take and continue if they radiation effects occur in late pro-phase and ana-phase. If it occurs ahead of that you can probably get cells to stop dividing, is this any significance to us in the clinic- no. In meiosis youve got to think the same thing can happen as in mitosis. That is to say you can get inversion, you can get cross over, the only difference, of course, is when youre dealing with individual cells within the epithelium dividing the likelihood that one or two cells having had a change occurring in them causing any sort of effect on the living organism is relatively minuscule. The most effect that occurs in somatic tissues are probably going to be at most resulting in cell damage. A lot of the change is going to actually wind up causing no change in the cells. Obviously if you make those similar types of changes in cells through meiosis, are going to become part of the living human being and at least be bringing half the information for the next generation in meiosis. There is a greater likelihood of it causing an effect. Having said that, if you look at what people tend to be most concerned about when it comes to the effects of radiation, people always tend to be most concerned about the effects on cells that are going to be producing offspring. The non-somatic cells, those are actually not the ones were most concerned about because we throw that apron on somebody and thats the end of the discussion. We are still most concerned about the effects of radiation on somatic cells, the ones that are not going to be producing the next generation of Americans. Because most of the changes that occur are going to not replicate, or they may actually restitute. Or, theyre not going to have an effect on the functioning of the cells or, at most, youre going to kill a cell or two. Does that really matter when you have hundreds of millions of billions of cells- no. Every time you use your hands to puck up something youre probably killing some cells. Thats just part of the normal function of epithelial cells, a turnover, a wear and a cell death. But, we are concerned about those cells that may actually in sufficient number have been changed. That they are going to do things that theyre not supposed to be doing, of course what were concerned about is neoplasia. Which brings us back to, thats why such things as alpha radiation and beta radiation are of more concern to us than gamma and x-radiation. But, having said that, even though x-radiation is an extremely weak carcinogen, it gets much worse press than what it is actually able to do.
So, in summary, ionizing radiation can either by direct of indirect effect cause wares entruxle. As I said, most of them will repair. There will be a break and they will re-join. Some of these breaks may result in structural rearranging, in deletions, and loss of part of the chromosome, and again, that is what we tend to be most concerned about. It will occur in other molecules in the cell as well, but were not generally concerned about that. Now, loss of part or complete chromosomes may result in changes in the cellular status quo, which may lead to aberrations of function or death of the cells. Although this sounds very dramatic, this last part thats not one we care about. Well it isnt, youve got millions and billions of cells, you think one or two of them, more or less, will make a difference. Its those that have the status quo that were concerned about. Now, the effects of radiation vary with dose. LET and the stage of the cell cycle. Generally speaking, the higher the dose, the more likelihood youre going to have changes, the more likely that you might have a massive change, one that actually becomes visible in the cell now. The levels of radiation that we are using in diagnostic radiation are, in comparison to radiation in general, relatively low doses and relatively limited as you say, they are not given to the whole body which is of much greater concern to us than the limited areas and they are low doses of radiation. Theyre affected by LET (linear energy transfer), the more of the energy that is transferred within the same tissue, the more likely youre going to be to see a macro cell change in there. As I said, to see the alpha particles and the beta particles are the ones that will most likely cause those. X-radiation has a relatively low linear energy transfer and therefore deposits its energy in various parts of the body, so that in most cases its not going to have any effect at all because its not going to be involving a large proportion of them, and as I said it will also depend on the stage of the cell cycle which will interest the people doing research, but is of no value to us whatsoever. Because we will have some cells that will be sensitive in parts of their cell cycle and well have ones that will be in non-sensitive parts of the cell cycle. Those cells that are more rapidly diving will tend to be distributed amongst that, those that are not rapidly diving will generally not be in the areas where their most sensitive. Which is why such things as nerve cells, for example, or muscle cells are relatively radio resistant in comparison to such things are epithelial cells.
Guidelines for the use and regulation of radiation: we are dealing with electromagnetic waves that vary by wavelength and by energy. Electromagnetic radiation encompasses a large part of electromagnetic spectrum, and light and x-ray are part of electromagnetic spectrum. They have similarities , light coming out of all light bulbs is polychromatic. That is , it comes out at various wavelengths. X-radiation coming out of tube is also polychromatic even though we cant see it as color. Proportionality equation:
Energy increases as wavelength goes down- E=1/
Light and x-radiation have several other things in common. Both light and x-rays can pass through most forms of glass, except leaded glass in x-ray suites. Light doesnt pass through wood, but x-rays do. When you open a film packet there is a thin lead foil inside. It prevents back scatter radiation from reaching the film. If you turned the packet around so that the lead foil is facing the tube the result wouldnt be blank, but less dense image. Because some of the radiation has passed through there so the theory that lead blocks x-radiation is not absolute. Light and x-rays have the ability to affect film. Both also have affects on tissue. X-radiation has affects on two areas: somatic growth and genital or reproductive areas. To reduce these effects we have certain rules, regulations and guidelines. The federal government deals with the production or import of x-ray equipment. Whereas state codes deal with the use of the equipment. Maximum Permissible Dose (MPD) for occupationally exposed workers. Maximum allowable dose (MAD) is the same thing. MPD=n-18(5rem)where n=age and 18 because no one allowed to operate equipment is under age 18. REM=radiation equivalent mammal. Which for our purposes is almost the same as a roentgen (level of energy). We are allowed to receive 5 REMs per year. Now we use scieverts instead of REMs so MPD=n-18(5 centi-scieverts). Educational or occupational workers are allowed to receive a higher dose because the make up a smaller amount of the gene pool (expendable). Non-occupational persons may receive one tenth of occupational workers. In the industry there are radiation workers and non-radiation workers. In health care there are radiation workers, non-radiation workers, and non-workers (patients). We are always looking for ways to decrease radiation exposure amounts, one way is for an x-ray table to not be used for more than one investigation simultaneously. The operator should keep as far away from beam at all times. No body part of anyone else, other than the patient, should be in the x-ray beam. Holding devices should be used with weak patients and children. If parents, grand-parents or assistants are called in for aid they must wear protective clothing (ie. Head, aprons, gloves etc.). Do not use a finger to hold film intra-orally. Do not hold the tube during exposure (requires patient cooperation if necessary). Lead gauntlets must have 1/4 mm of lead if using up to 15D KVE x-rays. Docimeters are used to measure amounts of radiation and must be worn by certain staff members. In Iowa the operator must be 9 feet away from beam, any other state the rule is 6 feet. Clinical indications for making radiographs must be established before any radiographs are made (see Kodak handouts). Only make radiographs that are required for treatment. Automatic timing exposures must be used. Always explain to the patient what you are doing and why you are doing it. The beam must be columnated by law and cover only a designated area (2.7 cm on skin area).
May 25, 2001
Jay Pronk
Production of the Radiograph
We’ve talked about the production of X rays and the interactions of x rays and of course the ones that we are interested in the are ones that interact with the patient that is in the beam that we are trying to make the image of. So we have the interaction, we have the beam that has information in it and what we need to be able to do is record that information so that we have a diagnostic image. This brings us to image receptors. We are now in a stage where we are going into an age of digital radiography although most of our imaging is analog still. But in this lecture and the time being we will focus on analog imaging…that is to say film based imaging. Currently digital imaging lectures take place the senior year and that is where they are staying at the time being and where you will be getting it. The reason we are not going into that yet is that we are not yet other than in our sim clinic using digital imaging because there are problems using digital imaging on the dental side. This is because the manufacturers on the dental side have yet to develop a standard to which all digital imaging must adhere.
Now when we deal with analog film based receptors we have non-screen film and screen film and you will see what the difference is in a moment. Nonscreen film, our standard film in the dental college, come in standard sizes and adhere to the American national standards institute standards sizes of 00 through to 4. Now we do not use all of those sizes in the clinic. We use zero which is sometimes referred to as pediatric film. 1 and 2 are generally referred to as periapical film although they are used in other types of radiographs. And #4 size films which are used in occlusal radiographs. The numbers refer to different sizes. #2 size the one people think of as the standard dental radiograph size. #1 size film is just a little shorter and about the same length. #0 size film is a little shorter than the #1 size. The #4 size film is considerable larger than the standard #2 size film because it rests on the occluding sides of the teeth. I don’t expect you to memorize the sizes….this is just so that you have some idea. You just need to understand that these film sizes are standard sizes and it doesn’t matter if you are dealing with a #2 size it doesn’t matter where in the world you are that is the size. This is one of the problems we have at the moment with digital imaging….not one manufacturer makes their receptors the same size as anyone else.
The films can also be used for so-called bitewing radiographs. The films are turned sideways plus there is a #3 size film that is strictly used for bitewings. Bitewings are those that you have probably had where you bite on a tab which makes an image of the crowns of the upper and lower teeth on one radiograph and it is predominantly used for caries detection although it is used in terms of periodontal bone exams as well.
Most of the films that we use in N. America are currently still Kodak. I say currently because if they screw-up many more times there are going to be other manufacturers. They have begun to do things in there manufacturing that they never seemed to do before. Plus they seem to be reorganizing-I swear- their company every two months so about the time you figure out whose on first someone else is running the show with no one else knowing what is going on. Having said that….those other films that we use….if you look at the boxes….E means that they are a speed group E. There are various speed of radiographic film just as there are many speeds of photographic film….400…800. (excessive sneezing which rendered this portion inaudible) (even more sneezing, I think someone may be dying) There are two numbers that follow; the first is a designation of the film size. The second is of how many films there are in a film packet. Almost all of the film packets in the college will have a 2 in this column.
The history of nonscreen dental film goes all the way back to right shortly after X rays were discovered in 1895. The first dental film was made on Eastman NC roll film. All they did in those days was take photographic film and wrap it in black paper and rubber dam to prevent moisture from getting at it. The first prepackage film that was commercially available was in 1913 and that was by Kodak. They simply put the film inside a wrapper so that the dentist would not have to do this. There still wasn’t that much difference between photographic film and this. From there on there have been various changes going toward the more modern type of film that were designed to be exposed by x rays because there is a difference in the sensitivities and the emulsions depending on what you are expecting it to be exposed to. There were introductions of pieces of lead to prevent backscatter. And from then on most of the changes were in the speeds of the film. Kodak has this annoying habit of renaming films so that when you go back and referred to a radiotized film the bring the old name back so that you have to write the year….
As far as speeds of film are concerned if you took a unit of one-whatever it took to expose the regular dental film in 1919 to get a specific density, you see that as the various films were introduced there was a reduction in the dose required for the same point. As of six months ago it took 1/48th the dose of 1919. So when we are talking that are current films are faster we are talking about considerable amount of less radiation. You could, in effect make 48 exposures for the patient to receive the same dose as in 1919.
So the films come in prepackage boxes. There are various sizes of boxes with information on how many are contained within the box. The film packets themselves have a front and a back. And on the back it will tell you such things as ‘opposite side towards tube’ meaning make sure you have it oriented the other way. You see that there are various ways in which these things can open. They also color code them so you can see just what speed you are using. On back it will also tell you what type-speed of film you are using and it will tell you if you have 1 or 2 films in the pack because that is important for you to know. You need to know if you have 2 films in a pack because if you don’t know and you put them both in the processor you will ruin them. Almost all films in the college are ‘duplitized’…that is to say there are two films per pack. Also there is a dot which indicates orientation. Kodak puts an embossed dot in the corner. We make a radiograph as if we are standing outside a patient’s mouth looking at the patient and we have the film image the same way. Therefore the dot is convex towards us. In the exercise today you will be asked to describe where the dot is. The dot is always in the same corner on radiographs. Only the number 4 has it in an opposite corner. If you get a copy of a radiograph you should be able to orient the image to its proper location.
Now we have the packets as either plastic film packets or paper film packets. There are advantages and disadvantages to each one of them. The plastic one obviously protects the film much better from saliva. The paper ones tend to be a little more comfortable in the patient’s mouth although if they are both placed properly it shouldn’t be a problem. You will see both types in the building but most are paper. These aren’t plain paper though….they are somewhat resistant to saliva. It a patient had a lot of saliva it is possible that there may be some saliva contamination.
As I said earlier the film packets either come in 1 or 2 film per packet. Outside of the actual film is black paper. It servers two purposes. First it protects the film from light exposure. Two, it allows for the films to be pulled out of the packet. Outside of the inserted black paper is lead foil. It is there to prevent backscatter radiation. Today in exercise you will be proving that backscatter radiation does exist. The paper packets versus the plastic packets tend to open slightly differently. The plastic packets are almost as if you were opening an envelop. The paper keep changing and sometimes are like opening a zipper or picking up the area and tearing off the entire back.
There are dfferent manufactures and they all don’t have packets that open the same as Kodak. This one happens to be Flow. Flow Dental is a component of Wolf. Flow is Wolf spelled backwards…..rather something out of the fifties how they got their name….no seriously. It is similar but the black paper does not go all the way around. There are other sizes as well….this one I brought back from Germany. The far left one is the standard #2. These here are Dupont. This is a more or less #0 film but this is one we don’t have in N America and I never did figure out what the good of this one was. It is bigger than a standard #2 periapical but smaller than a #4 so all I can see is you are wasting a lot of radiation….I kept asking if this was veterinary film but couldn’t seem to get an answer. They open differently as well. Be cautious about packets from abroad because some do not contain the lead foil and can get backscatter radiation and that will degrade the image. Dupont cuts the corner off their film for orientation. It cuts off the opposite corner of where Kodak puts the dot. Dupont doesn’t sell dental film in N. America anymore.
All of those are nonscreen film. That is because you expose the film in the packet directly to X ray radiation. There is another type of film called screen film. Screen film requires a cassette and screens. Those are the films that are used inside the flat.. uhhh boxes. Virtually everyplace else in the college uses this type of film. Only intraoral radiographs use nonscreen. So there will be some form of a cassette that has a front and a back to it. The front is made of a material that is relatively radiolucent. The back will not be radiolucent. It has an opening where you load the cassette. It has a loading of locking mechanism that pushes the film tightly to the screen. When you open them up you have inside of the cassette the film and on either side you have screens that contain phosphorus which gives off light that interacts with the film and not the direct beam. Any screen is basically made up of the same matter. There is a polyester base. It is relatively thin as you can see. 0.007 inches. On one side or on both sides is a subbing layer that holds the next part to the polyester base. You will have in there the emulsion. The emulsion is the active component of the film. There are double emulsion films and single emulsion films that only have it on one side. We don’t use any single emulsion films. The emulsion is a gelatin material that is relatively hard. As you process this it becomes softer. On the outer side of the emulsion is the supercoat that does one or two things depending on what you are using. One is it physically protects the film that is being handled during processing. Second is it contains a dye that washes out during processing but makes the film somewhat less sensitive to various wavelengths of light. This allows various lights as safe light in the darkroom.
Inside the emulsion is silver bromide. Silver bromide crystals have various orientations depending on the speeds of the film. Also sulfide sensitive inclusions that used to occur naturally in the gelatin of cows and Kodak is the largest buyer of bones from domestic animals. This is now specifically added in controlled setting. The emulsion consists of gelatin, silver bromide which is the sensitive material, the silver is what is precipitated.
Black felt is usually included around the opening of the cassette to not allow light to enter the cassette. The screens consist of many layers too. You have the main layer which is the phosphor layer that is going to interact but you have to have that on a base. Generally there will be a reflecting layer. Some do not have reflecting layers. A supercoat also is included because these too can be damaged easily. This protects the phosphor layer underneath. Don’t damage the screens because they can cost you between $200 and up….emphasis and on up. Now, what happens on the screens is this. You have materials that are in there that are struck by X ray radiation. These discrete packets of energy or quanta have considerably more energy than a photon of light. A photon of radiation strikes the screen and the phosphorus of the screen take up that energy they then give it off just as quick as they take it up-instantly. Since a photon of light has less energy than a photon of x radiation, when the x radiation photon gives up it energy it takes multiple photons of light to equal it. The screens thus multiply the exposure. The film doesn’t care what it was struck by. You are increasing the exposure of the film though and reducing the amount of radiation required Screens are a method of reducing the exposure radiation. There is a trade of though and we will see that in a moment.
Some film will be exposed to X ray radiation. The phosphorus glows when it is struck and light goes off in all direction but the light causes less defined image which is a negative trade off of screens. This also shows the importance of a tight contact between the screen and the film. It is a little more complicated than that but since some light goes out, some light goes to the back of the screen and causes even more fuzzy image. If you consider that you don’t have a good screen/film contact then you will have a poor image and less defined image. You need to know defined and definition in order to describe attributes of imaging. Therefore you can have a more distorted image. Always tell Rosemary if you drop a screen because of these previously mentioned reasons
(he then goes into descriptions of mammograms and interpretation about these previous concepts)
There are some manufacturers that use films that get rid of a third problem. We’ve had the fact that you have the light spreading out. Then you have the fact that it is reflecting off the back and spreading out. Well, then you have the problem that some of the light that is produced in the front screen passes through the film and actually not expose the front layer of the emulsion but the back layer of the emulsion. This is called crossover and it is a further spreading out and decrease in the definition of the image. There are some films that have a layer that is called an anticrossover layer. It has dyes in it that during the time the film is being exposed, light cannot cross through and expose emulsion on the other side. That die is later washed out of the image. We don’t use those just because you have to increase the dose to get exposure.
Films have different speeds and this is almost always due to the size of the crystals. A larger crystal will give you a faster film because and given crystal requires a photon to strike it to activate it. The larger the crystals the less defined and more grainy the image. Same thing happens in screens. Screens have different speeds. Initial screens were calcium tungsten screens and these are the standard which everything else is compared. The manufacturer would make screens of different speeds and the intermediate screens were described as ‘par’ screens. Faster screens were called high speed, fast speed, some name that describe the speed. Slow screens are generally called detail screens because they had smaller crystals of phosphor. Some manufacturers would speed up screens by just putting more phosphor in. The problem is the only way you can do this is by making the screen thicker but then some of the phosphors are further away from the film and the light will have a chance to spread out further. Therefore larger crystals or thicker screens will result in faster screens but less defined images. That was until about fifteen years ago until there was a change in the screens that were used and that is going to rare earth elements for the screens. The new screens came out of the lanthanide series. These new screens are referred to rare earth screens from the lanthanide series. This is because they tend to be gallinium oxide. They have a material that actually activates the phosphor. So you have to know what type of screen you always have in there.
The difference between the old phosphorus and the new phosphorus is the conversion efficiency. The rare earth screens have a reduction in the amount of radiation required while at the same time maintaining the image quality. Now then you also have to make sure that you protect your films in the same way that you were buying your films that you were going to be using while going off on your vacation you would not leave them sitting in the back seat of the car in the driveway to make sure they were well heated up because that is going to fog your films. The same thing with x ray film as well. There are various dispenser types. These dispenser types are designed to protect the films inside. They tend to be lead lined or stainless steal in order to protect the film from scatter radiation. Of course it is a good idea not to have your films in the same room as you will be taking the exposures in anyways. Do remember that you have to protect your films from stray radiation, heat and chemicals and kerosene. So don’t store your films in a basement that contains kerosene because it will tend to ruin the films. Chemical fumes do tend to interact with films. Also physical abuse of your films…. how you store them…. you don’t stack things on them. If you use films that come in big boxes you don’t stack them but store them on end. Those of you doing exercise one today will see the effects pressure has on films.
May 30, 2001
Landon Rockwell
There are several factors that are under your control with the dental x-ray machine, not necessarily every dental x-ray machine will have all of these factors adjustable. Those factors affect four things that we talk about with respect to the image especially when we are talking about them on film. As we get to digital imaging further down the road it gets more complex because there are other factors we havent talked about. Remember we are staying with analog at this point.
There are four factors that we talked about with respect to analog is film, density is one of them. Density in very simple terms is how dark is the film, of course that is a non scientific term. When you are talking about density there has to be way of talking about it that someone else at some point distant understands what youre talking about. Density pertains to the ability of the film, the radiograph if you will, to block the transmission of light. Density is a logarithmic measurement, imagine you have a thousand photons of light striking that film 1000 to the log base 10 is 3 and for every 1000 photon of light that struck the film only 10 of them got through that would be 10 to the log base 10 which is 1, so 3 minus 1 is 2 so the density would be 2. Density is the difference between the base 10 log of the intensity of light striking the film and the base 10 log of the intensity of light that actually passes through, in every day terms you are talking about how black or dark something is, that is what density really means.
Contrast is very simply the difference in density between any two adjacent areas. Contrast affects your ability to see things on the image. Detail is defined as the ability to see fine structure in the area under observation. Obviously some of these factors affect other factors. In this particular case you can see one area well and another area not so well so you would say that one area has good detail and the other area has poor detail. The factor most effecting detail in this case is contrast, although there are other factors that effect detail as well. Detail is the ability to see fine structures in the area under examination. Definition is how sharply defined the entities are so if you have poor definition you will have poor detail and poor contrast because they blend into each other and all interweave. Basically definition means that you have relatively long transition between and area of one density and another or a relatively short transition, in other words is it sharply defined or is it not sharply defined. Those are the four we are talking about: density, contrast, detail and definition. They are influenced to a certain extent by a lot of factors that are under your control some of which are under your control at the unit or control panel.
There are a lot of thing at the control panel that are under your control one of which is kVp, this is written lower case k upper case V and lower case p this is the proper way to write this. The capital v is for Volta, which comes from (?). kVp stands for kilovolt peak. Most of the time in the dental building we use the setting 70 kVp. kVp will affect density because one the things kVp will do is will affect the amount of radiation coming out of the machine. So if you start at a given setting and to increase the kVp and you maintain every thing else the same you will get an increase in the exposure rate, A rough guide for 15 kVp that you increase you will double the exposure rate so at the same unit of time and the same Ma you will get twice as much radiation from your machine. That is only a rough guide, but its good enough to use in the clinic if you are making adjustments. So if you went from 60 kVp to 75 kVp you would double the exposure rate. If you went from 75 to 90 kVp you would again double the exposure rate. If you went from 60 to 90 roughly speaking you would quadruple the exposure rate. And since exposure will effect how much of whats on the film is effected and that is what gives you the blackness it increases density. KVp very much effects contrast and that is one of the major things that kVp effects that we try to optimize.
Step wedges are made of pure aluminum and the idea is you make an exposure through the step wedge and if you look at it from one end to the other its thicker or thinner depending on which direction you are going in. The amount of radiation getting through under the thicker end is less than is getting through the thinner end so the film under these various steps are receiving different amounts of radiation therefore on processing you will have different densities. The thin end will get more radiation than the thick end according to this pictorial representation. On our diagnostic machines we generally work in the ranges between 50 and 90 kVp. There are machines that come with fixed kVp, that means you cannot adjust. The majority of the machines that in the building are variable KVp because in an institution like this, its something that we need. If you notice there are a series of rectangles of different grays representing that part of the film was under the step wedge. Where radiation made it through more of the step wedge there is an increase in density on the film. The range that you can discriminate between black and white areas are classified as long scale contrast or short scale contrast. In the long scale contrast you can discriminate a longer range before the film looks all white or all black then a short scale contrast. So long scale and short scale contrast is just how far or not how far you can go out on the film before you loose the ability to distinguish between two adjacent areas. Dr. Ruprecht refers to this contrast as functional contrast. If the contrast difference between 2 adjacent areas is difficult to differentiate then this is referred to as poor functional contrast. You can see there is a trade off going from high to low kVp. At the high kVp you can see more of the steps, which means that you can see more information on the radiograph, but the contrast between the rectangular parts is poor. Where as the lower kVp you cannot see as many of the steps, but the difference you have between the steps is better. We dont make radiographs of step wedges for diagnostic purposes we make radiographs of patient where that type of contrast is scattered randomly over the individual. So what we do is try to take something that is going to give us parts of both ends that is why most of the radiographs are done at 70 kVp. That gives a reasonably long scale, not as long as 90 kVp, but longer than 50 kVp and we get a reasonably good contrast. Its a trade off and that why we use 70kVp.
The next thing under our control is Ma. There are machines where you cannot vary the Ma, they come as a fixed 7,8,10 or 15Ma. These are the 4 setting, but mostly likely it will be at 10 or 15Ma. On our machines we have a switch that allows us to change the Ma between 10 and 15Ma. These are such minor changes that we rarely go to 15Ma. There are a few reasons to change to 15Ma, but we usually stay at 10Ma because its better for our machines because we dont get quite the same blast of energy at the target at the same time. These are small Ma compared to what is used on the medical side. They dont go below 25Ma and can go on to 3000-4000Ma, so we are talking about totally different Ma settings. Ma will affect density, Ma is the current across the tube and so directly effect the number of photons that will be coming out of the tube. If you increase the Ma you will get more exposure meaning you will have a high exposure rate and therefore, if all the other factors stay the same you will expose more and you get an increase in density. Contrast is not affected a lot by Ma providing that you dont markedly over or under expose. We will see this when we get to the H&D curves. Generally speaking if you follow the manufactures directions Ma will not have much of an effect on contrast.
Time is almost the same as Ma, its the rate you are getting the number of photons coming out and obviously if you increase the time at a lower rate you would still get the same number of photons coming out. Time is the setting you can adjust on all the machines. There are various types of dials that you have on the time setting. Some are written in fractions of seconds. The ones we have upstairs you will notice that below one second there are written in fractions of seconds only in the sense that they are written in 1/60 of seconds they are written in impulses. You have to be able to adjust the time. Time also effects contrast and density. Since time and Ma are closely related we often put them down as Ma's. This stands for millamperage and time expressed in seconds. Ma written properly is upper case M and lower case a. we can adjust these 2 relative to each other as long as the product of Ma and time in seconds is the same and everything else remains the same the exposure will remain the same. It is just like rain if you get a lot of rain over a short period of time or a little rain over a long period of time you will still get the same amount.
The fourth thing under your control is distance. You can position the machine closer or farther away and we also have long and short PID that controls the distance for different techniques that are used. The most standard distances used in dentistry are either 8 or 16 inches focal film distance. If you change distance this will effect the area of exposure, so if you increase the distance you would decrease the exposure. If you wanted to keep the same exposure you would have to compensate by some of the other factors. This is just the inverse square law. If you took a point source flashlight a put it on a tile wall and it covered one tile, then you doubled the distance the light would spread out over 4 tiles but each tile would only receive ¼ of the light because the flashlight would not be putting out more light per unit time. We basically have two standardized distances that we use, these are used for different techniques and people refer to these as long and short cone technique. This is a hold over term when cones used to be used instead of PIDS. The technique you will be using is 16 inches because we use the paralleling technique. Distance will affect density but doesnt have much effect on contrast. You will notice that I am not doing much about detail and definition they are only a little effected by these factors, but they are effected by something else and this is the film that used.
Film has an effect on detail, contrast density and definition. Film very much effect density because films come in different speeds. Just like you photographic film where different speeds mean they have different sensativies and require more or less light to get the same density. Same thing happens with x-ray film they require more or less radiation. You have a fast film with the same amount of exposure it will wind up denser, if you have a slow film with the same amount of exposure it will wind up less dense. Contrast is effected by the film. Its a matter of how sensitive the silver bromide crystals in there, therefore how much difference are you going to get in density in neighboring areas by slight differences in the exposure. That is more or less film based with the exception of kVp also effecting contrast. Detail is something you get when you buy the film, that is something the manufacturer makes that has for the most part to do with the size of the crystals that you have. Silver bromide crystals will give you an image on processing as a result of precipitation of silver. Definition the same thing, it is something that the manufacturer determines.
There is usually trade offs between detail and definition with respect to how much radiation is required. In general you can say that something that has good detail and definition then the manufacturer will require more radiation exposure to get the image. You balance of f the need you have for the need for that detail and definition and how much against how much exposure the patient is receiving. Obviously you may have the need for certain information, soyou have to pick the right film to give you that definition and detail other wise there is no point in making the exposure. On the other hand if you don't need it then you dont use that. That comes up more often for us in general radiology with the screen film combination and less often with the film we are using without screens. Once we decided what we are using for the area we dont generally use different speeds on dental film. The way you determine the qualities of film, the density and contrast is by making an exposure with a step wedge, which will give you some image.
You will plot the amount of exposure that each part under the film receives and that will be done as a logarithmic term. Then you will measure the density that corresponds to each of those areas. Density is already a logarithmic function. This is referred to as a H&D curve or Hunter and (?) curve. This gives you the characteristics of a film and you can compare one film to another. What you have is log relative exposure and density curve. You plot the exposure against the density you will see that even when there is no exposure the is a certain amount of density over in the toe of the curve. That is because even if you stripped off everything, so there is no silver bromide precipitated on the film you would still have the base that is the film base itself and emulsion that will give you a certain amount of density. The base will block a certain amount of light. You will always have this plus even though you may have no exposure of silver bromide when you put in the chemicals some of the silver bromide will be converted to silver and bromide and that silver will be precipitated. There will be some chemical interaction between it even if there hasnt been activated by radiation so we call this base plus fog. Base being the film base, which is the emulsion fog, which is the activation of silver bromide through natural occurrences or just effective of chemicals on them. You see that as exposure increases you get a increases in density, This tends to be a gradual increase until you get over to heel of the curve them suddenly it shows up quite strikingly. As exposures increases and then in the shoulder of the curve it rounds off again. Here in effect you havent got sufficient amount of silver precipitate for it to be a noticeable difference in the density until you get to the point where you can really start to see the differences. Here the film is so black that as you add more silver to it will not make it any blacker. Our exposures are always designed to be in this area around two. What I mean by that is we want to make sure that we make our exposure; say you are making a radiographs of the hand the those parts of the film that are behind the areas between the fingers will receive more radiation and will wind up quite black and those behind the fingers will get very little radiation and will wind up fairly clear or white if you will and those behind the soft tissue will be intermediate and all that will be differences in the amount of exposure that that part or the film receives and you want this to translate in to striking differences in densities because that is what our image is based on.
Solorization is a reverse process where you actually start getting less density. This process is used in duplicating films in the clinic. The film has been solorized by the manufacture. Lay the film that we want to make a copy of and the duplicating film on top of it and put a lid on it and the light come from below and light goes through the image on the radiograph and the parts that are clear will receive a lot of light exposure and we dont want this to be black so we want a film that has reversed itself. This process is called solorization.
Films will have different curves. There is a difference in these 2 triangles that I have drawn depending on where they are on the curve. Down here you see that an increase in exposure you get very little increase in density and over here for a little increase in exposure you get a large increase in density. You want to make sure you image is in this part of the slope. Use the manufactures recommendations and use exposures factors they tell you and use exposure factors we have posted and those are designed to put you in that part of the curve. Different slope or gradient further up translates into large differences in blackness or density for small differences in exposure that is what you want to see because this is what will give you your image. There are 3 different speeds of dental film E, D and F and there are about 40 different speeds on the medical side. We have narrow and wide latitude films the difference between the narrow and wide latitude is the narrow is for small differences in exposure you get a striking difference in density where wide latitude film for small differences in exposure you will only get a small difference in density. Which means with a wider latitude you have more range in setting your exposure before you completely under or over expose or get a film that is unusable. Here you only have a small range before you are under or over exposing and the other you have a wider range. That theoretically would make it easier to screw up the film but the results are hard to interpret because you dont get striking differences in densities in various areas. If you have something that has only a subtle difference you may not be able to see it here. You have to decide what you are looking at and what differences you are expecting and what area you want and what the abilities of you technologist are when deciding on what type of film.
This is a chart that shows how kVp, time, distance and film relate to contrast, detail, density and definition. KVp does have an effect on density its major effect is on contrast and does not have much effect on detail and definition. Ma, time and distance have there major effect on density. They dont have a lot of effect on contrast unless you under or over expose the same thing for detail for these two. Distance does have some effect on detail and definition as you will see in a few minutes as we get in to geometric effects that will affect the ability to see fine structures. Film does have an effect on density and a major effect on contrast, detail and definition.
This brings us to geometry. One of the things affected by geometry is definition, how sharply you transition from one area to another. Depending on how close you get you object to the film you will get differences in edge gradients. Take what happens from the 2 extremes, a photon coming from one extreme will define the edge of the target as being here and a photon coming from the other end will define the edge as being here, almost the same place. If you move the object away from the film they project the edge at slightly different places so the net result will be not as sharp an image. You want to try to get the object and the film as close together so you get good definition and reduce the edge gradient. If you are getting edge gradient you want to get the object close to the film, sometimes this is not possible as with the root of a tooth. You cant get the root any closer then the intervening soft tissue. You can over come this problem by using ratio geometry. You can move the source of radiation back and this can also reduce the edge gradient. This is why we generally use 16inch focal distance rather than 8 inches.
Depending on where the source of radiation is and how close the object is to the film we can more or less magnification. Generally speaking you dont wont magnification. You want the radiograph as representative of the structure as possible especially in endodontics. You are using the radiograph to tell you how far the instrument should be going. You will always get a little magnification, but use geometry to minimize it. You will always get 1 to 1.1 percent of magnification. The size of the focal spot will also effect definition.
The focal spot is the area in the tube that produces the radiation. You want the smallest focal spot possible to give you a sharp image however if you have a point source it will generate to much heat and burn a hole through the tube. We have to compromise between a small focal spot for sharp image and a large focal spot to distribute the heat so we use a .3 and .6 up to a 1.5 cm focal spot. Most of the ones in dental machines are .3 or .6. This will give us a reasonable good image. You want to make sure your geometry is correct so you get the least amount of distortion of the main objects under consideration, since we dealing with three dimensional anatomy something is always going to be distorted. If you have something oval and you dont have it lined up in the beam it can wind up being round same thing if something is oblong can come out square. This is mainly important when we are looking at lesions because something that is rounded is more likely to be cystic. Something that is elongated can be neoplasm. We want to minimize the distortion.
Scatter radiation is produced by the interaction of x-rays with matter. One of the main places that cause scatter radiation is the tongue. Some the radiation will go thru the film hit the tongue and come back to the film as scatter radiation. The distribution of radiation on the film should correspond to what has been in the path of the beam. Scatter radiation is randomly put all over, it is like taking a piece of translucent gray plastic and laying it over the image. This has an effect of decreasing contrast. The film packet has lead foil that helps decrease backscatter radiation. Backscatter radiation tends to be called soft radiation and is stopped by the foil. We also have scatter radiation on extra oral radiographs. For example a cranial stat for orthodontic and orthonathic purposes. This also decreases the contrast and makes the radiograph harder to read. To decrease backscatter first, you decrease the size of the area for the volume being radio graphed. Second, you may do is use the grids, now grids come in stationary and mobile types. What a grid consist of is a series of alternating lead strips and spaces. The spaces in between are generally filled with a plastic type substance to allow the transmission of x-rays. The lead strips and spaces are identical in size. The grid is placed between film and the source of radiation the idea is that scatter radiation is going in at an angle and it is not going to pass between the spaces between the grid. Most of the primary radiation is parallel to that so about half of the radiation will pass thru the grid. Grids do two things: it does decrease the scatter radiation of the film so it improves the contrast. It also reduces the amount of primary beam that reaches the film so in order to get the same amount of radiation we need to increase the exposure. The amount you have to increase will depend o the characteristics of the grid. You may have ratios of 6 to 1, 12 to 1, 8 to 1, the greater the ratio the less the scatter radiation and the greater the primary radiation is reduced. Most grids are focused because the primary beam is a spreading beam, you try to take these and have them aligned in such a way they are aligned with the pattern of the primary beam. What that means is that you have to use it at the proper distance. If you are using a focus grid it will tell you at what focal film distance you should be using it. Standard focal film distance is 40 cm or it might be something like 5 ½ feet for cephlameteric purposes, you have to make sure you have it set up so if you move it closer or farther you wont get grid cutoff. You also have to make sure you are coming at the right angle, which is 90 degrees to the center of the grid, and make sure the grid is not backwards. We use grids for our cephlametric images the grid we use is the stationary grid. We use a moving grid for a panoramic. When using the grid you will also get thin lines on the image, unless I point them out you wont be able to see them. With a R-bucky grid, the grid actually moves during the exposure. As the grid is moving across the beam every part of the film will have equal amount of time. This will cause the grid line not to be seen. What happens when you are producing a radiograph is you get activation of silver bromide for x radiation that gives you a latent image on the film. Silver bromide in the emulsion has been activated and contains information, but it is latent so you cant see it. You then put it in developer, remember this is called processing not developing; you get reduction of silver bromide to silver. That gives you the black areas where there has been activation, but it still wouldnt give you a good image. You then have a wash on a manual processing if it is on an automatic processing you dont have a wash because the rollers that transport film thru various chemicals baths actually squeeze out the chemicals. You then have the manifest image, but you cant see it real well because have a lot of green film that hasnt been effected by the developer. You then go into the fixer, what this does it removes silver bromide and it doesnt matter if it has been activated or not, it will remove it and that is what gives you the clean image as it removes the green image. Then wash it. This is an important wash because if you dont wash the film properly at this point you will have residual fixer on it and that will be oxidized while it is sitting there in you archives and as you go back in two years and look at it is no longer a diagnostic image. Then you make sure you dry them properly because all the material on the film is gelatin and is soft, and if it isnt dried properly before you handle it you can scratch that and damage it very easily. Black areas are where you had silver bromide that is activated and that was then precipitated by reduction in the developer. Silver and bromide goes into solution that gives you the black areas, the areas that werent activated and not worked upon by the developer, those areas had the fixer worked on them and striped of the silver bromide giving you the clearer area.
Radiology
June 6, 8:00
Eric Rossow
Today we are discussing standard tomography not computer tomography. Pantomography is the underlying principle of making various images that we do make up in the clinic. Tomo is greek for cut so tomography is making an image of a cut. You take a section(cut) out of a patients body in focus and blur the other areas so you can select a slice within the patients body and see it clearly depicted without the superimposition of other images.
Synonyms for tomography: Laminagraphy-most popular one that might still be heard.
Planigraphy
Stratigraphy- used in geology
Body layer radiography
Sectional radiography
Zonography- Subset of tomography
Basically if you take a radiograph of any part of the body you are going through layers of the body. All of these structures will be superimposed upon each other. Ruprecht arbitrarily demonstrates this with three colored entities on the slides.
If you came at a different angle with the x ray those three entities are projected on different parts of the film than at the original angle, and would be free from each other on this film. However anywhere along the beam you are still going to have images superimposed on it all of the structures closer to the source of radiation or further from it.
Your taking a complex three dimensional entity and making it into a two dimensional which is why you need to know your anatomy.
As you rotate the source of the radiation of your beam and the film(image receptor) around a given point(the red point) the given point is projected onto the same part of the film, but the other parts would be projected onto different parts of the film relative from where they were projected to from other angles. Basically those images at the fulcrum point have been projected onto the same spot on the film so you are building up an image while other areas are blurred. These areas that are not at the exact fulcrum point have their images blurred because they have constantly been projected onto different parts of the film during the exposure so that the only thing that is seen sharply on the film is the layer that you are interested in.
So if you took a phantom like a wedge and you had on its sloped surface a series of wires lying there and you set your fulcrum at the level where a particular wire is and you made an image of it that wire would show up well defined but as you get progressively further from that wire you see that they are blurred. If the fulcrum wire were at a lower level and underneath another wire you would now see the image of the fulcrum wire through the other wire because the other wire is blurred. You can blur osseous structures that are in the way of the one you want to see so you can see the one you want better to see if there are changes in it. To do that it requires a more complex machine than the ones we do lab exercises with. He then describes the machine he had at the U of Saskatchuwan. With it you can set exposure factors.
You can change the speed of the cassette which is connected to the source of radiation so that as you rotate around the fulcrum point the fulcrum point is going to be on the same spot on the film. But if you changed the speed of the cassette instead of having it move as is you disconnect it, damming radiation for the time being, (this is a hypothetical scenario) and you pushed the cassette at constant speed but at a different speed than the area at the fulcrum point is being projected, say at the speed the green dot is being projected, you will end up projecting the green spot on the same area on the film. You are changing the speed of the cassette to correspond to the speed at which another layer is being projected. You then build up (make it the defining image) that image even though you are rotating around a different layer(fulcrum). Just make sure the film moves at the same speed as the beam does at the layer you are interested in. It does not have to be the layer that is the fulcrum point.
The TMJ is a tough structure to image and you cannot make an undistorted image of the TMJ making a straight projection geometry image. He shows some blurred tomograph images, representing different sections, that were made with straight projection Linear tomography. You can see smaller subtle changes with this imagery. When you are using unidirectional imaging you are going to blur structures. You want to make sure when you are doing Linear than you move across the structure. Bones lines run in all directions and when you blur in one direction you are not blurring in another direction and that is why we have multidirectional imaging, which we will talk more about later.
Zonography is not truly a synonym. In tomography the arc that is moving is generally at a minimum of 30 degrees and may go as far as 45. It is a fairly wide arc. The further the arc goes the thinner the slice that is in focus will be and that is called tomography. You may want a thicker slice in focus so you will go with a thinner arc and that is considered zonography because instead of a layer you are getting a zone.
Machines have choices on the degrees you will go.
So linear tomography goes in the same direction whether you are using a large or small arc. You can move the machine or the patient to go across the structure you want but linear is somewhat limited. If you have two lines on structures running at 90 degrees to each other and you image in the direction of one of those lines you will blur that line but not the other. You can blur both if you run 45 degrees to them but the body has many lines and it is not that simple to unwanted images.
So then you want to, throughout the exposure, go across the images in various directions so that you are always blurring some of them. The net result will be that they will all be blurred and only the layer you are interested in will not be blurred.
Instead of moving in a line you are moving in a circle around the fulcrum point in many directions, blurring it effectively. The way you decide how thick or thin the layer is going to be is by adjusting the radius. The larger the radius, the greater the arc and the thinner the section and vise versa.
That is a relatively noncomplex motion.
You can have spiral units where you are moving circularly but you are varying the radius throughout the procedure. You are getting multidirectional movement but because you are varying the radius you are also blurring the edges and top and bottom of the slice. This is a little more complex motion.
You can also have an oval movement which is just a variation on circular.
A more complex motion is hypercycloidal in which you have a series of ovals in different diretions. You are varying not only your arcs but also your directions so it is complex.
All of these are designed to remove the superimposed anatomy that you dont want to see.
We do not use standard tomography much in the health care profession anymore. That has been replaced by CT. But we do use a modified form of tomography and that is Pantomography or panoramic tomography. When you have a structure that is not in a flat layer like the jaw that is in a elipse you have to be able to get it on one flat film so you make some changes.
Pantomography in the initial stages used to have a fixed source of radiation. The film was on a turn table and the patient was on a turntable so you could project through one part of the body you were putting the image of the jaw on a film. Problem with this were that the jaw is not semicircular so you did not get the type of image you would expect and patients get dizzy so that method was not popular.
The next method kept the patient in place and the source of radiation and the receptor moved around the patient but when you did that you got a slice like this instead of like this(?)
So the next principle that was used was disconnecting the motion of the film from a rigid connection with the source of radiation and they moved it at a different speed and got a different plain in focus. We want more than a plain. We want to vary throughout which plain is in focus so we then get a motion of the film such that its speed is changing in such a way that it is corresponding to the jaw that you are interested in. However a problem still exists to where you are not coming through the various layers in the direction that you want even though you may be shifting so you then must move the fulcrum point throughout the motion.
So you started out with standard tomography then you disconnect the film so that you can move it a different speeds to where the fulcrum point is and you vary this speed somewhat. Then you also vary where the fulcrum point is because by doing both you can follow the arch of the jaw around. Various slices can be chosen to look at with this method.
The panorex was one of the first pantomographic imagers made. It did move the fulcrum point as well as the film within the carrier then halfway through the exposure it shifted the patient. Now the fulcrum point was moved and a path was made that corresponded to the arch. You had to move the patient to get the entire thickness of the mandible throughout the exposure. You moved the film through a chamber called a camera and as this chamber moved the film moved within the chamber as it rotates around the patient. It then changes its speed by doing this therefore picking up another layer so by the time you got all the way around the patient the film is now at the other end of the chamber. So if you change the speed of the cassette you are changing which layer it is that is being projected onto the film in order to overcome the technical difficulties you have when you go around the arch. (Train Analogy regarding different speeds with regard to each other). No longer anymore panorexes in the college of dentistry.
The fulcrum point can move throughout the motion to give you an image that corresponds to this arching of the jaw.
Panelipse is another type of imager that had a curved descent that was flexible which was against the principle of what descent should be but it also rotated and inside the mechanism it had one bar that moved this way and then one that would move it at 90 degrees so that as the two of them were rotating through each other you had this type of motion. The net result is that you have this v path for you fulcrum point.
The machine in this building is the OP 100. It does the same thing and the fulcrum has a path distinct to this manufacturer.
He now rips on the advertising of pantographic images. We will not need to now this for a test unless he is ridiculous.
Production of Radiographs --- Processing (continued lecture from the 30th)
You have your film that has had activation of silver bromide to silver either by light or by x rays. When you do this you get a latent image that cant be seen at this point. You then use a developer which reduces activated silver bromide selectively to silver. Bromide goes out into solution. You then wash it to get rid of the developer so you dont carry it over so much into the fixer. You than have a manifest image but you still have all of the green parts that werent developed so the image is still one that you cannot use. The film then winds up in fixer and the fixer than removes any silver bromide it encounters. Now you actually have an image but you than need to put it through a wash to get off the fixer because if fixer does not get off the film will discolor the film after a couple years. Then you want to make sure you dry it so that the emulsion which is gelatin and the silver bromide and the sulfide specs hardens again so that you can actually handle the film. During processing the gelatin that forms part of the emulsion becomes soft and you can damage it vary easily if you dont allow it to dry properly. At that point you have your final radiograph.
At the time of processing the emulsion swells and throughout there are variations of its swelling and shrinkage. The reason it swells is because it is gelatin but we are interested in the swelling because its swelling allows the chemicals to get into the emulsion and get at the silver bromide so that either you reduce it in the developer or remove it in the fixer. In automatic processing the swelling and shrinkage is less variable than when doing manual processing.
Every developing solution is different so it is important to use appropriate chemicals for the film you are processing. Generic developer will contain some type of reducer. Almost all of them will have hydroquinone, some have metanole or pheonodone in there as well. Hydroquinone selectively goes to silver bromide that has been activated. It is slow acting so follow directions for time. Metanole and pheonodone are nonselective and they will take silver bromide that has not been activated and also reduce it to silver. They make sure you have sufficient density in the film that you shifted on an H and E curve out of the tomoheal into the slope where youcan actually see the differences. Some developers have those and some dont. There will then be a restrainer of potassium bromide. It is the bromide part that you want because when you start off with new developer and you have fresh reagents they will react quickly and the bromide makes sure the reactions are not so fast that you over develop. There is also accelerator or activator that does the opposite and removes bromide from the solution so that you dont slow down the reaction so much. They keep the reaction moving at the same range of speed. Ther is also a solvent (water), a preservative so they dont oxidize, and some put in a hardener so the emulsion does not become too soft. That is a generic developer.
Generic fixer contains a fixing agent. There is hypo which is sodium or amodium disulfate. There is also neutralizer which will be sulfuric or acetic acid and the reason you have that is because the developer is basic and you will carry some of that over into the fixer and fixer is acidic in nature and if you carry that over you will change the pH so you need a neutralizer. Solvent( water), preservative, and some have hardener.
June 8,2001
Chad Schwitters
Were making a change now in what we are covering in that up to this point weve dealt with radiation physics, biology, protection, photochemistry. The videotape should be available, perhaps you could remind me, on the paralleling technique which will lead you into the last part of the lab exercises. We are now moving into the start diagnostic phase of radiology, so you could say were changing from radiography into radiology. Anatomy the most important part of diagnostic radiology is knowing the anatomy. It doesnt matter if you are looking at radiographs or in the clinic palpating a patient, if you have no idea what the anatomy is, you have no idea what you are doing. For those you who are starting to get your anatomy, this is your not to subtle reminder that anatomy is where its at. Go back and review it& Anatomy is power.
Were going to be looking at normal radiographic appearances and some terminology that we will be using in the interpretation of radiographs. Notice I used the term interpretation and not diagnosis. Diagnosis is not what we do with radiographs. Interpretation is what you can see from one test. Diagnosis is the responsibility of the clinician caring for the patient. In many cases that will be you, but there is a clear distinction between what you are doing with the radiographs and with what you are doing clinically.
Dia (through) gnosis (knowledge)involves knowledge of various aspects of the patient and your knowledge of the underlying disease processes and the knowledge of all that weve covered in radiology. All of that together will lead you to your final result. Diagnosis is the result of gathering knowledge via history, clinical exam, and many other investigative procedures that may be deemed necessary. So take everything into account, which is much further and much more widespread than what we are doing with radiographs. The other procedures mentioned may include radiology. So you gather all this info together, the knowledge gained from the interpretation of all these tests, and look at all of the results to gain a comprehensive diagnosis. One may not be consistent and you need to decide which are the out-liers. It is possible that a radiology investigation may provide all the information necessary to make a diagnosis, that is to say you could look at a radiograph and say yes that is whatever , and there are certainly disease processes that can be very clearly identified on a radiograph, but that is not making a diagnosis, that is still making an interpretation. It may seem like you are getting all the info from the radiograph, but you still need to correlate that with the clinical findings, before the diagnosis is made. Examplethe anteater and the petrified log.
Radiolucentadjective used to describe a substance that permits the transmission of x-rays (Relative term that describes the object and not the image). This is similar to translucent , which deals with visible light.
Radioopaqueadjective used to describe a substance that does not readily permit the transmission of x-rays (relative terms that describes the object and not the image).
Contrastdifference in the density of two neighboring areas on the radiograph. It is part of the image and a result of the object.
Densitythe blackness or ability to block the transmission of light (of the radiograph, not the object).
Detailvisibility of fine structure of the object under investigation. Can you see fine structure? yes or no. If you can, that yes fine detail. If not, poor detail. Related to definition.
Definitionthe sharpness of the fine structure of the object under investigation. Detail is can you see the fine structure and definition is how sharp is it . Obviously those two are closely relatedif it isnt sharp then you are not likely that you will see the fine structure. These separate terms exist for sake of reference.
Distortiondifference in shape of the image compared to the object. An ovoid object may project circular etc. Technically speaking, we will always have a little distortion. Our radiograph will be slightly enlargedthat is distortion. We have a 2D image of a 3D objectthat is also distortion. We tend to ignore those forms of distortion.
Brackets around dentin & cementum indicate that they are normally indistinguishable due to the thinness of the cementum. Cementum is actually slightly more radiolucenttherefor below dentin on the spectrum.
Brackets around cortical bone cancellous bone, and calculus represent the mass effect. They have the same radiolucency but ordered by masslarge object appears more radiopaque. This is same for soft tissue and body fluids. Cartilage is a soft tissue.
White restorations are all over the spectrum. It is dependent on what molecules/atoms are in the material. Recently, manufacturers are using fillers to make them more radiopaque.
Radiopaque objects (amalgam) appear less dense (white) on the radiograph.
Lamina dura is a thin layer of cortical bone.
Anterior teeth appear as a cross section through the middle of the tooth.
Posterior teeth do too, but multiple cusps and roots make it more complex.
Blunder bustflare of apex, incomplete development, like a funnel.
In the mouth cusps are seen at same height. On radiograph they appear at different heights due to angle at which the image was made.
Molars have abrupt end to pulp chamber due to multiple root canals.
We know the common anatomy of the teeth, but variations are not uncommon.
Alveolar process and socket:
Periapical radiographs to look at the bone and tissue around the teeth.
Alveolar process is not alveolar bone , it is a process of max or mand bone.
Lamina dura ( Lamina dura used to describe lining of max sinuses etc.)hard layer. Extends around apex, but may not be seen on the radiograph.
Irregular shaped roots (dumbbell) make image more complex. You can see multiple edges.
You dont see the facial or lingual surfaces of the lamina dura, due to tangential angle.
Periodontal ligament spaceradiolucent line on the radiograph between the lamina dura and tooth, 1mm and constant width. It may be seen even where lamina dura disappears.
Crest of alveolar process (usually 1mm apical to CEJ)shape will be determined by space between the teeth. Anterior mandibular teeth have relatively narrow space. The crest therefor comes to a point at its height 1mm below CEJ. Anterior maxillary teeth are large, with a larger spacethe crest of the process may or may not begin to form a table . The intermaxillary suture is the radiolucent line between the central incisors. It is W or V shaped and variable. As you move posteriorly, teeth are bulkier and the spaces between the teeth are larger. A table may be seenvery thin in both the max and mand, but may be thicker and easier to see in the mandible. The surface of the table is equidistant (parallel) to and 1mm from a line drawn between the CEJs.
Mandible Radiology
Angel Sheriff
6/11/01
Inf Alv Canal/Vascular Canals
You only see one border of the inf alv canal. The convexity of the canals are above that in almost all the cases. So that you don’t think that it is below that and think that you can undertake a surgical procedure and transect the nerve.
Inf alv canal - sweeps downward and forward and as you move forward you are expected to see a somewhat smaller diameter; more posteriorly - becomes somewhat larger because it has more contents and gives off arteries
** if you only see one line it is almost always going to be the inferior border**
Relative height in comparison to the apices of the teeth varies somewhat, but it will generally be inf or just running across the apical 1/3 of the root - part of variance will be due to position taking radiograph and part to location of the canal
Appearance is one of a looped appearance on the outside, so that is not an abnormal structure
Inf alv canal is a lg vascular canal - have main canal with branches given off that go to surrounding teeth, gingival, and bone in general
Appears as a radioluscent line that has a thin radiopaque border; same thing goes for other vascular canals; radiopaque border may be so thin that you cannot see it. That is important to know these canals are in the area so you don’t confuse them with a fracture line, which looks like a radioluscent line - look for radiopaque lines to know the structure is a vascular canal; bone is an extremely vascular tissue - storehouse of Ca and must rapidly exchange Ca
In posterior mandible canals are less apparent b/c the bone is bigger and wider there
Mental Foramen
One facial aspect of mandible - region of premolars (2nd premolar generally); opens in a superior and posterior direction - sweeps upwards and backward; won’t necessarily see as a well-defined structure b/c we would have to be coming down long axis of canal and that is not how we take our radiographs - we normally come at 90 degrees or somewhat more anteriorly
Appears like a radioluscent area and sometimes can see the inf alv canal coming up to it
**Generally speaking, we should be able to the mental foramen as far ant as the distal aspect of the 1st premolar and as far post as mesial aspect of 1st molar - as far as ht is concerned, it can be seen anywhere from inf of roots of premolars to 1/3 of the way up the roots - knowing where it is located will make you less likely to confuse it with something that is pathological
Alveolar process can resorb and sup aspect of the mandible as reduced to the level of the of the mental foramen - vague pain on chewing b/c of pressure on the nerve
Lingual Foramen
Terminal aspect of inf alv canal - two alv canals come to together here
Midline structure - helps determine where midline is
Radioluscent area with thick radiopaque border around it - come at it at a 90 degree angle and you should see it with a fair amount of regularity
Elongation of canal is due to coming at it at a different angle
Variation from pt to pt and in technique causes variation in the appearance of lingual foramen
Mental Triangle
Located on facial aspect of mandible - variable radiopaque area - variable b/c it is best seen when coming at a steep angle, but that is not the angle we are coming at. Like a pyramid and it looks blended into the surrounding area. Depending on angle, it is seen better or worse - not something that we are looking for, so we don’t make radiographs to pick it up - if you see the inf border of mandible then you know you are coming at a steep angle and the mental foramen should be fairly pronounced
There is more trabeculation seen toward the superior and anterior aspect than the inferior and posterior aspect
Submandibular Salivary Gland Fossa
Posterior part of body of the mandible; below mylohyoid muscle attached to mylohyoid ridge find depression for 2/3 submandibular salivary gland; demarcated by mylohyoid ridge - pronounced mylohyoid ridge = pronounced submand salivary gland fossa, and vice versa
Mylohyoid ridge -- extends from about the 2nd molar to the 2nd premolar; radiopaque structure
Internal oblique ridge - structure on internal aspect of ramus; appears continuous with mylohyoid, relatively difficult to see b/c not very pronounced
External oblique ridge - external aspect of ramus, more or less the ant border of the ramus; more easily seen on most radiographs b/c more pronounced; looks like a curved, elongated, triangular chisel-shaped structure that runs ant and inf; projects relative to teeth at varying heights due to the angle at which you make the radiograph
Topography and surface anatomy of mandible add to appearance you have
Surface is not smooth-it will be going in and out b/c of location of teeth and other anatomical structures such as musc attachments. Appears more radioluscent in thinner areas and you have to make sure not to confuse as pathological; some areas are more radioluscent b/c the bone has come together between the teeth so that you have less structure to go through and it appears radioluscent, so don’t confuse it with something pathologic.
June13th, 2001
Jessica Slater
The recording for the lecture was started a couple minutes into the lecture, therefore the transcription begins mid-sentence.
Ö in the maxilla will be a radiolucent area, either oval or circular, you may or may not be able to see this, there will be radiopaque lines on either side, which is consistent with a vascular canal, in this case is the Nasalpalatine canal. It runs up and opens superiorly in the anterior medial aspect of the nasal fossae, or on either side of the septum as two foramina. Those are the pteraseptal(?) foramina that are in the superior aspect of that canal structure, so itís a Y-shaped canal structure that runs up from the nasalpalatine. You may also see in the same radiograph another radiolucent line that has on either side a radiopaque line. In this case this is not a vascular canal, this is a suture. Each of these represent the cortex of the edge of this or that maxilla, and it will run through and be superimposed on the two halves, nothing to do with each other except that they are in the same area. The incisive foramina are considered to be normal in size if it has a diameter in a horizontal direction up to ten mm. Less than that would be considered normal, beyond that we start to consider the possibility of it being abnormal because there is a cyst that may develop in this structure. A cyst is basically a pathological cavity that is lined by epithelium that contains a clear semi-fluid or semi-solvent. There are a lot of different types of cysts that have different characteristics. This one would be a nasalpalatine canal cyst. Up to ten mm is normal though. The border that you have on this structure may be thick or it may be thin because it is the edge of bone. This is the cortex of bone because you do actually have this thing forming in that area between the two maxilla. Itís more or less oval in this particular case, notice that here there is no border to be seen. There may or may not be a border to be seen, and thatís because what you really have in this case is what is almost like having a straw, that is running up the palate up to the nasal structure. If you think about it, although you might be able to see the opening of the straw if it is tilted somewhat, it doesnít necessarily have a superior and an inferior border. It kind of borders on the two distal halves. The superior part will only be giving you an appearance that is actually sort of a lip that is forming to give you an edge. So it may not always have an anterior opening onto the surface. See the superimposition of the teeth that are there, you would expect to have a normal periodontal ligament space and a normal ?(inaudible) because this is a normal structure and it is unassociated with these. See where your teeth are and if you remember from your anatomy where the incisive foramina are, it is in the palate, it is not even in the same plane as the teeth.
Ruprect covered a couple examples illustrating different size borders, sutures visibility, superimposition, etc.
Generally speaking when you have an edentulous patient the appearance of the incisive foramina will be somewhat larger, and that is probably because there is being bone removal here and here as a result of disuse osteoporosis. Osteoporosis means you have less bone per unit body mass. And it is disuse osteoporosis because the reason for it is that there isnít as much function here because the teeth are gone.
As a rough guide you can use the size of central incisors to estimate the size of the incisive foramina. The average size of a central incisor is 8mm from mesial to the distal. Remember that it is not 8mm that is the upper limit of the incisive foramina, it is 10mm.
In another example he explains that there is a radiolucent area that is superimposed over the image of a tooth but you could see through it the normal periodontal ligament space and the normal lamina dura.
As you move around the arch, the general relationship of structures, since youíre dealing here with a 3-D entity, will change on the image. That is called parallax (?) or chronological rule. You may have heard it referred to as the SLOB rule, same lingual, opposite buccal. As you move in one direction, the relative position on the radiograph of those two, it will appear that the one that is further away from you is moving with you relative to the one that is closer. So the two will be superimposed as you move distally the root that is more to the lingual will move relative to the other one in the same direction, hence ìsame lingual, opposite buccalî. What happens here, the midline is here, were not coming directly to the midline, we have shifted so that we are coming more or less to the lateral incisor, so that we have moved distally, this object, which we know is on the palate, which is more to the lingual, has had its image move with us relative to the image of the central incisors. So instead of being projected from the middle it has moved with us and is therefore superimposed over the apex of this tooth. Then you can see the rest of the anatomy is still there and it tells you that this is nothing pathological. It has shifted with us from the midline, and it helps you to determine that what you are dealing with is something anatomical. You personally have to know what anatomy is there so that you can even consider that what youíre dealing with is something anatomical. When youíre looking at the incisive foramen as you move over to the region of the lateral incisor you should expect it not to be projected at the midline, but over the apex of the central incisor. Now thatís an anatomical structure, its important to know it for what it is because there may be other entities in the area that youíre going to have to differentiate. Rarified osteitis (?) is one of them, it is a term that we use that encompasses abscesses cysts, or granulomas because in the early stages those cannot be separated and we donít separate the granulomas and the abscesses, but we can start to determine when something is a cyst. As a result a pulpal necrosis, as a result of caries and or fracture of the root, this tooth does not have a pulp, and we now have an inflammatory part, thatís rarified osteitis, those do not represent images of the incisive foramina.
We will have a couple lectures next year over incisive canal cysts, he showed a slide where the incisive foramina measured ~20mm and this is well beyond what we consider normal.
Also in
this area we have the anterior nasal spine. Push up under your nose,
itís the hard part that you have there. If youíre
making a radiograph of the area that would be projected off the
superior aspect of the radiograph, not to be confused with an
incisive foramina cyst. This is on the facial side. In some
radiographs you will be able to see the radiolucent line of the
intermaxillary suture.
We may also pick up several components of the nasal structures; crux, cartilage, fossae. May pick up part or all of one or both inferior conchae with the inferior meatus.
When were coming through at a direction like this we are going to pick up part of the nasal septum, remember the component parts of the nasal septum are the septal cartilage, the perpendicular plate of the ethmoid bone, and the vomer. Cartilage is a radiolucent tissue but you have to remember that when you are coming through at this direction, first of all you are going to be coming throught a large extent of cartilage. So even though it is a relatively radiolucent tissue it is not completely radiolucent and what you have on either side of it is air, and therefore by contrast this will appear to be radiopaque as a result of contrast and as a result of the bulk that youíre going through at the long axis. On the lateral aspects of the nasal fossae you have turbinate boneÖ (mumbling)
In the midline region, you have the central incisors and the lateral incisors, and you see here a line that is more radiopaque, the soft tissue of the nose. Up here weíre picking up the lower aspects of the nasal fossae on each side, and there is the septum in the middle. The nasal structures, the way to think of the septum and what youíve got in here in two dimensions, think about it like an anchor. The shaft of the anchor is the septum and the arm of the anchor being the floor of the nose. Here you have a radiolucent area, the anchor is below this, this is the inferior conchae which makes this the inferior meatus.
The osseous, soft tissue, and cartilaginous parts of the septum, (the shaft of the anchor) were illustrated on slides. Then on the floor of the nose (the arms of the anchor) can see both the soft tissue components and the osseous component of the inferior conchae. The osseous component being part of the turbinate bone covered by mucosa.
As we move around the maxilla, there are depressions. The one that is in the midline area, that is to say between the midline and the canine, is the incisive fossae. That is the one that is the one that is generally going to be seen in more pronounced fossae. Distal to the canine we have the canine fossae, that is a somewhat less pronounced fossae. It is a matter of topography and knowing that you may have what appears to be an area of increased radiolucency in this region because of these. Again, remember your anatomy when interpreting radiographs, all it is, is a depression on the facial aspect of the skull. So depending on how much of a depression there is youíre going to see more or less radiolucency.
Then also, we sometimes start picking up a radiopaque line across the top that is the floor of the nose. Besides over the midline area that being the arms of the anchor, as we move more posteriorly it starts to become a more horizontal line across the top part of your radiograph. The floor of the nose actually represents the jxn of the floor of the nose and the lateral wall of the nose. This jxn shows up as a line and as we start to pick it up we start to see the floor of the nose and we also start to see the anterior border of the maxillary sinus as it starts to become horizontal. And the superimposition of that line that we call the floor of the nose and the anterior border of the maxillary sinus is referred to as the Y-line of Ennis. It is a landmark that we use for the canine region, except for here at Iowa where the line almost always ends up being in the region of the 1st premolar.
This is the border of the maxillary sinus, and you can see that it not as distinct a line, it tends to appear as an undulating border, wavy, and almost a discontinuous border. The maxillary sinus is also referred to as the maxillary antrum. The maxillary sinus starts out as an out-pouching that comes from the middle meatus, between the inferior and middle conchae, and this starts to enlarge and just digs out if you will, or excavates the body of the maxilla as the individual gets older. The enlargement is often not one smooth border, but it looks as if it is a series of joinings. Septa may be obvious on our radiographs from this sinus too.
The floor of the nose (radiopaque lines), the Y-line of Ennis (the anterior border of the maxillary sinus) and maxillary sinus are all illustrated on the same radiograph by Ruprect. Roots of the teeth are seen through the image of the maxillary sinus, not within.
Radiology
Dr. Axel R.
Holly Vanhofwegen
Pantomography
The first and primary point to be made about pantomography is that there will be distortion in every radiograph because it portrays in 2 dimensions an elliptical plane. In addition to that, the ray is angled upwards 5 to 7 degrees.
Anatomy:
Anterior part: The plane will be thinner than in the posterior part, thus it will be portrayed differently. There are blurry apices of the teeth, generally in the anterior mandible; this is because of the narrower plane portrayed in the anterior region. The nasal fossa will be seen, but not well depicted because the anterior part is caught.
Nasal septum: Seen as a midline structure usually, and will not be in focus lateral to that. Inferior concha: Radiopaque; soft tissue surrounding also seen; surrounded and filled by air space, providing contrast.
Soft tissue of nose: Variable, dependant on patient positioning.
Maxillary sinus: Occupies bulk of maxilla. Floor of the orbit is LOWER than the roof of the maxillary sinus. The image can be superimposed on the lower part of the orbit. The superior aspect of the maxillary sinus is usually arching over the inferior rim of orbit.
Zygomatic process of maxilla: Thick radiopaque line because it is thick dense bone. Behind it is the zygomatic arch, which is not as radiopaque because it is thinner. These bones are superimposed over other structures as well.
Zygomaticotemporal suture: Between zygomatic bone and temporal bone. It is L-shaped, and not seen always. Do not mistake it for a fracture.
Orbital ring: Seen in a posterioanterior direction. Usually seen in a young person because the skull is smaller. It is a thick osseous structure, and seldom seen in adults.
Infraorbital canal: Radiolucent line with two radiopaque lines, indicative of a neurovascular canal. It is at a 45 degree angle going superiorly, with the posterior part projected upwards. Don’t confuse with a fracture.
Pterygopalatine fissure: In the pterygopalatine fossa. It is an inverted tear shaped structure at the posterior aspect of the maxilla; also the anterior border of the pterygoid process of the sphenoid bone. Ortho uses this for a routine landmark.
Pterygomaxillary fissure: Moderately well defined structure.
Pterygoid process of the sphenoid bone: Blurry; vague added radiopacity. Well out of focus.
External auditory meatus: Seen on 90% of the images as a radiolucent circle.
Soft tissue of the ear may be captured as well.
Posterior to ramus of mandible: Air space of nasopharynx and oropharynx. Ear lobe is seen because of its contrast to the surrounding air.
Styloid process of the temporal bone: May appear as a radiopaque linear structure, and a calcified stylohyoid ligament may appear from it.
Hyoid bone: Relatively small structure; radiopaque. Just inside the plane of focus, it is a horseshoe shaped structure that is flattened out. Especially with this structure, ghost images will be projected onto the contralateral side. This structure is discontinuous because it passes out of the plane of focus (too posterior).
Vertebral Column: Can be seen in 3 places in a pantomograph. Vague radiopacity because it is out of focus, but on the edge of the image it may be seen. Also, this depends on the size of the patient and the angle of the ray.
Cornoid process: The process off the mandible diverges laterally, occasionally just out of the plane of focus.
Condyle: What appears to be the superior surface of the condyle on the image is generally the superior surface of the medial aspect of the condyle. Remember that the angle of the ray projects upward, placing the most superior surface of the condyle somewhere inferior to the false zenith in the image.
Glenoid fossa: 95% not depicted. Surface depressions on the ramus are normal anatomic variations.
Inferior alveolar canal: Sweeps downward and forward. Gets progressively smaller.
Floor of the nose: The floor of the nose and the lateral wall of the nose comes together to form three lines on your radiograph.
Soft palate: Fairly thick mass; may have a pronounced line that shows the oropharynx.
Tonsillar Hyperplasia: Seen on radiographs, but not used for diagnosis.
Mental triangle: Not as well defined; vague, radiopaque area. All objects are reflected on the other side as blurry images.
Positioning patients: Want bulk of arches in focal point. ~1.3x magnification causes distortion.
Motion: Appearance of discontinuity of bone (fracture).
Rotated head: Not uniform magnification so one side looks larger than the other.
Too far forward: Images of ant teeth are narrower
Too far back: Teeth out of focus, but portrayed as wider.
Chin too downward: Very pronounced pointy chin.
Chin too far up: Square appearance of jaw.
Oral Radiology Notes
Danielle Wingrove
CT and CT Anatomy
We do need to understand what is on the CT images because we will run across them in some of Dr. Ruprecht’s lectures as well as some other lectures.
How are CT images made? Computed Tomography (a dated term). MRI is a form of tomography. The images are still made using x-rays. You do still have an x-ray beam but the type of beam is changing. Now we are going to the cone beam. You do have a receptor but instead of a film receptor you now have a photoreceptor, that picks this up as digital images. This beam of x-rays passes around the patient, it is a different direction than standard tomography. You do still get a slice of the object you are looking at but how the slice is made is different than standard tomography. This machine has a table which the patient is on , and you have a gantry which includes both the receptor and the source of radiation. The gantry is capable of tilting and this allows one to position the patient differently and therefore get a different image that some more conventional types where the gantry does not move.
Gantry: Conventional CT- you have the receptor and source moving around but you can’t see it and you have the patient moving through the gantry. Usually it goes around 270 degrees and then it goes backwards. Does not continuously go around. Here you have to put patient in then move the gantry once around and then move that patient and have it go once around so you get a series of discontinuous slices.
Spiral or helical CT- keeps going around 360 degrees and does not go backwards. Put the patient in the machine and get a continuous stream as the patient is fed through the machine.
Patient is made up of pixels, like that of a monitor. Each pixel or area of a patient has a attenuation coefficient which is related to radiopacity, or how much radiation is attenuated by that area of the patient.
The patient is made of a series of these pixels and now you have radiation passing through this area you get a total attenuation for that area or for the path of photons. You get the total attenuation by adding everything that is being passed through by the beam. In CT you pass through many directions and get many attenuations for each directions. The computer will the construct a gray scale for each of the attenuation areas of the patient. Water is at 0 where as compact bone is at the top of the list and air/gas is the bottom of the attenuation list. Have a range from 1000 to -1000. You can separate out certain soft tissues that standard radiography cannot.
The CT film- these are print outs from a laser printer not a film printout. These printouts are from a data set collected from the receptor and interpreted by the computer to produce the image.
Axial Slices- Making slices that are 90 degrees to the long axis of the body. Somewhere on the film you will get what is a scout image (sorry I didn’t see what he was pointed out because I was too busy taking notes) Like a lateral ceph, here is where you decide where the other slices are going to be. The other information around the image is different depending on which CT unit you are using but the one he showed in class included the scan #, or slice #, the date, the MAS, the kV, the slice thickness, the gantry tilt, the table position, a ruler to measure objects in the film, and some say some sort of direction so you know which is front, back, left or right. Also some images give the age , sex of the patient as well as if the image was made with contrast or not, this is when a dye is injected to determine where blood vessels are. In the bottom right there is a WL or WC which is windowing and labeling or windowing and centering. With labeling you can take the attenuation coeficients and spread it out over a gray scale. The WL tells how large the range is of the gray scale and it will give different images with different values. A lower WL gives more contrast in soft tissues but bone looks chunky because you put pixels together to bring the W down to a lower number. This is volume averaging.
The Centering is when you have a scale and determine where to center the image on that scale. This can change the density of the image.
Now he just went through some images of axial slices and what you can see when moving through slices.
Coronal CT- Making images parallel to the long axis of the body. This is mainly used for looking at the paranasal sinuses. He showed an image with no eyes or appeared to have no eyes but this is because a person’s face is not flat. The eyes are more deeply inset than other features on the face so as we move post. we will see the eyes in different slices. With the coronal CT you can see some differences in the soft tissues but we don’t do much with soft tissues in coronal CT. Here we begin to see streak artifacts. We see these mainly in the mouth where materials such as amalgam will completely attenuate the beam and leave a blank image where those beams would be. These look like streaks coming off the teeth.
Sagittal CT- Don’t use very often at all. If we do it is for the TMJ.
2D reconstructed CT- Helical are reconstructed. We tell the computer to stack the data then make slices through this data in a different direction. You can see some chunks where the data has been stacked, just know this for what it is and not a fracture or some other abnormality. You do this mainly when you can’t get the patient in a position to get the slice you want or if you happen to see somethng on a previus sereis that you want another slice to look at it in a different way.
Isodent slice- pick an area and show all areas that have the same attenuation coefficients.
2D reconstructed panoramic and orthognathic- Take axial slice, say the mandible, and trace a line in the middle of the arch. Get almost a pantamograph image. The computer then generated orthoradial cross section through the jaw- this can determine the size of the bone, where the contacts are, what the length of the tooth is and how close one may come to the canal perhaps when placing an implant. One can then determine the length and width of the implant needed for each individual so you can avoid important structures.
3D reformatted CT- stack a series of slices and tell computer to remove all entities that have a specific attenuation coefficient. Example is to remove all the soft tissue of the face so we can look at just the skull- this can help oral surgeons in determining where fractures are or any other structures they may need to see.
Rotation around the axises- with the computer we can rotate the image along the x,y or z axis to see any portion of the image. This too can aid oral surgeons in some complex cases of fractures.
CT surface and volume rendering- here you look at just the osseous structures and then can add the soft tissues. An example was a child with hydrocephaly and we can see the skull structure and then see how the soft tissue can fit over that osseous structure. One can sometimes even do virtual surgery to see what the results will be before the surgery is even performed.
Disarticulated parts- we can remove any one entity and look at that individually. An example is to take just the mandible and look at that by itself. We can the rotate this entity and see all angles of the one entity that we are interested in.
6/25/2001
Oral Radiology
MRI
MRI or magnetic resonance imaging is more complicated than CT because of the multiple different ways of imaging companies’ use interpretation software. The basic principal is that instead of using x-rays you are using high magnetic fields and radio frequency waves. In the body within all the elements you have protons, we are interested in hydrogen mainly. In the body the hydrogen protons will be aligned with their long axis of spin in any direction (random). When you put the body in a high magnetic field, between .5 and 1.5 tesla (10k times earths magnetic field), the long axis of the spin, that is to say the axis of rotation, of these protons will align with or against the magnetic field. As was stated when these protons spin on their long axis they wobble like a top. This wobble has a frequency, referred to as the Larmor frequency, which is in the radio-frequency range. This frequency depends on the strength of the field that is applied to the body. In the MRI machine the field is not uniform throughout, there is a gradient, and so depending on the protons position it may have a slightly different frequency.
If you then pulse that proton with a radio frequency that corresponds to its Larmor frequency one can get the proton to flip onto its side. With the axis of rotation shifted 90degrees, as the proton spins it sends out a magnetic pulse that is picked up by a surface coil. The surface coil is like that found on the secondary side of a transformer. As the wave crosses the coil it delivers a voltage, the strength of which is used as a determinant for the number of protons in that area. After a certain amount of time the protons will go out of phase and realign themselves with the magnetic field.
The computer sets-up the body in a series of X, Y, and Z gradients so that it is broken into discrete parcels of information. These parcels are called volume elements or voxels, from which you will be getting specific signals, so that the computer can determine where it got that signal from and the strength of the signal. Because of this method the computer can reconstruct the image from any of the three axis, by assigning a certain gray level to corresponding signal strength.
Imaging
On an image, which is usually black and white there will be a TR and a TE. TR stands for repeat time while TE stands for echo time, because you don’t perform these just once. You do it multiple times to build up a sufficient image, because it allows you to get a higher signal to noise ratio. These factors also give an indication of what type of image you are looking at. We are going to focus on T1 and T2 weighted images along with Proton Density.
T2 is the time constant that describes the rate of loss of transverse magnetization. This loss occurs because the magnetic moments of adjacent atoms interfere with each other causing the nuclei to diphase. A T2 weighted image is acquired using a long repetition time between radio pulses and a long signal recovery time. A tissue with a long T2 produces a high-intensity signal and is bright in the image. T2 wieghted images are commonly called “water images” because water has the longest relaxation time and therefore is the brightest in the image.
T1 is the time constant that describes the rate at which net magnetization returns to equilibrium by the transfer of energy. A T1 wieghted image is produced by a short repetition time between radio pulses and a short signal recovery time. Because T1 is an exponential growth time constant, a tissue with a short T1 produces the brightest image. T1 weighted images are called “fat images”, because fat has the shortest T1 relaxation time and the highest signal relative to other tissues. High anatomic detail is possible in this type of image because of the high contrast.
Proton Density is the measure of loosely bound hydrogen nuclei. The higher the concentration of these nuclei, the stronger the net magnetization at equilibrium, the more intense the recovered signal, and the lighter the MR image. Tightly bound bone produces a poor unusable image because the hydrogen nuclei are not free to align themselves to the field. Soft tissue and water hydrogen are more freely organized.