03/04 Image Formation

Transcribed by Anam Khalid July 3, 2014

[Radiology] [2] – [Image Formation] by [Dr. Alan Friedman]

[7] – [Ionization][Dr. Friedman] – Okay, good morning. Worst thing about a … nothing worse than a two-hour lecture except a two-hour lecture at 8 o’clock in the morning. Anyway, what we’re going to be covering today … last time we covered the production of the x-rays in the x-ray tube and some of the devices inside the x-ray tube. What I’d like to do today is review some of the major topics that we covered last time because I know we went very quickly and I need to clear up some information. I think your class president found an error in one of the slides so I’ll point that out to you. Only one error out of quite a few slides is pretty good. We were talking … and then after we review some of the information about production of x-rays, we’re actually going to go ahead and see how the image is produced, how we can get the best possible image for the best diagnostic purposes. Remember, as dentist, we’re imaging head and neck areas and especially for beginning students, general dentists, we’re imaging teeth. So, whenever we say the object ... the object is the tooth. So we’ll cover that. We spoke about ionization. Ionization is the removal of an electron off a neutral atom. Certain energy, certain types of electromagnetic energies can ionize tissues because they have enough energy. They have enough energy because they’re very short wavelengths and the short wavelengths have a high amount of energy. And that’s why radiation, x-radiation is harmful due to its ability to ionize tissues.

[8] – [Properties of Radiation]Properties of radiation, very quickly, they have no mass. Commonalities, speed of travel, 186 miles per second and no weight. They have no mass. The differences: what’s the difference between light and x-rays? The wavelength. X-rays have short wavelengths. You have short wavelengths, you have penetrating ability. Because of the shortness of the wavelength and the frequency, they have a short wavelength but high frequency. So the penetrating ability has to do with the wavelength and the frequency.

[10] – [Wavelengths]And these are just an example of short … by the way, all of the … most of these slides, most of these diagrams are in the textbook that I pointed out last time. I believe they’re in VitalBook. Dr. Frommer’s textbook is in VitalBook, very easy reading. There may be 35 pages to the chapter but out of the 35 pages there are about 27 diagrams. So, it’s really not that bad. You just read over the information. Any of the information that you’re having difficulty with.

[13] – [Properties of Radiation]And of course, these are the properties, we spoke about them. They travel at the speed of light.

[12] – [Electromagnetic Spectrum]And x-rays belong to a grouping of radiations called electromagnetic radiation.

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And again, a spectrum is a grouping from the long wavelengths on this side and short wavelengths on that side and x-rays form this category over here.

[16] – [The Dental X-ray Tube and It’s Components]Okay … again all of this information we covered. I just want to pick up on some of the information. Just to review, we went over some of this material very quickly and I threw a lot of terms out at you. The different components of the x-ray machine … of the x-ray tube. Remember the x-ray tube is a little glass tube that’s evacuated. It has … it has different circuits in it. It has a high-tension circuit which is at the anode. It has a low-tension circuit, which is at the cathode. The anode contains the sleeve or stem and the tungsten target. The tungsten target. Tungsten is chosen as a target material because it has a high atomic number. So what happens is, as the electrons bombard it, it can go into the atom and it bends and veers off course and that’s Bremsstrahlung radiation. It can also knock off one of the inner shell electrons, we’ll see that in a moment, and that’s characteristic radiation.

[17] – [The Dental X-ray Tube and Its Components]And the port is the opening. There’s an aluminum filter. Very important. What is the aluminum filter do? What is the purpose of the aluminum filter? We know that the x-rays coming off the anode, off the target, have different wavelengths. Why do they have different wavelengths? It has to do with a multiple Bremsstrahlung. Each time an electron, a high-speed electron goes into another atom, it veers off course, there’s less energy. So the initial Bremsstrahlung has short wavelengths. Those are the ones that we want to use. The aluminum filter removes the long wavelengths so that there’s less radiation to the patient. What’s another reason we have a heterogeneous type of radiation coming out of there? It has to do with the sin wave of the alternating current. Remember the alternating current goes up. We’ll see that in a moment. It goes up to 90 kVp. Well at 90 kVp, when it reaches that peak, we have very very, very very short wavelengths, high-energy radiations. As it’s going up the peak, lower and lower wavelengths. We need to remove those. So by filtering out … the term filtering is removing the long wavelengths. What is the ultimate effect of the aluminum filter? Protection for the patient. Everything is protection for the patient. We can get the least amount of radiation with the best possible film. Inherent filtration means what’s built into the machine. The glass in the tube removes some of the wavelengths as it leaves the porte. And also the oil surrounding the x-ray tube. The oil is there to dissipate some of the heat. Remember heat is very important in this process. 99% of the energy of the high speed, kinetic energy of the high speed electron is converted to heat. Only 1% is useful radiation. Also, heat is important in which part of the x-ray machine? In the x-ray tube? The cathode. Because when you turn your machine on, what are you doing? You are heating the tungsten filament. By heating the tungsten filament you’re boiling off electrons, thermionic emission effect. Those electrons are in the form of an electron cloud. We’ll see that in a moment. Very quickly.

[18] – [The Dental X-ray Tube]

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That’s the x-ray tube …

[19] – [X-ray Production]… and these are the terms we spoke about. Thermionic emission effect. What part of the x-ray tube does that occur at? It occurs at the cathode, the little tungsten filament. The amount of voltage is about 2 to 5 volts. How do we get 2 to 5 volts? Step-down transformer. Okay? Electron cloud is the electrons after they boil off. They’re not going anywhere because there’s no positive charge. Right now you’re just heating the filament. The way you get the positive charge is you walk out of the room press the exposure switch. When you press the exposure switch, that causes the anode to become positive. The electrons will shoot across at high speed. Now, what controls the speed of the electrons? Only one thing: kVp. The higher the kVp, the faster the electrons shoot across. What controls the amount of radiation or the quantity of radiation? Miliamperage at the cathode. Okay? So, when we increase the miliamperage, there are more electrons available to shoot across. Will those electrons be more penetrating? Not necessarily. It has to do with the kVp. Now, if we increase the miliamps, then we need less exposure time. That’s the miliamps seconds, which I’ll show you. There was an error on that calculation there. So, the quality of the x-ray, the quality has to do with the kVp. Quality is the penetrating power. The higher the kVp, the faster the electrons shoot across. The more energy, the more kinetic energy they contain, the Bremsstrahlung reaction will give off radiations of shorter wavelengths. The cathode ray, the cathode ray is the stream of electrons shooting across. When does that occur? When you make your exposure switch. The kilovoltage has to do, again, with the speed of the electrons coming across.

[20] – [The Dental X-ray Tube]Here is the x-ray tube broken down, again, into the cathode and the anode. We also have the aluminum filter here filtering the x-rays and we also have the diaphragm, which is a device which collimates. Collimation is important. Why? What does collimation do? If you say protect the patient, that’s the answer. Everything is to protect the patient. How? By making it a smaller beam as possible. We just want the little x-ray film in the patient’s mouth to get hit by radiation. And, so, we collimate beam. We can also collimate the beam with the position-indicating device, which is lead-lined. And there’s a circular one and there’s a rectangular one. We use rectangular collimating devices. Why do we use rectangular collimating devices? Protect the patient. How do we protect the patient? There’s about 55-65% reduction in the patient dose because of the rectangular collimator. Why? It’s smaller. Less radiation, less surface tissue is exposed.

[21] – [Rotating Anode]Okay? And, why don’t we turn the miliamps up to a high number? Then we’ll have a hundredth of a second of exposure time. Because it’s miliamps seconds. What is the limiting factor? Heat. Because if that little target area is so tiny and it’s bombarded with so many electrons, it’ll just boil. It’ll just heat up and get destroyed. And so,

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what we have in some x-ray machines, panoramic units, and extraoral units, catscan machines, we have the same setup. It’s all thanks to the guy Coolidge. He’s the guy that came up with the hot filament circuit. This is what we have today, a hot filament circuit. And, so, what happens is the anode is located here but there are several anodes and it spins around as the x-rays come out. And so in those machines we can crank up the miliamperage to a high number. What is the result of that? What is the advantage? Less exposure time, less chance of the patient moving, especially when we’re taking head films, or skull films. That’s the advantage of that.

[22] – [Activated Filament Circuit]So again, you walk into your office, you turn on your x-ray machine. What are you doing? You’re not producing radiation. What you’re doing is, you’re simply heating a little tungsten filament, which is embedded in a focusing cup called molybdenum. It’s hard to say. Molybdenum focusing cup. And what happens when you do make your exposure, the molybdenum cup becomes negative. It repels the electrons. So there are two things that are happening: the positive charge at the anode attracts the electrons and the molybdenum focusing cup focuses the stream of electrons on the smallest possible area. And we’ll see in today’s lecture why that’s important for image. The smaller the focal area is, the focal spot, the sharper your x-rays will be, the better detail they’ll have.

[23] – [Activated High Tension Circuit]And then when you make your exposure, that causes the electrons … we now have a cathode ray, which is a stream of electrons shooting across the x-ray tube at very high speed … again, the speed is controlled by the kVp … striking the tungsten target. And there are millions of atoms of tungsten.

[24] – [X-ray Production]When the high-speed electrons strike the tungsten target, then radiation is produced.

[26] – [Bremsstrahlung and Characteristic X-rays]How is radiation produced? Well, Bremsstrahlung is one. The high-speed electron at A enters into the atom and it is attracted to the nucleus. As it gets closer to the nucleus, the rotating electrons kick it out of there. When it kicks it out of there, it loses speed and it veers off course. Bremsstrahlung means braking or slowing down of the radiation. There has to be a transference of energy from the kinetic energy to the Bremsstrahlung radiation. Now that, in effect, A’ goes into another atom and then we have the same thing happening over and over again. Hence, different wavelengths of radiation. Hence, the need … we get a heterogeneous grouping of x-rays coming out of the machine. We’ve got to get rid of the long wavelengths. The short wavelengths are what we need. Another way that radiation is produced called characteristic radiation. In this case we have another electron at B coming into the atom and enters into the atom and instead of being veered off, it actually knocks the inner shell electron out. How much energy is needed to do that? 69,000 volts. That’s why we run … kilovolts. 69,000 volts or 69 kilovolts. That’s why we run our

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machines 70 to 90 kilovolts in order to produce some of this characteristic radiation. When the electron is knocked out, the k shell … only the k shell electron knocks out another electron you won’t produce characteristic radiation … knocks out a k shell electron, the other electrons in the atom cascade into the inner shells to fill those shells. And in the process they give off energy. The energy from going from a higher energy level to a lower energy level is in the form of characteristic radiation right here. Okay? So that’s the production of the x-rays.

[27] – [The Dental X-ray Machine and X-ray Production]Now we spoke about current alternating current, direct current, self-rectification, our machines in most of the clinics are current alternating machines. So what does that mean to you? You’re the dentist; you don’t need to know much about the electrical circuitry in here. But what that means to you is that you’re not getting a continuous stream of radiation when you press your exposure switch. You’re getting little pulses of radiation. In one second, there are 60 pulses of radiation. Our machines are calibrated, the exposure time, in impulses. So if you see in the clinic 30 impulses and you need to know how long an exposure time that is, you divide by 60. So 30 impulses is a half a second of radiation. Again, we use different impulses or exposure time because we can’t control the kVp and we can’t control the miliamps. That’s preset for us so it makes it easier. So if you’re going to do a radiograph of an anterior tooth you need less impulses, less exposure time, than a posterior tooth because the posterior tooth is thicker and we’re going to talk about density of the film in a little moment, and the contrast of the film. And rectification is just a conversion of alternating current to direct current. So our machines are self-rectified or half-wave rectified. Because half of the time they will be converted to direct current. So, again, a lot of these terms … consider how they affect you clinically and what needs to be done. Clinically, the only thing you need to know is that impulses … because of the alternating current.

[28] – [(sin wave slide)]And this is what it looks like. So you’re producing radiation in every cycle there are 1/60th of a second and 60 of these cycles in a second. And so at this point we’re producing radiation and then when we have the conversion and then we have no radiation here … but look at the sin wave. Up at the top of the sin wave, 90 kVP, or if you’re using 70 that top would be 70. The kilovolt potential would be 70. Right at that top point is when we’ll have the most energetic radiation because the speed of the electrons at that point will be higher because of the difference in potential there will be a higher potential energy difference. That’s another reason why we have to use filtration because we do have many many different wavelengths of radiation.

[29] – [Components and Functions of the Dental X-ray Machine]And again, some of the components in the machine. The step-down transformer. Where do we have the step-down transformer? In the cathode. Why? 2 to 5 volts. Converting 110 volts to 2-5 volts to boil off the electrons. So the step-down transformer is in the … controls the miliamperage, in order to get the boiling off of the electrons. The step-up transformer is in the exposure … when you make your

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exposure switch, the high-voltage or high-tension circuit. What do we need to do? We need to get a potential difference in order to produce high-speed electrons of about 70 kilovolts, 70,000 volts. So, we take 110 volts and we ramp it up to 70.000 volts using a step-up transformer.

[31] – [Other Parts of the Control Panel]Okay? Electronic timer.

[32] – [Dental Control Panel]This is the machine upstairs.

[33] – [The X-ray Beam]Just some terminology. Remember primary radiation is the radiation that hits the patient. Before hitting the patient it’s called primary radiation. So think about protection, radiation protection. The patient needs to be protected. Primary radiation is the main concern for the patient. We need the least amount of primary radiation. As operators, we walk out of the room. Are we concerned with primary radiation? No. What we’re concerned with is, radiation hits the patient and … scatter radiation or secondary radiation is what we have to protect ourselves from. How do we do that? Walk away a certain distance, stand behind a barrier and you get zero amount of radiation. Remember, if you’re behind the barrier, x-rays travel in a straight line. So the x-rays are not going to come out of the room and make a curve and zap you. That doesn’t happen. It’s in the cartoons. Doesn’t happen in real life. So, that’s the difference between primary and secondary radiation. The central ray, in a lot of textbooks will say, “Aim your central ray at the tip of the canine” when they talk about taking radiographs on patients. You’ll see that in a while.

[34] – [The Divergent X-ray Beam]Okay? Remember the x-rays, just like light, diverge. Divergence of the x-ray beam is not good. We don’t want the x-ray beam to diverge for several reasons. Again, protection of the patient. What happens if the x-ray beam diverges? More of the patient’s head is hit by radiation. So we have to collimate that beam and we have to keep … the way to collimate the beam is to use the longest position indicating device that you possible can have. So one of my students said, “well 16 inches is the longest I’ve seen, why don’t we use the 32 inch or a collimator that’s as long as this room?” Because studies were done that after 16 inches, there isn’t any depreciable reduction in the divergence of the beam. The other reason we’ll see in our lecture today, if we get to it, we will, is that divergence of the beam will cause magnification of the image. So, if you are aiming at something and you’re aiming at a tooth that’s 25 mm long, if those x-ray beams are diverging, that tooth on the imaging device, on the film, will not be 25 mm long. It’ll be magnified. Magnification of the image is not good for several reasons. It distorts the cavity or the bone levels that you’re looking at and also it produces a shadow around the tooth called a penumbra, which we’ll talk about in a little while. Okay?

[35] – [Secondary Radiation/Scatter Radiation]

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Secondary radiation, that’s the radiation after hitting an object. So, there’s a lot of scatter radiation with the plastic cones that we spoke about.

[36] – [Filtration and Collimation]Very quickly, filtration, what’s the purpose of filtering the x-ray beam? To protect the patient. How? Remove the long wavelengths. There’s a thin piece of aluminum. What the aluminum does is it absorbs the long wavelengths, doesn’t allow them to penetrate. The short wavelengths couldn’t care less that there’s a thin piece of aluminum. They just shoot right through that. And so those are the only one that will get through to hit the patient and we can get a good image with the least amount of radiation by removing them. What are the federal regulations as far as numbers, memorization? 1.5 mm of aluminum, if you’re less than 70 kVp machine. More than 70 kVp, 2.5 mm of aluminum. What’s collimation? Restricting the size of the beam to the smallest possible beam. What’s the purpose of collimation? Protection of the patient. Okay? Less radiation hitting the patient. How do we collimate the beam? We spoke about that. The lead diaphragm which is basically a piece of lead that’s put into the … you can unscrew that and actually look into it. You can look into it in the clinic as well but don’t look into it when someone’s pressing the button. Okay? Look into it. You’ll see two things. You’ll see a little circular or if it’s a rectangular collimator, a rectangular piece of lead. There’s a little rectangle in the center. Only the x-rays will come through there. Now what happens to the short wavelengths when it’s the lead? They’re removed. Okay? Difference between lead is lead is a better absorber than tungsten. And also the lead diaphragm is thicker. There are no regulations on the thickness of the lead because only purpose of the lead is to get the x-rays through a little tiny hole so we can collimate the x-ray beam. Okay? If you have tungsten that’s too thick, it’ll remove all of the wavelengths, including the short wavelengths. So you gotta think about that. Quality and quantity of the x-rays. The quality of the x-rays has to do with the penetrating power. It has to do with the kVp. The higher the kVp, the more penetrating, the more quality of the beam will be increased. Quantity of x-rays is controlled by the miliamperage. The number of electrons that are boiled off and that’s controlled at the low voltage, low tension circuit. Half value layer, we spoke about kVp. KVp is an electrical term. If you wanna know about the penetrating power of your x-ray machine in your office, you have to look at a number called HVL. When you get your x-ray machine and you look at the little label, it’ll say it has an HVL of 2.75 which is probably what you need because 2.5 mm of the aluminum filter and about another 0.25 of inherent filtration. So if you’re x-ray inspector comes to your office and says your HVL is 1, that means your x-rays are not penetrating enough and you’re gunna have to ramp up the exposure time which is something you don’t want to do. So that’s something that’s going to be … so it’s a measure of the penetrating ability of the x-rays. It’s not an electrical term.

[37] – [Rectangular Collimation]These are some of the position indicating devices. Again, collimation, half filter.

[36] – [Filtration and Collimation]

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Now this is where we had the little error. If you notice, the question asked how many miliamps do you need. Okay? And this little s is not needed. Okay? So if you lost any sleep over this, which you shouldn’t … anyway, there is no s after that, okay?

[44] – [Image Formation]Alright, so now we know how x-rays are produced. We’re ready to take our machine, put a film in the patient’s mouth, we’re gunna aim that … and we know the radiation has been filtered and it’s been collimated and we have a PID and so we reduce the amount of divergence of the x-ray beam and we’re ready to take our film. Okay? So what are some of the characteristics of an acceptable radiograph? I mean some of these things you can think of yourself. You don’t want a film that’s too dark. You cant see it. You cant make out the enamel, the dentin. So that’s not something you want. And so that’s number three. You don’t want too dark a film. How do you get too dark a film? Too much exposure time. Okay? One of the things you’ll find out in later lectures you can get too dark a film when you process your films and you keep them in the developer too long, they’ll be too dark. How do you get a light film? Underexposure. You’re supposed to use 10 impulses, you use 4 impulses. That’s because you didn’t change it from the patient before, it was a five-year old kid. You get a light film. Why do you get a light film? Because with that amount of exposure time you’re not getting enough energy hitting the film. Any energy that hits the film will convert as you’ll se in your processing lectures, converted to silver in the developing solution. So, another way you can get a light film and I see this all the time in the clinic: students have a perfect set of x-rays, one film is light. On that one film they had their collimator too far away from the patient. The distance is very important because x-rays, as they travel a distance, they will lose some of their energy, just like light. And that’s called the inverse square law, which we’ll talk about. You want the minimal amount of enlargement and distortion. Enlargement is actually magnification of the image which is an equal magnification of the image. Distortion is where one part of the image is larger than it should be or smaller and the other part vice a versa. So, how do you … and we’ll see all of this in a couple of minutes, we’ll go over that. How do we reduce the enlargement? We have to restrict the divergence of the x-ray beam. The x-ray beam, if it diverges, will cause a magnification of the image. And we want detail sharpness. We want to be able to see the enamel and right next to it, the dentin. We have to be able to see a clear distinction of the two. One of the major causes of un-sharpness and a lot of the un-sharpness we’re going to talk about, theoretical. Because the eye won’t see it. But one of the major causes of a blurred image … does any one know? A blurred image? Movement. Movement. So if the patient is moving or if you’re collimator is moving and you see this in the clinic. You have a moving collimator. Don’t have the patient hold the machine. Have someone fix it. Okay? But movement will cause a decrease in sharpness of the image. One thing I forgot there … contrast. And we’re going to talk about this … contrast here. Contrast has to do not with the degree of darkness on the film, which is density. How dense a film is. It’s the differences in degree of darkness on a film. The contrast is the differences, the scope of shades of white, certain things are very white on the film, okay? Things where the x-rays don’t penetrate at all,

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okay? So if a patient has a crown, that area will be very white. Because all of the x-rays are absorbed by the metal and none of them hit the film. Any time the x-ray radiation hits the film; it converts the silver halide to silver, turning it black. So the space between the teeth will be black. The pulp chamber, where the pulp is soft tissue that will be black. So those differences are very important when viewing a film. What is a cavity in a tooth? What does a cavity look like? Well, if you see a tooth with the enamel and the dentin and there’s a radiolucent ... Radiolucent means light goes … x-rays go through that area. It means that the tooth has been destroyed by bacteria. So when the x-rays go through there’s no enamel or dentin to absorb it. So the way you find a cavity is look for radiolucent area. How do you find an infection at the tip of the tooth? Periapical infection. Do you know what these things are? Periapical means around the apex. The way you can tell there’s an infection, patient has a large filling and then they’re complaining of pain, you take what’s called a periapical film. What periapical film means is that you’ve got the entire crown, root and apex, the bone below the apex. And what are we looking for? We’re looking for a radiolucency at the apex. What does that mean? Well it means the bone has been destroyed by infection. So that tooth will probably need root canal therapy or extraction. How do we know that? Because of the different densities and contrast and that’s how we diagnose these things. So it’s very important for diagnosis to have the proper density, which is the degree of blackness on the film and the proper contrast.

[45] – [Image Formation]and we can control these things with the x-ray machine. There are ways of controlling them. So again, the definition of density: degree of blackness on a radiograph. You want a radiograph of the proper density, not too dark, not too light. The contrast is difference in densities between adjacent areas. Now, what is a high contrast film? A high contrast film is you see black and white on the film, no shades of grey. Black and white. A low contrast film is there are all kinds of shades are there. You have some white, some black, that’s low contrast. Because the differences in density between the white and the little light grey are basically the same. That’s a low contrast film. Now, what controls the density of the film? We’re gunna talk about that in a little while but think about it. The density is the darkness of the film. So, kVp, the higher the kVp, becusae you have more penetrating x-rays coming through, the higher the kVp the darker the film. So if you’re film is dark, one way to adjust that is to decrease the kVp. Again, we can’t do that because our machines are preset but you need to know the factor. What about miliamps? Miliamps, the number of electrons that are produced. Well, if you have a lot more electrons produced, then more of those electrons hit the film. So if you increase the miliamps, not necessary penetrating power, but you increase the miliamps or you increase the exposure time, the longer the exposure time the more x-rays will hit the film. The other factor is distance. The closer you are to the patient, the higher the density is going to be because x-rays travel. You move the collimator away, keeping every factor the same, the exposure time, kVp, miliamps … you move it away, those x-rays have to travel a distance, the energy will dissipate. So some of the factors that control the density of the film, kVp, miliamps, and distance, and exposure time. And

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all of those are pretty self-evident. And as far as contrast, that’s a little tricky. The only thing that controls the contrast in our machines is the kVp. Now, the higher the kVp, you can write … I have the students write this down but you have no notebooks so you can’t do this … but think about it. The higher the kVp, you make a little diagram, high kVp, low contrast. High kVp, low contrast. Low kVp, high contrast. So that’s the way it works. Again, if you have high kVp, low contrast and vice a versa. This is gunna get a little confusing so I’ll try to break it down for you.

[46] – [(contrast gradient diagram slide)]so this is what we’re gunna do. We’re gunna take an aluminum step wedge. Okay? It’s a piece of aluminum that has different thicknesses. At the right hand side, the aluminum is very very thin. On the left hand side it’s very thick. And then we’re going to shoot x-rays through this at different kVps. Look what happens at low kVp at 40 kVp, we have some of the x-rays in the thinner part of the aluminum step wedge will go through because it’s very thin. Even at 40 kVp. But most of them will not go through, they’re absorbed by the thicker part of the step wedge. So what kind of a film will you call this? Low kVp, high contrast. White and black, big contrast between those. As we increase the kVp, the x-rays have to go through varying thicknesses again. But when you have very high kVp, what happens is there’s variations in the x-rays going through. Some of them, again, most of them will go through the thin area and then more and more and more as you go up. So what you have here is a low contrast. There’s black and a little less black, little white. So you don’t have that sharp contrast. So this is called a low contrast film, high kVp, low contrast film. When you’re looking for cavities in the patient’s mouth, you really want a high contrast film. So you can see the enamel, the dentin, the cavity, this is theoretical. What we do to make it easy is we go right in the middle. We go about 65-70 kVp so we get the best of both worlds. Again, a lot of this is theoretical. The human eye can’t detect it. But from a scientific basis, this is what happens and we’ll go over this in a second. If you’re looking for early bone changes or periapical … you don’t have to know this for this course. But if you’re looking for bone changes, periapical pathologies, things like that, you probably want to be in the higher kVps. But again, go right in the middle you get the best of both worlds. You see that contrast. The contrast is important. Enamel, dentin, periodontal ligament which is black, we need those contrast. If we have contrast that is similar contrast we’re not gunna be able to see that. So we do need differences in the degree of blackness on the film. So again, if you look at this over here you notice you have your step wedge, the x-rays are going through and at A, do you think A is high kVp or low kVp? Low kVp because those little squiggling lines are longer. These are shorter wavelengths. See? These are shorter wavelengths. So what happens with that type of film because the kVp is low, you’ll either get penetration or no penetration. There’s no in-between. And so that’s a high contrast film and the bottom one is a low contrast film. High kVp, low contrast. Got it? Now I’m really going to mix you up. In a lot of the exams, they ask you about the scale of contrast, the scale. Okay? This is a short scale ... this is going to mix you up. This is short scale contrast. This is high scale. Scale. Okay? So try to get that in your mind. I’m not going to try to … you’ll get mixed up. So think about it and you’ll see. A high kVp, low contrast, but long scale contrast.

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Okay? Long scale contrast. Okay? I’m gunna go onto the next thing because I see everyone is …

[47] – [Overall Density and Contrast is Determined By:]Okay? So the contrast has to do with the transmission of x-rays through an object as we just saw. Changing the kVp is the main factor and it depends on object density. We’re lucky we’re only x-raying teeth and bone. So, we have a fixed … the tissue that we have has a fixed density. We know that. So the only way to control the contrast of the film is with kVp. So film contrast we’re going to cover when we talk about processing of films. All of these things have to do with the amount of x-rays hitting the film. And when we process them, there are different factors. So, for our purposes, density or darkness of the film is controlled by several factors but contrast, one factor, kVp. KVp is the only factor in contrast.

[48] – [Image Detail and Definition]Now, that takes care of the density, darkness, and contrast of the film. Now we need to see how we can get the sharpest image with the least amount of magnification. And that’s the object to make the proper diagnosis. So we want a film with detail. The visual quality of a radiograph depends on definition or sharpness. Now what are the sharp areas on the film? You see a tooth. The tooth itself is the umbra, the sharp area. You won’t see, but theoretically, around the edge of the tooth is an un-sharp area called a penumbra. The penumbra is the un-sharpness or blurring that surrounds the edge of a radiographic image. A lot of students come down to 1A, when they’re there, “Doctor Friedman, show me the penumbra, I want to see a penumbra …” I can’t show it to you. It’s again … it’s a theoretical thing. We need to reduce that penumbra with the x-ray machine in order to get the sharpest image. So we want to keep the penumbra as small as possible. That fuzzy area. Penumbra actually comes from two latin words: “pen” is almost and “umbra” is shadow. So there’s a shadow around the outside of the object that you’re imaging. So how do we keep the penumbra as small as possible? And we’re going to go through these steps. Small focal spot. Use the smallest focal spot you can. Do we have control over the focal spot? No. it’s in the machine. The manufacturer of the machine selects the focal spot. The smaller the focal spot is, the sharper the image is going to be. So, in our current machines, the focal spot, focal area, is about 1 by 3 mm. You don’t have to memorize that. 1 by 3 mm, which is a small area. Why don’t we use smaller than that? Because of the problem of heating. So we are … we can’t get any smaller than that. Why is the smallest focal spot very important? We’ll see that in a moment. The angulation of the target, if you remember I showed you on the diagram. The target is not perpendicular to the x-rays … to the electrons coming across. It’s tilted at about 20 degrees. There’s a 20 degree tilt in there. And geometrically what that does is the effective focal spot … we’ll see the diagram in a moment … is smaller than the actual focal spot. And so by doing that little twist, we can get the focal spot a little smaller without changing the actual physical size of it, which is restricted by the amount of heat that the x-ray machine is putting out. Okay? And what else do we want to do? Increase focal film distance. We want to have the longest distance we have. What is focal film distance? The distance from the focal point to the film in the patient’s

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mouth. FFD. The longer that is the more parallel the x-rays are going to be. The more parallel the x-rays are going to be, the less magnification and the less penumbra will be formed. We’ll see diagrams of all of this. And we want to decrease the object film distance. So if we’re taking a picture of a tooth. If this is the tooth, and this is the film, we want the film as close to the tooth as possible. The object, which is in our case a tooth, film distance, as short as possible. Why? Because as you get further away, the x-rays penetrate and in order to hit the film they have to cross the distance. What happens when the x-rays penetrate the tooth and have to travel this distance? They again diverge. So we get magnification. The closer you are, the less magnification of the image. And for those of you who get advanced, we’re gunna talk about this later. The buccal cusp of a premolar and the lingual cusp of a premolar, which do you think will be sharper on the film? Again, theoretical. If you can see this, you got super eyes. What do you think will be sharper? The one closer to the film, or the one further away? Closer. So the lingual cusp will be sharper. That’s a piece of information you’ll never use in your life. Okay? But theoretically this is what they tell you. If you read a textbook, they make a whole big deal about it but as a practicing dentist, who cares? You can see the cusp. Look in the patient’s mouth if you want to see it. Okay? But that’s a theoretical thing. Okay? So those are the factors that we need to use in order to get the sharpest possible image for the best diagnosis. We’ve already controlled the radiation. We protected the patient. We’ve collimated. We’ve filtered. We’ve done everything we possibly could do. Now we want to get the best image with the least amount and we have to do some things in the patient’s mouth: placement of the film and the other factors that are involved. Okay? So, again, smallest focal spot, that’s done by the manufacturer. You can’t go in there and chisel away your focal spot. You’ll break the machine. Leave it alone. The angulation of the target, also done by the manufacturer. And the smaller the focal spot the sharper the image is going to be. Increased focal film distance, the reason for that is the longer the film distance is the more parallel rays hit the film, less divergence of the x-ray beam. The less divergence of the x-ray beam, why is that important? Less magnification of the image and less penumbra. The penumbra is increased when the magnification is increased. And again the object film distance, the tooth has to be as close to the film. As the tooth gets further away, the x-rays have to penetrate. And as they penetrate, they will diverge and you will get the magnification of the image.

[49] – [Image Penumbra]We’ll be taking a break in a second. Okay? And, again, here’s your source of radiation. This is the recording plane. In our case, it’s the film. This is going to be a tooth. And what happens is the more parallel your x-rays are, the less of the shadow will be formed. If your x-rays are divergent and hit them at an angle like this, that increases the penumbra. We need to keep the penumbra as short as possible. Okay?

[50] – [Image Detail and Definition Depends On:]Okay. So this is just a repeat of what we just spoke about. The image detail and definition depends on size of the focal area at the anode. The smaller the focal area, the sharper the image. The focal film distance. The longer the focal film distance is,

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the shaper the image is going to be. The shorter the object film distance is, get that film as close to the tooth as possible, the less magnification, less penumbra, the sharper the image is going to be. Another thing you want to do, and this you’re going to get in your lectures on taking the actual … putting the film and you’ll actually see teeth and … I don’t have any of those films for you. You want to position that object and film parallel to each other. So when you take periapical films, there are two techniques that you’re going to learn. One is the paralleling technique. One is the bisecting technique. What is the paralleling technique mean? It means the film is parallel to the long axis of the tooth. Look in the patient’s mouth, you have a lower molar, take your film and place it here. If you have an upper anterior that’s tilted, ten to fifteen degrees, take your film, put it in the patient’s mouth so it’s parallel. Why is that important? Because as the x-rays travel through they come in at right angles. If they come in right angles, they will record the size of the tooth exactly as the size of the tooth. So if you have a tooth that’s 25 mm long in the mouth, you know that, okay? You put the film parallel to the long axis. These are the teeth, here’s your film, parallel. You direct your central ray perpendicular to the film, the size of the tooth 25 mm will be recorded as 25 mm geometrically. What happens if you don’t shoot at perpendicular? You come in at a steep angle. That’ll cause foreshortening. You’ll learn all of that. So it’s important to avoid elongation, magnification, and foreshortening of the image. Factors in choosing the focal film distance. Something called the inverse square law, which we’re going to talk about after the break. Movement, object, film and tubehead, tubehead drift. So, to get the best possible film, we need to reduce the movement. Obviously, if your collimator is moving that is a no-no. If an inspector comes to your office, what they’ll do is they’ll get to your x-ray machine and they’ll aim it, they’ll go right over the operatory. I’ve had this done about 25 times in my office. They’ll put the machine and they’ll start telling me about kVp and I said, “okay, thank you.” But you can talk to them about that because he was telling me about the HVL and I disagreed with him and he said, “How do you know that?” I said, “I’m a dentist, I know.” Okay? So he’s going to put the collimator in different areas, the tubehead. And he’s going to look for drift. If there’s any drift in that machine that’s a violation. That’ll cost you $10,000. But the good news is, you don’t have to pay. They’ll give you a chance to fix it. So here’s the thing, if you know beforehand, you know that the inspector’s coming to your office fix it before he comes. Don’t have the aggravation. You do that before he comes, you put the collimator anterior teeth where the patient is, move it over to this side, posterior teeth, you see it’s drifting, usually the way to fix that is simple. You call someone in or if you’re handy, you tighten a couple of screws and that prevents the movement of the tubehead. And again, that will cause the most un-sharpness on the film. All of these other things that we spoke about: magnification, small focal … most of those are theoretical and you get them on exams. But if you have a film where the patient moved or the tubehead moved, you’re gunna have a blurry image, it’s gunna be a retake. Okay? And viewing conditions. This is important. You went through every single step. You’ve collimated. You’ve filtered, you this and you’re doing all this stuff. And you put the film in the right place and your kVp and your miliamps was correct and your exposure time and you went in and you developed it correctly. You have a perfect film. Then you go over to a faculty member and he takes the film and he goes

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over to the window and he looks at it like that. Or he goes up to the light … you’re gunna see that all the time. You’re losing a tremendous amount of information. What you really need, and if you come into 1A and the viewing room and it’s a little dark in there, it’s romantic, we have candles … no we don’t. the darkened conditions are so that the way we diagnose a film is we look at a viewbox. And all of this is going to be useless in a couple of months because we’re going digital. They’re ripping out all the viewboxes. So if you want to see a viewbox, come in there, or you can look at it in a catalog. So you have a viewbox and you put the films on there you don’t want any light coming in from the sides. Because what we’re getting information is the light being transferred through the film so we can see the contrast. That’s the enamel, it’s very white and wait a second, right at the junction between the enamel and the dentin, there’s a radiolucency. That’s not supposed to be there. How did that get there? That may be a cavity. Or you look at the tip of the root, and you’re looking and you see a nice area of the root … perfectly, nice contrast … but you see a circular area around the root. That’s probably an infection, an abscess, and infection, something like that. So you need the proper viewing conditions, they’re very important. But after having said that you’re going to have faculty upstairs that say, “let me see your x-rays” and don’t tell them that x-rays are invisible, a form of electromagnetic radiation, they don’t like that. Okay? They will take the films and you can go and watch and they’ll hold them up to the light, “Yeah, yeah I think that may be a cavity. Go drill that tooth.” And you drill the tooth … there’s nothing there. So, hold it up to the viewbox. And you should never drill a tooth based on one film. Okay? Because there are a lot of things that look like cavities that may not be cavities. You’ll learn that a little bit in this course, things like cervical burnout or indirect pulp capping, we’ll learn all of those things. So we’ll take a few minute break. I think we can get out a little early today. But I need a five-minute break for myself. When we get back we’ll continue with this, we’re going to cover, again, some of the factors that give us the best quality film, what we can do to get that.

[51] – [Effective/Actual Focal Area]Okay … I think the mic is on … I hope I shut the mic when I went to the restroom. And if you hear a babbling brook on your itunes, that’s my way of calming … I don’t remember if I shut it off so, anyway if you hear that … okay. So let’s continue with the … how do we get sharpest image with the most definition and detail? One of the ways we get that if you remember the list of objectives there is to have the smallest focal spot possible. The smaller the focal spot, the sharper the image due to the decrease in penumbra. The actual focal area measures about 1 by 3 mm. but because of the tilt, the way the manufacturer puts this, remember, this is the cathode, the electrons will come across here. This is the copper stem. And embedded in the copper stem is a little piece of aluminum … excuse me, tungsten … okay. There’s a little piece of tungsten in there called the focal spot or target. And by tilting it we actually have the … the x-rays will come out of here after striking the tungsten due to the Bremsstrahlung and characteristic radiation. And we actually have what’s called an effective focal spot. The effective focal spot is always smaller than the actual focal spot due to the tilting. So we don’t have the problem with overheating because the size … the actual focal area is larger than the effective focal area. But we

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get the advantage of having the smallest focal area to get the sharpest possible image. Okay? So that’s one of the factors.

[52] – [FFD and OFD]another factor we spoke about … focal film distance and object film distance. That’s the focal point there. This is the object. So the focal film distance from the focal point, the tungsten target to the film should be the longest possible distance to reduce the magnification. And the object film distance … okay? Object film distance needs to be as short as possible. Okay? So you’re looking at this and you’re saying, “Dr. Friedman, why don’t we move the film closer to the tooth?” Well this is an upper tooth, if you looked in the patient’s mouth, there’s a palate, the vault and the shape of the palate. If you get your film closer to the tooth, it’s no longer parallel. Okay? So in the paralleling technique … no technique is 100% effective. Think about it. When you’re doing a lower molar and you’ll learn all of this. The lower molars have no inclination to the buccal or lingual. They’re coming straight out of the jaw. So if you take your film and put it in the patient’s mouth and it’s right up against the molar, you have satisfied all the requirements. The film is parallel to the long axis of the tooth and you also have this short object film distance, which will cut down on magnification and penumbra. Why can’t we do that with anterior teeth? That looks like an anterior tooth. Has to do with the shape of the palate. The closer you get to the tooth, the less parallel you’re going to be. So this is a diagram of the paralleling technique. This is a tooth here, that’s the film, and it has to be placed … there are certain anatomical constraints that we have in placement of the film. If you place the film closer, then that’s called the bisecting technique. I’ll show you that in a moment. Okay? So, remember, a lot of this is theoretical. The human eye cannot tell the difference if it’s 2 mm away, if it’s 3 mm away. A lot of these are theoretical considerations.

[53] – [FFD/Recessed Target]Okay. Also when purchasing a … now you understand this a little better … when purchasing an x-ray machine … this is, by the way, the tubehead. That’s the tubehead here and that’s your collimator or position indicating device and of course you have your filtration and your lead diaphragm here. This is the focal point here. Now the distance from here to the end of the position indicating device is measured. So let’s say you’re looking at it and your collimator is 8 inches. So that’s an 8-inch focal film distance. However, you can have the electrical component of the anode can be recessed back in the machine. Some machines do that. What is the advantage of that? Well you have a longer focal film distance which is what we want. But we don’t have this monstrosity sticking out at the patient. Look at that, it looks like a cannon. These long 16-inch collimators. So understand that before you set up your exposure times in your office, and this will be given to you by your manufacturer, the exposure time has to do with the focal film distance. Because the longer the focal film distance the more exposure you’re gunna have to give. Because the distance is longer. So we have the advantage of long focal film distance but we have to change our exposure time. We’ll set that up because of the variable of the distance. On the shorter collimator, you need less exposure time because the x-rays travel a shorter

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distance. So we’re going to talk about that when we see the inverse square law in a moment.

[54] – [Paralleling vs. Bisecting Techniques]So these are the two techniques used in clinical dentistry for radiographing teeth. Two techniques. Paralleling technique. What does the word paralleling mean? Simple. The tooth is parallel to the long axis of the film. The film and the tooth are parallel to each other. Direct your central ray perpendicular to the film or the tooth and what does that do? It gives you a true representation of the size of the tooth. Geometrically, that tooth, whatever the size it actually is in the mouth, will be imaged that way in the film. The other technique which we’re going to quickly talk about because you’re gunna have a whole lecture on this … you actually place the film right up against the lingual surface of the tooth. So you do that in the patient’s mouth and what happens is that an angle is formed between the long axis of the tooth and the film and what you have to do as the radiographer, think about this, is figure out what the angle is by looking. Then, in your mind, you’ve gotta draw a line that bisects that angle … go that? Alright. And after you have that bisected line, you direct your central ray perpendicular to the bisected line. Got it? If you can do that, you’re a genius. Can’t do it. Every textbook that has a discussion on the bisecting technique will show you this diagram but then they’ll give you a number. They’ll say when you do anterior teeth, put your collimator at 50 degrees. For the anterior teeth. Molars, 30 degrees. What’s the problem with that? Well the problem is, I can show you five different textbooks written by professors of their colleges and they have different numbers. It’s a guess. So you will get the tooth on the film because the x-rays go through the tooth. The film is behind the tooth so you’ll see the tooth but you won’t have the true representation of the tooth. There’s a lot of distortion. So the best technique of course is the paralleling technique and of course that’s what we teach you. But there are certain cases where you cannot use the paralleling technique. For example, think about this, patient has some missing teeth and you suspect some kind of pathology in there, there’s a root left in there or there’s a cyst and you want to get a picture of the edentulous are. Which technique would you use? There’s only one technique. Because what’s the definition of the paralleling technique? The film is parallel to the long axis of the teeth. There are no teeth; you have to use the bisecting technique. So when you get to the preclinical lab, we’ll show you how to use the bisecting technique. But from an image standpoint, the best image would be using the paralleling technique. The least amount of distortion and magnification of the image. Okay?

[55] – [Intensity]Now, we spoke about the density and the contrast. There’s another factor here which is almost exactly as the density. Density is the darkness on a film. The darkness on a film has to do with the intensity of the x-ray beam. The intensity of the x-ray beam is basically the total energy contained in the beam. So, what is the beam intensity affected by? Several factors. Almost the same factors as density: kVp, the higher the kVp, the more intense the beam will be because there are more penetrating x-rays. The intensity is measured at the film or at the patient’s face. So

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the higher the kVp, the more intense, the more energy contained in the beam. The more miliamps, the higher the miliamps the more electrsons, the more electrons hitting the face, there’s a higher intensity of the beam. Exposure time, the longer the exposure time, okay? The longer the x-rays are hitting the patient, the more energy, the more intensity the beam will have. And the focal film distance. How does the focal film distance control the intensity of the beam? The further you are away, the less intense the beam is going to be. So, we see students coming in, they’ve got a light film. The reason the film is light, the exposure time is correct, the process is correct, they had their collimator instead of being next to the patient’s face, they had the collimator out here. And we calibrate our machines to be 1 or 2 inches away from the patient. You move away, even though you have the same miliamps, the same kVp, and the same exposure time, by moving that collimator away, you’re decreasing the intensity of the beam. So, basically, you don’t need to know this formula. Forget it. We’re not plugging numbers in, it’s the concept of quality times quantity. What’s the quality? kVp times the miliamps over the area that you’re doing. The larger the area is, the less intense the beam is going to be. And the exposure rate, not the exposure time. I’m not going to go into this too much because it’s a little complicated. What I need you to know is that the intensity of the beam is controlled by kVp, miliamps, exposure time, and focal film distance. Okay? The higher the kVp, miliamps, the increase in exposure time will increase the x-ray intensity. What about focal film distance? The further the focal film distance is, the less intensity of the x-ray beam. Now …

[56] – [Inverse Square Law]How do we measure the intensity? By something called the inverse square law. Now, you move the x-ray collimator away from the patient’s face. It’s supposed to be 1 inch away from the patient. You move it to 2 inches. So you’ve doubled the distance, correct? You double the distance. So, in order to get the same intensity, what will we have to do to the exposure time? Someone said 4, which is correct. But I’m from Brooklyn. I say, “Wait, you double the distance, you double the exposure time.” That makes sense, right? But it doesn’t work scientifically. The intensity of radiation varies inversely with the square of the distance. So what you’re going to do is intensity is equal to 1 over the distance squared. 1/distance2. And I’m going to show you how we use this. Again, I’m not throwing this stuff in for you to know, physics. This is a clinical situation, which you’ll be faced with many, many times in the clinic. One of the clinical considerations of the inverse square law has to do with the proximity of the collimator to the patient. You think, “Well, I’m just moving it back, I just moved it a little away.” Well, if you moved it an inch away, double what it was before, you’re actually decreasing the density by 1/4th. So you’d have to increase your exposure time 4 times. If you have 10 impulses, in order to get the same film, with the same density, contrast, you would have to increase your exposure time. And I’ll tell you how we clinically do that in a moment. So what happens is the further away you get, if the focal film distance doubles, the exposure time has to quadruple. Now, when do we use that? Very simple. You buy an x-ray machine and it has an 8-inch collimator and Dr. Friedman said you get better pictures with a 16-inch collimator. So what do you do? You call up the Schein or whatever … and you

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order another … they unscrew … you order a 16-inch collimator. Boy, I’m gunna get beautiful pictures here. Alright? And then you don’t change the exposure time. What do you think your films are going to look like? They’ll be extremely light because those x-rays are traveling a longer distance now. So you wanna change the exposure time. You got beautiful films with 10 impulses for anteriors. So what do you have to change your exposure time to get the exact same intensity and the same film? You have to change it to 40. Now, here’s a patient getting 40 impulses. Boy, you’re really zapping that patient. 40 impulses. That’s a lot of impulses compared to 10. Which patient is getting more radiation? They’re getting the same amount of radiation because the distance traveled, the intensity measured at the patient’s face will be exactly the same. So, if you have to change or you’re in an office and you got this long collimator, it’s 16-inches long because the dentist was there before said the longer the collimator is, the less divergence, the better the image is going to be. But you don’t like it, it’s bumping into the wall. It’s hard to work with. You unscrew it; you put in an 8-inch collimator on there. Okay? Now, the x-rays don’t have to travel that distance. So, 40 impulses. What do you gotta do? 10 impulses. So even though you double the distance from 8 to 16, you don’t double the exposure time if you’re going from 8 to 16. You quadruple the exposure time. And the same thing if you move three times away, if you triple the distance, okay? If you triple the distance. 1/distance2. You gotta do whatever that distance is, you gotta square that distance there. So if you’re going from one area, three times the distance, probably it’s 1/9th the exposure. You’re never gunna do that because it’s only 8 and 16 inch collimators. So from a clinical standpoint you won’t do that. So, as the distance increases, intensity decreases and therefore the exposure time must also increase. What’s the factor? How much do you have to increase it by? The square of the distance. So if you’re going from an 8-inch to a 16, this is a typical board question, I’ll tell you what it’s going to say. You came to your office, you decided to change your collimator from 8-inch to 16-inches, the old exposure time for proper density and contrast was 12 impulses. What will be the new exposure time to get the same density and the same picture? Remember, if you increase the distance, you have to increase the exposure time by a factor of 4 if you’re doubling that. Okay?

[57] – [Inverse Square Law]And this explains the inverse square law a little bit. This is 2 times as far away as this is. And look what happened. Because of that distance, you’re getting divergence of the x-ray beam. So here you can see the intensity is a little darker because of the distance, getting lighter over a larger area. So, to compensate for that doubling of the distance, it’s 1 over the distance2. So the inverse square law does have clinical applications when you’re changing your position indicating device or when you’re taking films, make sure you’re as close to the patient’s face as possible. Because our machines are calibrated and your machines will be calibrated by the type of film that you use. We’ll talk about that later. But also by the distance you are away from the patient.

[58] – [Movement = Blurred Image]

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Ah! We see teeth. I finally showed you teeth. I got thousands of pictures of teeth and you’re going to see thousands of pictures of teeth but we wanted to show you what happens when a patient moves. We don’t have detail. We can’t make out the difference between this and this. They all look the same. The density that you see here, this is an opaque, a radiopaque object. That’s how we get the term, the white area is radiopaque. Can anybody guess as to what that is? It’s some kind of metallic object. Is it gold or silver? Can’t tell the difference. Because both of those will absorb radiation. So the patient can have a silver amalgam filling or they can have a gold inlay or onlay, you can’t tell the difference. If a patient has a composite filling, that will not be as opaque. The reason is, the composite fillings allow more radiation to go through. So you’ll see this when you learn about the diagnosis. But what happens, x-rays go through the tooth, they hit the film. No x-rays are going to hit the film in this area. Why? Because the crown or the filling has absorbed the x-rays. What do we see here? That’s the pulp of the tooth, the pulp chambers of the tooth. Why are those radiolucent? Because soft tissue. The x-rays can readily go through that soft tissue area. And right behind this third molar, you can see the bone structure, the trabeculation. But again, because of the movement, that’s the number one way of getting an un-sharp image … all the other ways are theoretical … is some kind of patient movement or collimator drift. We’re speaking to someone about, they were gong through the clinic and they see the collimators are moving. If the collimators are moving, that’s usually due to the fact that if you’re taking an upper film on a patient and you have your chair very high. You try to lift your x-ray machine. As it gets to the top, it will drift down because it’s reached its maximum height and it’ll just fall down. So, lower the chair, number one. If the student is taking a radiograph on a molar on this side and then you come in and you’re taking a molar on this side, sometime you have to move the entire arm apparatus to the other side. If we just move part of the arm, it will drift. So before you go ahead and take out your tools to fix these things, try to adjust the arm, try to adjust the chair. But if there’s any movement, don’t have the patient hold the collimator up against the face. That’s a big … the radiation police will get you for that. Okay? And you can see all kinds of things going on in the clinic … I was telling one of the students about, the student kept missing the film because of the rectangular collimator. You know, if you miss part of the film, that’s called collimator cutoff. So what’s the effect of that? Half the film is not diagnostic. Patient’s getting radiation and you’re not getting any information. So, you know these collimators screw off. So what the student did is unscrew the collimator. He understood that that’s collimating and if you unscrew it, the patient is showered with radiation. So he didn’t miss the film but the patient got a radiation shower. So don’t do that, okay? That’s movement.

[59] – [Viewbox]Okay. And again, the last thing I just wanted to show you very quickly and we get out a little early is the proper viewing conditions in a darkened room. You have your set of x-rays and there’s even a magnifying glass. Some of these things like incipient lesions which mean beginning lesion are very difficult to see. Don’t drill a tooth based on one film. Always … we had a patient here on, what’s today, Thursday? … we had a patient on Tuesday and the student dismissed the patient. We looked at

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the panoramic film of the image and there was a … looked like a fracture in the lower jaw. And the patient was let-go. Now that could be an artifact because the patient had no history of trauma, had no pain, had no swelling. So why did we diagnose that and I have the film if anyone is interested. Why did we diagnose that as a fracture? Because it looks like a fracture. Are we going to do any surgery? No. you know what we’re going to do? We’re going to get the patient back next time and we’re going to take another film. That’s what we’re going to do. Because if it’s an artifact it won’t show up in the exact same place. If it is a true fracture, it’ll show up. So, again, even in a cavity, you see something that looks like a … again, you need the proper density and contrast, if it looks like a cavity, many times you’re tempted to go ahead and drill that tooth and many times it may not be a cavity. So be careful about that. Okay? So I expect to see in the news next week, beaches, and people with Dr. Frommer’s textbook on the beach … no. But again, please. To tie all of this together, if you have the time, because the information I gave you, if you understand, it is sufficient for exam purposes. But if you want to tie all of this together and get another view of what I spoke about, that’s a very good textbook. You can use the White and Pharaoh as well. Have a great weekend. Okay? Okay.

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Documents

03/04 Image Formation