Download File - Logan Class of December 2011

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2 types of radiation
Particulate radiation is natural radiation. It’s the stuff that is radioactive. It’s the stuff
used in the radiology in the nucleus medicine department.
Radioactivity is based on half-life. When you want to know how radioactive something
is, the world refers to it by half-life. So he may ask a question: how many half-lives will
it take for a substance to go below 13%. What you do is start with 100%. The first halflife is expressed as + ½. So you start off with 100%. Then you go down to 50%. That’s
your first half-life. The second half-life you go down to 25%. That’s called +1/2 2. The
third half –life you’re down to 12.5%. That’s below 13% so the answer to that question
would be 3 half-lives.
The other type of radiation is electromagnetic EM radiation is man made radiation, Xrays. Another word for x-rays is photons. A couple of things most people screw up on
this test more than anything else do:
 X-rays, electrons, and photons
 Inner shell and outer shell electrons
So when we go through this, I’ll be saying electrons and photons. When I talk about
photons I’m talking about after the electrons have produced the photons.
The way we make x-rays in an x-ray tube is with an anode and a cathode. This anode and
cathode together is known as a diode. Let me back up a little bit. I want to say that EM
radiation is more powerful and more penetrating than particulate radiation. Then we talk
about gamma radiation and x-radiation, the only difference is their source. Understand
that thee x-ray source is man made and the source for gamma rays is natural.
He’ll ask some questions about spectrum here. Notice that you have the frequency on the
right and the wavelength on the left. You have the radio frequencies on the top of this
chart, with visible light in the middle, and the x-rays and gamma rays down below. Now
he can ask 100 questions on this. You can be looking at the frequency on there. He might
ask if radio frequency is higher or lower than x-rays. Or he might ask about the
wavelength. For example, is the wavelength of x-rays higher or lower than visible light?
So notice that wavelength and frequency are indirectly proportional. So x-rays have a
greater frequency than visible light.
Remember this formula velocity = frequency X wavelength or energy = velocity X
frequency/wavelength.
Now let’s talk about how EM radiation is produced. EM radiation is produced by the
diode. The cathode has 2 little springs inside of it and the cathode works kind of like your
toaster. This little spring, when mA hits it gets red hot. This is called thermoionic
emission. It forms an electron cloud around the filament. The cathode is negative. The
electrons produced around the cathode are negative. The anode or the target is positive.
Now his diode in an x-ray tube is not alternating current, it is direct current. The way we
get from AC to DC is called rectification. Rectification is the process of changing
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alternating current to direct current. The electrons travel from the negative cathode to the
positive anode at half the speed of light. Now when the electrons hit the anode, which is
made of tungsten, a phenomenon occurs and x-ray photons are produced. In third part of
this lecture, we’ll tell you exactly how this happens. This production of x-rays is your
primary radiation. Now when this phenomenon occurs you have the heat boiling off the
electrons, it is only 1% efficient. 99% of the energy is wasted as heat. Only 1% of the
energy makes the photons.
X-ray properties
These are the properties of x-rays:
He has asked many questions about the properties of x-rays
 They are highly penetrating, invisible rays which are electromagnetic
radiation
 They are electrically neutral
 They can be produced over a wide variety of energies an wavelengths
 The photons themselves as they pass through your body produce heat
 The photons travel in straight lines. There is nothing manmade that can
change the course of a photon
 Now I told you the electrons travel at half the speed of light. The photons
travel at the speed of light
 They can ionize matter
 They can fluoresce
 Is all this coming out of the book? NO. All of this material comes from a
reference manual on your syllabus called Buchong. It’s in the library.
 They can’t be focused
 It affects photographic film. So don’t put film through the x-ray machines at
the airport
 They can cause changes in matter by ionization and excitation
 They produce secondary and scatter radiation
This is a question that a lot of people screw up. It is the electron in the tube that produces
primary radiation. It is the photon that produces secondary radiation.
 They cannot be compacted, subdivided, ionized, focused, or reflected.
 They can be absorbed, attenuated or compensated and they can penetrate
matter.
 They have no mass
 Photons are neutral
The next thing that we will talk about is density and contrast. I know that a lot of people
get this mixed up but hopefully we’ll straighten this out for you.
When you look at an x-ray you see darkness on there. That darkness that is on the film is
the density of the film. The contrasting density between those dark areas (the grays and
the whites) represent the contrast. And we’ll show you how that works.
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Density
The controlling factor for density is mAs (the mA (current) and the time); it is how much
radiation you are receiving. The influencing factor is kVp because that’s what is driving
the mAs into your body. You don’t need know the other influencing factors, but we will
talk about them a little later. If you give more mAs, the film gets darker. Density is a
quantitative measure.
The controlling factor of contrast is kVp and contrast is a qualitative measure.
Now he’ll ask questions about contrast. If you can memorize the relationship between
terms used to describe contrast and kVp you should be okay. When he talks about high
contrast, we are talking about low kVp. If you have one film shot at 60 kVp and another
at 80 kVp, when you look at those in lab you will see the high contrast. It means few
shades of gray which means increased contrast, or short scale contrast. Low kVp shots
apply to all extremity work. Now when you talk about low contrast, you are talking about
high kVp. Think of a chest radiograph, where you have many shades of grey. You are
gaping away forum a black and white image to a grey image period. Many shades of gray
give you a decreased contrast also known as long scale contrast. He will simply ask you a
question; he won’t be telling you if it is high kVp or low kVp, he might use the term low
contrast, or the term long scale contrast. So you have to know if it is high or low kVp.
When you are looking for fractures you want a high contrast image. Black and White
shows osseous structures best. Now if you want to look for a tumor in the lungs, you
want the low contrast, or high kVp, because if it were black and white, you would not see
a tumor. You need many shades of grey to distinguish things like pleural cavities,
interstitial markings, pneumonia, which is about the same density as interstitial markings.
If you are using low kVp there is no way to distinguish between the two. If you shoot a
chest image at only 50 kVp you won’t see pneumonia, but if you shoot at 110 kVp it will
jump right out at ya.
Inverse Square law
I1/I2 = D2/D1
Dr. G uses the term mAs conversion. If you are at 40 inches using 20 mAs and you go to
80 inches, you have to use 4 times the mAs to make the film have the same density. Now
the inverse square law states that the further the tube is away from the patient, the less
radiation they receive. It’s inversely proportional. Even though you kick up the mAs,
intensity, the skin dose is less. It is a hard concept to get.
It is because of wavelength. The shorter, softer wave lengths are not hitting you. I like to
use the analogy of a garden hose. If you are shooting the house with the softest stream
you have, you have a lot of spray that comes off. If you back that up, then all that spray
hits the ground and all you are left with is the hard stream.
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He’ll ask some questions about intensity. He’ll ask questions using MR (milliroengtens),
a measure of radiation. He’ll say if we shot a chest x-ray at 40 inches and it was 20 MR,
then we went back to 80 inches and shot it, and then use the inverse square log to figure
out the new MR. Just use the formula.
The 15% rule
It states that if you increase the kVp by 15%, you have to decrease the mAs by 50% to
make the film have the same density. Similarly, if you decrease the kVp by 15% then you
have to double the mAs to get the same density. Why would you want to do this? You
would not want to do this when you have a patient with Parkinson’s disease, and they are
shaking and you take your super tech and you say you need 60 kVp using 50 mAs. You
realize that 50 mAs means 50 mA for 1 sec. In 1 sec, you are going to get a very blurry
film because the patient is moving. If you cannot stay still you need to cut the time. What
you can do is increase the kVp by 15% and this will decrease the time by 50% (1/2 sec).
This might give you a shorter time so there is not motion on the film. The radiology rule
says you can do this two times. Increase 15% kVp, cut the time in half. If this does not
work do it again.
Half Value Layer
I have seen questions about half value layer. Just know that half value layer is what a
radiation physicist will do when he comes into check the equipment. He is checking the
quality of the kVp and what the half value layer states is that the amount of aluminum it
takes to decrease the intensity of the beam by 1/2. They will come by a year later and
perform the same test using all the same things from before to make sure that it does not
take any less or more aluminum to decrease the intensity by 50%.
Three Main Parts to an X-ray machine



the control consul
the generator
the tube
We have already mentioned that the x-ray tube is a diode.
The generator
There are 2 types of generators. Now Dr. G talked about high frequency generators and
about how great Bennett high frequency generators are.
There are high frequency generators and there are single phase generators.
What’s the difference between a single-phase unit and a high frequency unit? This is my
analogy. A single phase unit shoots radiation at you like a machine gun. A high
frequency unit shoots radiation at you like a laser beam. Now understand that all x-ray
machines are plugged into the wall. They use alternating current. Now they rectify that
into direct current. When they change it into DC, one of the waves is lost. You have a
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positive wave that goes up and a negative wave that goes down. Well, they just get rid of
the negative wave, so that the current is coming at you all in positive form. Alternating
current is 120 cycles per second. So, when they cut off all the negative parts, you are only
getting 60 pulses per second. This is called single phase half wave rectified.
AC, the wave comes in and only at the tops of the wave is radiation being pushed out.
This is a single phase half wave rectified, 60 cycles per second. So, then they discovered
that that is not good enough. There is not enough radiation coming out. So, then they
figure out a way to flip the negative and make it a positive through another type of
rectification. So, now in example B, you have single phase, full wave rectification. This
full wave rectification, down at the bottom, gives you 120 cycles per sec. This machine
gun effect, up and down 120 cycles per sec is called ripple. When we talk about ripple,
we talk about single phase equipment. Now when they talk about high frequency, high
frequency units have about 1-2% ripple. The way high frequency works, when it is
energized, it reaches its peak and it is a straight line that goes across period. I remember
Dr.G saying that bennet high frequency tends to have the lowest ripple of any High
frequency (HF) machine out there. We have to know this because they do a test called the
spinning top test. They use the spinning top test to check the timer. I have images of the
spinning top test in the lab portion in the lab portion. So if you have a question about it,
if you don’t get it here I’ll explain it a little bit better in the lab. It’s a disk with a little
pinhole in is spun. As you spin that thing, let’s say you set the time on 0.1 seconds and
you have 120 cycles/second. How many little dots should you see? 12. (1/10 of 120).
Now he could ask you a question about a half wave rectified unit. If he did a spinning
top test on a half wave rectified with the timer set at 0.1 second, how many dots would
you see? 6. If you did a spinning top test on a high frequency unit, how many dots
would you see? You would not see any dots. You would see an arc. Is a spinning top an
adequate piece of equipment to check the timer on a high frequency unit? No, because in
a 360 degree circle and only 1/10 of the circle was taken, how are you going to figure that
out? It would be good to check on second (full circle) or half a second (half circle). Any
questions? So to decrease the ripple you decrease the frequency. Did he talk about 3
phase systems? Jus so you know, and I’ve only seen a couple of questions about this,
before they came out with high frequency, and when they found out that single phase was
not good enough, they had what was called three phase. Three phase uses 3 power cords
coming into a generator as opposed to 1. So what they did with the ripple is they stacked
them on top of each other.
The tube
If you’ve not seen an x-ray tube, here is one. This is what’s inside the metal housing.
This is the cathode on this end, which is negative. This is the anode on the other end.
The anode spins. It spins at 3000-3500 rpm. If this blows in your x-ray machine, it will
cost you $5000. They used to have stationary anodes. The problem with stationary
anodes was heat loading. They got too hot. So they started spinning the anode. What’s
the anode made of? Tungsten. What’s the melting point of Tungsten? Pretty hot! Now
you always know the anode side and the cathode side. These are stupid little things but
I’ve seen them on a test. The cathode side has on wire going through into it. The anode
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end has tow wires going into it. One is a positive wire, and the other wire goes to the
motor that spins the anode. If you don’t know which end is which, look at the number of
wires going into each end.
The angle of the anode
There’s an angle on these anodes. If you look close you can see it on the one being
passed around. The average angle is about 14 °. Now, I’ve seen on tests, answers like
12, 14, and 15. Now he won’t have all of those listed. He’ll say the average angle is 5°,
15°, 30° or 45°. So now that the average angle is about 14°, plus or minus a couple that
is the angle on the anode.
He might have you calculate heat units. As the electrons hit the anode it gets hot.
Remember he said 99% of it is dispersed as heat. The machine is only 1% efficient. The
way you calculate heat on a single phase unit is multiply the kVp and the mAs. To
calculate the heat units on a high frequency machine (there’s no ripple so the number has
to be bigger) is to multiply the kVp, the mAs, and constant 1.42. I’ve also seen the
constant on a test being 1.35. So I’ve seen a couple of different numbers. I don’t know
what he has in the book it’s 1.42. So that’s how you calculate the heat units of a HF unit.
Now getting back to the x-ray tube. The tube has a glass housing on it. The purpose of
the glass tube is to provide a vacuum. Just like a light bulb if you break the vacuum on a
light bulb the filament will burn out. A brand new anode is smooth. When the anode fails
it starts to get rough because it gets too rough and little pieces of tungsten are flinging off
of it. When that happens it’s called plating. The melting point of tungsten is 3410 ° C.
You can actually see it on the inside of a glass housing if the anode gets too hot or blows.
Anode heel effect
The area that the electron hit is called the focal spot. If you look at the side view of the
anode, notice the angle on it. Where you see the arrow, that is the heel of the anode. It’s
the part that points down, the very bottom of the anode. Now there’s a phenomenon that
occurs, and I have four different sources that say four different reasons why. But we’ll
go over what Dr. g told you. As the electrons come from the cathode and hit the anode,
and they have that interaction, and photons are produced, more photons are produced on
the cathode than the anode side of the tube. The reason why is because of absorption of
the newly formed photons that come out of the thicker part of the anode. On the anode
end of the tube there’s more absorption of photons. That’s why there are fewer photons
that come out the anode side of the tube. Another way to say this is fewer photons come
out on the anode side, more come out on the cathode side. This is because of the
absorption of the thicker side. What they are saying here is that in this area there’s more
absorption of photons, that’s why there’s a lesser percentage of photons coming out.
Some questions he may ask about this (guaranteed) if you x-ray somebody’s femur; the
femoral head is thicker than the knee area. Will you have the femur head under the anode
side or the cathode? The cathode. If you x-ray somebody’s lumbar spine, will you have
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the cathode over L-5 or L-1? L-5. Have you done your room drawings? Keep that in
mind. He’ll take of 25% if you don’t remember to account for the anode hell effect.
When you go down to the x-ray room there’s a reason the upright bucky is on that side of
the room. It’s the anode hell effect. Another question he may ask. Look at the
percentages down below. What difference is there in radiation or mAs? What
Difference is needed to see a change on the film?
If you have 10 mAs and you increased it to 11 would you see a difference on the film?
No. Dr. G says it takes about 10% more (or less) to see it on film. Will you be able to
see the anode heel effect on a film? Is there a 30% difference between one end of the
film and the other? YES.
Line focus principles
This example represents a 12° angle on an anode. We’re using the large focal spot or the
large filament of the cathode. What that represents is a large focal spot or the electrons
coming in and hitting an angled surface. When those electrons hit an angled surface and
produce photons, there’s a primary beam given off that’s smaller than the actual electron
beam hitting the target. As the angle increases the effective focal spot, and as the angle
decreases the effective focal spot decreases. This is why you use a large focal spot or a
small focal spot when x-raying a patient. When you use a small focal spot you get a
small effective focal spot (more detail). This leads to the subject of penumbra. If you
compare this to B over here, we see that A used a large filament, and in B we used a
smaller filament. So your effective focal spot is smaller and more concentrated when you
use the small filament. Basically, this says you are controlling how big your primary
beam is. The smaller the primary beams the more detail you’ll have. So that is why you
have to use a small focal spot when you x-ray someone’s chest or extremities. Why
would you use a small focal spot for everything, go back to your heat units. Look at the
image on the left. The surface area of the large focal spot is bigger. So all factors remain
constant. If you use 100 heat units dispersed over a larger area, you will not heat up the
anode as much. If you sue the same 100 heat units on a smaller area you could burn a
hole in the anode. That is why you do not use a small focal spot when you x-ray the
lumbar spine. You could probably get away with a couple of shots doing that (small focal
spot), but if you do it routinely, you will be spending 5000.00 for a new tube.
Let’s talk about the penumbra. Umbra means image proper. The (PE) part before that
means almost. Now if you look at the green area on there this area represents your
patient. This is the tube. This line back here is the film. The distance between the tube
and the film is the FFD (focal film distance). Back in 1980 they came with other sources
than focus, so you may see terms like, SID or source image distance, meaning the source
is the tube, the image meaning the film. When we use digital radiography we do not use
film, we use an image plate. So, FFD and SID are synonymous.
The term image is synonymous with the term film. He is going to ask a lot of questions
about this. I do not know what he is going to ask exactly, but you need to get this concept
down. As you increase the object film distance, or the distance between the patient and
the film you get magnification. The image on the film becomes larger. Think about the
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analogy of a flashlight and you hand. You’ve got the flashlight in your hand and you
have the wall behind you. As your hand gets closer to the wall, the image, the shadow,
gets smaller. And as you move you hand away from the wall to the flashlight the image
or shadow gets larger. The same phenomenon occurs with x-rays. Now if you look at the
shadow produced on the wall, the further from the wall you get, the more blurred the
shadow gets. That blur is penumbra. So, as we move the object away from the film the
penumbra increase and the umbra increase also. This is simply magnification. Penumbra
is an unsharpness. He is going to throw all of these at you. You have got to be prepared to
answer these. The best way that I can explain it is a flashlight, your hand, the wall, and
the shadow. If my hand is 1 foot away from the wall and 2 feet away from the light, the
shadow is going to be a certain size. As I move the light further back the shadow is going
to get smaller. So he is going to ask something like: everything else remaining constant if
you increase FFD or move the tube away from the film will your image have more or less
penumbra. You will have less. This is one reason why you shoot chest films at 72”,
because you want to have as much detail as possible. In hospitals today the y shot chest
films at about 10 to 12 feet.
It all depends on your generator. The generators here have a max mA of about 300 to 350
mA. Units that can shoot at 10 to 12 feet have mA of 1200 and 1500. So you can shoot at
12 feet and still keep your time down to .1 sec or so. In a hospital when you do
angiography, or an arteriogram, they shoot the vessel at about 9 feet, to get the detail. Just
so you know (he won/t ask you about this). If you think about it geometrically, using a
large or small focal spot, there are differences in penumbra between the anode side and
the cathode side.
As the photons come out of the tube itself they go through a collimator. What is the main
purpose of a collimator? It is radiation protection for your patient. It limits the exposure
to your patient. State laws require (federal law) that you must show at least 3 borders of
collimation on your films. If you are not you are breaking the law.
We are going to go back to the cathode real quick. On this cathode you will notice the
filaments are housed inside a little indentation. These are focusing the electrons (as well
as possible) toward the anode. This is called a focusing cup. It is made out of nickel or
molybdenum. The purpose of the focusing cup is to direct electrons not photons to the
anode. I have seen him ask questions about the focusing cup. He will have one answer:
none of the above. He asks the question using the word photon instead of electrons. So
you are lead into thinking it focuses photons at the anode. NO. It does not it focuses
electrons. He really interchanges photons and electrons in his questions so be aware. \
?
Inside of your x-ray tube (we’re talking about off focus radiation now) when electrons go
across, they don’t all hit the anode. The focusing cup does its best to direct the electrons
to the anode, but it doesn’t do a very good job. Those electrons hit other things inside the
x-ray tube. It hits the bolt that holds the anode. It hits wires inside the tube. It hits
different things. So when electrons hit tungsten of the anode they produce characteristic
or bremstralung radiation. When the electrons hit other stuff those produce their own
characteristic or bremstralung radiation too. When it does that, more than just the
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primary radiation of tungsten comes out. There is other primary radiation from other
components that also come out of the tube. When that happens that is call off focus
radiation. Another term he likes to use is second source radiation, meaning the first
source is the tungsten and the second source is the other stuff in the tube. Where do you
see this now? You’ve all had full spine x-rays done. Have you ever noticed the x-ray is
collimated down to your spine? But you also can see a shadow of your ears on the
image. Or you can see the shadow of the traps of the shoulders. You can see the fat
folds. The primary radiation is giving you the image of the spine. The off focus
radiation is giving you the image of the ears and shoulders etc.
I’ve seen questions where they’ve got a full spine film and an arrow pointing to the
shadow of the ear and what causes this? It’s the second source or off focus radiation.
Another term he uses is extra focal radiation. You can see in their image how this
happens. It comes off of all the components in here. The collimator does its best to
collimate the primary beam. Another use of the collimator is to block the second source
or off focus radiation.
The next subject is grids
Does everyone know what a grid is? The primary use of a grid is to absorb scattered
radiation. When an x-ray goes through a patient, scatter is produced. But when the
scatter comes out of the patient it is call remnant radiation. Primary radiation goes in and
remnant radiation goes out. In that remnant radiation there is scatter radiation. The grid
cleans up the scatter. A grid works like mini blinds. When you have the sun coming
directly through you get the primary radiation coming in. When you close the blinds you
still get primary radiation, but the areas above and below the space block it. A grid
works in the same way. Now you have different grid ratios. The most commonly used is
12:1. The grid ratio is the height of the lead strip divided by the distance between each
strip. So this is the way grid looks. In the center of the grid, the lead strips are straight.
As you go out to the periphery, the strips angle. As the beam comes out it does not come
straight out; it diverges. The grid is set up to have the divergent rays come through it.
This is called a focus grid. If you look at this grid, these strips in the center are straight
up and down, and as they go out to the edges they angle. They angle in accordance to the
x-ray beam from the tube. This is called a focus grid. If you have a dedicated 40 inch
focus grid, can you use it at 72 inches? No, because the divergence is different. This was
on a test 3 trimesters ago. What happens if the grid is flipped upside down? What would
it look like? The center part, since the strips are straight, will allow the primary beams to
come through. The divergent beams are now running into backward angled strips of the
grid. The image will have black stripe down the middle and nothing (gray and white) on
the periphery. This is how Dr. G puts films into your test for the midterm and the final.
What he does is when you go down to the lab and Atherton or Mather or Rocky are doing
the workshops with you and you do the spinning top or you have a grid, well they will let
a student x-ray the grid. The student doesn’t know there is a tube side to the grid, and
they will have the tube side toward the table and produce the image. When Dr. G sees it
he’ll say “MY, that’s a screw-up”. And then it shows up on your test. Also one time
students were down there and the big cable that comes down got under the collimator.
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And when the film was developed there was this big thing with cross-hatched stuff on it
coming through the film. That was on the midterm. That time was when 30 students
were supposed to be in a lab and only 5 showed up. Those 5 were the only ones that got
it right. Do you have images in your classroom of all the labs that were done? You may
want to network with your classmates and look for bizarre things like that because he will
use them on a test. Now we’ll talk about x-ray film. X-ray film is a piece of polyester
plastic called DMT, which stands for dimethylterphthalate. That’s what the base of x-ray
film is made of. Now on this base they have an emulsion. This emulsion is made from
animal fat. Its gelatin and they take this gelatin and throw in these silver bromide crystals
(95 % is silver bromide and 5% is silver iodide) they spray it on both sides of the film.
That’s what x-ray film is made of. Just an historical note, there’s a hospital called St.
Anthony’s hospital. Back in the early day of radiology they didn’t have polyester plastic.
They put the emulsion on a glass. SO if you go into the museum of x-ray, the first images
were put on glass. Then when they came out with plastic, they used the same emulsion.
That emulsion, instead of being fat based or gelatin based was nitroglycerin based. And
that hospital burned down in the late 60’s because somebody was smoking in the film
library.
Now when you shoot an x-ray, what produces the image on the film, light or x-ray? It’s
light. 99% of the image that is developed is produced by light. About 1% is from
radiation. The screens in the cassette produce light. When the photons come in they
interact with the phosphors that are in the screens. The screens emit light. This light
produces the image on the film. Before you develop an x-ray, you’ve expose it. Prior to
developing it, it’s called latent image. I think I remember the question correctly, “is a
latent image diagnostic?” The answer is NO, it’s undeveloped. It’s in the dark and you
can’t see anything.
The screen in the cassette contains phosphors. This is why you have so many types of
companies. Here at Logan you have Kodak. You notice that the Kodak screens are
called lanex. Lanex is short for Lanexium. That’s the name of the phosphor. They
discovered these phosphors. An Indian discovered these (on a historical note) out in the
field. The gamma and UV radiation coming in from space would interact with these
phosphors that were in these big rocks. When it was pitch dark at night, these would
glow. And the Indians worshipped these as gods. Now it was Thomas Edison in hi lab
that had a radiation source and different types of phosphors in the lab. And one night
when he energized the source he saw these phosphors glow. Then he put together how
the phosphors and the screens could produce an image. Now this is how these companies
compete. They have different types of phosphors. Have you seen the experiment where
they open the cassettes and expose them? You’ve seen the glow they give off. Some are
green. These are called orthochromatic Green emitting phosphor or simply
orthochromatic.
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Dr. Guebert also talked about PARALAX. Look at a cassette. You have a tube side to
the cassette. As the x-ray photons come true the patient and the remnant radiation comes
through, it goes to the grid, goes through the table and by the time it hits your cassette it
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will hit the tap screen first and that screen will emit light. It will go through the film and
that photon will hit the bottom screen and that bottom screen will shoot light back up.
That photon is coming in at an angle and so will produce a latent image on that film.
That same photon will produce another latent image. It is impossible for these images to
be perfectly superimposed. That lack of superimposition is called PARALAX. It will
interact with the emulsion on one side and the same photon will interact with the
emulsion on the other side. Now guys were are only talking microns here Ok? But in the
early days when the film was thicker PARALAX was a problem. The best image you
could get was blurred.
CHEMISTRY
When a film is processed this is a reduction reaction. When the latent image goes in the
developer it is called a reduction reaction. That is why they called those things reducing
agents. Phenidone is a reducing agent in the developer that rapidly produces detail
shades of gray. Hydroquinone is a reducing agent in the developer that slowly produces
heavy densities. He will not ask questions about the activator but will ask something
about the restrainer, potassium bromide (which decreases reducing agent activity, antifogging agent). I guarantee he will ask a question about the hardener, glutaraldehyde
(hardens emulsion, reduces gelatin swelling). So the two reducing agents, the restrainer
and the hardener.
Most processors are 90 seconds. So he will ask you some developing time. How long
does the film stay in the developer? 20 to 25 seconds. What would happen if the film
would stay in the developer too long? It would get too reduced, it would get too dark.
What would happen if too much oxygen (room –air) stays contact with the developer too
long? The developer gets oxidized. Also know the purpose of water in the developer
solvent.
Let’s talk about the fixer. In a 90 sec. Processor your x-ray film is in the fixer for about
20 sec. When I say about 20-25 sec. For developer and about 20 seconds for fixer it all
depends on the temperature. As the T is hotter, things seem to cook a little faster. So
heat it up, and your film will get darker. If your film comes out too dark out of a
processor, your technique can be perfect but if your developer T is too hot, it could come
out black.
Now let’s get back to oxidizing in the developer. If your developer is oxidized it will not
be as efficient so your film will not be reduced. It will produce a light image.
Now let’s talk about fixer. The main clearing agent is ammonium thiosulfate (removes
undeveloped silver halides from emulsion). You walk in the dark room a Monday
morning after that processor has been sitting there all weekend. It smells like ammonia in
the dark room. The other thing he likes to ask about is the activation: acetic acid
provides acidic pH, stops reduction). If you go to the fixer tank itself you can smell
vinegar. The hardener is potassium alum (hardens emulsion). The preservative is
sodium sulfite (maintains pH). The next thing your film goes through in your processor
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is your wash tank. And it just washes all the chemicals off (about 20 seconds of
removing developing and fixing chemicals). And finish with dryer for 20-25 seconds
(evaporates water and hardens emulsion).
He will ask questions about manual processing. They separate out the clearing agents
and your hardeners and those are called stop bass.
All these times should add up to about 90 seconds.
Characteristic curve (also called H and D curve)
This is a graph of the density and the relative log of exposure. The bottom of the curve is
called the toe and then you have the straight portion and the top portion is the shoulder of
the curve. The toe of the curve is controlled by the PHENIDONE (the reducing agent in
your developer). And what phenidone does is produce the shades of gray in your film.
So if you look at the relative log notice that the shoulder will show darker and that the toe
will be lighter. So phenidone controls the toe part of the curve of the gray area and the
hydroquionone produces your darker density. That’s why you have 2 reducing agents in
your developer. One produces the shades of gray and one produces the shades of black.
Why do we have an H and D curve? We have them because we have different types of
film out there. Here at Logan the film that we use in our cassettes is G type film. The
film that we use in the 36 inch cassette for long spines that is your H type film. Your G
type film with the screen that they are in produces a 600 screen speed. These are your
black border cassettes. The G type film is in your yellow cassettes both gray border and
black border the G type film in the gray border cassette with those screens produce a 100
screen speed. So gray border G type film = 100 speed. Black border G type film = 600
speed.
So if I shot an x-ray using 60 mAs on a 100 speed gray border cassette and now I want to
shoot the same image on a black border cassette, how much mAs should I use? It’s 600
speed. It’s 6X faster than the 100 speed so 10 mAs.
The film screen combination in your 36 inch, the H type film is 1200 speed. We have
different types of film speeds. Here we are talking about the same type of film in the
gray border and the black border cassette. The only difference is the speed of the screen.
So in the situation we are not comparing gray border and black border. What we are
comparing here is your film from your G to the H. SO you got G film in your gray and
black and H type film in your 36 inch. Those films have different properties. One is able
to react to light faster than the other. So the one that gets darker first is your faster film.
So in this example you can see that film A gets darker first before film B. The diagnostic
range of density on the film that is what our eyes can see is .25 to 2.5. You have 2 films
in front of you. One has a density of 2.5 and the other a density of 5. It looks the same
degree of blackness to us. We can’t see the difference.
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There is another type of film we use here. This film is your very special film that is
called insight. Insight is used in chest x-ray. The system that they have here for insight
film in the screen is about 250 speed system.
If you look at these films up here, A and B, I hope you can understand that film A will be
more just black and white. It’s not covering a very broad range of density so it will have
very few blacks and whites. Now, your B type film is covering more ranges of density so
you’ll have more degrees of gray on that type film. Now when you look at the insight
system, the insight system produces many shades of gray, very many. This is called film
latitude. The film latitude is that film’s inherent property to be able to produce many
shades of contrast.
Different types of interactions
You have 4 main types of interactions. We will talk about what happens inside the x-ray
tube and these involve electrons with the electrons hitting the anodes. There are 2
interactions that occur. The electron binding force of tungsten is almost 70. So when
you are using kVp less than 70, less than the binding force of the target you are shooting,
you are looking at Bremstralung radiations^. These occur when the incident electron
goes around the nucleus. It is coming in at half the speed of flight. It goes around the
nucleus and when it does that it give out energy. When it does that it produces an x-ray
photon.
The second type of radiation occurring inside the x-ray tube is when you reach the
binding force of the material you’re hitting. Now since different materials have different
binding forces they call this characteristic radiation. This is characteristic of the binding
force of tungsten. So when you shoot an x-ray on 70 kVp and upwards you now have
both primary radiation of both bremstralung and characteristic. When you shoot an x-ray
using greater kVp than 70, your primary radiation consists of those radiations. So
remember, if he mentions characteristic radiations or bremstralung radiations, these are
radiations that occur from incident electrons. It happens in x-ray tubes.
Now, with Bremstrahlung and characteristic you have now produced the photon. The
next 2 interactions happen in your body. The first type of interaction is call Compton
interaction. This is what the scatter is. The photon interacts with the outer shell electron.
When the photon goes through and interacts with the cells in your body it produces
scatter. It is knocking of an outer shell electron.
So what happens to the electron? It becomes a free radical ion in your body. So with an
x-ray, what is the most damaging? The x-ray or the free-radical? The free radical.
Now, when that photon comes in your body, there is another thing that happens. It gets
absorbed. That is called photoelectric absorption. When that occurs, it knocks out an
outer shell electron. The outer shell covalent bond rolls in and fills that in. And when
that happens a second photon is produced.
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So, if he asks you anything about secondary radiation, secondary radiation is a
phenomenon of photoelectric absorption. So what does that mean? For an instant when
your body gets and x-ray it becomes an x-ray tube.
The confusion can occur for the student at the level of: “is it produced in the tube or in
the body?” “Does it involve an electron or a photon?”
Is the photoelectric effect increased… ( so the more absorption you have which happens
at lower kVp you have more absorption and less Compton scatter), when you use higher
kVp, you are decreasing the absorption simply because the binding energy of your bones
won’t be able to stop the radiation, so you’ll have more incident interaction with outer
shell electrons. SO the higher kVp will produce more Compton scatter.
Last moment stuff
You have five densities that show up on an x-ray. You have air, water, fat, bone, and
metal. (Mnemonic: All Women Find Big Men). When you take an x-ray of all of these
things, air and water will who up black on your film and that is called radiolucent. So
your lungs are radiolucent. Now bone and metal are radio opaque and will show up
white on film because the light will all be absorbed. Fat is considered to be in between
Your ID camera in the dark room when you flash the ID on the film, this is a medical
document.
The light in the dark room is 15 watts and has to be 4 feet from the counter top.
And you all know how to work your Supertech. You’ll need it for the exam. The
important thing to know about it is your correction factors. The supertech is set up for
40’. So if any images require 72’ then you have to incorporate that correction factor. If
he asks you anything about an osteoporitic factor or an Arnold Schwarzenegger type man
(muscular) there are correction factors for those on your supertech.
The 10 day rule is form the start of the menstrual cycle. For example, a y/o female
measures 43 cm, correction factor in the room is ------.