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X-ray interactions with matter can be understood as the following.

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There are four major interactions which are very critical for diagnostic

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radiology and nuclear medicine. Among the four interaction,

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the classical scattering, also called as Rayleigh or elastic scattering,

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Compton scattering, photoelectric effect, and pair

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production.

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Among these four, the main area of interest are the

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Compton scattering and photoelectric effect because these two

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type of interaction are the dominant way of creation of the

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images in diagnostic radiology and nuclear medicine.

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I'm not going to talk much about classical scattering because they're

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usually at a very low energy interaction, and they

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don't penetrate through the body. They just absorb the surface of the body.

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Because of that, we try to remove those low energy X-ray

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beam, which I'm going to explain later.

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So therefore, classical scattering is not of interest for our discussion.

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Similarly, pair production only occurs when the energy is

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higher than the one MeV, which is not, again, of our interest.

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We are going to focus on these two, Compton scattering and photoelectric effect.

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What is photoelectric effect?

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The photoelectric effect is an interaction of incident radiation or

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photon with inner shells of electrons of any matter.

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When this interaction occurs, when the external radiation

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can knock off an electron, or by transferring all its

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energy, it can eject an electron, and that is called

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ejected photoelectron

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as that is equal to the kinetic energy and minus

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the binding energy. So it can show in this image down here,

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an 100 kV incident photon is interacting with matter,

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and this particular atomic structure, the energy is sufficient to knock

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off an electron, and the electron moves out with certain

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energy. There's a difference between the binding energy and the

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energy incident, and that is called the photoelectron.

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More importantly, when this particular electron is knocked off, there is a

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remaining empty shell. In nature, what happen is the electron from

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a higher shell will jump into the lower

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shell. By doing so, there releases a difference in the energy

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called characteristic X-ray. So we're going to see that the empty

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shell immediately filled with electron from outer orbital,

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resulting in emission of characteristic X-rays.

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And those energies are very unique.

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For example, in a iodine matter, when radiation interaction, the

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K-shell energy is about 34 keV, L-shell electron is

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about 5 keV, and M-shell is 0.6 keV.

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I'm going to bring this together when we do the radiography on how the X-ray

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imagings are formed.

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One of the fundamental relationship is the energy of the

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binding energy is proportional to the atomic number squared,

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Z squared. So the photoelectric effect leads

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to either characteristic X-ray and/or auger

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electron. Why is this important? We're going to discuss later when we

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discuss mammography physics.

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The probability of the photoelectric absorption is given

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as follows. It is proportional to Z cube, that is the

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atomic numbers cube, divided by

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energy cube. Means greater the energy of the

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incident radiation, lesser the chances of act-- happening with

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the photoelectric effect. Whereas the lesser energy

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means there is lar-- more chances of having this photoelectric absorption

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probability.

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This explains why contrast decreases as

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higher energy X-rays are used in imaging process.

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Again, I'm going to bring this tie it up together with image contrast

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later. But the photoelectric absorption is important to

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understand. It is one of the key interaction mechanism of radiation

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with matter, and especially when the atomic number of the

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interaction material are small and the energy is low, the

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chances are much higher. And that also brings back to the

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contrast agents we use, like iodine contrast and so forth thereforth.

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If you look here, the photoelectric effect of tissues, on the

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left-hand side, the x-axis is X-ray energy, on

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the y-axis is mass attenuation coefficient.

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If you look through, for a tissue, the absorption is very

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high at the low energy and goes down

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dramatically.

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Now, if you add a contrast material, you see this type of a

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spectrum. And in this type of a spectrum means around

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this area, it's called the K-edges, the energy is

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more absorbed, therefore, the signal spikes up, increases,

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and that's why you use contrast agent, to enhance the image

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quality. And the two commonly used contrast

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agents in X-ray imaging is barium contrast and

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iodine contrast. It's pretty same for fluoroscopy,

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radiography, and CT. And this is the reason why,

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because they have a K-edge around in a convenient

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area, which is in the area in between the dynamic range we use.

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On the right-hand side, if you'll see, again, this thick is listed in

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the keV energy, and on the y-axis is the

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percent of photoelectric absorption

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and also the percent Compton scattering on the right-hand side.

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If you look here, for most of the diagnostic energy range,

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this is the range between 10 to 100 keV.

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In fact, we generally go up to 60, 70 keV, that's all.

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In this case, the majority is this Compton scattering is

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very high in the low energy and decreases as we go

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along. But there is a Compton energy comes into-- Compton

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scattering comes later. But these things, the sodium iodide and

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lead, are discussed in nuclear medicine.

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I'm not going to discuss here, but this is the range of interest we are interested

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in photoelectric effect for tissue.

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So if you look in the attenuation coefficient, soft tissues

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or bone, what happens here is trying to match

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with respect to the absorption, the type of

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interaction of the radiation with the tissue, and how does

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that translate to imaging. This is an image of a chest X-ray, and

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this is an imaging of a mammography.

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In mammography, it is all soft tissue.

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There is no bony structure. So a lot of this is done in the

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photoelectric range here.

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Whereas a chest X-ray range includes both a wide range of

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tissues from bone to soft tissue to air.

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We work in the range of the Compton area.

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So for most of the energies which we use in imaging,

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the predominant interaction is photoelectric absorption or

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Compton scattering.

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How do we make an image? This is showing a lateral

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X-ray of a knee, and the way it can be done

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is the detector can be placed according to the patient, wherever you want,

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and then the X-ray tube is automatically positioned to get an image.

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You can see there is a contrast between the bony structure and the

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soft tissue structure, but within the soft tissue structure, you don't see the

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difference between muscle and soft tissue and so forth.

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Just to give you an idea. Therefore, we

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need to get an image for diagnosis.

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We want tissue contrast.

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There are four major categories of materials and tissues

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that produce tissue contrast in radiography.

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The four major categories are air, adipose or

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fat tissue, soft tissue, and bone.

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So the image contrast depends on the radiographic technique we used.

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What technique we use will determine the tissue contrast.

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As you can see here, this is a complex image.

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The chest X-ray is one of the complex imaging technique because it has to

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balance between the bony structure, the lung air,

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and also the other soft tissue. So it should accommodate

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all these things when it does the imaging.

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Whereas in mammography, we're typically imaging the fatty

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tissue and glandular tissue or the soft tissue, so there are no bony

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structure, and that makes a determination of what type of technique we are

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going to use.

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This is the basic radiography system component.

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Typically, this is a tabletop X-ray system, radiography system.

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The basic component are the X-ray tube,

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and we have aluminum filters for the reason I'm going to explain later.

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We have a collimator, which will adjust the X-ray field

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size to the anatomy of interest we are imaging.

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Underneath the patient, we have what is called an antiscatter

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grid.

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We also have a photocell, which will automatically adjust

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the technique based on what it is seeing, the patient

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thickness, and it's called as automatic exposure control.

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Then we have a detector. We have detectors from film screen,

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older days, to digital detector, and I'm going to discuss all these

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things as we go along. So we have source on one

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side, detector on the other side. Typically, for a

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tabletop radiograph, the distance between the source to the

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detector is about 100 centimeter or approximately 40

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inches. That's a typical distance for the basic

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X-ray of the abdomen or any of the body part

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when the patient is lying on a tabletop.