Transcription
So we've looked at the photoelectric effect, which is the primary interaction responsible for providing anatomic detail within our radiograph. Now let's look at the primary mechanism for scatter within the patient, known as Compton scatter.
Now, scatter is not what we want. It provides patient dose and it reduces our image quality by decreasing our image contrast and increasing the noise within the image. So, as we've looked at, there are three things that can happen with incident x-rays: they can be transmitted and reach our x-ray detector, they can be attenuated and cast a shadow on our x-ray detector, or they can be scattered. Primarily, they're scattered through the Compton effect or Compton scatter.
Now, these scattered x-rays, when they hit our detector, they are not congruent with that incident x-ray beam. So the signal on the detector provides us no anatomic detail from our image. All it does is decrease the contrast and increase the noise within our image.
So let's have a look at how Compton scatter occurs. We have an incident x-ray here coming from our x-ray tube. These are either Bremsstrahlung or characteristic radiations at varying different energies in our x-ray spectrum. That incident x-ray then interacts with an outer shell electron in our patient's tissue here. Now, that x-ray, unlike the photoelectric effect, doesn't confer all of its energy to that electron. Only some of that energy is given off to this electron. We still release that electron from the outer shell here, and this is still called a photoelectron. This is what's going to apply those to the patient's tissue again through the process of linear energy transfer, which we'll look at shortly.
Now, this x-ray that collided with the outer shell electron will scatter at an angle, known as our scatter angle. The angle between our incident x-ray and our scattered x-ray here, this theta here, represents our scatter angle. Now, this photoelectron will have an energy that is proportional to the scatter angle. The larger the scatter angle here of this Compton scattered x-ray, the larger the energy, or the more the energy of this photoelectron. This energy of this Compton scatter is determined by the incidence energy and the scatter angle here.
Now, you may be wondering, what about the binding energy here? Well, in tissues, our valence electrons, the binding energy is so small that it's actually inconsequential in these calculations. What we need to know is the larger the scatter angle, the more energy that's being conferred to this photoelectron, and the less energy this Compton scatter has. So, as we increase the scatter angle, we decrease the energy of this Compton scatter. You can think about it as a billiard ball. If we have a glancing blow, very little of that energy has been transferred to the photoelectron. If we have a large collision with a high scatter angle, we lose a lot of energy in that collision, and most of that energy is released in the form of this photoelectron's kinetic energy.
Now again, we can draw this on the graph, and you'll see that Compton scatter is quite different to the photoelectric effect. In the photoelectric effect, we saw the likelihood of the photoelectric effect decreasing exponentially with increasing photon energy. We know that in a more dense tissue, this photoelectric effect will increase, and in a higher atomic number tissue, the photoelectric effect will increase even further.
Now, Compton scatter, in diagnostic energy ranges, has very little dependence on incident x-ray energy, unlike the photoelectric effect. There's a very small decrease in the amount of Compton scatter that happens as we increase our incident photon energy. Compton scatter's likelihood is largely dependent on the density of the tissue, or the electron density within that tissue. It has very little to do with atomic number because as our atomic number increases, so does our number of electrons, and that electron density remains roughly equal. So, Compton scatter is largely dependent on density, has very little dependence on photon energy.
Now, you can see at a roughly 30 keV photon energy, the amount of photoelectric effect and the amount of Compton scatter occurring is roughly equal, or at least the probability of these events is roughly equal. As we increase our photon energy, the contribution, or the interactions provided by Compton scatter, are much more in number compared to our photoelectric effect. So, as we increase our KVP, as we increase our photon energy, the signal provided to our x-ray detector has much more contribution from Compton scatter, relatively speaking, as opposed to the photoelectric effect. We get more noise within our image and less contrast as we get higher and higher exposures here with higher photon energies. So you can see that changing our photon energy changes the proportion of Compton scatter to the photoelectric effect.
So, the last interaction that we're going to look at is called Rayleigh scatter, or elastic, or coherent scatter, that occurs largely at lower than diagnostic energy ranges. Compton scatter and photoelectric effect are two concepts that you need to have clearly in your mind. They will come up over and over again in exams, and I've asked this in multiple different ways in the SAQ question bank that I've linked down below. So, if you're studying for your Radiology Part Ones or your registry exams, I'd highly recommend going and checking out that question bank. So, I'll see you all in our next talk where we will look at our last interaction, Rayleigh scatter. Cheers.