Transcription
Let's spend some time now having a look at the process of filtration, which is an incredibly important topic when it comes to our x-ray physics module.
Now, you'll see later on in this module, we're going to examine the process of X-ray production at the anode, and I've mentioned this a couple of times now: Bremsstrahlung radiation and characteristic radiation.
Now, this diagram I've drawn here is a diagram we're going to look at in some depth when examining Bremsstrahlung radiation. What I want you to see here is that we can accelerate electrons towards the atoms. This is a Tungsten atom within our anode, and it's the interaction between these electrons and this Tungsten atom that is going to produce X-rays of varying energies.
Now, you'll see the x-rays with longer wavelengths have lower frequencies, and when we looked at electromagnetic radiation, the lower the frequency of a wave, the lower the energy of that wave. As these x-ray interactions get closer and closer to the nucleus of our atom, the frequency of the x-rays produced increases, and that increase in frequency correlates to an increase in x-ray energy.
This is what's known as our unfiltered Bremsstrahlung radiation curve. It's all the x-rays that we have produced on our anode through the process of Bremsstrahlung radiation. The vast majority of those x-rays or photons that have been produced are low-energy photons. These photons will interact with matter within our patient, releasing electrons from the atoms within our patient. Those electrons, known as photoelectrons, will infer dose to our patient.
In these low-energy x-rays, they are much more likely to be attenuated and not reach the X-ray detector. So, they're going to contribute to patient dose but not contribute to our image that is received on the detector. So, these are x-rays that we don't want.
Now, what we can do is filter these x-rays. We can place a sheet of material between the anode and our patient, and we can reduce the number of x-rays that reach our patients.
Now, you may be wondering, why are the low-energy x-rays being taken out of this x-ray spectrum, but we're still keeping these high-energy x-rays? This is the fundamental concept that we need to understand when it comes to filtration. The process of filtration occurs via an effect called the photoelectric effect.
The photoelectric effect is defined by this function here. This Tau here represents the likelihood of the photoelectric effect to occur. If we were to increase the density of our filter, we would increase the likelihood of the photoelectric effect to occur. We would increase the likelihood of x-rays to be attenuated. If we increase the atomic number of our filter, we often use an aluminum filter, which has an atomic number of 13. If we were to increase that to a Tungsten filter with an atomic number of 74, we would get much, much more attenuation of these x-rays because our atomic number is exponentially proportional to the likelihood of the photoelectric effect to occur.
This is something that remains the same for set filters, so it doesn't change. What does change is the energy of the X-ray photon interacting with that filter. If we were to increase the energy of the X-ray photon interacting with our filter, we would decrease the likelihood of the photoelectric effect to occur because this is divided by energy. As energy increases, our denominator increases, and our likelihood decreases. So, our low-energy x-rays give a low denominator, a higher likelihood of the photoelectric effect to occur, and that's why our filters preferentially filter out our lower-energy x-rays.
Filtration is the process of removal of x-rays from an x-ray beam, and it does this by removing preferentially these lower-energy x-rays because of this photoelectric effect. The photoelectric effect attenuates x-rays; it absorbs the entire x-ray energy and releases a photoelectron. That x-ray is completely absorbed and won't reach our patient, and because lower-energy x-rays are preferentially absorbed, we get these higher-energy x-rays going towards our patients.
This is what's known as the filtered Bremsstrahlung X-ray Spectrum. This is a concept that we are going to go over many times again in this course, so use this as a starting point, a conceptual framework that we can build on throughout the course. I also use this formula over and over again in the past paper question bank that I've created. I've linked that in the top line of the description. If you're studying for your Radiology physics exams, I'd highly recommend going through that question bank. They're high-yield questions, and I take you through the answers one by one, showing you how I would go about answering those questions.
Filtration occurs at many different points within the X-ray beam. We can broadly classify filtration into two separate categories. The first is inherent filtration.
Inherent filtration refers to the inherent filtration that occurs within our x-ray tube, and this is something that we generally can't change. It refers to the glass envelope, the conducting oil, and the X-ray window within our x-ray tube. All of these will attenuate the beam to some extent. Our glass window is predominantly made of silicon, which has an atomic number of 14, which is similar to aluminum that we will use in other filters, which has an atomic number of 13. So, this glass window attenuates some of those lower-energy x-rays. The conducting oil will also have interactions with this x-ray beam, and the photoelectric effect will occur to some extent there. And x-rays leaving our x-ray window, depending on what that window is made up of, whether it's beryllium, an atomic number of four, or whether it's aluminum, atomic number of 13, we will get some inherent filtration here. Filtration that we can't really manipulate because it is part of the X-ray tube itself. In our next talk, we're going to look at a process called collimation, and there's a mirror within our collimator that will also contribute to inherent filtration.
Now, we have inherent filtration and we have added filtration. This is fairly self-explanatory. This is filtration that we add to our x-ray beam. Inherent filtration comes to a value of about 0.5 to 1.5 millimeters of aluminum equivalent. Inherent filtration is the equivalent of us placing an aluminum sheet that is between 0.5 and 1.5 millimeters thick. Now, we can add another sheet of metal there. It could be aluminum, it could be beryllium, copper, tin, to further attenuate those low-energy x-rays. Our goal here is to reduce those low-energy x-rays, to remove them from our beam, because they contribute to dose but they don't contribute to our image.
Both inherent and added filtration change our x-ray spectrum, and we have a full section later on in the course completely dedicated to the various different factors that can change our x-ray spectrum. But you can see here, our yellow graph represents our unfiltered x-ray spectrum. We've got loads of these low-energy photons here. Now, we can never really create this unfiltered curve because in order to create x-rays, we need our x-ray tube, because we will have some inherent filtration within our tube. So, you can see that with inherent filtration alone, we reduce these lower-energy x-rays. We will remove such a large portion of these lower-energy x-rays. We can then add more filtration, place another filter between the X-ray source and our patient, and that will further reduce the X-ray Spectrum.
What we're doing here is we're changing two factors. We are reducing the photon number, the area under the curve, and that's what's known as our x-ray beam quantity. We are reducing the number of photons that reach our patient. We are also increasing the average energy of our x-ray beam. If you were to look at the yellow graph here, this linear graph, our average energy would be somewhere down here because the majority of these photons are low-energy photons. As we remove those lower-energy photons, we increase the average energy of our spectrum. Again, as we add more filtration, we increase again the average energy of our beam. This is what's known as x-ray beam quality. We are increasing the X-ray beam quality; we're increasing the penetrability of our x-ray beam.
An important thing to note here is that we never reduce our maximum photon energy. These high-energy photons are still making it through. Generally, we get less x-rays, but we still get some photons within our maximum energy range here. So, filtration increases our x-ray beam quality whilst decreasing our x-ray beam quantity, which is a process that we will go over again in a future talk.
Now, we can add a special type of added filtration known as a compensation filter or an equilibrium filter. The purpose of these filters is to get an even exposure to our x-ray detector. This is what's known as a wedge filter. We can attenuate more of the beam over the thinner part of our patient's body and allow more x-ray beam intensity to pass through over the thicker part of our patient. So, if you take this example here, we have an ankle, which is thicker than our foot here. Our ankle will attenuate our x-ray beams more, and fewer x-rays will reach our detector. So, we can use our wedge filter to allow most of these x-rays to go through, and they will be attenuated more by our thicker part of the patient. Over the thinner part, over our toes and our forefoot here, we can attenuate this x-ray beam more by this compensation filter, and then we get less attenuation in this thinner part of our patient. Hopefully, what that does is gives a more even exposure to our x-ray detector.
This is what's known as a bow tie filter. It's often used in CT scanners. Our x-ray tube is rotating around our patient, and the thickest part of our patient, even if this x-ray tube was rotating around, would be the center of our image. The sides of our patient here are thinner, so we can attenuate that beam more because it's got less tissue to go through, again allowing for more equal distribution of the exposure to our detector.
Lastly, this is what's known as a trough filter here, where our mediastinum, we're looking at the top of a patient here. Our mediastinum has our heart and our great vessels, our sternum on the front. Our peripheries have our lungs. The mediastinum is much more attenuating to x-rays than our lungs. So, we can attenuate the periphery more by these thicker parts of our trough filter and attenuate less over the mediastinum in this thinner trough here on our filter. So, our compensation or equilibrium filters allow for more even exposure to our x-ray detector by compensating for the attenuation that is going to occur within the patient.
So, that's about all we have today for filtration. We've got inherent filtration and added filtration, and in an x-ray beam that has an energy of 70 kVp, we want that inherent and added filtration combined to be the equivalent of about 2.5 millimeters of aluminum. Now, it's important to remember that compensation filters fall under the category of added filtration, and we can use them to compensate for the different thicknesses of patient tissue, all the different densities of patient tissue that we are imaging, and create an equal exposure to our detector.
So, in our next talk, we are going to be going over the concept of collimation, a process where we can reduce our patient dose while simultaneously creating a better image, a win-win situation. So, I'll see you all in that talk. Goodbye, everybody.