Transcription
So up until now, we've looked at the X-ray machine and the various different constituents of the machine itself. We've looked at the X-ray circuit that powers the X-ray machine. We've looked at X-ray production within the X-ray tube, as well as X-ray beam geometry, filtration, and collimation of that X-ray beam. We've also seen the X-ray spectrum and how changing various different factors affects the X-ray beam quantity, as well as X-ray beam quality.
These X-rays are now heading towards our patient, towards the tissues that we are trying to image as radiographers and radiologists. Now, there are three separate interactions that happen within the diagnostic energy range that I'm going to cover in the next three talks. And we're going to start with the most important to us, the photoelectric effect. We'll then move on to Compton scatter and Rayleigh scatter to round out the three talks within this X-ray interaction with matter talk series.
So let's start with the photoelectric effect. We see that these X-rays here, coming from our X-ray tube containing multiple different X-ray energies, are heading towards our patient. Now, three things can happen. One, the X-ray can be transmitted, pass right through or penetrate the tissues that we are trying to image. There is no interaction with this X-ray and the tissues itself. There has been no energy deposited here. That X-ray has gone through unscathed, reaching our X-ray detector.
The second thing that can happen is X-ray beam attenuation, absorption of that X-ray without any X-ray reaching our detector. And this is the process of the photoelectric effect that we're going to look at today. The third interaction that can happen is what is known as scattering here, where our incident X-ray is deflected at an angle, either through the Compton scatter or Rayleigh scatter interactions.
So today, let's start by looking at attenuation here, or the photoelectric effect. So what exactly is the photoelectric effect? Well, we have an atom here within our tissues, within the patient that we're imaging, and this is the incident X-ray beam that's coming from our X-ray tube. It's either Bremsstrahlung or characteristic X-ray radiation heading towards the patient tissue. Now, what happens is this X-ray will collide with an electron within that atom, and all of that X-ray's energy will be deposited into that electron. The entire X-ray energy has been annihilated there. All of that energy is now being conferred onto this electron.
That electron will then be released if this energy of this incident X-ray is higher than the binding energy of that electron. If it's higher than the binding energy, that electron is released, and the release of that electron is what's known as a photoelectron. And it's this photoelectron that is traveling through the patient's tissue that is going to confer the most dose to the patient. You see, when an X-ray travels through a tissue, it only interacts when it comes into a direct collision like this. When an electron travels through a tissue, because it's negatively charged, and X-ray has not got any charge, but an electron does, because it's negatively charged, it can repel surrounding electrons. And that is how we infer dose onto the patient through a process known as linear energy transfer, which we're going to look at after these three lectures.
So what energy does this photoelectron have? Well, we can use this formula. The energy of the photoelectron is equal to the incident energy of our X-ray minus the energy required to release that electron, the binding energy of that electron. So this photoelectron has the energy of this incident X-ray minus the binding energy of that electron. Now, we've created a vacancy here, and as we've seen in our characteristic radiation production talk, that vacancy will be filled by an outer shell electron. So here, an L-shell electron is dropping down from a higher energy state to a lower energy state, and that loss in energy is released in the form of a characteristic X-ray.
So you may be wondering, doesn't this characteristic X-ray then go off and head to our detector? And that is a valid thought. But there is a reason this characteristic X-ray will never reach our detector. When we looked at characteristic X-ray production, our target material was tungsten. It had an atomic number of 74. When we're dealing with tissues, X-rays interacting with matter within a patient, our average atomic number is seven. The binding energies here are much, much lower in patient tissue as opposed to our anode target material. And we know this characteristic X-ray being released is the difference between the binding energy of our K and our L-shell electrons here. This energy of this characteristic X-ray is very, very small, less than 1 keV, and that X-ray will be attenuated within the patient's tissues and won't be able to reach the detector.
So the photoelectric effect differs from characteristic X-ray production in a couple of ways. The most important of which is that the photoelectric effect has an incident X-ray that releases an electron. In characteristic X-ray production at the anode, it's an incident electron that is releasing this electron from our K-shell. So we've created an ion here. Our atom has one less electron than it had previously. We've created a low-energy X-ray, and we've created a photoelectron that is going to convert those to our patients.
Now, what is the probability of the photoelectric effect to occur? Now, we've looked at this before when we looked at X-ray beam filtration, when we preferentially removed lower energy X-rays by adding a filter in our Bremsstrahlung curve. And we looked at this formula here. The probability of a photoelectric interaction to occur, this probability is proportional to the density of the patient's tissue, the atomic number of that tissue to the power of three, and it's inversely proportional to the energy of that incident X-ray coming towards our patient.
So the denser a tissue, the more likely that X-ray beam is to be attenuated. If we look at bone, bone casts a shadow on our X-rays because more photoelectric effect is happening there. The higher our atomic number, the higher the probability of the photoelectric effect to occur is. When we give contrast, iodine, it has a high atomic number. We get a big shadow cast on our X-ray there because that high atomic number of the iodine is increasing the likelihood of the photoelectric effect to occur.
Now, perhaps counter-intuitively, as our energy of that incident X-ray increases, we get less and less photoelectric effect occurring. Now, the way I like to think about X-ray energies interacting with matter is by way of an analogy. Now, if you've ever looked at a ceiling fan, if you've been in a hotel and there's that fan on the ceiling and the blades are rotating at a set speed, think of those blades as the electrons rotating around our atoms within the patient's tissue. Now, if I was to hold a ball and throw it towards that fan, think of the ball as the incident X-ray heading towards our patient's tissue. The slower I throw that ball towards the fan, the more likely that ball is to hit one of the blades of that ceiling fan. The faster and faster I throw that ball, the higher and higher that X-ray energy, the more likely it is to miss the blades of the fan and hit the ceiling. The same happens here. As we increase our X-ray energies, the less likely it is to hit one of those electrons within our target material.
Now, we can represent this graphically, and we can see that in tissue here, as the photon energy increases of our incident X-ray, the likelihood of the photoelectric effect to occur decreases. And you can see how mass attenuation coefficient on the y-axis of our graph, this is something we're going to go over in a future talk. This is an exponential change on this axis. So although this looks quite linear, this decrease, this is actually an exponential decrease in the likelihood of the photoelectric effect to occur.
Now, you can see here I've included iodine with a higher atomic number. We see that atomic number to the power of three increases the likelihood of the photoelectric effect to occur. Because iodine's atomic number is so much higher, the likelihood of the photoelectric effect to occur is so much higher. Now, you may be wondering, what is this strange jump within the iodine here? And this is what's known as the K-edge.
Now, what is the K-edge? Well, the K-edge is the energy level at which our K-shell electron within the iodine here will be released, the binding energy of our K-shell electron. At energies below that range, we cannot release that K-shell electron, so we are not getting the photoelectric effect occurring there. As our X-ray energies are higher than that K-shell binding energy, we then make available a whole load more electrons for the photoelectric effect to occur. We get this sharp increase in the likelihood of our photoelectric effect to occur, and we can use this to our advantage when we use iodine in contrast studies. This is the diagnostic energy range here. We get the benefit of that K-shell or that K-edge occurring, increasing the likelihood of the photoelectric effect to occur, increasing the contrast between our iodine and our surrounding tissues.
Now, you may be wondering, in tissue, why don't we have a K-edge here? Well, because our atomic number is so low, and that K-shell binding energy is so low, our K-edge would be right down here at 1 keV. It's so small that we don't see it on this graph. We get this drop-off according to this probability formula here, where our likelihood of the photoelectric effect to occur decreases exponentially as photon energy increases.
Now, this becomes really important to note, especially when we start looking at Compton scatter and Rayleigh scatter in the diagnostic energy ranges. And it's the differences in the density and the atomic numbers of the various different tissues within our body that provides us with the anatomical detail in our image. And in our ideal world, we would only want either X-ray transmission or the photoelectric effect to occur to get a perfectly crisp image. Unfortunately, scatter events occur that decrease our contrast within our image, decrease our spatial resolution by increasing the noise in the image, and that is dominantly occurring by the Compton effect, which we're going to look at in our next talk. So I'll see you all in that talk. Goodbye, everybody.