Transcription
Finally, having spent some time looking at the X-ray tube, the X-ray circuit, as well as X-ray beam geometry, we can get into the meat of our X-ray physics module by looking at how we actually go about creating X-rays within the X-ray tube.
Now, as I've mentioned, there are two separate mechanisms that we create X-rays at the anode. The first is via Bremsstrahlung radiation, and the second is characteristic radiation. Now, in this first talk, we're going to focus specifically on Bremsstrahlung radiation before moving on to characteristic radiation in our next talk.
So, where exactly are these X-rays produced? Well, they're produced on the actual focal spots in the anode here, and they're not just produced on the anode surface; they are produced within the anode, like we've seen in our anode heel effect talk. So, we have our cathode producing electrons, our tube potential accelerating those electrons, those electrons striking our anode, and X-ray production occurring at this anode.
A really common question that comes up is, what happens to the energy of those electrons that strike the anode? Now, less than one percent of that energy is then converted into X-rays. The rest, over 99% of that energy, is converted into heat at the anode, and we've looked at the various different mechanisms the anode has in order to deal with all of that heat production.
Now, of that less than one percent that is converted into X-rays, the majority of those will be Bremsstrahlung radiation. So, if we have a look at our Rutherford ball model of the atom here, and we have an electron that has come from our cathode, it's being accelerated towards our anode via our tube potential, and it is going to strike our target material. Now, this atom represents the target material of our anode, which is most commonly tungsten. Now, this isn't technically a tungsten atom; there aren't 74 protons here, and there aren't 74 electrons, but it's a diagrammatic representation of tungsten.
Now, the energy at which this bombarding electron will strike the anode is determined by the tube potential. Our primary and secondary circuit have created a tube potential that accelerates those electrons towards the anode. Now, this electron will be coming at a specific kinetic energy and will be interacting with our target material.
Now, one of the major differences between Bremsstrahlung radiation and characteristic radiation is that in Bremsstrahlung radiation, there's an interaction between the striking or bombarding electron and the nucleus of our atom. In characteristic radiation, we get interaction of this striking electron with the initial electrons of our target. So, what I've done here is I've removed the electrons from this diagram in order for us to not get confused here.
Bremsstrahlung radiation, this is really important to remember, is the attractive force between this electron and the positive nucleus of our target. Now, Bremsstrahlung in German, I'm told, means "breaking," and you'll see why that is used to describe this type of radiation. This electron is coming in at a specific kinetic energy and will experience an attractive force between the positive nucleus and that negative electron.
Now, this electron, as it gets attracted to that nucleus, will slow down and change direction. If we're driving a car and we turn, we're going at a constant speed, and we turn the car; the car will slow down as we turn, and our body will feel like it needs to carry on in the other direction. That's a similar thing to what's happening here in Bremsstrahlung radiation. As this electron changes its course due to that attractive force, that electromagnetic force, it loses some kinetic energy, and in a closed system, energy needs to be conserved. And that loss in kinetic energy is then released in the form of Bremsstrahlung radiation.
So, let's have a look at three examples here. The first is, we have an electron coming in at a specific kinetic energy. It comes close to the nucleus and slows down as it experiences that electromagnetic force. Now, the closer this electron is to the nucleus, the stronger that electromagnetic force. The distance between the electron and the nucleus is inversely proportional to the force that is experienced between the two. So, the closer we are, the more loss of kinetic energy we get, and the greater the Bremsstrahlung radiation energy is that is released. This Bremsstrahlung radiation energy is proportional to the amount of kinetic energy that is lost by the bombarding electron.
Now, if an electron coming from the cathode was further away from the nucleus here, that attractive force would be less than in our previous example. There will be less loss of kinetic energy here, and the Bremsstrahlung radiation energy that is released will also be less. The opposite is also true: if an electron was to strike the nucleus and lose all of its kinetic energy, the Bremsstrahlung radiation that is released will be equal to the energy of that bombarding electron.
Now, there's another way to represent this graphically, and we looked at this diagram when we looked at our filtration talk. Now, here on our y-axis, we've got photon number. Photon, our electromagnetic radiation, the number of photons. And on our x-axis is our photon energy, the energy of that Bremsstrahlung radiation. Now, we have electrons coming from our cathode towards our target material here. Now, the way I've drawn this, the color coding here represents the strength of the electromagnetic field that is experienced between the negative electron and the nucleus of our atom. These don't represent the electron shells here; these represent distance away from this nucleus here.
Now, the more protons within our nucleus, the more Bremsstrahlung radiation that will be generated. So, the higher our atomic number of our anode target, the more Bremsstrahlung radiation that is produced. The kinetic energy, or the energy of these electrons that are being accelerated towards our anode, as I've said before, is determined by our KVP, our kilovolt peak. Here, the number of electrons that are heading from our cathode to our anode is directly proportional to our current and our exposure time, our MAS, and it's exponentially proportional to our KVP.
Now, if an electron was to strike the nucleus, as we've said, all of that energy will be converted into Bremsstrahlung radiation, and that photon energy here will be equal to the energy of that bombarding electron. Now, as we head out more and more to the periphery of our target material here, the Bremsstrahlung radiation that is released will have a low photon energy. But the surface area here, the likelihood of an electron to be interacting at a longer distance, is much more than as we get closer to the nucleus. The nucleus is a very small part of our atom, therefore, very few photons are produced at these high energies, and much more occur at these low energies.
Now, low-energy electromagnetic radiation has a longer wavelength, and high energy has a shorter wavelength, a higher frequency, and therefore a higher energy, just as we discussed in our electromagnetic radiation talk. Now, this is what's known as an unfiltered Bremsstrahlung radiation spectrum. Here, we've got this linear spectrum where we've got lots of low-energy photons and fewer high-energy photons.
Now, as I've said before, if we were to place a filter between our target material and our patient, we would preferentially filter out these lower-energy X-rays, X-rays that contribute to patient dose but don't contribute to our image. And that occurs via the photoelectric effect that we've looked at before. As the energy of these X-rays increase, the likelihood of our photoelectric effect to occur decreases. We preferentially attenuate the lower-energy X-rays, and higher-energy X-rays are more likely to go through our filtered material. Now, this filter can represent our inherent filtration, our glass envelope, our conducting oil, and our X-ray tube window, all the added filtration that we place between the X-ray tube and our patient. Again, this is a really important formula to understand.
Now, if we were to look at this graph from head on, this is the kind of graph that comes up in exams over and over again. And in some countries that have short answer question-based exams, drawing this or explaining this graph is very important. And even in MCQ exams, knowing where these graphs intersect, especially with our x-axis, is incredibly important. So, this is what's known as our filtered Bremsstrahlung spectrum, and I just want to take you through a couple of points that are core knowledge you need to understand.
This maximum photon energy is determined by our KVP. Nothing else determines this energy here. If we were to change our target material from tungsten to another type of material, this maximum photon energy would not change. This is purely determined by the energy that these striking electrons have coming from our cathode to our anode. The number of X-rays, the area under this curve, is determined by our KVP, our current, our target material, and our filter addition here, all of which we're going to look at in more detail when we look at the X-ray spectrum.
It's very difficult to create an unfiltered X-ray spectrum because we have some inherent filtration. So, most of the X-rays below a photon energy of 12 will not reach the patient here. So, when asked to draw this graph here, we don't want to include any X-rays below that energy level.
Now, there are multiple ways in which this specific question can be asked in exams, and what you need to do is understand how this spectrum is generated, understand the process of Bremsstrahlung radiation, and then when you're asked about what a change in KVP or a change in target material or generator waveform does to the spectrum, you can go through the steps in your head and using the logic and reasoning by understanding this process, you can see what will happen to the spectrum. And if you want to practice these types of questions, check out the question bank that I've linked in the top line of the description. I go through all the different ways in which these questions get asked in exams.
So, now we've looked at the production of Bremsstrahlung radiation and the Bremsstrahlung spectrum. Let's shift our attention now to characteristic radiation, combine those two spectra, and get our X-ray spectrum proper. I'll see you all in that talk. Goodbye, everybody.