Transcription
In the last talk, I mentioned that there are five main factors that we can manipulate in order to change the X-ray spectrum. Changing those five factors are going to either have an effect on our X-ray beam quantity or our X-ray beam quality.
Now, the X-ray beam quantity is represented by the red here, the area under the graph, the number of photons within our X-ray beam. And the X-ray beam quality refers to the average energy of our X-ray beam, represented by this green line here. So, these are the five factors we're going to go over today: filament current, our tube potential, filtration, both inherent and added, anode target material, and our generator waveform.
So, let's start with the filament current and see how changing the filament current affects our X-ray spectrum. Now, when we're talking about the filament current, we are talking about the current that we run through our tungsten filament on our cathode. Now, when we run current through this tungsten filament, we make electrons available on the surface of that tungsten filament in a process that we've covered called thermionic emission. Now, the more current we run through that filament, the more heat is generated in that filament coil, and the more electrons that are available to then be accelerated towards our anode.
Now, I've said it before, but it's a good point to remember: we are not accelerating, we are not changing the energy of those electrons that are at the surface of our cathode here by changing our filament current. The only thing we are changing is the number of electrons available. It's our tube potential that will then accelerate those electrons towards our anode.
Now, if we have a look at this diagram, our Y-axis represents our tube current, the number of electrons that are going from our cathode to our anode, and our X-axis here is our filament current, the current, the mA, the milliamperes that we are running through our tungsten filament coil here. Now, ignoring this first little bit of this graph, we can see that the graph, at a certain kilovolt percentage, is relatively linear here. And importantly to note here, our change in filament current results in a proportional change in tube current.
Now, tube current is the number of electrons that are flowing across our X-ray tube, and that is proportional to the number of X-rays that we produce. So, it logically makes sense that as we increase our filament current, we increase our X-ray number, the quantity of our X-ray beam. We are doing nothing to the energy of those X-rays; that is our tube potential that changes the energies. So, our quantity increase is proportional to our filament current increase.
So, let's have a look at that on our X-ray spectrum graph here. In green is a low current X-ray spectrum. As we increase that current, we increase our photon number, we are increasing the area under the curve, increasing our X-ray beam quantity. We are not changing the average energy of this X-ray beam spectrum. So, a change in filament current has no effect on X-ray beam quality. This maximum photon energy does not change as our filament current increases. And you can see that our characteristic X-rays remain the same. It's still the same target. If it was tungsten, like in this example, our L shell dropping to our K shell, M shell dropping to our K shell, is independent of the current. It's specific for our target material.
The second thing that we can change is our tube potential. Now, our tube potential is what accelerates electrons from our cathode to our anode. So, increasing our tube potential will increase the energy of those electrons heading towards our anode. Not only does it increase the energy of those electrons, but it also increases the number of electrons, the number of electrons on the surface of that filament. As we increase our tube potential, we pull more of those electrons away and send more electrons towards our anode.
So, let's take an example here. Here is our X-ray tube. We are running a potential difference between our cathode and our anode here at a set filament current, let's say four and a half. We can see that a tube potential of 60 versus 80 versus 100 kilovolts results in a marked difference in tube current, the number of electrons flowing across this space here. So, at 4 mA, our 60 kilovolt tube potential results in a relatively low tube current. As we increase that kilovolt to 80, we get a significant jump in our tube current, and an even more significant jump when we go to our 100 kilovolt tube potential. And our relationship between our tube potential and our tube current is exponential. An increase in tube potential results in an exponential increase in our tube current.
So, that shows that increasing our tube potential will increase the area under the curve, our X-ray beam quantity. Now, what exactly does it do to our X-ray beam quality, the average energy of our X-ray beam? Well, I've said to you before, if we have a tube potential of 80 keV, there will be a set number of electrons going across towards our anode. Now, an increase in that tube potential will result in two differences. One, there will be more electrons, which we've just looked at. Now, we've increased our X-ray beam quantity. But those electrons will also have a higher energy. Our X-ray spectrum now has shifted to the right. We've increased the average photon energy here.
So, have a look at this graph here and see how it changes between 60 keV and 100 keV. Our maximum photon energy has increased there, as well as our area under the curve has increased. So, not only does tube potential increase our X-ray beam quantity, it also increases our X-ray beam quality. So, we can see here, we've got four different spectrums from 60 keV all the way up to 110 keV. The maximum photon energy corresponds to the tube potential that we've created. Those electrons that collide directly with the nucleus on our target material lose all of their kinetic energy, and that is released in the form of bremsstrahlung radiation at an energy that is equal to that tube potential.
Now, you'll see our characteristic X-rays again do not change. We haven't changed our target material, but our average energy, if we go from purple to green to red to blue, has increased. Our X-ray beam quality has increased, and you can see the area under the graph has increased, our X-ray beam quantity, or our photon number, has increased.
Now, when we were looking at characteristic X-ray production, we saw that tungsten had K-alpha and K-beta characteristic X-rays. And you know that the K-alpha characteristic X-rays are between 57 and 59 keV here. And you may be wondering why this 60 keV photon energy here doesn't have any characteristic X-rays if our characteristic X-rays come at 57 and 59 keV. Now, this is a question that's come up a couple of times, especially in the New Zealand and Australian exams, the advanced imaging technology exams. And I've included that in my question bank along with an answer to that question. So, if you want to practice these types of questions and see how these questions are asked in exams, then go and check out the question bank that I've linked below. It's a great way to get ready for your part one exams.
Now, let's go on to our third factor, which is filtration. Here, we've seen this graph. We've seen how adding a filter preferentially filters out lower energy X-rays through the photoelectric effect. Now, when we look at bremsstrahlung radiation, like there's an unfiltered bremsstrahlung radiation curve here, we can see that the majority of photons are low energy photons. Our average energy of this spectrum here will be low because then most of our X-rays are low energy. Our number of X-rays is also high because we haven't filtered out any of these lower energy X-rays. As soon as we add a filter like that, we reduce the number of X-rays. Our X-ray beam quantity has decreased. The photon number has decreased. But what we've done is we've taken out those lower energy X-rays, and actually, our average energy for this X-ray spectrum has increased.
Now, so filtration is an interesting one. It reduces our X-ray beam quantity, reduces the dose of X-rays to the patient, but it increases our X-ray beam quality. It increases our photon energy, or our average photon energy. So, let's look at it on our graph straight on. You can see how an unfiltered spectrum, an inherent filtered spectrum, and an added voltage spectrum all result in sequentially less and less or fewer and fewer photons. We have decreased our X-ray beam quantity. Simultaneously, though, we have increased the average energy of each of these X-ray spectrums. As we filter our X-ray beam, we preferentially remove lower energy X-rays and increase the average energy of our X-ray spectrum.
The fourth thing that we can change is our target material. We've been specifically looking at tungsten, but you can change that target material to create different X-ray spectra. So, we may have a target material with a high atomic number or a target material with a low atomic number. Now, bremsstrahlung radiation production is proportional, it's actually exponentially proportional to the atomic number of a target material. As the atomic number goes up, there's more of an attractive force between those negatively charged striking electrons and the nucleus of our atom. We get more slowing down of those electrons and more release of bremsstrahlung radiation.
So, we can see here, a low atomic number target has fewer X-rays within the X-ray beam. It has reduced the X-ray beam quantity. Our maximum photon energy here has not changed. That is determined by our tube potential, regardless of what our anode target material is. When an electron is accelerated at a specific tube potential and collides with the nucleus of one of those atoms in our target, it will lose all of its kinetic energy and release a bremsstrahlung radiation that is equal to that energy. That's independent of the atomic number of our target.
Now, as we increase our atomic number, we increase the number of X-rays, and that increase is exponentially proportional to the atomic number of our target material. Of note here, and what we haven't seen in other factors that affect our X-ray spectrum, is that our characteristic X-rays will change. Our characteristic X-rays are named as such because they are characteristic for that specific target material. These discrete energy levels are fingerprints for the target material. And actually, it's this difference in the characteristic X-rays that we can use in other fields in science. If we're looking at planets that are far away and we see the electromagnetic radiation that is released, the characteristic radiation that is released, we can determine the atomic structure of those planets based on the characteristic X-rays. It acts as a fingerprint, a unique discrete energy pattern for specific atomic number atoms.
So, a change in the target material largely leads to a change in X-ray beam quantity. It has very little impact on the X-ray beam quality. The only real change in energy levels is this change in characteristic radiation that we see here, which will be separate for different types of atomic numbers.
The last factor that we can change is our generator waveform, our electricity supply to our X-ray tube that is creating our tube potential between our cathode and our anode. Now, in our actual practice, this is something that we won't be changing. Our generator waveform will generally be set. But this is the type of question that comes up over and over again in exams because it requires knowledge of the X-ray circuit, it requires knowledge of how our X-ray tube works, and it requires knowledge of X-ray production at the anode and the changes that that has on our X-ray spectrum.
So, let's have a look at what we're talking about when we talk about waveform. We've seen this now when looking at our primary and secondary circuits, that our kV ripple is the difference between our kVp, our kilovolt peak, and our baseline here. Now, when we have single phase, two pulse rectified, or a full wave rectified wave, we get fluctuations in our voltage that range from zero to our kVp. Our electrons that are being accelerated towards our anode fluctuate in their energy levels. They will have periods of time where they reach our kVp, but for most of the time, those electrons will have lower energies.
Now, lower energies of those electrons going towards our anode will result in a lower energy X-ray spectrum, a lower beam quality. Now, when we overlap those ripples more and more, or we use a high frequency inverter, we reduce that kV ripple. We reduce the difference between our kVp and our lowest kilovolt in this current year. Now, when we talk about a kV ripple reduction, we are talking about improving that difference between our cathode and anode. The more we reduce our kV ripple, the more constant our flow of electrons is from the cathode to the anode.
So, let's have a look at a graph here. A high frequency waveform that we would like to use in our X-ray spectrum provides a direct flow of electrons from our cathode to our anode at a set energy level with very little ripple between our maximum kV and our lowest kV. So, we've got a lot of photons being produced, a high X-ray beam quantity. We saw that a higher kVp results in an increased X-ray number. And when we use a higher frequency waveform, our kVp will remain higher for longer. When we have a 100% ripple waveform, there are periods of time where our kVp will reach a peak. That's why we have photon energies that are the same here for this example, 100 keV. But there are long periods of time when our voltage will be lower than our kVp, and that will result in a reduction in the number of photons, as well as a reduction in the average energy of those photons.
So, more ripple results in fewer X-rays and lower energy X-rays, a reduction in X-ray beam quantity as well as a reduction in X-ray beam quality. So, this is a lot to get your head around, and I'd encourage you to learn these well, like the back of your hand. These without a doubt will come up in exams, and that is asked in multiple different ways. So, you really need to become familiar with a couple of factors where our spectrum ends in terms of photon energy and how manipulating these five factors can have an effect on our X-ray beam quality as well as our X-ray beam quantity.
So, in our next talk, we are going to be looking at how X-rays interact with the patient's tissues when we expose them to this X-ray beam. So, I'll see you all in that talk. Goodbye, everybody.