Transcription
Let's move on to our final components of the X-ray circuit, the filament circuit. Now, our primary and secondary circuits, their function was to create a tube potential across our X-ray tube that was in one direction and continuous. Our filament circuit can almost be seen as a separate circuit in which its function is to control the amount of thermionic emission that happens at the cathode.
Now, the filament current serves two major purposes. The first is to determine the current flowing through the filament, and the second is to determine which filament we use. Now, you would have seen there's a large filament and a small filament, and changing between the two will affect our effective focal spots and ultimately our spatial resolution on our X-ray tube. So, let's have a look at the circuit itself.
We have our wall power here, our low voltage, about 220-volt alternating current, that is coming to the hospital. Now, the first thing we can do is select our current that is flowing through the circuit. Now, this is what's known as a rheostat, which is a variable resistor. We've done nothing to change our voltage here. So, our voltage, which equals our current times our resistance, our voltage remains the same. We've got constant voltage coming in here. If we were to vary our resistance, change our resistance, our current would have to change proportionally in order to keep the voltage the same. So, increasing our resistance will decrease our current, and vice versa.
We can then select which filament we want to use, our large or our small filament, within the cathode, within that focusing cup of the cathode, and then we come to what is known as a step-down transformer. Now, we saw that between our primary and secondary circuits, we had a step-up transformer, which took low-voltage current and changed it to high-voltage current. Here, we are doing the opposite. We are taking high-voltage current, it's still our wall current, and reducing that voltage further. We started with current at about 220 volts; we're going into the range of 7 to 10 volts here.
Now, a step-down transformer that goes into a high-resistance circuit, a high-resistance filament there, will actually lead to an increase in current. This step-down transformer is actually increasing the current on this side of the circuit. We've got a lot of coils with a large changing electromagnetic field. That large changing electromagnetic field operating over a smaller number of coils, but increasing that current. That's a fact that we just need to learn: that step-down transformation actually results in an increase in current. We are dealing with two separate systems here. People often get confused by saying, "But if we decrease our voltage, we need to decrease our current in order to compensate for that." That's not the case; we're dealing with separate systems here.
So, what have we done? We've selected our current through our rheostat, which is a variable resistor. Increasing our resistance will decrease our current, and vice versa. And we've chosen the filament that we want to use. We are now supplying that to our cathode here, and depending on the amount of current flowing through our cathode, that will determine the number of electrons that are available through the process of thermionic emission for acceleration towards our anode.
Now, important to remember, this filament current does not accelerate our electrons from our cathode to our anode. It's our tube potential that does the acceleration. That's our primary and secondary circuits. What the filament circuit does determines the number of electrons available. So, we saw this tube potential in our secondary circuit talk, where changing the KVP will increase the tube current exponentially. Now, what happens when we increase our filament current?
Now, this graph is not a perfect graph, but what I want you to see is that changing our filament current from 4.5 to 5 changes our tube current in a linear fashion. These graphs may look exponential, but at these low tube currents here, we have a bit of an exponential graph here. But as we reach tube currents at a set point here, we see that these graphs are actually linear graphs. So, after a specific tube current, changing our filament current is actually a linear relationship. Increasing our filament current is proportional to the tube current increase. So, the change in filament current will be proportional to the number of electrons that are available.
Now, when we look at X-ray production and the X-ray spectrum, we'll see that changing filament current will change the X-ray quantity. It will do nothing to the X-ray beam quality or the energy of the X-ray beam. And that change in X-ray beam quantity is directly proportional to the change in filament current. So, that's often a question that comes up in exams: Does filament current change X-ray beam quality? And the answer is no. Does it change X-ray beam quantity? Yes. And what is the relationship there? They are directly proportional. And it's those types of questions that I've included in the question bank that I've made and linked below if you want to check that out.
So, we've seen now that the X-ray circuit serves multiple functions. We take low voltage to high voltage, alternating current to direct current. We change that fluctuating current into smooth current. We can select the KVP by changing that on our primary circuit. We can select our exposure time. We can determine the filament current, which is directly proportional to the number of electrons available. And we can determine whether we use our large filament or our small filament. All of these changes will either change the number of electrons going from the cathode to the anode or the energy of those electrons going from the cathode to the anode.
We've now got those electrons striking our anode, and that is where X-rays are produced. And finally, in our next talks, we are going to be discussing the process of Bremsstrahlung radiation and characteristic radiation, and how the energy and the number of those electrons going towards our anode actually change the number and the energy of the X-rays that we produce at the anode. So, I'll see you all in our next talk, where we look at Bremsstrahlung radiation, and we can finally see how we go about creating those X-rays that head off towards our patient. I'll see you all there. Goodbye.