Transcription
So we've looked now at the X-ray tube and the various different constituents that make up the X-ray tube, and we've examined what happens to those X-rays after they leave the X-ray tube, prior to interacting with the patient. Now let's turn our attention to the circuit that is required to power this X-ray tube and the various different components that we can change in order to manipulate our X-ray beam.
So, in today's talk, I'm going to be looking at what's known as the primary X-ray circuit. Our X-ray circuit itself can be separated into a primary low voltage circuit, a secondary high voltage circuit, and a filament circuit or a tube circuit here. Now, over the next three talks, I'm going to look at each one of these individually, showing you the various different things that we can change and showing you the components and their functions within those circuits.
Now, the X-ray circuit serves multiple functions, and the major thing that it has to overcome is the difference in hospital electricity supply versus the X-ray tube requirements. The electricity supply to the hospital is inadequate to create X-rays in our tube, and we need to manipulate that current in order to create the requirements for the X-ray tube. Now, the first issue with hospital supply is its low voltage. Depending on what country you're in, it's about 220 volts. We need that voltage to be in the order of kilovolts, 50 to 100 times the voltage supply to the hospital.
The second issue is that our current supply to the hospital is alternating current. It goes in both directions, at current alternates over time. Now, when we are trying to provide a tube differential from our cathode to our anode, we want current flowing in one direction. We don't want the current to alternate. We don't want electrons going from the anode towards our cathode.
Now, the second issue is not only is that current alternating, but it's fluctuating like this. We want a constant stream of electrons towards our anode. We don't want a fluctuating stream. So we need to convert that current into direct current, and we want to smooth out that current so that it is a constant stream, it's not fluctuating over time.
Another issue with this current is that it's a set voltage, and we want to be able to manipulate that voltage, change the kVp in our X-ray tube, depending on the type of image that we are trying to create.
Now, the primary and secondary X-ray circuits work together in order to increase this hospital voltage supply to the order of kilovolts. So let's have a look at the primary X-ray circuit.
Here is our primary X-ray circuit. Our hospital supply here is this lightning bolt. This is where the hospital voltage, the 220 alternating voltage, is coming into our primary X-ray circuit. Now, this, I promise, is the most simple I can make the X-ray circuit while still maintaining the detail that is required for your understanding. So let's have a look at the primary X-ray circuit and examine the various different components that the current comes into contact with in this circuit.
Now, this is our hospital wall supply, and the first component is known as the line monitor. Now, the line monitor ensures that we are in fact getting 220 volts from the hospital electricity supply. If this voltage was to change for any reason, we would be multiplying that voltage out. In this moment, we're going to look at what's known as a step-up transformer, and we multiply that voltage. And if we're not certain that 220 volts is coming from the wall, then we could be exposing the X-ray tube to a larger or smaller tube potential, and ultimately exposing our patient to a different X-ray energy than we are dialing in in our machine. So this line monitor, if there is any difference in voltage coming from the wall, it's linked to what is known as a line compensator, and that will compensate and make sure our voltage is what we think it is.
Now, after that, the current comes into what's known as an auto transformer. Now, the auto transformer is a single coiled transformer, and what it does is it allows us to select our kVp. It allows us to vary the voltage going into the primary X-ray circuit. Now, you'll notice that this is labeled kVp, killer voltage peak. We're dealing with volts here, not kilovolts, but this is the dial that is on our machine, and it has been calculated for this step-up transformation that we're going to look at now. So although we are changing volts between about 100 and 400 volts, the number on our machine will be the kilovolts that we are supplying to our X-ray tube.
Now, an auto transformer uses the process of self-induction. Depending on which coil we place our kVp selector on, that will cause this electromagnet to self-induce at a certain voltage. Now, that voltage then continues to our next component, which is where we determine our exposure time. Now, the exposure time is an incredibly important part of our X-ray circuit because it determines the amount of time that our patient is exposed to X-rays. Now, it's very difficult to hold down an exposure timer manually for the required amount of time, and we generally have an electronic exposure timer where we dial in the exposure time prior to taking the X-ray. Now, exposure timer can also be linked to the current that is going through our filament, a mass timer, or we can have an automatic exposure control where our exposure to our detector is what determines our exposure time.
Now, our voltage is heading into what is known as our step-up transformer, and our step-up transformer is what brings our low voltage current into our high voltage current. The first half of the step-up transformer forms part of the primary circuit, and our second half of the step-up transformer forms part of our secondary circuit. Now, we're going to look at the step-up transformer next, but before we leave this image, I want to mention this last component here, which is known as a circuit breaker. Now, this is a safety mechanism here. If the voltage was too high within the circuit, we would have what is essentially a fuse box that would break the circuit and prevent us from creating X-rays with energies that are too high.
So let's have a look at our step-up transformer. Now, as I've said, the primary function of the step-up transformer is to increase the amplitude of our current, increase the voltage of our current. We want to go from volts to kilovolts. Now, current running through these coils in our primary side of our step-up transformer will create an electromagnet in this magnetic core, that's through the process of electromagnetic induction. Now, because this current is alternating like this, it's switching in direction over and over again, our magnetic field that we've created is also switching in direction. That magnetic field will then interact with the coils on the secondary side of our step-up transformer, and the moving magnetic field, like we've looked at in our electromagnetic force torque, that moving magnetic field will induce a current in the secondary coils on our step-up transformer. We can use this formula here: the voltage in the primary circuit over the voltage in the secondary circuit, so the proportional relationship between the primary and secondary voltage is equal to the number of coils, or the number of turns in our primary side, over the number of coils or number of turns in our secondary side, and that is how we can calculate what our input voltage will be converted to in our output voltage.
Now, we still have a problem here. We've increased our voltage to the levels that we require for our X-ray tube, but this current is still alternating. We need to now rectify that current, convert that current into direct current, and then we need to use a generator in order to smooth out that current, and we get a constant flow of electrons from our cathode to our anode. And that is the primary function of the secondary circuit, which we're going to look at in our next talk.
Now, before we move on to that secondary circuit, I want you to see here that the controls we are changing, selecting the kVp and selecting our exposure time, those are in the low voltage parts of the circuit. We are never manipulating anything within our high voltage part of the circuit, and this is a safety measure here. We are only ever exposed to the low voltage part of our circuit, both our primary and our filament circuits.
Now, changing these components comes up over and over again in exams because they have a major effect on our X-ray spectrum. Now, in the question bank that I've created and linked below, we examine closely the X-ray spectrum and how changes in our kVp and our exposure time manipulate that X-ray spectrum. So if you're studying for exams, those are high-yield questions and are highly recommended checking that course out.
So now let's move our attention to the secondary circuit, where we change that alternating current into direct continuous current in our X-ray tube. I'll see you there. Goodbye.