Transcription
Hey, welcome back to another explainer. In this video, you'll learn everything you need to know about the X-ray tube, which will fall nicely along the other videos I've made on X-ray production and the different types of radiation that are produced.
So, if I had to summarize this whole video into a few words, it'll be this: there are two main components of an X-ray tube, the cathode and the anode, which together we call them the diode. You know, because there's two of them, diode. And by the way, the diode is just another name for a semiconductor device, which is basically a device that conducts electricity, allowing a flow of current electrons from one direction to another.
Anyway, so the cathode is the negative part and is what emits electrons that go towards the anode, the positive part. And this interaction of those electrons coming in from the cathode and hitting the anode is what generates X-rays. That's basically what's going on. Now, I know that's already more than a few words, but now that you have a general idea and overview, I'm going to take you through each component and tell you what it does and what its purpose is. So, by the end of this video, you should be an expert in the anatomy of an X-ray tube.
I've put timestamps below so you can skip around or maybe return to a section that you needed, so use that as you will. And also, stay tuned for the end where I'm going to challenge you a little by giving you some practice questions to see if you really understood everything. All right, let's get into it.
First, starting off with the cathode. So, the cathode is the negatively charged electrode, and it's the source of electrons within the X-ray tube. It's usually made of a material called tungsten, which is a metal with a symbol W (I don't know why) and an atomic number of 74. And at the tip of the cathode is where you find the two filaments, where we have the small filament and the large filament. The small one is used for the fine focus setting, and the large one for the broad focus. If you have no idea what fine and broad focus are, it basically controls how fine your final image is. Don't worry about it for now, I'll make another video and link that below once it's ready.
Now, the material of the cathode is important because it's able to withstand the thermal stress of repeated heating cycles without significant evaporation or deformation of the filament, because it's strong enough to maintain its shape and size during X-ray production, which can get quite heated. So, what happens is that there an electric current pass through the coil, causing the cathode filament to be heated, and this causes the electrons from the tungsten material to be boiled off and ready to be deployed. And this process is called thermionic emission, which makes sense, right? Because "thermionic" has something to do with heat (thermo), and "emission" just means being discharged or emitted. So, thermionic emission is the electrons being emitted from a substance at very high temperatures.
Now, remember I said the cathode is negatively charged? Well, our electron is also negatively charged, so you can guess what happens next. Next, these loose electrons are then rushed towards the anode at very high speeds, the energy being determined by the potential difference across the cathode and anode, which we set by our KVP. And so, the cathode is positioned within the X-ray tube in a way that the electrons emitted are precisely directed towards the target area on the anode. And by the way, the small and large cathode filament are housed within what's called a focusing cup, which, as the name suggests, focuses the beam of electrons directly towards the anode. And this is kind of what it looks like.
So, if, for example, I'm trying to X-ray someone's finger, I need the highest resolution possible, then I choose the fine focus setting. That is, the smaller filament will be activated. And then, let's say my next patient is coming in for an abdomen X-ray, whether or not select the broad focus, and so the larger filament will be used. There's different reasons why you'd pick each, but again, not relevant right now. I'll cover that in another video.
So, now let's talk about the anode, the positive end of the tube. It's a metal disc, also usually made of tungsten, although it could also be made of molybdenum or rhodium. So, when these high-energy electrons coming in from the cathode collided with the anode, this is what creates the X-rays. The way I remember which electrode is positive or negative, I think of where the electron starts off, which is at the cathode. And if the electron is negative, what would the cathode have to be for this interaction to make sense? Well, it would also have to be negative, right? Because two negatives repel, and that's what causes the electrons actually to be pushed away from the cathode and go towards the positive end, that is, the anode. That's not exactly the reason why, but it's just how I like to remember it.
Now, the difference here is that the anode is actually on a rotor, meaning that it rotates. And it's like this because when the electron collides with the anode, it also generates a lot of heat. So, to counteract that effect, the anode rotates to spread the heat over a large surface area, which prevents it from overheating in one single spot. And this is important because it prevents the anode material from melting, which, as you can imagine, can compromise the production of X-rays.
Once the X-rays are produced, they're then guided through what's called an exiting window, which can go through various filters and collimators before it reaches the patient or detector. More on that a little later.
Now, all of this is encapsulated by a vacuum-sealed glass container, and it's this vacuum that allows the electrons to freely move from the cathode to the anode without colliding with any air molecules, because if it did, it would cause them to scatter and interfere with the production of X-rays. So, this glass has to be made of a high-strength, heat-resistant glass that can withstand the stresses inside. Also, the glass acts as a barrier, protecting the tube's internal components from contamination.
At the bottom of the glass envelope is where you find what's called the exiting window, which is the part that's designed to allow the generated X-rays to exit the tube with minimal absorption and scattering, making sure that most of those X-ray photons reach the target, which in this case is the patient or the object being imaged. So, that's pretty self-explanatory. Beryllium is actually a common material used for the exiting window because it firstly has a low atomic number of four, which means it has fewer electrons and therefore less interactions and absorption of the X-rays, but it's also lightweight and has a high melting point, making it capable of withstanding the physical and thermal stresses it had encounter during a day-to-day operation of the tube.
Okay, but how strong can this glass really be? I mean, it's glass at the end of the day, so it can't just be exposed out like that. And that's why the glass envelope is actually housed in a metal casing or tube housing. And this tube housing is lined with a thin sheet of lead of about 2 to 3 millimeters, because lead, with its atomic number of 82, is an ideal material for absorbing any unwanted X-rays. So, we never actually see this glass envelope, I mean, unless it's opened up for service and whatnot.
And what's interesting is that this glass envelope is sitting within oil in the tube housing. Can you guess why? Give you three seconds. A few moments later. It's to reduce heat. That's right. See, you're getting it. Yes, we're still talking about the heat that's produced, which, by the way, that's now two distinct ways that the tube is minimizing heat: first being the rotating anode, and now the glass envelope being bathed in oil in the metal housing. Again, because if the heat is too high, we can't generate those high-powered X-rays that we need to penetrate our patient's tissues.
Now, the tube housing has another function, and that is to act as a lead shield, absorbing the X-rays going in all other directions, cuz remember, we're only interested in the X-rays coming out of the exiting window, and we can direct those to exactly where we want them. And so, in an ideal world, where there are no X-rays leaking from the X-ray tube, they're only coming out from where we want them to. But spoiler alert, we don't live in an ideal world. So, there are going to be X-rays that go and scatter in other directions, which is what the metal housing is trying to prevent.
The legal limit of radiation leakage from an X-ray tube running at its maximum voltage should not exceed 1 milligray per hour at 1 meter from the anode, which is kind of easy to remember, right? It's 1-1-1-1-1 milligray, 1 hour, 1 meter. So, you know, even at the best of times, there's going to be some leakage. But note that the 1 milligray per meter per hour is the legal limit, so ideally, it should be a fraction of that.
Now, there are two other main components of an X-ray tube, which technically aren't part of the X-ray tube itself, itself, but a part of the system that we often collectively call the X-ray tube, and they are filters and the light beam diaphragm. Let's start off with filtration.
Now, to fully understand and appreciate the use of filtration, you first need to understand the different types of X-ray production, where we have characteristic and Bremsstrahlung radiation, and in particular, the latter. And I've made a separate video on that, so I'm not going to go through it now. I'm assuming you know what Bremsstrahlung is for this next part, but if you don't, I've linked them down below for you.
All right, welcome back. Now, what you got to understand is that the energy of our X-ray beam, which, as you know, we determine and set by our KVP, isn't constant. Recall what KVP stands for is the kilovoltage peak, meaning that it's the peak that this beam of energies will reach, and that actually majority of those range of energies fall below that, which is what this graph is all about. These lines are the characteristic peaks, don't worry about them for now, and this hill-looking thing is a range of Bremsstrahlung radiation produced, where these lower energy X-rays are quite weak, and so they're not actually contributing to the image. They're just adding to the patient's absorbed dose, or rather, entrance skin dose (ESD). So, wouldn't it be great to just filter out those lower energy X-rays and keep the big ones? Well, it's your lucky day, cuz yes, we can. And that's basically what filters do. We can look at the range of X-ray energies that are produced, and we can then go in and strategically filter out certain parts of it.
The most common filtration materials are aluminum and copper. You may have seen or heard "2.1 millimeters of aluminum or equivalent at 70 KVP," which basically means that if you have an X-ray tube using up to 70 KVP of exposure, there shouldn't be any less than 2.1 millimeters of aluminum or filtration used, and that any less is now allowing too much of those low-energy X-rays in. Now, there's many types of filters, I just talked about the basics, but legally, every X-ray tube should have some level of filtration.
All right, now let me tell you about the light beam diaphragm, or LBD for short. Last one, I promise. So, at this stage, the X-ray has been produced, and we went through all the different components that are involved, and the beam has even gone through a filter to remove the useless lower energies, in effect hardening the beam, and now it's ready to be directed to the patient. And this is where the light beam diaphragm comes in, where we can further direct and collimate the beam to suit exactly where we want it to hit. This is the part you see on an X-ray tube, the big cube box looking thing with those two little knobs on either side or on the front, depending on the manufacturer. The collimator knobs are used to, well, collimate the beam, and you can actually rotate the LBD around an axis too, which could be useful if your patient isn't lined up straight, so you can just straighten the beam by rotating the LBD.
The clever part of this is that there's a little light bulb inside the box, strategically placed to show where the actual radiation beam would be. You know, because X-rays are above the visible part of the electromagnetic spectrum, so we can't really see it with our naked eye, but the LBD allows us to, so that's pretty cool. And note that as you change the collimation, there are metal plates that move around at the base, which you can see if you just tilt your head underneath at the base of the tube, which, by the way, I recommend you do the next time you're on site or in the pack rooms.
All right, that's it for this one. Thanks for watching. And if you stayed this this long in the video, then well done. I know it's not easy listening to my voice for that long. Anyways, as promised, here are some of the practice questions. So, pause the video or take a screenshot. I'll wait. See how you go with them, and I'll put the answers down in the description below. If you still feel a little rusty on any part, I don't blame you, it's a lot to take in, so just refresh the page and take another listen.
Now, click here to watch my video on the anode heel effect, which you don't want to miss out on. All right, bye for now. Stay curious.