Transcription
So now that we've looked at the cathode and reviewed the process of thermionic emission, we have electrons available at the tungsten filament surface, ready for acceleration towards our anode. So let's now shift our attention towards the anode. And if you have a look at this diagram I've drawn, the cathode here, side on, we've got our electrons that are then being accelerated towards our anode. We're actually looking at the anode head on here. So if we were to look at how these fit in within our x-ray tube, here is our cathode, our electron stream, accelerated electrons due to a voltage difference or a voltage potential between our cathode and our anode. And we see those electrons striking our anode. And this point at where they strike is what's known as the actual focal spot on the anode.
Now, the anode is made of tungsten, and it's the interaction between these accelerated electrons and the tungsten that generates our x-rays. So this actual focal spot is the site for x-ray production. Now, the vast majority of the kinetic energy of the energy that these electrons have will be converted into heat. Only one percent of that energy will be converted into x-rays itself.
Now, this anode is actually angled slightly to allow us to get x-rays leaving the x-ray tube perpendicular to this electron beam. If this anode was exactly flat on with our electron beam, the geometry would work out as such that we would have to have our patient at an angle to this x-ray tube. And this angle, amongst other things that we'll look at soon, allows x-rays to leave the tube perpendicular to this x-ray stream. So if we were to cut this anode in cross-section to view that angle, this is what it would look like. Here is a 2D diagram, and we can see our cathode on its side, the electron beam going across towards our actual focal spot on our anode. This is our anode in blue here. Now, this angle here is the angle I was talking about. It's known as the anode angle, and we can change that angle to change some of our x-ray beam geometry. Now, some textbooks, you will see measure the angle here on the anode itself. Other pieces of text will draw a line down from the anode here and measure this angle. Those angles are the same, as long as these two lines are parallel, these angles are the same.
Now, the more we increase our anode angle, the larger our actual focal spot gets, and the larger our field gets, and the larger our effective focal spot gets. All of which we're going to look at when we look at the line focus principle, when we examine some of the geometry of x-ray beams.
I mentioned that the vast majority of the energy of these electrons will be converted into heat. 99% of that energy is converted into heat. Only one percent is converted into x-rays, of which most of those is a type of x-ray called Bremsstrahlung radiation, and a small percentage of those will be called characteristic radiation. Two separate topics that we will look at in some depth.
So, the anode has a problem that it has to deal with all of this heat generation, and it's a lot of heat generation. And the anode needs to have a couple of mechanisms to cope with this heat. Now, this is a very common question that comes up in all exams. When I was collating the various different exams, this was a top 10 question. It came up more often than it didn't come up. So you really need to know these heat coping mechanisms that the anode has.
That if we were to look at our x-ray tube again from the outside, you can see that the anode itself sits on a stem, and there's an anode body here. Now, there are two motors here that use electromagnetic induction to rotate this anode. This is what's known as a rotating anode. So in red here, I've marked the actual focal spot, and if that anode were to rotate, you could see that we would create a focal track all the way around our anode. We would then spread that heat generation over a larger surface area just by rotating the anode, and this is the primary mechanism for which the anode can deal with all that heat production. We can get what's called a stationary anode that doesn't spin, but we can't get long exposure times because that anode will be building up heat on one focal spot here. When we rotate the anode, we spread that heat over a larger distance, and that's why when I showed you this diagram, we're looking at the anode now. Friends, on this dotted area is what's known as our focal track. So that's one of the mechanisms that the anode uses.
The second is that it is actually made of tungsten. And we saw in our tungsten filament, tungsten has a very high melting point. It's able to tolerate lots of heat while maintaining its structural integrity. Not only that, it's a good conductor of heat away from the anode. Secondly, I mentioned that the anode angle changes, changes the size of this actual focal spot. If we increase our anode angle, we can tolerate episodes slightly more heat at the anode.
Next, and we will look at in our next talk when we're looking at the x-ray tube, there are a couple of mechanisms in which heat is moved away from this focal spot towards our tube housing. Now, the main way that heat is moved away from the anode is by radiation. There are a couple of heat transfer mechanisms. I'm sure you've heard of radiation, conduction, and convection. The vast majority of heat moving away from this actual focal spot is done by radiation.
Now, what actually is radiation? Well, radiation is electromagnetic radiation, and it falls just above the spectrum of visible light. It falls in our infrared electromagnetic wavelength. If you cast your mind back to our electromagnetic spectrum, when you're standing next to a fire or if they if you next to a flame that comes up and you feel that blast of heat on your face, that heat that you feel immediately, that's radiation, that's radiating heat. If you were to hold a metal rod into the fire and over time that rod heated it up until you feel the heat in your hand, that is conduction. Molecules or particles passing on heat energy from one another. Conduction. If you were in a room that had a fire in the corner and the air in that room started to warm up and change where cool air was replaced with warm air, that is convection. The movement of air or liquid. So we will see both conduction, convection, and radiation within the x-ray tube, but radiation is the major mechanism for heat removal from the focal spot.
We can then add a substance into the surface of this anode known as uranium. So we put 10% uranium into the surface here, and it prevents surface cracking of the tungsten anode here. That alloy that we've now created can tolerate the heat much better and keep that smooth anode surface. There's also a pool of oil that surrounds the vacuum, the glass housing that the cathode and the anode are in, and that can conduct heat away from the anode itself.
We can also change some factors that we control when producing the x-rays. We can expose the anode to electrons for a shorter period of time. If we reduce our exposure time, there'll be less heat production at the anode. We can decrease our filament current, the amount of electrons that we are producing, as well as decrease our tube potential. Both of which will reduce the amount of heat at the anode. Now, obviously, that comes as a trade-off with the parameters that we need in order to get an adequate image to make a diagnosis.
So you can see there are multiple different mechanisms by which the anode tolerates that heat, and the anode is a topic that comes up over and over in exams, and it's where a lot of the action happens. It's where we produce our x-rays. It's where we determine our x-ray beam geometry, and our anode angle has a big role to play in the spatial resolution of our final image. So the anode is a really important structure to note.
So we're going to take a step back now and look at the x-ray tube as a whole. The major components are the cathode and the anode, which we've spent some time on. Now, I just want to tell you about the other components before then heading on to x-ray beam geometry, where we look at how changing the anode angle will change our actual and effective focal spots, as well as our field size. And we're going to cover a topic called the line focus principle, which is incredibly important in our x-ray physics module. So I'll see you all in the next talk where we look at the x-ray tube. Goodbye.