Transcription
I mentioned in the previous talk when we were looking at the anode that changing the anode angle will have a significant effect on both our effective focal spot as well as our field size and consequently the resolution of our image. And this all falls under the category of X-ray beam geometry. So I want to take a moment now to dive a little bit further into the geometry of the X-ray beam and show you how we can change certain parameters in order to get the geometry that we want for our patient.
So if we have a look here at a simplified diagram, we have our cathode here that has produced electrons which are being accelerated towards our anode. Now I've just used the bottom half of the anode here for this diagram. Now this first cuboid that's going from the cathode towards our anode represents our Electron Beam, this path of electrons going towards our anode. So this here is the Electron Beam or the bombarding Electron Beam where those electrons strike the anode. This red region here is known as the actual focal spot. It's actually where x-rays are produced and where heat is produced.
Now x-rays are produced at that actual focal spot and head out towards our patient. And this is represented by our effective focal spot, what is the effective x-ray beam that is heading out towards our patient. Now this is a mid-effective focal spot, and I'll show you in a second how the focal spot changes throughout our x-ray field. But it's really important to know these three structures: the bombarding Electron Beam, the actual focal spot, and the effective focal spot.
So if we look at this from side on, this is the same diagram here, just a little bit more simplified, a 2D diagram of the cathode. This is our Electron Beam. This here where the Electron Beam comes into contact with our anode is the actual focal spot, and this here heading off is our effective focal spot.
Now I showed you here the anode angle will change our effective focal spot, and there's a way we can calculate that which is known as the line focus principle. The line focus principle is not complicated. It shows us the relationship in the longitudinal direction between our anode angle and our actual and effective focal spots. This is the formula that we use: sine theta is the effective focal spot over the actual focal spot. And we are talking about the width of the effective focal spot in this direction and the length of the actual focal spots in this direction here. So if we were to measure this length where the Electron Beam comes into contact with our anode and we were to multiply that by sine theta, by the angle, our anode angle, we would get this width here of our effective focal spot.
Now as we lower sine theta, as we decrease our anode angle, we will narrow our effective focal spot. It's simple trigonometry that you would have done at high school. We know that this angle here is the same as this angle here between our anode and our effective focus. By coming down, this is the angle here. Sine theta is the opposite over hypotenuse. So the opposite is our effective focal spot here, effective focal spot over hypotenuse, our actual focal spot. If we were to multiply that actual focal spot by sine theta, we would get our effective focal spot.
So let's look at that in a more practical sense. We'll start on the right-hand side here. This is what we've just seen: our Electron Beam making our actual focal spots, giving us our effective focal spot at a specific anode angle. If we were to take that same Electron Beam but reduce our anode angle, you can see how we have narrowed our effective focal spots. And in a future talk where we talk about geometric blurring, you will see why a small or narrow effective focal spot is good when trying to create sharp images with good spatial resolution and little geometric blurring. So this is one way which we can narrow our effective focal spot by decreasing our anode angle.
The second way we can do it, you see here our anode angle on the left is the same as our anode angle on the right, but we've got a much narrower effective focal spot. And the way we've done that is we've selected our smaller filament on our cathode. You will remember we had a long and a short filament on our cathode. If we were to decrease the height of our Electron Beam here, we have then decreased this point of contact between the Electron Beam and the anode itself. We've decreased our actual focal spot length here, and in turn, from the line focus principle, we have then narrowed our effective focal spots.
Now not only does changing the anode angle affect the effective focal spot size, but it also changes something that's known as field size. You can see here I've got a large anode angle on the right and a small anode angle on the left. A large anode angle gives us a wider effective focal spot, and our small anode angle, a narrower effective focal spot. This is with the same Electron Beam coming in. But x-rays, as I've mentioned, are released isotropically in 360 degrees. And our larger anode angle here will mean our actual field size, the area of x-rays that are heading towards our patient, is larger.
So if we are imaging a large body structure, if we need to image the entire abdomen, we might not be able to use a small anode angle because our field size is not big enough. We might need to increase the anode angle at the expense of increasing our effective focal spot, increasing our geometric blurring, but we've got a larger field to create that larger x-ray. So it's a trade-off between field size and effective focal spots. And not only that, we mentioned that increasing our anode angle, increasing the actual focal spot, allows the anode to tolerate heat more often, and that's another reason why we may need to use a larger anode angle.
So these are the two major things that change when we change our anode angle. We change our effective focal spot width and our field size. And when you asked about the anode angle, these are generally the answers you give within exams. And this question I've actually gone over multiple times in multiple different iterations within the past paper question bank that I've created, linked in the first line in the description below.
Now this is not the full truth. Not only does our effective focal spot size change and our field size change, we also get a change in the phenomenon known as the anode heel effect. And we're going to look at that in our next lecture. And the last thing that is important to note is that this effective focal spot size that we are drawing here is for the midline of our x-ray beam, or the midline of the field. This effective focal spot actually changes as we move from either the anode side or the cathode side of our x-ray field. That geometry changes. And this is a little bit of a difficult concept for many people to get. But when we talk about effective focal spot size, we're talking about the average effective focal spot size, the one in the middle of the field.
Now if you look at this diagram here, the anode angle is the same for all of these anodes, and the Electron Beam is exactly the same with the same actual focal spot. What I've done here in the middle is showing you the effective focal spot that we calculated with our line focus principle. But as we head closer to the anode side of our field, we see that that effective focal spot gets narrower and narrower. We get less geometric blurring towards the anode side of our image. The opposite is true as we head towards our cathode side. Our effective focal spot gets larger and larger. And you can see why it's an effect of the geometry of the angles heading out towards our anode side versus our cathode side.
So it's important to just remember in your head, and this kind of question catches a lot of people out in exams, that there's not only a variation in effective focal spot size when we change our anode angle, but there's also a variation in effective focal spot size throughout our field. Closer to our anode, it gets smaller. Closer to our cathode, it gets bigger.
So in the next talk, we're going to look at the anode heel effect, a really important concept to wrap our minds around and something that we can actually utilize as radiographers and radiologists in our everyday practice. So I'll see you all there. Goodbye.