Transcription
When we were looking at the factors that we can change in order to reduce the amount of scatter within an image, I showed you the air gap technique, where we increased the distance between our patient and the detector. And some of those scattered photons no longer hit the detector. And I say that came as a trade-off, because when we increase that distance, we increase what is known as magnification, and we increase the amount of geometric blurring or geometric unsharpness within the image. And I didn't elaborate any further then, and I want to take some time now to discuss these concepts of magnification and geometric blurring.
Now, in order to understand this, we need to define a couple of terms. The first is what's known as our source. That's the focal spot. That's where we're producing x-rays via Bremsstrahlung and characteristic radiation. And those x-rays are heading out towards our patient. Our patient in these equations is what's known as the object, and our detector is what's known as the detector or the image. Now, the distance between our focal spot and the patient itself is the source to object distance. The distance between our object and the detector is our object to image distance. And the total distance from the source to the detector is our source to image distance.
Now, we've looked at examples where we are dealing with a point source of our x-rays, our focal spot, that has no size. And that's an ideal world where we would produce x-rays from a point source. Now, we see that that's not the case. Our focal spot has some size. And x-rays produced at this part of the focal spot are released in an isotropic manner. They're released in all directions. And you can see that x-rays released here will cast a shadow on our detector here. And x-rays released on that same spot will also cast a shadow on our detector here. And if we were to look at this side of the focal spot, we see that the shadow cast by this region of the patient is different, solely due to the fact that those x-rays are coming from a different point on our focal spot. And it's this concept here, this blurring here, that is known as geometric blurring or geometric unsharpness. It's a function of this focal spot having some size. Now, this region here, where our shadow is blurred, is what's known as our penumbra of the image. Now, we want that penumbra to be as small as possible. The geometric unsharpness or geometric blurring to be as small as possible. And I mentioned that when we magnify an image, we get more geometric blurring. So let's have a look at what that looks like itself.
If I were to decrease the magnification in this image, the way I could do this is to bring our object closer to our detector. We decrease that air gap. As we decrease that air gap, you can see that our image has gotten smaller. The magnification has reduced, but our penumbra, as well, has gotten smaller. So just moving that patient closer to our detector here, see what it does to our image. It decreases the size of the shadow here, but also decreases that geometric blurring. And what I've done here is I've increased our source to object distance and decreased our object to image distance.
Now, when we look at magnification, we can calculate the magnification by dividing our source to image distance over our source to object distance. So it goes without saying, if we increase our source to image distance, we increase the magnification of our object. If we decrease the denominator here, we decrease the source to object distance, we will increase our magnification. So let's have a look at what that looks like here. We've increased our source to object distance, we've increased our denominator, therefore we've decreased the amount of magnification. And what that's done is it's decreased our geometric blurring. This is the exact example that we looked at here. So, super common question when it comes to exams, how can we decrease our magnification? Well, we can increase our source to object distance, or we can decrease our source to image distance.
So let's have a look at the other example. Let's now decrease our source to image distance. We can reduce our magnification. So as we bring that detector closer to our source again, we decrease the magnification and we decrease the geometric blurring within our image. So we can see that magnification and geometric blurring are related. As our magnification increases, our geometric blurring also increases, and vice versa. But it's not the magnification itself that is causing the geometric blurring. We've looked at this previously. It's the fault of the focal spot that causes the geometric blurring. If this focal spot was a point source and we magnified our image, there would still be no geometric blurring. It's the focal spot that is causing this geometric blurring.
And right at the beginning of this course, we looked at the focal spots and how we could change the size of the effective focal spot. Now, there were two main ways that we changed the width of this effective focal spot. The first thing that we could do is decrease our anode angle. As we decrease that anode angle, our effective focal spot gets smaller. As our effective focal spot gets smaller, our geometric blurring gets smaller. The second thing that we could do is use the shorter filament, the smaller filament, a narrower electron beam going towards a smaller actual focal spot, giving us a narrower effective focal spot. Now, these come at a trade-off because we get a reduction in our field size, but we get better image quality because of this narrower or smaller effective focal spot.
So we've seen now, as the focal spot increases, our geometric blurring increases. Our penumbra gets larger. The smaller and smaller that focal spot, the closer it gets to a point source, the less geometric blurring, the sharper our image. And we've seen that if we move our object closer to our detector, or if we increase the distance between our source and our object, we get less and less geometric unsharpness or geometric blurring. So we can actually then go about calculating geometric unsharpness. It's a function of the size of the focal spot. As our focal spot increases in size, we increase our geometric unsharpness. As we can see here. And it's a function of the object to image distance, the distance between our patient and the detector. As we decrease that distance between the patient and the detector, we decrease our numerator here, we decrease our geometric unsharpness. And it's a function of our source to object distance, the distance between our focal spot and our patient. As we increase that distance, we decrease the amount of geometric blurring.
And if you go back and have a look at magnification here, you can see how this is related to magnification. How magnification plays a role in amplifying the effects of the focal spot. And that's why we times the focal spot by this equation here. Magnification doesn't cause geometric blurring, but it amplifies the geometric blurring that is caused by the focal spot itself. Now, these type of equations come up often in exams and are asked in multiple different ways. A couple of which I've included in the question bank below. So go check that out if you are studying for a radiology physics exam. Otherwise, I'll see you all in our next radiology physics talks. Until then, goodbye everybody.