Transcription
We've looked at multiple different concepts now when it comes to x-ray beam geometry. We've seen how changing the anode angle changes both the field size and the effective focal spot size. We've also reviewed the anode heel effect and how changing the anode angle, our source to image distance, as well as our field size, all change the anode heel effect. And finally, we've looked at the process of filtration, both inherent and added filtration, and briefly touched on the concept of compensation filters. So, multiple things have happened to our x-ray beam, and they still haven't yet reached our patient.
Now, this is the last concept that we're going to touch on prior to x-rays finally reaching our patient, and it's what's known as collimation. Now, collimation is the removal of x-rays from our x-ray beam to allow only a specific portion of those x-rays to head towards our patient. So, let's have a step back here and see the entire system as a whole. We've got our x-ray tube here where x-rays are being produced. You can see how x-rays are produced in all different directions, isotropic, produced in 360 degrees. Some of those x-rays will make it out of our x-ray window here. They've had to go through our glass envelope, our conducting oil, and our x-ray window. All of these are inherent filters of our x-ray beam. They then go through our added filtration, a sheet of metal that we can place to further filter our beam. It now enters what is known as the collimation machine or the collimator here.
Now, these gray blocks here represent lead sheets that will attenuate our x-ray beam. Lead has a high atomic number, and we saw with the photoelectric effect, the higher the atomic number, the more likely the photoelectric effect is to occur, the more likely those x-rays are to be attenuated. So, these x-rays do not reach our patient because they are being attenuated by this lead sheet or our collimator. If we were to look underneath this collimator machine from our patient's view here, this is what those collimating sheets would look like. Two parallel sheets of lead that can move in and out like that, and another two parallel sheets of lead that can move in and out like this. Those sheets moving reduce or expand our field size. They determine how many x-rays make it through towards our patients.
Now, you may be wondering what this light source is here. Well, we have a light source within our collimator machine that shines light onto a mirror. This line here represents a mirror. This mirror is 45 degrees to the light source, and the light will bounce off in parallel to our x-ray beam. So, the light that is exiting our collimator is congruent with the x-rays that are released from the collimation machine. We can't see these x-rays being released from the x-ray tube; they're not in the visible light spectrum. And this light allows us to see where we are collimating the x-ray beam. It matches the x-ray beam. Now, if you've ever seen someone getting an x-ray, you will see that square of light on their body. That is the collimator. That is showing exactly where the x-ray field is.
Now, our collimators, the more we collimate an image, the more we reduce the x-ray exposure to the patient. Now, if you have a look at this diagram here, collimation does two things. Here is an uncollimated image. We have our source of our x-ray, our patient or our object that is being imaged. Here is a mass or a bone or something that we want to image within our patient, and this is our x-ray detector. Now, x-rays can do three things when they interact with the patient. They can be transmitted, they can pass through the patient and hit our detector. They can be attenuated by the photoelectric effect and not reach our detector, or they can be scattered via a process called Compton scatter. Now, scatter gives a signal on our detector that is not congruent with the x-ray that was coming in. It adds noise to our image and reduces contrast.
If we were to collimate this x-ray beam, reduce the amount of x-rays that are reaching our patient, we get a much narrower field of view here. Our x-ray field is smaller. These scattering events on the side of the patient are no longer occurring, and they don't contribute to the noise that we see here on our image. So, collimation reduces the amount of scatter with our image, and what that does, it gives us better spatial resolution, there's less noise, and better contrast because we're not getting this scatter contributing to noise. So, collimation improves our image. It gives us better spatial resolution and better contrast, as well as decreasing the patient's dose. These regions of the patient is no longer exposed to x-rays. So, it's a win-win here. We reduce dose and we get a better image.
The one thing to note here is collimation does nothing to our x-ray spectrum. It doesn't change the energies of the x-rays that are reaching our patient. You mustn't get as confused with what happens in filtration where we attenuate the lower energy x-rays. These x-rays that are reaching our patients still have that same x-ray spectrum that we are generating at our anode here. And that's all there really is to collimation. It's not a difficult or complex topic to understand, but it's one of the most powerful tools we have when actually taking x-rays. We get better images, and the patient has less dose. It's something that if we can collimate, if we can reduce our field size, we must do it in any patient that we are imaging.
Now, I've shown you a collimator that is a square like this that can create various different rectangles or squares. There are multiple different shapes of collimators. You can get circular collimators and collimators that are designed specifically for specific x-rays that we are trying to take. So, now our next talk, we are going to loop back round and look at the x-ray circuit that powers our x-ray machine. We'll look at the primary circuit, the secondary circuit, and the filament circuit. After that, we can finally move on to the frame strung and characteristic radiation production of x-rays that I've been talking about for so long. So, I'll see you all in that next talk where we look at the x-ray circuit. Until then, goodbye.