Transcription
So, if you look now at the factors that we can change in order to reduce the amount of scatter contribution to our image, and the last thing I looked at in our previous talk was anti-scatter grids. I want to spend a little bit more time here looking specifically at anti-scatter grids.
So, what exactly is an anti-scatter grid? Well, it's a grid that we place between our patient and the detector. We have an x-ray source with our primary x-ray beam. We then have an exit or remnant x-ray beam heading towards our detector. Now, that exit x-ray beam has either primary transmitted photons or scattered photons.
Now, an anti-scatter grid has multiple septa of generally lead, a highly attenuating material to x-rays, that are separated by what is known as an interspace material. Now, those x-rays that are parallel to our grid will pass through the grid. They'll be unattenuated and hit our detector. Those that are not parallel to our primary x-ray beam, AKA scattered photons, will be attenuated by these lead septa within our anti-scatter grid. Now, some of the scatter will make it through that hasn't been scattered at a very large angle and will still contribute to the exposure at the detector, but we've greatly reduced the amount of scatter going through to the detector.
The primary photons that make it through to the detector is what's known as our primary transmission factor. Some of these primary photons will hit the grid septa. They will hit the top of the grid septa, so we will get a loss in some primary photons. The scatter transmission factor talks to the amount of scatter that makes it through our anti-scatter grid. Now, we want our scatter transmission factor to be well below our primary transmission factor, and that's what an anti-scatter grid does. It predominantly attenuates scattered photons.
Now, you can see here, this grid is what's known as a linear grid. All of these septa are parallel to one another. Now, we know that x-rays don't pass like this. They don't pass in a straight line. They diverge from our x-ray source. X-ray production at the focal spot is an isotropic release. They're released in all angles and they diverge like this. So, what we actually need is a grid that's called a focused grid. Now, we do get linear grids, but generally we use focused grids where the outer septa are angled to account for that divergent x-ray beam. So, the septa now better match that divergent primary x-ray beam, and we don't get cut off of these outer regions on our detector because this divergent x-ray beam would then be attenuated much like a scattered photon would be if this grid was linear.
The first thing to note with a focused anti-scatter grid is that we need to have a set focal length here, and generally the focal length is 40 inches or 100 centimeters here. Now, we can see that if we weren't in that focal length, then we would get cut off of our primary x-ray beam. And there are multiple different types of cut off when looking at grid setup in our x-ray detection systems. So, these are the four main types of cut off that we get. The first that I've mentioned is off-focal grid cut off, when we place that anti-scatter grid at a focal length that is incorrect for this specific focused grid. We can see that these outermost regions now are attenuated by the grid despite being part of the primary x-ray beam. Secondly, if we have that grid off-center from our x-ray source, our x-ray source needs to come directly through the center of this grid. If we have it slightly off, we're going to get cut off again on these outer regions of our grid. We can have off-level grid cut off if we're taking a mobile x-ray. So, your patient's in a bed, and the mattress is slightly squishy, and the way that the patient is lying on that grid turns the grid slightly, we're going to get cut off again. These are all issues that we need to think of when using grids in our x-ray systems. Another off-level grid cut off can actually be how we angle our x-ray beam towards the patient. Now, when we are taking a chest radiograph, we can angle up and down like this depending on how tall the patient is or where our detector is. And if our x-ray grid is lying in this orientation, the septa are going this way on the short axis, and then we were to angle those grids, they would cut off our x-ray beams. We need to make sure that the grids are lying on this angle, longitudinal, on the long axis, so that when we angle our x-ray beam, those grids don't cut off the x-ray beam. That's another off-level grid cut off that we can get. And obviously, if we place the focused grid upside down, they are no longer congruent with that primary x-ray beam, and we get a large amount of cut off on the peripheries of our grid.
So, let's have a look at the x-ray grid itself and define a couple of terms. The first is the height of the grid itself. Now, this can be a little bit confusing because you would generally think of the height as the thickness of the grid, but the height is determined by the length of these grid septa here. The thickness of the grid we call the height. Then we have what is known as the grid septa here, which is generally made of a highly attenuating material such as lead here. And those grid septa are separated by what is known as an interspace material. Now, ideally, this would be air, just allowing x-rays to pass by freely, but air doesn't give us that structural integrity of the grid itself. So, we generally use aluminum, which has an atomic number of 13, or carbon fiber, that will let through most of those primary x-rays.
Now, the thickness of our grid septa we call T here, and the thickness or the width of our interspace material we call W. Now, if we were thinking about scattered photons trying to make it through this grid septa and the angle in which they would need to be in order to make it through, we can see that changing some of these parameters will change the number of scattered photons making it through. The first thing we could do would be increase the height. As we increase the height of our grid itself, fewer and fewer scattered photons would make it through. The second thing we can do is decrease the width of this interspace material. The narrower and narrower this space here, the fewer scattered photons that make it through. And this is what's known as our grid ratio. Our grid ratio determines that scatter transmission factor, the number of scattered photons that make it through our grid. Now, grid ratio is the height of the grid divided by the width of the interspace material. You see, it has nothing to do with the thickness of the septa itself. As we increase our height, we increase our grid ratio. We decrease the number of scattered photons that make it through. As we decrease our width of the interspace material, we increase our grid ratio again and decrease the number of scattered photons that make it through.
We can talk about a term called grid frequency. If we were to add our septal thickness and our interspace material width here, we would get the distance between one septa and the following septa. Now, the combination of these two distances, if we were to divide one by that distance, we would get our grid frequency, the number of grid septa per unit length.
Now, we can see that as we increase our grid ratio, fewer and fewer photons make it to our x-ray detector. Our exposure to that detector decreases, and we can calculate a factor known as the Bucky factor. Now, Bucky factor was much more important when we were dealing with screen-film radiography. You'll remember in screen-film radiography, we had that characteristic curve and that linear region, that region of latitude was very specific exposures that our x-ray detector required in order to give us a usable image. And placing a grid between our patient and the detector would change the exposure that our x-ray detector experienced. So, our Bucky factor is the increase in exposure that we need to apply to our patient in order for our x-ray detector to have the same exposure that it would have had without a grid.
So, let's look at this a little bit more closely. The exposure without a grid, if we weren't to put a grid between our patient and the detector itself, the x-ray detector would experience a specific exposure. If we were to then place a grid between the patient and the detector itself, we would decrease the number of photons reaching our detector. In order to keep the number of photons the same, we would need to increase the dose, increase the exposure to the patient to allow for more of those primary transmitted photons to make it through and get the same exposure at our detector. And this is what's known as the Bucky factor.
Now, you may come across a term of a Bucky grid. The Bucky grid and Bucky factor are completely different. The Bucky factor is that increase in exposure in order to get the same exposure in our detector. A Bucky grid is what is known as a reciprocating grid, a grid that moves like this. Now, you'll see that if we were to have a grid called a stationary grid that stayed still, we would get small lines on our radiograph because our septa here would cause shadows on our grid. Now, a Bucky grid reciprocates like this. It moves like this, and that movement prevents those lines being formed on our grid. The Bucky factor is that increase in intensity that we require to get the same exposure on our detector.
Now, we can use our grid ratio to determine the Bucky factor that we require. As the height of our grid gets bigger, or the width of our interspace material gets smaller, we require more and more exposure in order to get those same x-ray exposures at our detector. And you can look these factors up. You don't need to commit these to memory, but as a grid ratio increases, our Bucky factor increases.
Now, when we looked at collimation, we were reducing the amount of scatter and we were reducing the patient dose. Grids are not the same here. Grids reduce the amount of scatter reaching our detector, but it comes at a cost. We need to increase patient exposure and ultimately increase patient dose in order to get those same exposures in our detector. So, it gives us a better image, but it comes at the cost of increasing the dose to the patient.
So, I've spent three whole talks now looking at scatter, and you might wonder why dedicate so much time to scatter itself. And in my experience, going through multiple past papers, there were two major themes at the top and above the rest: one was scatter, and one was the factors that influenced the x-ray beam. And you'll see in the question bank that I've linked below, I've dedicated a whole section just to questions regarding scatter. It comes up over and over again in exams. And if you're studying for your part one exams or your registry exams, I'd highly recommend going and checking out those past paper questions.
In the next talk, I'm going to look at magnification and the concept of geometric blurring or geometric unsharpness. You saw that when we use the air gap technique to reduce our scatter, we got magnification. And I said there was an issue with that, and that issue was geometric blurring. So, join me in that next talk where we're going to look at magnification, look at geometric blurring and unsharpness, and see how the focal spot itself is actually contributing to that unsharpness. So, until then, goodbye everybody.