Transcription
Hello and welcome back. Now, in our last talk, we looked at four different factors that influence the amount of scatter within our image: the tissue density, the tissue thickness, our field size, and the X-ray energy.
Now, today we're going to look at how we can reduce that scatter contribution to our final image. And a lot of it will be related to those four factors that we looked at in our last talk. So, if you haven't watched the last talk, go watch that first, then follow it up with this talk here.
Now, if we look at our X-ray overview here, we have our X-ray tube, our collimator, our patient, and our detector here. We call this X-ray beam prior to hitting the patient our primary X-ray beam, and the beam leaving the patient the remnant or the exit X-ray beam. And it's the exit X-ray beam that hits our detector here.
Now, we can change multiple factors either in the primary X-ray beam, or within our patient, or within this exit X-ray beam in order to reduce scatter in the image. Now, the most important thing that we can do to reduce scatter is collimation here. Now, remember, collimation is using these shutters here, these lead shields that narrow down our field of view. We can image just the part of the body that we want to image and not expose other areas of the patient to ionizing radiation.
So, if we are interested in just this part of the patient here, we have all of this tissue on the flanks of our patient contributing to scatter, decreasing contrast, increasing noise within the image, ultimately leading to poorer spatial resolution. If we collimate that field, we decrease the field size, we reduce patient dose because these regions of the patient are not getting ionizing radiation anymore. Only the region that we are looking at is exposed to X-rays. Not only do we decrease patient dose, but we have eliminated all of this scatter here. All of the scatter events that were occurring in these regions of our image no longer are occurring. And this is the best way that we can reduce scatter. The scatter reduction and dose reduction to the patient here is proportional to the field size reduction. So, not only are we decreasing patient dose and scatter, but we're improving our image. And if you're ever asked in an exam how can we reduce our scatter, collimation should be your first answer because it has that patient benefit as well.
The second thing we can look at is X-ray beam energy. And I mentioned in the last talk that this is a difficult concept for people to get their heads around. And I'm going to spend a little bit of time here trying to explain why increasing X-ray beam energy actually increases the amount of scatter contribution to our final image.
Now, we've seen that the photoelectric effect decreases exponentially as photon energy increases, and Compton scatter just gets this slight decrease at low photon energies. Photoelectric effect predominates, and as that energy increases, our Compton scatter is what contributes most to attenuation within our image.
Now, another way to represent this is percentage contribution to attenuation here. At low photon energies, the photoelectric effect predominates when it comes to attenuation. As we get to higher photon energies, we get more and more Compton scatter proportionally. You can see here, at 15 keV, the photoelectric effect contributes to about 60% of our image. If we just go to 30 keV, that is completely switched. Compton scatter is now contributing to about 60% of the attenuation within the patient.
Now, a useful way that I like to think about this is taking our primary X-ray beam and thinking of it as a light that we are shining against the wall. Our primary X-ray beam is casting a light on the wall. Now, if we want to image anything, if we want any detail within our image, we need some attenuation. We need to place something in that primary beam, that light source, to cast a shadow onto the wall.
Now, think of the photoelectric effect as placing a piece of paper in front of that wall that is completely opaque. It blocks all light photons from reaching that wall and casts a definite shadow on that wall. Now, think of Compton scatter as placing a baking sheet in front of that light source. It allows most of the light through, but it diffuses that light. That shadow that was caused by the photoelectric effect, that once had a crisp border, now has a fuzzy border. That baking sheet has caused some scatter within that image.
Now, if we were to increase the intensity of that light that we are shining, and our photoelectric effect was to decrease, that paper that was once blocking all of that light becomes more and more see-through. It allows more light photons, primary photons, through onto the wall. Now, what is happening to that shadow that was cast on the wall? The shadow is getting dimmer and dimmer. The contrast between the shadow and the primary beam is getting less and less. We're losing some of that contrast, we're losing some of that anatomic detail.
Now, the rate at which that paper becomes more see-through is far greater than the rate at which our baking tray becomes see-through. Now, at a certain point on this graph, it's here, there will come a point, an intensity and X-ray energy, in which our baking sheet will become more opaque than the sheets of paper that we initially placed. Our photoelectric effect has decreased so much that that paper has become more and more transparent, and the attenuation of that light is coming more from the baking sheet than it is from that paper that we initially placed there.
Now, our image has gotten worse. Our anatomic detail, the shadow that we're casting on that wall, has gotten less and less. And the attenuation of that light, at a certain point, happens more from the baking sheet than from the paper. Our Compton scatter is contributing to the image more than the photoelectric effect. And that's the way I like to think of it. When we think of increasing X-ray beam energy, yes, perhaps the pure number of photons in Compton scatter is decreasing, but the proportion of scatter compared to the photoelectric effect increases as photon energy increases. And the effect that that has on our final image, the shadow that is cast on the wall, means that we're getting lots of anatomic detail, and scatter is actually contributing more to the attenuation than photoelectric effect itself. And that's a good way to think about it: as we increase photon energy, scatter contributes more to the image.
The next thing that we can look at is tissue factors itself. And I said that thinner tissues scatter less. Now, there aren't often moments that we can actually decrease the thickness of tissue. And this generally only relates to mammography, where we can compress breast tissue, make the thickness of the breast tissue less. And that compression gives us much better contrast and spatial resolution within our image. It decreases the noise, increases the contrast. And this is so important when it comes to mammography because we need contrast. It's contrast that allows us to pick up microcalcifications and other pathologies within the breast. So, tissue compression can also reduce our scatter.
Once our scattered X-rays have left the patient, we're now in the remnant or the exit X-ray beam. There are two things that we can do to reduce the scatter. The first is what's known as an air gap. We can increase the distance from the object that we're imaging to the detector. Increasing this distance, this air gap, means that some of the X-ray photons, so scattered photons that would have hit the detector if the detector was closer to the object, now no longer hit that detector. And we've increased our contrast in the image. So, the further our X-ray detector is away from the object that we're imaging, the less scatter contributes to the image.
Now, there's a problem here because when we increase this distance here, what we're doing is magnifying our object. The shadow that is cast on our detector is much bigger. The image that we get is larger than the structure that we are imaging. And something known as geometric blurring occurs as we magnify objects on our detector. And in two talks time, I'm going to go over that magnification process and the concept of geometric blurring or geometric unsharpness. But for now, this is a way that we can reduce scatter contribution to the X-ray intensities hitting our detector.
The last thing that we can do is place what is known as an anti-scatter grid between our patient and the detector. We are producing X-rays in an X-ray source, they interact with the patient, and they can either be primary transmitted X-rays that are parallel with our X-ray source, or they can be scattered X-rays that come off at different angles from our primary X-ray beam.
Now, what an anti-scatter grid is, is it's a series of highly attenuating septa that are parallel to our primary X-ray beam. Transmitted X-rays that are parallel to these anti-scatter grids will pass through the grid and hit our detector. So, scattered X-rays that come off at an angle will be attenuated by these grid septa, and very few of them will reach our detector. This is a way of reducing scatter contribution to the exposure to our detector, and it's known as an anti-scatter grid.
Anti-scatter grids are something we need to know in some depth, and I'm going to go over that in our following talk, where we're going to look at anti-scatter grids in depth.
The ways in which we can reduce scatter within our image is a common exam question, and scatter comes up over and over again in exams. I've dedicated a whole section in the question bank that I've linked below specifically to scatter, that's how often it comes out. So, if you're studying for an exam, go and check out that question bank. Otherwise, in our next talk, we're going to have a closer look at anti-scatter grids. So, until then, goodbye everybody.