Transcription
Hello everybody and welcome back. Now, over the last couple of talks, we've been looking at the various different types of X-ray detectors and how they take X-ray energy and convert them into a radiograph.
Now, those X-rays that are striking our detector are either transmitted unattenuated X-rays that have passed through our patient, or their scattered X-rays coming from Compton scatter or Rayleigh scatter within our patient. And it's these X-rays that contribute to the signal intensity that our detector experiences. Other X-rays can be attenuated by the photoelectric effect, and it's this attenuation that gives us the anatomic detail within our patient. And the varying different shades of gray are reflections of the linear attenuation coefficient of the various different tissues within our patients.
Now, in the next three talks, we're going to be looking specifically at scattered. These scattered X-rays, we're going to look at what scatter is, how it contributes to our image, and in this talk, the various factors that influence the amount of scatter within our image. Then, in the next talk, we're going to look at the factors that we can manipulate in order to reduce the amount of scatter in our image. And finally, we're going to end off by looking closely at anti-scatter grids and how they affect the way in which we go about taking our radiographs.
So, in an ideal world, we would create X-rays from a point source at our focal spot here, and they would be released in an isotropic manner, diverging out towards our patient here. Now, these X-rays would either be transmitted through the patient or attenuated via the photoelectric effect. And if only these two interactions occurred, then we would get a perfectly crisp image with no geometric blurring and no background noise. X-rays that went unobstructed towards our detector would make dark regions on the detector. Those that had variable attenuation, either attenuated by the photoelectric effect or passed through via transmission, will have varying scales of gray within our image depending on the linear attenuation coefficient of those tissues those X-rays are passing through. And the regions are completely attenuated X-rays would appear white on our image.
Now, unfortunately, we have this concept called scatter, where some of these X-rays that either would have been transmitted or undergone the photoelectric effect have now been scattered at an angle to the primary beam. They no longer represent any information that is anatomically relevant in our image. So, what has this scatter done? Well, firstly, it has decreased the contrast within our image. We have scattered X-ray photons hitting the detector where there shouldn't have been photons hitting the detector. We've darkened our light areas and we've lightened our dark areas. We've decreased the contrast within this image. Not only that, but because this signal here on our detector does not correlate to anything anatomically, we've increased the background noise or the background haze on our image. That electronic signal is random. Now, it doesn't correlate to anything within the patient that we are imaging. Not only that, but scatter also contributes to patient dose. When we looked at the Compton scatter, we saw that that incident X-ray releases a photoelectron from the outer shell of our target material, and that photoelectron travels through tissue imparting dose on our patient through the process of linear energy transfer. And lastly, scatter also can go off towards staff or patients within the room. There's occupational exposure here, where the subject being imaged now becomes a source of radiation. Now, this can either hit surrounding patients, or this can go to other regions within the patient and apply dose there, despite us not actually imaging those regions. So, scatter reduces our image quality by decreasing our image contrast and increasing the noise in our image. It contributes to patient dose, and it contributes to occupational exposure. There's nothing about scatter that is desirable. In an ideal world, we wouldn't want any scatter within our image.
Now, when we go about thinking about scatter, we can use these equations to kind of conceptually grasp how much scatter is contributing to our image. When we look at the X-rays that are hitting our detector, we can separate them into primary transmitted X-rays, unattenuated X-rays, and scattered X-rays. And we can use this equation here to calculate what's known as our scatter to primary ratio, the contribution of scatter over the contribution of primary radiation to the exposure on our detector. The higher the number of scattered photons compared to primary photons, the higher our scatter to primary ratio, and vice versa. We can also use what's known as the scattered fraction, or the fraction of scatter, where we take the intensity of scatter hitting our detector over the entire X-ray intensity hitting our detector.
Now, these equations, it's really important to note that they do not equate to image quality. They give us an idea of how much scatter compared to primary X-rays are going to our detector here, but they are not foolproof. They are not ideal indications of the quality of our image. We're going to see just now that there are times when the absolute number of scattered photons hitting our X-ray detector will decrease, and our primary transmitted photons will increase, but our image quality itself will decrease. And the reason this is, we will look at it a bit more in detail later, is that these equations don't take into account the photoelectric effect. And it's the photoelectric effect, it's the attenuation of X-rays that gives us the anatomic detail within our image. If all of the X-rays were transmitted through our patient, we would get a completely black image. There would be no anatomic detail. It's the attenuation that gives us that contrast, that gives us the varying different structures within our image. So, just remember that, keep that in the back of your mind when we now look at the different factors that influence scatter within our image.
So, they're actually four main factors that increase or decrease the amount of scatter within our image, and we're going to go through three of them today. The one that we're not going to go through today is tissue density. Now, the denser a tissue, the more Compton scatter there is in that tissue, and the more scatter contribution there will be to our image. There's nothing that we as radiographers or radiologists can do to decrease the density of tissue. So, the density that we have in the tissue that we're imaging is set, it's finite. It's important to note that a denser tissue will increase the amount of scatter, but is something that we can't change.
Now, the first thing we're going to look at is tissue thickness. We'll see how that affects field size and X-ray energy. These are the three main variables that contribute to scatter within our image. So, let's start by having a look at tissue thickness. And this is fairly self-explanatory. If we are taking a radiograph of a hand, a thin tissue, there is less distance of tissue that the X-rays have to travel through. If we're taking an X-ray of a thigh, it's much thicker than a hand. The distance that these X-rays have to travel before going through the tissue is larger. In a thicker tissue, the likelihood of a scatter event to occur is higher, and the likelihood of a photoelectric effect to occur is higher. So that does two things: it increases the amount of scatter, our scatter proportion increases, and it decreases the amount of primary transmitted X-rays. So our primary X-rays decrease, our scatter to primary ratio goes up, our scatter contribution increases. In a thinner tissue, there is more transmission and less scatter because of that reduced distance that X-rays have to travel. A fairly self-explanatory concept here, and we'll see that we won't need to use anti-scatter grids in really thin tissues because it's actually not that much scatter to get rid of. The thicker the tissue becomes, the more scattered it is within our image.
The second thing is field size. You know, we've looked at the concept of collimation reducing our field size, and this is perhaps the most important thing when it comes to scatter because when we collimate an image just to the region of interest, we decrease the amount of patient dose because these flanks on the image here are not being exposed to X-rays anymore. Only the region of interest is being exposed to X-rays, and these regions now aren't contributing to scatter. We've got no scatter events occurring outside of our field of view. So, decreasing our field size not only decreases the amount of scatter, but it also decreases the patient dose.
Now, we can combine these two factors into this graph. Our Y-axis is our scatter to primary ratio. The higher that ratio, the more scattered it is in the image. And our X-axis is our field size. These three different graphs represent three different tissue thicknesses, and we can see that for all tissue thicknesses, as we increase our field size, our scatter to primary ratio trends upwards. The increase in field size results in an increase in scatter. Not only does an increase in field size increase scatter, but an increase in tissue thickness has a marked increase in the amount of scatter or the scatter to primary ratio within our image. So, increasing field size and increasing tissue thickness all increase the scatter contribution to an image.
Now, I've left X-ray energy to last because this one is a little bit more difficult to conceptualize. The higher our X-ray energy is, the higher the half-value layer of that X-ray beam. That quality of the X-ray beam has been increased, and we get more transmitted X-rays through our patient. We've seen that an increase in X-ray energy leads to a drastic drop-off in the amount of photoelectric effect and a gradual drop-off in the amount of Compton scatter. So, as we increase our X-ray energies, the actual number of scattered photons decreases when we're looking at Compton scatter. But in relation to the photoelectric effect, Compton scatter predominates. Now, we are losing some image quality as we increase our X-ray energy from the fact that more X-ray beams are being transmitted and less of being attenuated by the photoelectric effect. And when we looked at our ideal situation at the beginning, it was that transmitted versus photoelectric effect that was giving us that detail in our image. Now, we're getting more transmission and less photoelectric effect. We're reducing that detail within our image, and we've still got lots of scatter here. We're not getting that drastic drop-off in Compton scatter in our image.
So, if we have a look at a graph here, this is the equation that we looked at when we looked at the likelihood of the photoelectric effect to occur. There's an exponential decrease in the amount of photoelectric effect as energy increases. We get this drop-off. We get a slow, gradual drop-off. Now, the scale is not linear. So, this common sense scatter is actually relatively linear as photon energy increases. We get a slight reduction in Compton scatter. So, one would think that if we're reducing Compton scatter, our scatter contribution to the image would be less. Now, there's two ways to think about this. One, our contribution of Compton scatter to attenuation, comparing to the photoelectric effect, actually increases as photon energy increases. The difference between these two lines increases here. So, the amount of detail in our image as a result of Compton scatter is technically increasing as that photoelectric effect is decreasing away. The second point to note is that as we increase our X-ray energies, if we don't adjust for those intensities by changing the current in our filament, we are actually increasing the number of photons. We saw that an increase in X-ray energy leads to both an increase in X-ray beam quality as well as an increase in X-ray beam quantity. So, that increasing energy, if we don't account for it, we will actually get a higher absolute number of Compton scatter. So, increasing our X-ray energy actually leads to a higher contribution of scatter to our image. We get reduced image quality, and this is a concept that is a little bit difficult to get your head around.
Now, in our next talk, we're going to be looking at the various factors that we can manipulate in order to reduce the amount of scatter within our image, one of which being changing the photon energy of our X-ray beam. So, I'm going to discuss this concept a little bit more in that talk.
So, to summarize, increasing X-ray beam energy, increasing tissue thickness, and increasing field size all increase the amount of scatter contribution to our radiograph, and a reduction in scatter leads to a better image. So, I'll see you all in our next talk where we're going to look at the factors that we can change in order to reduce scatter in our image. Until then, goodbye everybody.