Transcription
So let's move on to the last of our x-ray detectors, our direct thin film transistor arrays. We've looked at our cassette-based systems, our screen film radiography, our computed radiography, and we've looked at our indirect digital radiography, our CCD chip, as well as our indirect thin film transistor array.
Now we're going to look at our direct thin film transistor array, which doesn't require scintillation. It's still a digital radiography system, but we don't convert x-rays to light first. We convert x-ray energy directly into an electronic signal. And you can see from this diagram here, it shares a common detector element here, a DEL, the same DEL that is used in our indirect thin film transistor array is used in our direct thin film transistor array.
Now the difference here is that we use what is known as an amorphous selenium layer here, and this is a semiconductor. Now our amorphous selenium has the ability to convert x-ray energy into an electronic signal. Now what we do is we create a charge differential across this amorphous selenium with a positive or anode region on the top of that amorphous selenium semiconductor where our x-rays are striking the detector, and a negative or cathode end of this amorphous selenium which interacts with our TFT array.
A specific property of this semiconductor is that when x-rays strike this layer, it can create what is known as ion pairs. Now we traditionally think of current flowing as electrons flowing, which is the correct way to think about it. But when an electron flows through a semiconductor like this, we also get what is known as an electron hole flowing in the opposite direction. The movement of electrons creates electron holes moving in the opposite direction. So what happens is negatively charged electrons flow towards this positive end of our amorphous selenium semiconductor, and our electron holes, or the electron pair that is being created, flow towards the negative end here, towards our TFT array, and that electron hole can be measured as current on our TFT array.
The main difference here is that we haven't created light photons within a scintillation layer. And what happens is when we create light photons, we get spreading out of that light photon. That hasn't happened here. Where our x-ray strikes this amorphous selenium is where the electron pair is created, and directly because of this charge differential, that electron will flow in the direct line of that x-ray. We haven't got lateralization of those photons. We've got very good specific spatial resolution here. Where that x-ray hits this semiconductor is exactly where that electron hole will be going towards our TFT array. We've got excellent spatial resolution here.
The issue comes is that this semiconductor is not great at dealing with high x-ray energies. It only really works at the lower ends of the diagnostic x-ray spectrum. That's why a system like this is ideal for something like mammography. In mammography, we need really good spatial resolution and we need good contrast. But because of the compressed breast tissue, we are able to use a lower quality x-ray beams, lower average energy of our x-ray beams, and this system thrives in that setting because we get great spatial resolution, but we don't have the problem of the semiconductor not being able to deal with those high x-ray energies because we're using lower x-ray energies.
The other thing here is when we looked at our DELs in our indirect thin film transistor array, we saw that that photodiode indiscriminately led electrons down onto the entire DEL here, and there were regions that are not sensitive here that weren't contributing to the image. Now what we can do in our direct digital radiography systems is mold that electric current onto the sensitive area on a DEL. That full factor that we created is less of an issue here because we are creating these current differentials. We can ensure that those electron holes that are created are molded onto this sensitive area. So we get great signal for the x-ray energy that we are creating here. We don't get lots of signal onto this non-sensitive area here.
Again, when those electron holes come down onto this sensitive area, that charge is stored within the capacitor. And after our x-ray has been exposed, we can then sequentially close these TFT switches and read out the charge that has been stored in our capacitors here. That charge is then amplified and sent to our computer, where each DEL represents a specific pixel on our image, and the amount of current that is stored within each capacitor correlates to a pixel value, a grayscale value on our image.
In all of our digital radiography systems, our indirect, our direct, as well as our computed radiography systems, we have actual pixel values on a computer screen that we can manipulate. It's not like our screen film radiography where there's a characteristic curve and once we have our film, there's not much we can do. Here on our digital systems, we can invert the film, we can change the contrast and change the exposure. We have a lot more data that we can manipulate in order to change our image.
So hopefully by now, you've got the broad overarching categories of the x-ray detector systems and how they differ from one another. And hopefully, you've gained an appreciation for some of the benefits versus the drawbacks of the various different systems. And that's the type of question that generally comes up in these exams. Sometimes you're asked about specific processes that we've gone through in each of these talks, but often you're asked to compare the different systems, compare indirect to direct, or compare digital radiography to computed radiography. And these might be talks that you need to go over again and again because there are subtle differences between the two. Here we're dealing with amorphous selenium. In our indirect thin film transistor array, we were dealing with hydrogenated amorphous silicon. There are subtle differences between the two. And hopefully, after you've gone through these a couple of times, you'll really start to get a good understanding for how we go about detecting x-rays, processing them, and creating our final image that we see on our computer screen.
So to round off this x-ray course, I want to talk about scatter, something that we have mentioned multiple times, but I haven't actually showed you how we go about reducing the scatter in the image and improving our image quality. So that's what we're going to be looking at next. So I'll see you all in those talks. Goodbye everybody.