Transcription
So, we've spent some time now looking at screen-film radiography and the characteristic curve of a specific screen-film. Now, let's shift our attention to the digital radiography systems, and I'm going to start by looking at computed radiography.
Now, computed radiography falls under this broad, overarching category of digital radiography. And when we're talking about digital radiography in this sense, we are talking about the final radiograph that we create. We create a digital radiograph on a computer screen where grayscale pixel values represent each pixel on our image. Now, we can further subdivide that into computed radiography systems and digital radiography systems.
Now, these digital radiography systems talk about the way in which we process those X-rays, and we do that through digital systems. Now, when we talk about computed radiography, up until the final step, everything here is analog. Here, we are using a manual process, and at the last moment, we use something called an analog-to-digital converter where we convert all this analog signal into a digital signal, and then we can create our digital image.
Now, computed radiography uses a cassette. It's a cassette-based system, and we can carry around that cassette for many months after we've taken the radiograph, prior to processing that in a process which we're going to go through today and digitizing that image, unlike our digital radiography systems where a digital image is created almost immediately.
Now, there are two processes I want to go through today. The first is acquiring our latent image, exposing our cassette to X-rays and forming what is known as the latent image. The second process is processing that latent image into a digital image. So, let's have a look first at the structure of the cassette itself and how we go about creating a latent image.
Now, this is our cassette. Our gray area here is a cassette that we place behind the patient that will be exposed to either transmitted or scattered X-rays. Now, this computed radiograph detector is made up of barium fluoro bromide crystals. They're amorphous crystals. They don't have any set structure. There's no repeating structure between the crystals. They're randomly orientated within this computed radiography cassette.
Now, the barium fluoro bromide is doped by what is known as europium. Now, when we looked at our screen-film radiographs, we saw that our silver halide crystals were doped by a silver sulfide, and they have disrupted those crystals. Now, the same thing happens here. When we dope this cassette with europium, when we introduce europium into the barium fluoro bromide or the barium fluoride halide crystals, we just wrap the structural integrity of those crystals and we create what is known as F-centers here.
Now, these F-centers are made up of the fluoride atoms within a barium fluoro bromide crystal, and what we get is these relatively positively charged fluoride atoms known as F-centers. Now, a disrupted crystalline structure like this develops what is known as a valence band and a conduction band. So, the valence band is a lower energy state that has a lot of electrons available, and the conduction band is a theoretical energy level in which we would need to apply energy to the system in order for electrons to reach that conduction band.
Now, we can get into the weeds here, but I really want you to get the broad, overarching understanding of what is happening here. When we expose this cassette to X-rays, energy is put into the system, and electrons from our valence band can then be released to our conduction band. Now, that release of electrons causes europium two-plus to be oxidized to europium three-plus. It's lost an electron. Oxidation is loss of an electron. That electron, which has gained energy from these X-rays coming into our cassette, then goes to our conduction band and falls into what is known as the F-center. This positively charged fluoride atom has now gained an electron. It's been reduced. Reduction is gain of an electron, and it forms as a metastable fluoride atom here.
Now, what we've got is europium three-plus and a stable fluoride atom, an uncharged chloride atom, and this is what is known as our latent image. At this stage, we can carry around this cassette, and only when we process this image will we be able to see the digital image on a computer.
So, now we've created our latent image. These dotted lines represent three different periods in time: acquiring the X-ray, latent image formation, and then processing of that X-ray. This can happen in any stage once we bring the cassette to our CR reader, to our computed radiography reader. Now, this process can be reversed. We can release an electron from our F-centers, given energy into the system, and those electrons can drop down to a lower energy state and release energy when that electron drops down to the lower energy state.
Now, when we look at our latent image here, we have stored an electron within our F-center. Energy was required in the system into the conduction band to make this F-center here. That stored electron has potential energy here. It's got the potential to drop down to our valence band. We've stored that energy. We have created some memory within that image.
Now, what we can do here is shine red light onto our cassette. Red laser light in the wavelength of 700 nanometers, and that red light has enough energy to release an electron from our F-center here into the conduction band, which can then drop down into our valence band. It can reduce our europium three-plus into europium two-plus. It's gained an electron here. We oxidize our F-centers here. The electron drops down, and because it drops from a higher energy state to a lower energy state, we get electromagnetic radiation released. We've seen that in multiple different processes now throughout this X-ray course.
Now, that electromagnetic radiation that is released happens to be in the blue visible light spectrum. So, we shine red light onto our cassette, and the release of energy happens in the blue light spectrum, in about a 400 to 450 nanometer range. So, actually, what you can see we've done here is we've turned X-ray energy into light energy. We've created a signal that is light energy.
Now, we know that the conversion of X-rays into light is known as luminescence, and when we look at the intensifying screen in our screen-film radiography, that X-ray conversion into light was immediate, and we call that fluorescence. Here, there is a delay between the X-ray exposure and light production, and that delay means that this process is called phosphorescence. So, there's a difference between fluorescence, immediate conversion of X-ray to light, and phosphorescence, there's a delay in that X-ray energy ultimately leading to light being produced.
Now, when X-rays hit our cassette here, there actually is some fluorescence happening, but it doesn't contribute to the production of our image. The phosphorescence, the delayed production of light, is what gives us our signal to create our image.
Now, this readout process is done in a specific machine. We place the cassette in the machine, and we read out each individual spot on that cassette. So, this is what the machine looks like. We have rollers here that roll the cassette through the machine here. This gray region is our cassette, and what we do is we shine a red laser light onto a mirror here, and this mirror will rotate. Now, as the mirror rotates, our red laser light moves along the X-axis, along our cassette here. Now, we will scan the whole X-axis here, and then the computed radiograph will shift from these rollers, and we will get onto this next region of our mirror here, and again, we will translate right the way across to the start of our X-axis here. So, we sequentially read out these X-axes as we scan along the Y-axis of our cassette.
Now, this red laser light causes blue light to be released, and that blue light is channeled along these fiber optic channels towards what is known as our photomultiplier. So, blue light is released here, and some of the red light will also go along these fiber optic channels, and we have a filter here that filters out that red light. Only the blue light will go to our photomultiplier tube here.
Now, the intensity of that blue light is proportional to the intensity of the X-rays that hit the machine in the first place. So, the more X-rays hitting the machine will equal more blue light released. We have more F-centers that have been filled by those X-rays, and therefore, the intensity of the blue light that is released is increased.
Now, our photomultiplier tube does exactly that. It multiplies that blue light signal, and it sends it towards what is known as our analog-to-digital converter. That blue light signal that has been multiplied is then converted into pixel values. It's converted into a digital signal, ones and zeros, that we can then assign a value to that pixel on our computer screen.
Now, this computed radiograph can actually be read out multiple different times. We don't release all of those F-centers on the first scan of this. So, each time we read it out, our signal gets slightly less and less, and we can read this two to three times before losing all of those F-centers. If we want to clear this cassette completely, we shine a bright white light onto this cassette. It releases all of the electrons from the F-centers, and that cassette can then be reused for a completely different radiograph. So, it's important that we clear that cassette first prior to taking another radiograph, otherwise, we'll get a superimposed image of our first radiograph and our second radiograph.
Now, because we've created a digital image, our dynamic range of this image is much more than our characteristic curve on our screen-film. We have got specific pixel values for each one of these exposures on our computed radiograph cassette. So, we can see now, once we created our digital image, we can change the exposure. We can change the steepness of this graph. We can change the contrast within our image. We can manipulate those pixel values much more than we could do on our screen-film cassettes.
Now, we've looked at computed radiography and we've looked at screen-film radiography, our cassette-based radiography systems. Now, let's move on to digital radiography, where we look at our indirect and our direct digital radiography systems.
Now, knowing the difference between computed radiography, digital systems, and direct and indirect digital radiography systems is really important when it comes to exams. Something that I've covered in depth in the question bank that I've linked below if you want to go check that out. Otherwise, in our next talk, we're going to look at the process of insulation, which is foundational when looking at how indirect digital radiography systems. So, I'll see you all in that talk. Goodbye, everybody.