Transcription
So let's have a look at the first of our indirect digital radiography systems, that charged couple device system, or the CCD chip.
Now, a CCD chip has the ability to take light and convert that into a digital signal. In fact, it's the same chip that is used in video cameras, like the one filming me now. It's able to take the light coming from my face into the lens of the camera, convert that light into an electronic signal that can then be converted into an image.
Now, our CCD system falls under the indirect digital radiography systems because x-rays are required to be converted into light prior to an electronic image being created. Now, our CCD system itself has three separate layers. The first is the scintillation layer that we've looked at already, the cesium iodide crystals that form these long tubular structures that allow light photons to be funneled into a small section in this second layer here. So, this scintillation layer takes our X-ray and converts it into light photons.
The second layer in a CCD system is known as the coupling layer. The system is called a Charged Coupled Device. This is the coupling layer that couples our light photons to our CCD chip. Now, the coupling layer can be made of two different things. The first are fiber optic coupling. Now, if you're an 80s or a 90s baby like me, you would have had those lamps next to your bed with those fiber optic lights, and as you move that plastic wire, the light changes with that fiber optic wire. It's a similar concept here. The light striking this portion of the fiber optic coupling will be funneled down towards our CCD chip. The second type of coupling device is a lens that can focus that light down onto our CCD chip.
Now, we are taking x-ray photons, converting them to light photons, and then converting them into electrons. Now, photons and electrons are both quantum. They are packets of mass and energy, and ideally, we would like the mass and energy, the X-ray photons here, that energy to equal the electron energy here. We don't want drop off of that signal. And drop off of that signal is what's known as secondary quantum sync. Now, that's in fact what we do get here. Our X-ray energies are not the same as those electron energies here. We get drop off of those energies because of this coupling that is required.
Now, the second reason we get drop off is because we've got a really large area here. If we're taking an x-ray of a large body region and we need to focus that down onto a very small chip. These chips are small enough to fit in the camera. They are tiny, and we are focusing that light down, and there's always going to be a loss of some resolution whenever we need to take such a large area and focus it down onto such a small area.
If we were to flip the CCD chip and look at it front on, this is what it would look like here. There will be multiple small functional units on this crystalline silicon face. Now, these units here are what are known as dexels, and dexels correspond to the pixels within the image.
Light is being coupled from the scintillation layer to our CCD chip, either by the fiber optic channels or by the lenses that we use. And as that light strikes the silicon layer, electrons are released from the surface of the silicon. Now, the dexels here are separated by voltage gates here that have been etched onto this crystalline silicon. Now, the voltage that is run through these black lines here that separate each of these dexels prevent the electrons that have been released on the surface to spread into other dexels. They keep them within the specific spot.
Each dexel corresponds to an X-ray intensity that is hitting our scintillation layer. The higher the X-ray intensity here, the more electrons that are released within that dexel. Now, when we want to read out these dexels, we can change the voltage along these voltage gates and allow those dexels to be released into what is known as an amplifier, and then that signal can be digitized into a pixel value, a grayscale value that we can represent on our computer screen, ultimately making our radiograph that we've taken.
Now, the number of electrons again corresponds to the intensity of the X-ray here, and as we then read that digital signal, the more electrons that are available, the darker it will appear within that pixel. Now, the way we read out these CCD chips is row by row. We will allow this entire row to be read out, go through the amplifier towards our computer, and then all of these rows will shift down one, and we will read out the next one sequentially, over and over again. And each dexel again corresponding to a particular pixel.
So, what we've done here is we've taken X-ray energy, converted it to light, channeled that light down onto our CCD chip. The light hits that crystalline silicon, and electrons are released in proportion to those lights. Those electrons are housed within a specific dexel that corresponds to a specific pixel in our image, and those dexels are prevented from merging together by these voltage gates that are etched onto this crystalline silicon of our CCD chip. We read out those dexels sequentially and create our radiograph.
The second type of indirect digital radiography also converts X-rays to light, but the way in which we store and process that light energy is completely different to the CCD chip, and that's what we're going to be looking at in our next talk, where we look at thin film transistor arrays, where we can store electricity within capacitors and use a TFT switch to ultimately read out the signal that we created from the light photons coming towards our TFT array.
Now, knowing these types of systems, knowing the various different types of indirect digital radiography systems, as well as how they differ to our direct radiography systems, is essential knowledge for your Radiology Physics exams. And if you are studying for a Radiology Physics exam, check out the question bank that I curated in the first line of the description. Otherwise, I'll see you all in the next talk where we're going to look at our TFT array specifically in the indirect digital radiography system. So, until then, goodbye everybody.