Transcription
In the previous talk, we looked at our first system within the indirect digital radiography systems, our CCD chip, our charged couple device. Now we're going to look at the second indirect mechanism known as the thin film transistor array. Now, again, this is indirect digital radiography, so we still need to convert our x-rays into light prior to generating a digital signal. If we don't convert the x-rays into light, we are dealing with direct digital radiography.
So here again, we have three layers within our indirect thin film transistor array. The first being the scintillation layer, made of those cesium iodide crystals that channel those light photons onto a small area onto the second layer. Now, those light photons hit our second layer, which is known as a photodiode layer. Now, we've come across diodes before when we were looking at the secondary x-ray circuit. We saw that diodes allowed the passage of electrons one way through that diode. Here, we are converting light photons into electrons. We are releasing electrons down onto the third layer here, which is our thin film transistor array.
Now, this photodiode is made up of hydrogenated amorphous silicon here. Now, hydrogenated amorphous silicon is a semiconductor that converts light into electrons. We are converting light photon signal into an electron signal. Those electrons then head towards our thin film transistor array. Now, our TFT array, or flat panel array, is the same TFT array that we use in our direct digital radiography systems, which we're going to look at in our next talk. So, these three layers are tightly packed. They are touching one another. We don't want distance between these layers because that allows light to spread out. We want close contact between the layers where that light is hitting the photodiode, we want electrons to be produced, and we want those electrons to go straight down onto our thin film transistor array. So, we tightly pack them together like that.
Now, what exactly is a thin film transistor array? Well, if we zoomed in on it, we would have multiple of these small units packed in rows and columns. Now, these units are what is known as a detector element, or a DEL. Now, each DEL has three separate components. There are electronic components on a DEL and a photosensitive component on the DEL. The electronic components include this capacitor, which has the ability to store charge, and a TFT switch, which has the ability to release that charge from the capacitor itself. The photosensitive area on a DEL is where those electrons that have been released from our photodiode layer will then be channeled towards our capacitor for storage.
Now, again, the electron release from that photodiode layer is proportional to the number or the intensity of x-rays that are hitting our cesium iodide layer, and the charge that is stored within this capacitor is also proportional to the number of electrons that have been released. Now, what we do is we expose our detector here, our indirect thin film transistor array detector, to x-rays, and charge builds up within our capacitor here. Now, when we want to read out that x-ray, read out that signal, we will sequentially switch this TFT array switch here. The switch along these rows will be switched on, and the charge can then be released from this capacitor, and we can then again read out row by row on our TFT array. That electronic signal again goes through an amplifier, and we get a pixel value, a grayscale value for each one of these DELs in our detector.
So, this differs from our charged couple device, where light photons were converted into electrons, and those electrons were then read out as we change the voltage gates here. Here, the light photons are converted into an electron current by our photodiode layer, and that current is then diverted towards our capacitor, where it can build up, and we will release that current from that capacitor when we switch our TFT array here, and then we can read out our image and display it on our screen there.
Now, when we are looking at these three layers, we can see there are regions where x-rays will be converted into light. That light will then be converted into electrons by this photodiode semiconductor layer here, and those electrons will hit our electrical components here, or this purple region on our DEL, and those electrons won't be hitting the sensitive area on our DEL here and won't contribute to the image itself. Only electrons hitting this blue region here will be stored within our capacitor, and this is what's known as our fill factor: our sensitive area on a DEL over the total area of the DEL. The more that our sensitive area takes up of this entire total area, the better signal we get, and the better resolution we get. We don't get loss of electron signal on this purple area. And in newer systems, they actually place the electric components underneath this photosensitive area, getting rid of this fill factor. But for the most part, our fill factor is somewhere in the region of 80%. And the smaller and smaller we make our DELs, the more pixels we try and fit in, the more proportionally our electronic components will take up, and the lower our fill factor will get. We lose some signal there.
Now, when we look at our direct digital radiography systems, we will see that this fill factor is less important because we are able to channel our charge onto the sensitive area here. Here, in our indirect digital radiography systems, our photodiodes are not able to channel that charge onto the sensitive area. It's indiscriminate. It sends down electrons over the entire area of this DEL here, and our fill factor determines how much of those electrons will actually go to contributing to our image. So, that wraps up our indirect digital radiography systems: x-rays to light, light to electronic signal, electronic signal to a digital image.
Now, let's shift our focus to the final detector system, our direct digital radiography detector systems, where we take x-rays and convert them directly into an electronic signal. That electronic signal will go to the same DEL here, where we store that charge in a capacitor, and when we want to read it out, we switch our TFT switch here, and we read out those rows on our thin film transistor array. So, I'll see you all in that talk. Goodbye, everybody.