Transcription
So let's start our X-ray detection series by looking at screen film radiography. Now, for many years, screen film radiography was the mainstay of radiography, prior to the introduction of digital radiography. Now, I know many exams are starting to phase out asking questions about screen film radiography, and many of those exams say they are not going to ask screen film radiography directly. But I have seen multiple questions come up where they ask you to compare and contrast digital radiography systems with film-based radiography systems. And if you don't at least have a basic conceptual understanding of how screen film radiography goes about, then it's hard to compare the two systems.
So today, I want to take you through this process of screen film radiography, the process of creating a film that we end up putting on a light box and seeing our radiograph. So, the screen film cassette has multiple layers within it. Now, this cassette is the actual cassette that we will take around and place behind our patient, exposing that patient to X-rays and exposing the cassette to X-rays. That X-ray exposure on the cassette is what is used to make the dark regions on our film here. This film in the middle of our cassette is initially a transparent plastic film that we will deposit dark regions on, depending on how many X-rays are exposed to that region.
Now, this cassette, we can open up like this. The outer blue layer here represents the plastic housing of the cassette. Now, as we open up that cassette, in the middle of it is our film that we will sandwich in between these layers of foam and intensifying screen. Now, the outermost layer of foam here allows us to compress that intensifying screen against our film. The foam, as we close the cassette, squishes that intensifying screen in direct contact with our film, and it's crucially important that there's no gap between our intensifying screen and our film, and I'm going to show you why that's the case just now.
Now, our intensifying screen is what converts our X-rays into light. It's a center later layer, our screen foam radiography requires scintillation. So let's have a look first at this insulator layer, then at the film, and then I'm going to take you through the seven steps that we need to do in order to process this film itself. So, our intensifying screen is made up of what is known as gadolinium oxy sulfide, Gd2O2S. Now, gadolinium oxysulfide is a phosphor layer that converts X-rays into light, and it's an amorphous crystal structure. There's no organized pattern to the gadolinium oxysulfide. Now, what happens is an X-ray will interact with this intensifying screen, with the gadolinium oxysulfide, and those crystals will convert that X-ray into light, one X-ray photon into hundreds of light photons, and those light photons spread in a random direction in all 360 degrees and reflect throughout that intensifying layer. So, one X-ray is creating multiple light photons here, and it's these light photons that are going to interact with our film itself in order to deposit those darker regions on our screen film.
Now, here, this X-ray is being converted into light, a process called luminescence. And if we're being more specific, this X-ray is being converted directly into light, immediately into light, and that process is called fluorescence. If there's a delay in this conversion, that's what we call phosphorescence. So, here we are dealing with fluorescence under the broad term luminescence. Here is fluorescence. So let's move on and look at the next layer, our film within our cassette. You can see I have taken the film out of the cassette itself, and we have now cut the foaming cross-section here where we can see the inner plastic layer and the outer emulsion layer. Now, it's this emulsion that comes into contact with our intensifying screen. If we have a closer look at this emulsion, it is an organized crystal lattice of silver halide molecules. Now, silver halide molecules are a combination of silver bromide and silver iodide, and they form this organized crystal structure here where our silver atoms are tightly packed within these bromine and iodine atoms, and they are suspended within this gelatin emulsion here. Now, what we can do is add silver sulfide, and what silver sulfide does is it disrupts that crystalline structure, it disrupts that organized pattern and exposes some of these silver atoms. Now, these exposed silver atoms that were once sharing its valence electrons with the bromine and iodine that surrounding it, tight chemical bonds between the silver and the bromine and the silver and the iodine, that structure has been disrupted, and we've got a silver atom that is relatively positively charged, being exposed to external light. Now, it's the light coming from our intensifying screen that will react with these exposed silver atoms, and what that light does is it causes these silver atoms to go through a process of reduction, gaining an electron, turning that positively charged silver atom into a neutral, inert silver atom. That silver now will no longer form chemical bonds with the accompanying bromine and iodine. It will be inert, and it will deposit onto this plastic here. Now, that is the initial step in our screen film processing.
So let's go through these steps of our screen film processing. The first thing that happens is an X-ray enters the cassette. This X-ray has either been transmitted or scattered from our patient. That X-ray interacts with our intensifying screen, and fluorescence occurs, where we get this scintillator layer causing X-rays to be converted into multiple light photons. Now, the X-rays can either interact with this first intensifying screen or they can interact with this second intensifying screen. So we require less X-ray exposure to get silver deposition on this film. We've got two intensifying screens. The second thing is one X-ray is converted into multiple photons of light, another process which allows us to use less X-rays in order to generate our image on the film. Now, this process is called fluorescence, immediate conversion of X-rays to light. We can see that if we increase the width of that intensifying screen, the area in which those light photons interact with our film becomes bigger. We lose spatial resolution here, but we gain more signal from more light photons being produced in this intensifying screen. So it's a trade-off between generating lots of light photons and getting good spatial resolution here.
Now, the third step is that light from the intensifying screen causes the positively charged silver atom to be reduced. It gains an electron and forms a stable, inert silver atom. That process has formed a latent image. That film now has memory of the X-ray exposure. We only get silver atom deposition where X-rays have hit our intensifying screen. These are the areas that are going to be dark on our image. If you think about lungs on an X-ray image, lots of X-rays make it through the lungs as opposed to X-rays going through bone. We get dark regions where our lungs are, that's where silver has been deposited.
Now, the next thing we do is we can take that film out of the cassette and take it for processing. And the first solution we place that film in is an aqueous solution here. Now, this gelatin layer is porous to the aqueous solution, and there is a reducing agent within this aqueous solution. Now, those silver atoms that have been reduced from the light in our intensifying screen act as signals for where X-rays have hit the screen, and that reducing agent will use those inert silver atoms as a catalyst to reduce surrounding silver atoms. So there is lots of reduction of this silver surrounding these inert silver atoms. We get an amplification of the signal where the X-rays hit, and this is what creates that dark region on our film itself. Now, remember that film was initially transparent, and when we take the film out of the cassette, it's still transparent. That silver is not visible to the naked eye. It's this further process of reduction in this reducing agent, using those silver atoms as a catalyst to further reduce more silver, that creates those dark regions on our film. We can now take that film and place it in what's known as a fixative or an oxidizing fixer to dissolve the inactive silver halide molecules. We can now take that cassette and place it on a light box and get an image here.
So we can see on this image, the regions outside of the patient where X-rays have hit our screen film directly are dark here. Regions like this region here where the bone has attenuated, we've got lots of photoelectric effect occurring in the bone here. Our linear attenuation coefficient is high in bone. Not many X-rays will reach our film. Those regions of the film itself will remain transparent, will come through the foam, and that is the foundation for screen film radiography.
Now, the structure of the film itself will mean that that specific film has certain characteristics. And in our next talk, I'm going to take you through what's known as the characteristic curve, how we can use different films for different applications, depending on the type of radiograph that we are trying to take. So I'll see you all in that next talk. Goodbye everybody.