Transcription
Hello and welcome back. In the last talk, we looked at screen-film radiography, and I mentioned that the film itself has a specific set of properties that we can choose prior to taking our radiograph. And those properties can be mapped out on what is known as the characteristic curve. The characteristic curve is specific for the particular film that we take.
Now, don't get this confused with characteristic radiation. In characteristic radiation, we were looking at the X-ray energies released that are specific to a specific target material. They are characteristic for the target material of the anode. Here, we're dealing with something completely different. We are talking about these specific properties for a screen-film, this plastic film within our screen-film radiography. Those properties are characteristic for that type of screen film.
Now, why do we need to know about the characteristic curve? Well, when we look at this overview, we can see that screen-film radiography creates a physical copy that we cannot manipulate once we've developed that film. So, we need to know the characteristics of that screen film prior to taking the X-rays. In digital radiography, once we've created an image with grayscale values for each pixel, we can then mathematically manipulate those grayscale values in order to change our image. We've got some leeway after we've developed our image.
So, let's have a look at the characteristic curve itself. On the X-axis is what's known as exposure, and this is exposure that the film itself experiences. It's the transmitted and the scattered X-rays that are reaching the film. Either they've got high intensities or low intensities. Here, on our Y-axis is optical density. Now, optical density is a physical property of the film itself once it's been developed. The higher the optical density, the more opaque that region in the image is. The lower the optical density, the more transparent, or the more light is let through, on that region of the image.
Now, when we're thinking about the optical density, we are talking about specific regions on the image. You can think about it as pixel areas on the image. How much light are they letting through when we place it on a light box? Now, the way we go about calculating optical density is using this equation here. And T in this equation is what's known as transmittance. Now, transmittance is looking at the ratio of light that we have in our light box versus the amount of light that actually passes through the film once we've placed it on our light box. So, I naught here is the total light, the light from the light box, and I here is the intensity of the light that is allowed through the light box. If we've got a really see-through image on our film, most of the light will come through, and our T value will get closer and closer to one. The darker and darker region on our film is the less light gets through, and the smaller LT value gets. So, as our T value gets decreased, our scale here, this negative log 10 scale, increases. Our optical density increases.
So, regions of the film that have high exposure here will be darkest on our screen film. They will have high optical densities, low transmittance values. So, these regions on the film will be dark. If we are X-raying the lungs, lots of X-rays get through the lungs, lots of exposure happens at our screen film, and that becomes really dark there in our image. When we place that film on a light box, we don't get light coming through, and our lungs look dark on our X-ray. The opposite is true for the toe of our image here. We are not getting many X-rays reaching our screen film itself. Our transmittance value is high. Once we plug it into this formula, we get a low optical density value. A lot of light is making it through. And this region in between our toe, the bright regions of our image, and the shoulder, our dark regions of the image, is our linear region here. And this is what provides contrast and anatomical detail within our image.
Now, when we're looking at this graph, it's quite difficult to conceptualize what these values actually mean. And we can actually plug in real values that are much easier to understand here. Now, these percentages of light here is the percentage of light that is allowed through our screen film once it's placed on the light box. Here, we can see almost 100% of the light on the toe region of our graph here is let through the film. Those are the bright regions on our image.
Now, as we go up by one in our optical density, so we go from zero to one, one to two, two to three, we get a tenfold decrease in the amount of light that's let through our film. So, at an optical density of one, only 10% of the light from the light box is going through our film, and only 1% at an optical density of two, and almost nothing at an optical density of three. Now, this tenfold decrease luckily corresponds to how our eyes perceive light. Now, a tenfold increase in light to us is perceived as a doubling of that light. So, each one of these is a two-fold difference to us. So, optical density itself is a great proxy for how we perceive light. We will see this region as half as bright as this region. Anything that has an optical density of two will seem half as bright as an optical density of one. So, it's a good kind of measure in your mind to see what these values mean on these graphs.
Now, when we are selecting a specific film, we want certain characteristics of that film depending on the radiograph that we're trying to take. Now, the first characteristic is what's known as the speed of the film. If we shift our characteristic curve to the left, that's said to be a faster film. Now, there are certain radiographs where we want a faster characteristic curve. If we're taking a mammogram and we've compressed breast tissue, there's thin tissue, lots of X-rays will make it through that tissue. We may want a faster screen film where we can have lower exposures, more contrast in the image. And in mammography, it's really important to have contrast between the various different structures, and we can get our anatomical detail at lower exposures here. So, we have a thin piece of tissue, we can afford to use very low exposures, and we get really good contrast in that image.
The second property that we can choose in our characteristic curve is what is known as latitude of the curve. Now, look at these two separate characteristics. So, we've got an orange curve which has a steep linear region here, and we've got a green curve which has a much more gradual linear region here. Now, this area of latitude here represents that linear region, the change in optical densities. Anything below these regions of latitude are whited out on our film. Anything above it will be dark on our film. Here is where we get the contrast, the anatomical detail in our image.
Now, a steep curve or a narrow latitude provides a lot of contrast in an image. And if we are looking at a specific radiograph like a hand radiograph, where all we need to see is the difference between the bones and the soft tissue, we can have contrast between these two types of tissues. We can have a narrow latitude, and we get good contrast between the bones and the soft tissue. If we take a radiograph which has got multiple different densities in it, such as a chest radiograph, we've got bone, we've got lung, we've got mediastinum, soft tissues, they've all got different densities, they've all got different linear attenuation coefficients. We can't afford to have such a steep contrast because we need to differentiate between all those tissues. We need to increase our dynamic range of the radiograph.
Now, what do I mean by a dynamic range? Well, if you're ever taking a picture of someone with a sunset behind them, you may have had the phenomenon where you can either focus on their face, and the sunset is completely whited out. We can't see any clouds, any detail in the sunset, but we can see the detail in the person's face. Or we can focus on the sunset, and the person suddenly becomes a silhouette. We've got high contrast there, but very poor dynamic range, we've got a narrow latitude. We can see where the person is and where the sunset is very clearly. There's a big contrast between the person and the sunset, but we don't have that detail in the image.
Now, we can place what is known as an ND filter over our camera. We can reduce the contrast in the image, and then we can see the details in the person's face as well as their details in the sunset itself. We've reduced our contrast, but we've increased our dynamic range. We get much more detail within the image. So, depending on the type of radiograph we are trying to take, do we want something with high contrast, or do we need more dynamic range to get the anatomical detail between the various different tissues?
So, that's it for the characteristic curve. Remember, it's talking specifically to screen-film radiography. Now, from here on out, we are going to look at digital radiography systems, starting with computed radiography.
A common question that comes up in exams is to compare digital radiography to screen-film radiography, and you can use the drawbacks of this characteristic curve, that we need very specific exposures in order to create an image, as one of the drawbacks for screen-film radiography. And if you're studying for your part ones or your registry exams, I've linked a question back in the description below that you can go and check out. Otherwise, I'll see you all in our next talk where we're going to look at computed radiography. Until then, goodbye everybody.