System speed refers to an x-ray systems efficiency in converting x-rays to an image. This can be used to describe analog systems (film) or digital systems. System speed refers to the proficiency in which an image can be obtained by the image receptor and processed either through a film processor, or digitally through a computer.
Below is an image of film cassettes. Film is located inside of these light tight cassettes. Also inside the cassettes are screens that fluoresce (glow) when struck by x-rays. It is this light that actually creates the image on film. We will go into this more in detail in the Processing module.
This is actually a picture of a test tool but the concept for the screen inside the film cassette is the same. The x-ray film placed inside of the cassette is designed to be sensitive to a specific wavelength of light. In this case, the screens do indeed emit green light when struck by x-rays, which the film is designed to be most sensitive to. This light is actually responsible for the image after it is processed. There are different kinds of cassettes, such as regular and extremity cassettes. Regular cassettes are used for most exams. Extremity cassettes are used in cases where greater detail is desired, such as in a hand x-ray. So what are the differences between these 2 cassettes? A regular cassette has a speed of 400. Extremity cassettes have speeds in the 150 to 200 range. Because extremity cassettes are slower, they do create better detailed images. Since they are slower, more mAs is required to obtain the proper density of the image. Again, here is your trade off. In order to obtain detail, we had to use a slower speed system and more mAs. Even the slowest speed screen, however, reduces patient dose significantly over no screens at all. So in conclusion, film screens and increased system speeds are used primarily to reduce patient dose. Again, we will go over the image processing part in the Image processing module.
If you were to increase system speed, what would happen to the following?