for Computed or
Computer Tomography. As I mention on the IVP page, tomography
is actually a fairly old technique that has been used for years.
Basically, tomography is a way of using coordinated movement
of the x-ray tube (the part of the x-ray machine that actually
generates the x-rays) and the film to blur out objects in the
body both above and below the area of interest. This gives the
effect of cutting the body into "slices" just as a scientist
would cut things into slices to better view them under a microscope.
What makes C.T. so special is two fold. One, it is much faster
in turning out "slices" of what ever it is scanning,
or examining; and two, it shows things in thinner and more detailed
slices than regular tomography.
The way a Cat scan works is by sliding the patient through a large metal "doughnut" like device that contains one or more x-ray tubes mounted on rings inside the metal housing, and a set of detectors that "read" the x-rays mounted directly opposite the x-ray tube(s) on the same series of rings. The x-ray tube(s) are spun around the patient in a 360 degree arc, and the readings from the sensors are sent to a special computer which then uses that information to generate a picture of what the body would look like at predetermined layers or slices if you were to actually cut the body into slices parallel to ground when the patient is standing. Because the computer can "see" the difference in densities of the various tissues making up the human body to a much greater degree than the human eye can when looking at a regular x-ray film, the images it generates are much better at identifying soft tissue structures (such as different types of nerve tissue, or the difference between the kidneys and the muscles surrounding them). Still, even a computer can only distinguish so much, which is why many cat scans use contrast (x-ray dye) to help define the images.
The most common cat scans, or C.T. exams, are of the head and can be done either without using contrast, or both with and without using contrast (the only time I've ever heard of someone having a C.T. of the head with contrast but not without was where the patient had already had a scan done without contrast being used when the patient was first admitted from the Emergency Room. In that case, the patient went into respiratory arrest because of the injuries that caused s/he to be brought in and so the scan had to be finished after the patient had been stabilized). Some of the structures in the head that may be looked at in this way includes: the brain, the skull, the sinuses, and the face.
The second most common C.T. exam is of the abdomen, and while the terminology used to describe them makes it sound like the abdomen can be scanned either with or without contrast, in reality a scan of the abdomen always uses at least a "oral" contrast that is drunk to allow better definition of the digestive or gastrointestinal system. Other common cat scans include: the neck, the spine, and the pelvic region.
For more details on these exams, click on the appropriate link in the table below.
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