CAT scans take the idea of conventional X-ray imaging to a new level. Instead of finding the outline of bones and organs, a CAT scan machine forms a full three-dimensional computer model of a patient's insides. Doctors can even examine the body one narrow slice at a time to pinpoint specific areas. We'll examine the basic idea of CAT scans. While the computer technology involved is fairly advanced, the fundamental concept at work is really very simple

The Basic Idea
Computerized axial tomography (CAT) scan machines produce X-rays, a powerful form of electromagnetic energy. X-ray photons are basically the same thing as visible light photons, but they have much more energy. This higher energy level allows X-ray beams to pass straight through most of the soft material in the human body. (See How X-Rays Work to find how X-rays do this, as well as how X-ray machines produce X-ray photons).
A conventional X-ray image is basically a shadow: You shine a "light" on one side of the body, and a piece of film on the other side registers the silhouette of the bones.
Shadows give you an incomplete picture of an object's shape. Imagine you are standing in front of a wall, holding a pineapple against your chest with your right hand and a banana out to your side with your left hand. Your friend is looking only at the wall, not at you. If there's a lamp in front of you, your friend will see the outline of you holding the banana, but not the pineapple -- the shadow of your torso blocks the pineapple. If the lamp is to your left, your friend will see the outline of the pineapple, but not the banana.

The same thing happens in a conventional X-ray image. If a larger bone is directly between the X-ray machine and a smaller bone, the larger bone may cover the smaller bone on the film. In order to see the smaller bone, you would have to turn your body or move the X-ray machine.
In order to know that you are holding a pineapple and a banana, your friend would have to see your shadow in both positions and form a complete mental image. This is the basic idea of computer aided tomography. In a CAT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles. The computer takes all this information and puts together a 3-D image of the body.

Scanning Procedure
The CAT machine looks like a giant donut tipped on its side. The patient lies down on a platform, which slowly moves through the hole in the machine. The X-ray tube is mounted on a movable ring around the edges of the hole. The ring also supports an array of X-ray detectors directly opposite the X-ray tube.

A motor turns the ring so that the X-ray tube and the X-ray detectors revolve around the body (in an alternative design, the tube remains stationary and the X-ray beam is bounced off a revolving reflector). Each full revolution scans a narrow, horizontal "slice" of the body. The control system moves the platform farther into the hole so the tube and detectors can scan the next slice.


Photo courtesy Department of DefenseDoctors usually operate CAT scan machines from a separate room so they aren't repeatedly exposed to radiation.
In this way, the machine records X-ray slices across the body in a spiral motion. The computer varies the intensity of the X-rays in order to scan each type of tissue with the optimum power. After the patient passes through the machine, the computer combines all the information from each scan to form a detailed image of the body. It's not usually necessary to scan the entire body, of course. More often, doctors will scan only a small section.
Photo courtesy NASAA scanned liver slice

Since they examine the body slice by slice, from all angles, CAT scans are much more comprehensive than conventional X-rays. Today, doctors use CAT scans to diagnose and treat a wide variety of ailments, including head trauma, cancer and osteoporosis. They are an invaluable tool in modern medicine. For much more information about CAT scan machines and other medical scanners, check out the links on the next page.