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.
In this edition of HowStuffWorks,
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
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.
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.
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.
For much more information about CAT scan machines and other
medical scanners, check out the links on the next page.