Understanding How CAT Scans Function

Key Insights

CAT scans elevate conventional X-ray imaging to a whole new level. Rather than just capturing the outline of bones and organs, a CAT scan machine constructs a full three-dimensional computer model of the internal body. Doctors can examine the body one narrow slice at a time, allowing for precise identification of specific areas.

In this article, we'll explore the fundamental concept behind CAT scans. While the technology involved is quite advanced, the basic principle is actually quite simple.

The Core Concept

Computerized axial tomography (CAT) scan devices emit X-rays, a potent form of electromagnetic radiation. X-ray photons are essentially the same as visible light photons, but with significantly more energy. This increased energy allows X-rays to pass through most soft tissues in the human body. (Refer to How X-Rays Work to understand the mechanics of X-ray penetration and how X-ray machines generate X-ray photons).

A standard X-ray image is essentially a shadow: You direct a light source at one side of the body, and a film on the opposite side captures the silhouette of the bones.

Shadows provide an incomplete representation of an object's form. Imagine standing in front of a wall, holding a pineapple in your right hand against your chest, and a banana out to your side with your left hand. Your friend, who is looking only at the wall, won't see the pineapple if there's a light in front of you because the shadow of your torso obstructs it. But if the light is on your left, only the pineapple’s silhouette will be visible, hiding the banana.

The same principle applies in a standard X-ray image. If a larger bone is positioned directly between the X-ray machine and a smaller bone, the larger bone may block the smaller one on the film. To view the smaller bone, you would need to adjust your position or reposition the X-ray machine.

To identify that you are holding both a pineapple and a banana, your friend would need to observe your shadow in different positions to form a complete mental picture. This concept is the foundation of computer-aided tomography. In a CAT scan machine, the X-ray beam rotates around the patient, capturing data from hundreds of angles. The computer then assembles this information into a 3-D image of the body.

Scanning Process

The CAT machine resembles a giant doughnut tipped on its side. The patient lies on a platform that moves slowly through the machine's circular opening. The X-ray tube is attached to a movable ring surrounding the hole, which also holds a series of X-ray detectors positioned directly across from the X-ray tube.

A motor rotates the ring, causing the X-ray tube and detectors to revolve around the body. Each full rotation scans a thin, horizontal "slice" of the body. The control system then moves the platform slightly further into the machine, allowing the tube and detectors to scan the next slice.

In this manner, the machine captures X-ray slices in a spiral motion across the body. The computer adjusts the intensity of the X-rays to ensure each tissue type is scanned with optimal power. After the patient passes through, the computer compiles the data from each scan to generate a detailed image of the body. Generally, only a small section of the body is scanned, not the entire body.

Because CAT scans examine the body slice by slice, from all directions, they provide far more detailed information than traditional X-rays. Today, doctors use CAT scans to diagnose and treat a wide range of conditions, including head injuries, cancer, and osteoporosis. They have become an essential tool in modern medicine.