An x-ray of the chest, showing the heart,  lungs, and other anatomy, on x-ray film.

How is a CT Scan Different?

 A CT scan is three-dimensional. By creating and looking at several three-dimensional cross sections, or “slices,” of a body (like slices of bread) a doctor can not only tell if a tumor is present, but roughly how deep it is in the body. A CT scan is three-dimensional because the information about how much of the X-rays are passing through a body is collected not just on a flat piece of film, but on a computer. 

This data can then be computer-enhanced to create a much clearer image than a plain x-ray film. These cross-sectional images can then be used for a variety of diagnostic and therapeutic purposes.

A CT scanned image of the abdomen

How CT Scans Take Pictures

1. A motorized table moves the patient through a circular opening in the CT imaging system. 

Patient in CT Imaging System

2. As the patient passes through the CT imaging system, a source of x-rays rotates around the inside of the circular opening. (See image below.) A single rotation takes about 1 second. The x-ray source produces a narrow, fan-shaped beam of x-rays to irradiate a section of the patient's body. The thickness of the fan beam may be as small as 1 millimeter or as large as 10 millimeters. In typical examinations there are several phases; each made up of 10 to 50 rotations of the x-ray tube around the patient in coordination with the table moving through the circular opening. The patient may receive an injection of a "contrast material" to help the scanner “see” vascular structure. 

CT Fan Beam

3. Detectors on the exit side of the patient record the x rays exiting the section of the patient's body being irradiated as an x-ray "snapshot" at one position (angle) of the source of x rays. Many different "snapshots" (angles) are collected during one complete rotation. 

4. The data are sent to a computer to reconstruct all of the individual "snapshots" into a cross-sectional image (slice) of the internal organs and tissues for each complete rotation of the source of x-rays. 

A radiologist studies the images produced in a recent CT scan.