A computed tomography scan (CT scan, also called a CAT scan) uses computer-controlled X-rays to create images of the body. However a radiograph and a CT scan show different types of information. Although an experienced radiologist can get a sense for the approximate three-dimensional location of a tumor from a radiograph, in general, a plain radiograph is two-dimensional.
An arm or chest radiograph looks all the way through a body without being able to tell how deep anything is. A CT scan is three-dimensional. By imaging and looking at several three-dimensional slices of a body (like slices of bread) a doctor could not only tell if a tumor is present, but roughly how deep it is in the body. A CT scan can be 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.
The data from a CT scan can be enhanced to be more vivid than a plain radiograph. For both plain radiographs and CT scans, the patient may be given a contrast agent to drink and/or by injection to more clearly show the boundaries between organs or between organs and tumors.
(Left) A 1 mm spiral CT "slice" through the mid-chest region, showing both lungs. The white spot in the right lung is a suspicious nodule that could be biopsied to see if it is cancerous. (Right) A close-up view of 8 "slices" focused on a lung nodule. Compared with a traditional X-ray, a series of CT images gives the radiologist a much better sense of nodule size and its potential threat. Images courtesy of A. P. Reeves, Cornell University.
Recent technical advances in CT scanning dramatically increased its speed and effectiveness by a process called multi-slice or helical (spiral) scanning.
(Figure A:) Conventional CT scan. (Figure B:) Spiral CT scan.
Conventional CT scans take pictures of slices of the body (like slices of bread). These slices are a few millimeters apart. The newer spiral (also called helical) CT scan takes continuous pictures of the body in a rapid spiral motion, so that there are no gaps in the pictures collected.
(Left) Conventional CT scan without contrast showing possible tumor in the liver. (Right) Conventional CT scan of the same patient using contrast. Images courtesy of Dr. Peter Choyke, Department of Radiology, Clinical Center, National Institutes of Health.