Computed tomography (CT), pronounced tuh MAHG ruh fee, is an X-ray system used to produce images of various parts of the body, such as the head, chest, and abdomen. Doctors use CT images to help diagnose and treat diseases. The technique is also called computerized tomography or computerized axial tomography (CAT).

During a CT imaging procedure, a patient lies on a table that passes through a circular scanning machine called a gantry. The table is positioned so that the organ to be scanned lies in the center of the gantry. A tube on the gantry beams X rays through the patient's body and into special detectors that analyze the image produced. The gantry rotates around the patient to obtain many images from different angles. A computer then processes the information from the detectors to produce a cross-sectional image on a video screen. By moving the table in the gantry, doctors can obtain scans at different levels of the same organ. They can even put together several scans to create a three-dimensional computer image of the entire body.

Sometimes an iodine solution, called a contrast agent, is injected into the body to make certain organs or disease processes show up clearly in the CT scan. For example, the patient drinks a barium mixture to outline the inner surfaces of the stomach and bowel.

Doctors use CT scans to diagnose many conditions, such as tumors, infections, blood clots, and broken bones. CT also may be used to guide a biopsy needle into diseased tissue. In addition, it assists in treating some diseases that might otherwise require surgery. For example, doctors can use a CT scan to guide catheters (small tubes) to an abscess in the body and drain pus from the infected area.

Contributor: P. Andrea Lum, F.R.C.P.C., Abdominal Radiologist, Ottawa Civic Hospital, Ontario.