How does the procedure work?

In many ways CT scanning works very much like other x-ray examinations. X-rays are a form of radiation—like light or radio waves—that can be directed at the body. Different body parts absorb the x-rays in varying degrees.

In a conventional x-ray exam, a small burst of radiation is aimed at and passes through the body, recording an image on photographic film or a special image recording plate. Bones appear white on the x-ray; soft tissue shows up in shades of gray and air appears black.

With CT scanning, numerous x-ray beams and a set of electronic x-ray detectors rotate around you, measuring the amount of radiation being absorbed throughout your body. The scanner will take a set of images and then the table will move (axial scan). Sometimes the examination table will move during the scan so that the x-ray beam follows a spiral path (spiral or helical scan). A special computer program processes this large volume of data to create two-dimensional cross-sectional images of your body, which are then displayed on a monitor.

CT imaging is sometimes compared to looking into a loaf of bread by cutting the loaf into thin slices. When the image slices are reassembled by computer software, the result is a very detailed multidimensional view of the body's interior.

Refinements in detector technology allow new CT scanners to obtain multiple slices in a single rotation. These scanners, called multislice CT or multidetector CT, allow thinner slices to be obtained in a shorter period of time, resulting in more detail and additional view capabilities.

Modern CT scanners are so fast that they can scan through large sections of the body in just a few seconds. Such speed is beneficial for all patients but especially children, the elderly and critically ill.

Electron beam computed tomography (EBCT) is an alternative to conventional CT for measuring coronary calcium. In EBCT, an electron beam is sent to a target ring located around the patient. The beam creates x-rays at the target rings, which radiate through the patient to the detector on the opposite end of the scan tube. Because the machine has no moving parts, it can acquire as many as 20 images every second—fast enough so that it avoids any blurring caused by the beating of the heart. While EBCT and CT have been shown to have similar accuracy in determining calcium scores, EBCT is not as commonly available, as it is less versatile than the latest generation of high-speed CT scanners.