COMPUTED TOMOGRAPHY

Computed tomography (CT) is accomplished by passing a rotating fan beam of x-rays through the patient and measuring the transmission at thousands of points. The data are handled by a computer that calculates exactly what the x-ray absorption was at any given spot in the patient. The data can be manipulated in a number of ways, displayed on a screen, or photographed. Because the data points are in the computer memory, it is possible to "window" the image and obtain a number of filmed pictures without additional radiation exposure. The computers can even display the data as a three-dimensional rotating image, although this is rarely necessary for diagnosis. Compared with plain x-rays, computed tomography uses about 10 to 100 times more radiation.

On early computed tomography scanners, the x-ray tube rotated around the patient to obtain a single "slice," and then the table was incremented before another slice was obtained. Newer scanners allow the x-ray tube to stay on and rotate at the same time that the table is moving. This is called a spiral or helical scanner. The most modern scanners not only have the helical motion but also have multiple rows of detectors and can obtain up to 16 slices at once.

The appearance of tissues on computed tomography scan depends to some extent on the computer manipulation, but in general, the basic four densities on computed tomography images are the same as those in plain x-rays: air is black, fat is dark gray, soft tissue is light gray, and bone or calcium and contrast agents are white. One advantage of computed tomography is that actual x-ray absorption of a specific tissue can be displayed. The units used are Hounsfield units, and the density of water is zero. The greater sensitivity of computed tomography compared with plain x-rays allows areas of tiny punctate calcification to be seen.

Computed tomography scans are presented as a series of slices of tissue. The method is similar in principle to slicing a loaf of bread and pulling up one slice at a time to examine it. Thus computed tomography is a two-dimensional display of two-dimensional information, and objects appear where they really are in space. The scans or slices are shown as if you are viewing the patient from the foot of the patient's bed. Thus the individual's right side is on your left. This also is the convention used for the transverse images of ultrasound and magnetic resonance imaging (MRI).

Contrast agents, frequently used in computed tomography scans, are usually the same water-soluble oral, rectal, or intravenous iodinated agents used in other imaging studies. Intravenous contrast agents are common, being used in probably 75% of all computed tomography studies, and obviously carry the risk of contrast reactions discussed previously.

The appeal of computed tomography is that a large number of structures are visualized simultaneously. In a patient with abdominal pain, one computed tomography examination shows the liver, adrenal glands, kidneys, spleen, aorta, pancreas, and other structures. This allows the clinician to identify macroscopic pathology quickly.