Computerized Axial Tomography

Computerized axial tomography (CAT) is a diagnostic procedure that employs x rays in a unique manner. The CAT scan machine is computer controlled to assure accuracy in placement of the x-ray beam. Axial refers to the fact that the x-ray tubes are arranged in an arc about an axis. Tomography is a combination of tomo, from the Greek meaning "to cut," and graph, "to draw," a reference to the fact that the CAT scan image reveals a crosssection of the body or body part.

The tomograph was developed in England in 1972. After a number of years of fine tuning the apparatus, it became a part of clinical medicine that is widely relied on now. Prior to the development of the CAT, x rays were done on the familiar table by a single x-ray tube that passed the rays through a given part of the body and exposed a plate of x-ray film. That film had to be developed and then viewed by a physician. This form of x ray was displayed on a film plate that offered a one-dimensional view of the body part under the x-ray tube. If a different angle was needed, the patient had to be turned over. The CAT offers a number of improvements over the old method.

The CAT scan machine, often referred to as the CT machine, consists of a horizontal pad on which the patient lies. Sandbags are placed around him to insure that he lies motionless. At one end of the pad is a circular structure that contains an array of x-ray tubes. The patient lies on the pad which is advanced into the circle until the desired area of the body is under the x-ray tubes. The x-ray tubes are focused to provide a very narrow angle of exposure, approximately 0.4 in (1 cm). The first x rays are made after which the array of tubes rotates and another exposure is made, the tubes rotate again, and so on until x rays have been made from all angles around the body.

Each x-ray tube is connected to the controlling computer. As the x rays pass through the patient's body they fall upon a sensitive window. The image from each tube is fed into the computer, and this is repeated whenever the xray tube fires, which is from a different angle each time.

In this way, the x-ray image is projected into the computer from different angles. The computer constructs a cross-sectional image of the body each time the array of xray tubes has completed a revolution around the patient.

Following each x ray exposure the patient is advanced another centimeter into the machine and the process repeats. X rays are made and the patient is advanced until exposures have been made in 0.4-in (1-cm) increments for the length of the organ being examined.

The images from each x-ray tube are fed through a computer, giving numerical values to the density of tissue through which the beam passed. The computer uses the numerical values to reconstruct an image of the cross-section of the body at the level the x rays passed through. The image is printed onto a screen for the physician to see and on a panel of x-ray film.

Doctors examining CAT scan x rays. Photograph by Joseph Nettis. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.

The differences in tissue density give the CT scan its definition. The liver is more dense than the pancreas, bone is more dense than liver, and so forth. The structures appear in different shades of gray on the screen and the film. The film is printed as a series of cross sectional images showing, for example, the liver from top to bottom. Any of the images can be called up on the computer screen for closer evaluation if the physician needs to do so.

A CT scan is described is a noninvasive procedure; that is, nothing is inserted into the body. At times the physician may want more contrast or definition to a given organ and may inject a contrast medium to accomplish this. A contrast medium is a substance that is visible on x rays. The medium, injected into the blood, will concentrate in an organ and will outline the organ or a cavity within it. In this way, the size of a kidney tumor may be determined, for example, as may other forms of pathology.

Obviously, the CAT scan is a specialized form of diagnosis and is not practical for such cases as bone fractures. The procedure requires more time to complete than does the ordinary, one-dimensional x ray and is not cost-effective for simpler procedures.

For diagnosis of soft-tissue tumors, which are difficult to print on an ordinary x ray, the CAT scan is superior.

All x rays rely on differences in tissue density to form the x-ray image. Bone resists the passage of the xray beam more than muscle, which resists more than a softer tissue, such as liver. Thus, the x-ray image from a single x-ray beam is a plate somewhat like a film negative showing various tones of gray. Small differences in tissue density, as would be seen with a tumor in the liver, where both tissues are nearly the same density, would not be seen as two separate structures. The liver would appear as a uniformly dense organ.

The CAT scan, however, takes x rays from different angles and the machine is capable after several exposures to determine the slight difference in densities of nearly similar tissues. The liver will appear as an organ of a certain shade of gray, and a tumor within it will be discernable as a spot of slightly lighter gray because of the minute variation in density. Also, by finding the panel on which the tumor first appears and following it through to the panel on which it disappears, the radiologist can determine the size of the tumor.