Computerized tomography

(medicine) The process of producing a picture showing human body organs in cross section by first electronically detecting the variation in x-ray transmission through the body section at different angles, and then using this information in a digital computer to reconstruct the x-ray absorption of the tissues at an array of points representing the cross section. Abbreviated CT. Also known as computed tomography; computerized axial tomography (CAT).

An imaging technique which uses an array of detectors to collect information from a beam that has passed through an object (for example, a portion of the human body). The information collected is then used by a computer to reconstruct the internal structures, and the resulting image can be displayed—for example, on a television screen. The technique relies on the fact that wave phenomena can penetrate into regions where it is impossible or undesirable to introduce ordinary probes. Since wave attenuation and velocity are affected by material properties, such as temperature and density, information concerning these properties is accumulated by a wave packet as it travels along its path. The intensities and travel times of such pulses measured at a receiver represent such accumulations. Although a single such datum does not allow point-to-point variation to be inferred, when many rays crisscross a region the corresponding interrelated data contain constraints that allow an inversion to determine interior structures. Considerable computational power and mathematical sophistication are needed in dealing with indirect, incomplete, and imperfect measurements, notably the branch of mathematics called inverse theory. The term computerized tomography for this technology arose in medicine, where x-ray attenuation data along many straight paths confined to a plane were used to obtain a map of density structure in a body section. The term tomography refers to methods that display only thin slices or sections through an object or a human body. See also Inverse scattering theory.

Examining a person or object in the laboratory requires the use of a gantry, composed of an x-ray tube, an array of detectors opposite the tube, and a central aperture in which the person or object is placed. The rigid gantry maintains the proper alignment between the x-ray tube and the detectors. An electron beam is used, which is focused on stationary anode rings located in the gantry surrounding the patient, making it possible to generate considerable x-ray output within a very short time and to acquire images while the patient is advancing through the gantry. Up to 34 scans or slices can be obtained within 1 s. The computer reconstructs the image from the information collected by the detectors. In order to obtain enough information to calculate one image, at least 90,000 readings (300 pulses and 300 detectors) are needed.

In medicine, computerized tomography represents a noninvasive way of seeing internal structures. In the brain, for example, computerized tomography can readily locate tumors and hemorrhages, thereby providing immediate information for evaluating neurological emergencies. Another advantage of computerized tomography is three-dimensional reconstruction. It is most useful in cases of fracture of the hip or facial bones, helping the surgeon to do reconstructive surgery. Other medical imaging techniques that make use of computerized tomographic methods include magnetic resonance imaging, positron emission tomography, and single-photon emission tomography. See also Medical imaging; Radiography.

After the success of computerized tomography in medicine, its possibilities in other fields were quickly realized. In the earth, atmospheric, and ocean sciences it has supplemented, but by no means replaced, older methods of remote sensing. Seismic tomography is now an important tool for investigating the deep structure of the Earth, testing theories such as plate tectonics, and exploring for oil. Ocean acoustic tomography is applied to physical oceanography, climatology, and antisubmarine warfare. Atmospheric tomography finds applications to weather, climate, and the environment. See also Oceanography; Plate tectonics; Seismology.