Computerized Axial Tomography

I

Introduction

Computerized Axial Tomography (CT or CAT), non-invasive diagnostic technique using a type of X-ray device that provides a clear view of soft internal organ tissues in the body. CT is used to diagnose various conditions, in particular cancer. A CT scan is the computer analysis of a sharply limited, thin X-ray beam passed circumferentially through an area of the body, producing a cross-sectional image, or slice.

The modern CT scanner comprises five major parts. A high-speed X-ray tube cooled by oil, air, and water forms the X-ray source. Its X-ray detector, normally a bank of about 1,000 solid state-crystal microprocessors coated with caesium iodide, receives the attenuated X-ray signal as it passes through the various tissues and bones of the patient being examined. The signal is electronically converted to binary data, which is read by the computer—the heart of the CT imaging system. The CT has a gantry, a framework that is mounted in such a way that it surrounds the patient in a vertical plane, and contains a rotating sub-frame on to which the X-ray source and detectors are mounted. A patient table (or couch) is positioned perpendicular and axial to the gantry so that it is able to travel along that axis.

II

Development of CT

The invention of CT is accredited to two individuals who worked independently: Godfrey Newbold Hounsfield, a British electronics engineer, and Allan Cormack, an American nuclear physicist. They shared the 1979 Nobel Prize for Physiology or Medicine for their work. Hounsfield and the electronics company EMI had created a practical scanner in 1972 following work started in 1967. By the late 1970s there were hundreds of CT scanners in a number of countries. Today, in developed countries, a CT scanner can be found in many major hospitals.

III

How CT Works

Topographic images are produced by using an X-ray source and a detector moving in a coupled way relative to the patient. In CT a thin fan beam of radiation rotates in a circular or spiral motion around the patient. Thousands of projected X-ray signals are reconstructed by computer algorithms to produce digital CT images, displayed by a high-resolution monitor. In this way the whole body can be imaged from head to toe.

Patients can be scanned on an outpatient basis, without the need to stay in hospital. For a whole body scan a gown is worn; if the gastrointestinal tract is scanned a contrast agent, such as barium, is administered, for which the patient has to fast before admission. As with standard X-ray procedure diagnosis of pregnant patients would not be carried out using CT.

IV

Advantages for Diagnosis

Whereas an X-radiograph gives 30 grey scales related to the amount of film exposure, the CT image can differentiate to over 200 grey scales; because of this greater sensitivity, there is greater discrimination between tissue types, thereby making cancerous cells, tumour sites, small, fine bones, and arterial blockages easier to identify. Furthermore, because of CT’s high resolution, grey matter in the brain can be differentiated from white matter and from cerebrospinal fluid; soft organs like the pancreas can also be easily defined. All this would not be possible using conventional procedures. CT is used particularly for defining malignancy and its extent.

A further benefit of the CT scanner is the availability of a data set for an entire person. With powerful computers it is possible to re-format the image data to produce body slices in any orientation. Three-dimensional reconstruction techniques pinpoint the locality, size, shape, and orientation of an organ or lesion; these techniques are of immeasurable value to the surgeon.