CT stands for computerised tomography. The term CAT scan (computerised axial tomography scan) is less used by the medical profession nowadays. CT scans are a combination of x-rays and computers.
When X-ray machines were first developed, they were a marvelous way of enabling doctors to see breaks and abnormalities in bones. They were not so successful at showing up soft tissues. Furthermore they took pictures from only one direction at one point in the body, thus limiting how much of the body could be seen at any one time and at what angle. Modern day CT scanners take pictures of soft tissue such as tumours, and send X-rays from all sides – around the entire circumference of 360 degrees – with no greater amount of radiation than regular X-rays. The computer then builds these cross-sectional images up into a two dimensional slice. CT scanners can also take pictures at every point through the body, fractions of millimetres apart. This means not only that the picture is extraordinarily accurate, but also that very precise measurements are possible (eg. in the case of a tumour).
CT scanners can also guide a doctor during a biopsy and may help in surgery. By far the greatest application of the scanners is in the area of malignancies to assess whether there is a tumour, how big it is and how far it has spread. A CT scan can also be used after a stroke to tell what part of the brain has been affected. Scanning is invaluable for suspected cancers deep within the abdomen, such as in the liver.
Until a few years ago CT scanners were enormously expensive both to buy and to run, and only the main hospitals could afford to acquire them and to employ the skilled technicians needed to operate them. By the mid 1990’s, CT scanners were available in many suburban and regional x-ray centres, and CT scans are now routinely ordered by general practitioners to diagnose back pain, head injuries and abdominal lumps.
Having a CT scan is quick, painless and safe. Sometimes a dye is introduced into the body before the scan is started, either by injection or swallowing, so that the particular organ under investigation will show up more clearly.
For the scan itself, the patient lies on a narrow bench to which they are firmly strapped to prevent movement that would blur the picture, and the scanner moves over the body. The machine rotates around as it takes the x-rays and may be slightly noisy.
The latest development is helical or multislice CT scans. In this procedure the CT machine rotates constantly around the patient, and slowly spirals along the length of the patient to give a continuous scan cut through the patient like a fine corkscrew. Using this technique, extraordinarily detailed pictures of individual organs can be obtained. These may be as small as the middle ear, or as large as the colon. Instead of having the invasive procedure of colonoscopy, patients can now have a virtual colonoscopy using a helical CT. Overlying tissue that interferes with plain X-rays and conventional CT scans can be removed in this type of scan.
Helical CT can also be used to guide a surgeon to a particular area to perform a biopsy, assess a severely injured person to determine which tissues require surgical repair, view arteries and veins and the degree of blood flow in them, and show the fine anatomical detail of a tumour. This procedure is much faster than conventional CT scanning as the machine never stops.