The CAT (computed axial tomography) scan is one of the wonders of modern medicine, and doctors love to have the opportunity to demonstrate it whenever possible.

It is an advanced form of X-ray that produces a three-dimensional model of your organs and bones, or it can zero in on one narrow slice of your body. But unlike an X-ray, it eliminates shadows, thereby making detection and diagnosis easier - or so the theory goes.

Developed in the 1970s from the digital imaging-processing technology used for the Apollo moon landings, it quickly moved from NASA’s control centres to most hospitals in the West.

Originally intended for head imaging, today it is used for viewing soft tissue, bone and blood vessels. It is now the preferred diagnostic tool for diseases of the lung, abdomen, heart and blood vessels, and for cancers of the liver and pancreas. A CAT scan can also check for bone mineral density in early osteoporosis.

Although popular among doctors, the machine can cause dread to the patient, especially one who is claustrophobic. The scanner is a large, dome-shaped contraption into which the patient is wheeled while lying on a table. Parents are advised to stay in the room if their child is being scanned.

Sometimes, a special dye is injected beforehand, especially if your abdomen or pelvis is to be scanned. The whole process can take up to an hour to complete.

An X-ray tube is mounted on a moveable ring at the edge of the dome opening. The ring also carries an array of X-ray detectors. A motor turns the ring so that the X-ray tube and detectors revolve around the body; each revolution scans a narrow, horizontal slice of your body. At the end of the process, a computer pieces together all the separately scanned ‘slices’ to form a complete 3-D picture.

Not surprisingly, this technique exposes the patient to a far higher dose of radiation than a conventional X-ray (Lancet, 1992; 340: 299). Sometimes, the whole process has to be repeated if the patient has not remained absolutely still during the lengthy scanning procedure.

It’s often used to diagnose a hernia even though it has misdiagnosed as normal up to one-third of children who did have a hernia (BMJ, 1993; 10 April).

The use of CAT scans on children has been a cause of concern. One study found that the radiation doses from these machines are far higher than they need to be, and may be a contributor to cancer in later life. In addition, the settings are often not lowered when children are being scanned so that, as a result, they receive radiation levels five times higher than necessary to obtain a good-quality image.

In the US, where 600,000 CAT scans are performed on children under the age of 15 every year, it’s been estimated that, as a result, 500 children will die from cancer when they are adults (Am J Roentgenol, 2001; 176: 289-96).