The MRI (magnetic resonance imaging) scan is a screening technique that has been in use since the early 1980s. It can provide a detailed picture of your body's soft tissues, especially the brain and spinal cord. It can detect tumours, coronary heart disease, joint injuries or signs of multiple sclerosis, for example.
The MRI machine uses a powerful magnet to take electronic pictures of you while you are lying down inside of a massive cylindrical magnet, which can weigh up to 500 tons and is large enough to envelop the entire body. As the cylinder fits rather closely to the body, some people experience claustrophobia while being scanned. The noise made by the scanner is so loud that patients are given ear protection during the procedure, which can last from 20 minutes to one hour.
While you are enclosed within the magnet, a quick pulse is applied, creating a magnetic field that is some 50,000 times stronger than that of the earth (BMJ, 1991; 303: 205). The effect of this is to excite the nuclei of the atoms making up the cells of the body. These hyped-up atomic nuclei produce radio-frequency echoes, which are translated into images through a computer.
Is it accurate?
MRI is supposed to be fairly accurate for detecting multiple sclerosis. In one study of MS patients, MRI scanning was able to detect the disease with 95-99 per cent accuracy (Radiology, 1991; 178: 447-51).
But other studies paint a different picture. According to a medical textbook on CT and MRI screening, many of the early reports that MRI gave more detailed images than CT were 'overly optimistic'. The initial fanfare for MRI, which came from individual cases, was not confirmed by larger subsequent studies (BMJ, 1991; 303: 205).
Lately, MRI has proved to be less than accurate in detecting early prostate cancer (Lancet, 2002; 359: 1643-7) and coronary artery disease.
It is now thought that MRI is better than CT for visualising the brain and spine, as it can take views from the top of the head and front of the skull, and is better able to detect subtle changes. In contrast, CT scans appear to be better for studying the bones and calcium deposition, and any sorts of trauma - such as blows to a body part.
Is it safe?
Radiologists maintain that MRI is far safer than an X-ray and that no adverse reactions have ever been reported.
Unfortunately, this is not true. Accidents, injuries and even deaths have occurred during MRI procedures. In one poll of 10 departments of radiology across the US, the overwhelming majority of serious injuries linked to MRI were burns (Am J Roentgen, 1994; 162: 189-94). The heating effects can also cause future fertility problems in men, as sperm are rendered sterile when heated up to body temperature (Am J Roentgen, 1990; 154: 1229-32). Four separate studies also suggest that MRI can cause memory loss (J Magn Res Imag, 1992; 2: 721-8).
The most serious injury reported with MRI occurred when an oxygen tank near the magnet struck a patient in the face (Am J Roentgen, 1990; 154: 1229-32).
Before undergoing the procedure, it is vital that you tell the radiologist if you have any metal devices in your body, such as a pacemaker, metal prosthesis, implants or surgical clips. You must also remove any metal objects you may be wearing, such as jewellery, watches, rings and glasses; even zippers or metal buttons on your clothing could become a hazard. You are also advised to remove any credit cards from your wallet as the magnetic strip can be wiped by the magnetic field.
Most technicians fail to mention that many cosmetics, such as some eyeshadows and lipsticks, also contain metallic substances.