HEALTH / Second Opinion

MOST OF us have only a hazy understanding of the different kinds of radiation and are far from certain which of the machines in our homes and offices depend on magnetism, electricity, microwaves, and so on. So it is not surprising that we may be uncertain about the energy source used in airport security screens and its effects on cameras, their films and other electronic gadgets in our luggage. An increasing number of us, especially as we grow older, also have various mechanical and electronic gizmos inserted into our bodies to keep them functioning. How are these affected by radiation sources around us?

The experts seem to disagree about the effects of security screens on medical devices such as cardiac pacemakers and on man- made artificial joints, most of which include some large pieces of metal. In part, the uncertainty is due to the many different screening devices used and the reluctance of operators to give detailed information about their specifications. Nevertheless, tests have shown, rather surprisingly perhaps, that

metal hip and knee joints rarely trigger a security alert. With pacemakers the sensible advice is not to risk any upset; someone with a pacemaker inside their chest should ask to be excused the screen, accepting that this will mean a hands-on personal search.

Magnetic resonance imagers are another story. The MRI scanner gives doctors the best pictures yet of the inside of the body, especially of the brain and spinal cord, and it does so without using X-rays and so (as far as we know) without any hazard to health. The patient lies inside a massive hollow magnet and is then exposed to short bursts of magnetic fields and radio waves. These stimulate the nuclei of hydrogen atoms to emit radio signals which are detected and analysed by a computer, which then generates images of the body. The danger is that these magnetic fields may move any metallic objects inside the body of the patient or anyone nearby.

A recent review in the British Medical Journal listed the medical devices that might or might not be affected by MRI scanning. Pacemakers come at the top of the list: the metal power unit may be physically moved and the internal switch may be permanently affected, so patients with pacemakers should not be scanned. Any medical implant with switches, power units and so on is at risk.

People with metal hip or knee joints can enter a scanner safely - but the BMJ warns that 'the procedure should be stopped if

patients experience pain in the region of large implants'. Small pieces of metal are more easily moved, and this includes fragments of shrapnel from war injuries and the small metal clips used to block off aneurysms (bulges) in the arteries supplying blood to the brain. Some clips are made of an alloy unaffected by magnetism. Clips used in sterilisation procedures in women, and metal stents (tubes inserted into body passages to keep them open) may also be affected by a scanner. All-plastic implants are safe.

The list of potential hazards is kept under constant review, but the precautions can work only if people can tell their doctors that they have a metallic device or fragment inside them. But too often a patient has not been given precise details of the implant and so cannot provide this vital information.