Magnetic resonance imaging entails a strong static magnetic field and changing magnetic and radiofrequency fields. Problems arise from any metal objects present in the body.

With magnetic resonance imaging, the whole body is in the magnetic field, and sensitive organs cannot be “screened” - as they can in most techniques that use ionising radiation. For example, a foreign object in the head can interfere with the magnetic resonance imaging of any part of the body, down to the toes, and the referring doctor and radiologist must be aware of any potential source of interference even if it is physically distant from (and unrelated to) the problem being investigated. Cleaners, engineers, and anyone accompanying the patient during imaging are subject to the same risks: nobody known to harbour or suspected of harbouring any hazardous object should come near the imager. The National Radiological Protection Board's arbitrary recommendation in 1983 that magnetic resonance imaging should not be …