Characteristics of the application and its use
MRI devices for medical imaging are able to provide detailed images of different internal body structures within a relatively short timeframe. In comparison to alternative techniques such as X-rays and CT scans, MRI does not involve exposure to ionizing radiation, and thus avoids the associated carcinogenic risk.
Effects of HF, LF and Static Fields on the body and health implications
From a non-ionizing radiation protection point of view, guidance regarding MRI equipment focuses on the patients being diagnosed or treated within these machines and on the personnel supporting patients, using, cleaning or manufacturing the machines. The general public is less of a concern as these machines are usually found within access-restricted environments. As with all medical procedures, general exposure limits do not apply to patients. The treating physician will always have to make a balanced judgment between the expected benefits of the treatment and the potential adverse effects. However, ICNIRP recommends that account be taken of a patient’s tolerance to body temperature elevation and of the need to avoid nerve stimulation. Special attention should also be given to patients who are pregnant, with a recommendation that the duration of exposure is kept to the minimum. More information for patients is given in the ICNIRP statement on MR and patients.
In relation to personnel working near MRI devices, the main protection issue relates to the static magnetic fields. These can provoke sensory effects such as vertigo and nausea as a result of the generation of small electrical currents in the balance organ. This, in turn, transmits signals to the brain, providing different information to that obtained through vision, resulting in the unwelcome adverse effects. These transient effects may be annoying and impair normal functioning. Thus, for workers - which in this instance can include doctors, nurses and other health care staff - the recommendation is in some cases to limit the strength of the field so that transient effects such as vertigo and nausea do not occur, and in other cases to provide for a set of site-specific work procedures. In particular, the speed of movement within a static magnetic field should be limited, as body movement induces electric fields and reinforces the sensory effects described above.