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» MRI Safety

Exposures, limits, safe systems of work. This section explores safety issues in magnetic resonance imaging.

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Heat Sensitivity and RF

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Which two body parts are regarded as particularly heat-sensitive?

The lens of the eye, and the testes.

A patient with a normal thermoregulatory response to heat will experience a compensatory dilation of blood vessels, resulting in increased blood flow and so the heat is dissipated around the body and through the skin, with only a small discernible increase in core body temperature. Some tissues may be particularly sensitive to RF radiation. The lens of the eye is regarded as potentially sensitive because of its lack of a blood supply and its tendency to accumulate damage and cellular debris. The testis is also regarded as heat sensitive. Testicular temperatures are normally several degrees below body temperature and male germ cells are adversely affected by elevated temperatures.

Physiological and physical correlations which are associated with a decreased ability to adapt to an increased heat load include old age, obesity and hypertension. Various drugs, such as diuretics, tranquilisers and sedatives, and vasodilators, decrease heat tolerance. In addition, the thermoregulatory ability of infants is not well developed; pregnant women may also be compromised in their ability to dissipate heat.

Another issue regarding heat deposition is the possibility of contact burns due to conductive loop pathways formed by the patient (and possibly wires or cables). Careful positioning of cables is advised (see the MHRA Guidelines), and intertwined hands, knee-knee contact, foot-foot contact and hands-at-sides contact is discouraged.

Further reading on this topic:
Books: MHRA Guidelines p20-21, 30

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