Safety

Abstract

MR is accompanied to exposure to three different mechanisms of interaction: the main magnetic field, switched gradient fields used for spatial encoding, and the radiofrequency fields used for signal transmission. The physical mechanisms are identical to those at conventional MR. Exposure levels in UHF-MR nevertheless reach values of concern necessitating appropriate measures to ensure safety of patients, volunteers, and operating and technical personnel. This especially applies to energy deposition by radiofrequency fields. Specific absorption rate increases with frequency, at UHF-MR the problem is exacerbated by the necessity to use parallel transmission to generate homogeneous excitation. Gradient-induced peripheral nerve stimulation depends on local changes of the magnetic field, and not on the main field strength. Secondary effects like induced currents when moving in the field do become appreciable at UHF-MR and lead to sensory effects like vertigo and nausea. For both of these exposure categories the physical mechanisms are basically known, serious disagreement arises about safety margins when defining exposure levels for safe application especially with respect to induced currents. The most contentious issue relates to safety of the main field itself. There are numerous reports about interactions of magnetic fields with living tissues, but so far no known biological effects have been identified as a definite health risk at currently achievable field strengths. From a scientific point of view this seems to be good news for the safe applications of UHF-MR, but translating this lack of evidence into regulatory guidelines is a tricky and contentious issue with potential serious consequences for the field.