Magnetic resonance imaging: Recent research on the biological im-pacts of static magnetic and high-frequency electromagnetic fields

Problem: in recent years, new studies have been published on biological effects of strong static magnetic fields and on thermal effects of high-frequency electromagnetic fields as used in magnetic resonance imaging (MRI). Many of these studies have not yet been incorporated into current safety recommendations. Method: scientific publications from 2010 onwards on the biological effects of static and electromagnetic fields of MRI were searched and evaluated. Results: new studies confirm older work that has already described effects of static magnetic fields on sensory organs and the central nervous system accompanied by sensory perception. A new result is the direct effect of Lorentz forces on ionic currents in the semicircular canals of the vestibular organ. Recent studies on thermal effects of radiofrequency fields focused on the development of anatomically realistic body models and more accurate simulation of exposure scenarios. Recommendation for practice: strong static magnetic fields can cause unpleasant perceptions, especially dizziness. In addition, they can impair the performance of the medical personnel and thus potentially endanger patient safety. As a precaution, medical personnel should move slowly in the field gradient. High-frequency electromagnetic fields cause tissues and organs to heat up in patients. This must be taken into account in particular for patients with impaired thermoregulation as well as for pregnant women and newborns; exposure in these cases must be kept as low as possible. excitation and preparation of the spin system or occurring during the movement of persons in spatially inhomogeneous static magnetic fields, always induces an alternating electric field. The electric field strength is proportional to the rate of change over time 𝑑 The induced electric field in turn leads to electric currents in conductive tissues. Due to this Effect, radio frequency (RF) electromagnetic fields cause energy absorption in tissue. The spatial distribution of energy absorption in the body depends strongly on the size, orientation, and internal tissue structure of the exposed body section as well as the frequency of the RF field. Theoretical and experimental studies show that absorption is maximal when the wavelength of the RF field corresponds to typical magnitudes of the body. Unfortunately, the wavelengths of the RF fields used in MRI are exactly in this resonance range. The energy absorbed in the tissue per unit mass and time is called the specific absorption rate (SAR in W/kg). It increases approximately with the square of the magnetic flux density B 0 of the static magnetic field, so that RF absorption is a safety-relevant Effect that must be carefully considered in high- and very-high-field MR systems. With increasing flux density B 0 , the wavelength of the RF field also decreases, so that local minima and maxima of the field distribution form in the body, which can lead not only to inhomogeneous RF excitation (i.e., varying pulse angles) but also to local SAR hotspots. So-called RF shimming is used to try to minimize these effects technically. The essential biophysical effect of electromagnetic RF fields is the heating of the tissue. It is determined not only by the local power absorption and the duration of exposure, but also very significantly by the thermal conductivity and perfusion of the exposed tissues and organs.