Here we summarize our brain mapping results of the induced magnetic field and forces acting on biological tissues in the head of 118 healthy human subjects exposed to the homogeneous static magnetic field of a 4 T MRI scanner. As far as our knowledge reaches, this study is the first to assess the distribution of magnetic field gradients and forces in a large cohort of human subjects, thus providing an important database toward better assessment of MR safety and potential physiological effects of static magnetic fields in humans. Our main finding is that the magnetic force induced by magnetic susceptibility differences at air/tissue interfaces in the head is highly significant across all subjects. As demonstrated by our correlation analysis, only in certain regions such as the right lingual gyrus was the distribution of magnetic force significantly affected by the orientation of the head, relative to that of the magnetic field.
The magnetic force applied on weakly magnetic/diamagnetic biological tissues immersed in a perfectly homogenous external magnetic field is proportional to the local field strength, the magnetic susceptibility of the tissues, χ, and the inhomogeneity of the local magnetic field (ie, the static magnetic field gradient, g), which reflects the geometry of tissue magnetic susceptibility differences, Δχ (7
The average magnetic force in the human head was maximal at the eyeballs and the orbitofrontal and temporal cortices (, ), consistent with previous calculations of the induced static magnetic field in the human head (8
) and previous findings in a single individual (7
). These regions include the hypothalamus, which controls body temperature as well as other autonomic functions, and the vestibular system that contributes to our balance and our sense of spatial orientation. Thus, our results are also consistent with the higher occurrence of tonic vestibular asymmetry, hyperreactive caloric responses, and spontaneous nystagmus documented after 30 minutes exposure to 9.4 T static field in a small group of MRI workers (18
). Vestibular dizziness has been shown to deactivate the middle/medial superior temporal area (MT/MST) (19
), and the same area was found to be associated with taste perceptions using different taste stimuli (20
). Thus, our findings on induced magnetic force across all subjects in the ITG () suggest that feelings of dizziness, vertigo, and “metallic taste” after exposure to high-field MRI could be associated with neurostimulation induced by magnetic force in the ITG and the other mentioned regions. However, at current magnetic fields no such symptoms were observed/reported in this cohort of subjects.
Lingual gyrus was the only brain region that showed a statistically significant linear correlation between Fz
and head orientation parameters (x-rotation, or pitch) (). This finding could be associated with the sensation of vertigo in the MRI scanner. The sensation of vertigo is frequently experienced when subjects move in and out of the MRI scanner, and could result from electrical currents induced in moving brain tissues exposed to magnetic fields (14
). However, when the head motion is perpendicular to the orientation of the magnetic field it can induce sensations of vertigo in humans and this has been postulated to arise from magnetohydrodynamic forces giving rise to similar effects as those seen in travel sickness (21
). Since the lingual gyrus was shown to activate in previous functional MRI studies on vertigo (1
), our findings on orientation-dependent induction of magnetic forces in the lingual gyrus suggest that the magnetic force acting in these brain regions could originate sensations of vertigo in MRI (3
), although these sensations would be much weaker comparing to that caused by motion in the static magnetic field (e.g., 1,3,4,6,22,23). Finally. This study does not support significant gender or aging differences in the magnetic force distribution in the human heads.
The amplitude of the imaging gradients and the acquisition bandwidth limited the accuracy of the measurements (estimated systematic error ≈ 3%). Furthermore, the measurements reflected average values within the imaging voxels. Thus, future studies using higher spatial resolution might allow detection of higher peak values for the magnetic force in the human head. This study evaluated the induced force during real MRI scan only; it does not measure the magnetic force while subjects entering or exiting the magnet, which at 4 T could be 300 times higher than that reported in this study depending on the motion speed of the body.
In conclusion, we assessed the magnetic force distribution in the head of 118 healthy humans exposed to 4 T magnetic field MRI using statistical parametric mapping techniques. We found that the induced magnetic force is highly significant in the eyeballs, orbito-frontal and temporal cortices, subcallosal gyrus, anterior cingulate as well as midbrain and brainstem (pons), regardless of subjects age or gender. The induced magnetic force was 6 × 105 times weaker than the Earth's gravitational force. While biological damage caused by the induced magnetic force in the brain during MRI scanning seems unlikely, at least under current strength of MRI technology, the potential effects of the magnetic force on brain function merits further evaluation since it could affect activation/deactivation signal observed with fMRI studies.