Abstract
The growing popularity of MRI in clinical settings and the innovative applications in e.g. MRI guided surgery has resulted in more frequent, longer and higher levels of exposure to the stray magnetic fields for employees. Especially the use of stronger field strengths in MRI has been associated with unwanted sensory
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stimulation and difficulties in task performance. In experimental studies as described in this thesis we investigated the effects of exposure to MRI-related stray static magnetic fields (SMF) and movement-induced low-frequency time-varying magnetic fields (TVMF) on behavioral changes in cognitive functions and vestibular related functions of postural stability and oculomotor performance. In addition, possible underlying mechanisms have been suggested. In two experimental studies, healthy volunteers were exposed to a 1.0 Tesla (T) SMF in the stray fields of a 7 T MRI scanner. In some of the exposed conditions an additional TVMF of 2.4 T/s was initiated before every task by performance of head movements. A broad range of tasks was assessed testing cognitive and vestibular performance. The subjects’ test results were compared to their performance outside the magnetic field (sham condition). Exposure to MRI-related magnetic fields seemed to decrease cognitive and vestibular related performance. In particular domains including attention and concentration, verbal memory and visual (motor) functions, and postural stability have been (repeatedly) identified. An important role of the vestibular system in evoking these effects has been suggested. Therefore, a mediating role for the vestibular system was investigated. Responsiveness of the vestibular system did not modify cognitive and oculomotor test performance upon exposure to MRI-related magnetic fields. However, a weak but significant indication was demonstrated for a modified postural stability by vestibular asymmetry upon exposure. Moreover, we demonstrated that behavioral response patterns on cognitive, postural and oculomotor tasks during exposure to an MRI-related stray magnetic field did not resemble those after direct stimulation of the vestibular organ by galvanic vestibular stimulation (GVS). Therefore, we cannot confirm nor exclude that the vestibular system plays a (mediating) role in MRI-related magnetic field induced behavioral changes. Several hypotheses for endpoint-specific underlying mechanisms have been generated; magnetic field induced Lorentz forces in the vestibular organ, movement-induced electromagnetic induction when present in the magnetic fields, sensory conflict/weighting theory between received visual and vestibular information, and limited information processing capacity following an overflow of information to be processed. These proposed underlying mechanisms are specific for different behavioral endpoints. In conclusion, the magnitude of the found changes in behavioral performance by magnetic field of 1.0 T and 2.4 T/s is small but of serious significance. Given the trend of scanning at ultrahigh field strengths (7 T and higher) exposure of personnel working with and around MRI scanners is supposed to further increase in the coming years. This can have serious consequences for employees, and especially for those employees who need to maintain a high level of precision and concentration e.g. surgeons performing MRI guided operations. Therefore, the knowledge as presented in this thesis should among others be used as a basis for the design of relevant control measures and policies to lower exposure and reduce the occurrence of behavioral changes for individuals employed under these conditions.
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