Working with MRI: An investigation of occupational exposure to strong static magnetic fields and associated symptoms

Magnetic resonance imaging (MRI) makes use of electromagnetic fields in the non-ionizing radiation frequency ranges. One of them is a continuously present strong static magnetic field (SMF), which extends up to several meters around the scanner. Each time an MRI worker performs tasks near the scanner, exposure to SMF occurs. Workers will also be exposed to time-varying magnetic fields (TVMF) as a result of movement through the spatial SMF gradients around the scanner. Exposure to MRI-related SMF and motion-induced TVMF has been associated with transient acute effects on sensory functions, referred to as MRI-related symptoms or sensory effects. These include sensations of vertigo, nausea, balance problems, light flashes (phosphenes) and metallic taste. This thesis addresses the intensity, frequency, duration and determinants of occupational exposure to MRI-related SMF and TVMF, as well as the occurrence of MRI-related symptoms among MRI workers and associations between these exposures and symptoms. Based on a nation-wide inventory it was estimated that almost 4,000 people in the Netherlands are exposed to MRI-related SMF and TVMF on a regular basis. Shift-based measurements of personal exposure to SMF and motion-induced TVMF were conducted among 271 MRI workers during more than 400 shifts, using portable magnetic field dosimeters. Peak and time-weighted average (TWA) metrics of exposure to SMF and TVMF were derived from these shift-based measurements. These metrics were strongly correlated (r ~ 0.7-0.9). Exposure levels were highest among MRI radiographers, scientific staff and technical staff. The type and magnet strength of the MRI scanner were important determinants of all exposure metrics. Highest exposure levels were measured at open up-right scanners and whole-body closed-bore scanners. Other factors associated with increased levels of one or more exposure metrics included (but were not limited to) the total number of subjects scanned, performing functional MRI or technical test scans, scanning high-care patients and administering contrast medication. Analyses of self-reported symptoms, which were collected with logbooks during over 600 work shifts, showed how frequently symptoms occurred among MRI workers and revealed a clear association between scanner strength and symptom occurrence. This association was strongest for vertigo and metallic taste, which occurred during 6% and 2% of the MRI shifts, respectively. Reporting of vertigo was significantly associated with all peak and TWA metrics of SMF and TVMF exposure, of which the association with full-shift TWA TVMF exposure was most convincing. Vertigo was estimated to occur during at least 5% of the work shifts at full-shift TWA TVMF exposure levels of 0.6 mT/s and higher. With the ongoing trend of using stronger MRI systems, workers’ levels of exposure to MRI-related SMF and TVMF, as well as the occurrence of MRI-related symptoms, are expected to increase. The exposure survey has enabled identification of highly exposed groups of workers and of work practices associated with increased exposure levels. These results provide guidance for development of exposure reduction strategies and control measures, and for designing exposure assessment strategies for epidemiological research