Exposure levels of radiofrequency magnetic fields and static magnetic fields in 1.5 and 3.0 T MRI units

Abstract

Magnetic resonance imaging (MRI) staff is exposed to a complex mixture of electromagnetic fields from MRI units. Exposure to these fields results in the development of transient exposure-related symptoms. This study aimed to investigate the exposure levels of radiofrequency (RF) magnetic fields and static magnetic fields (SMFs) from 1.5 and 3.0 T MRI scanners in two public hospitals in the Mangaung Metropolitan region, South Africa. The exposure levels of SMFs and RF magnetic fields were measured using the THM1176 3-Axis hall magnetometer and TM-196 3 Axis RF field strength meter, respectively. Measurements were collected at a distance of 1 m (m) and 2 m from the gantry for SMFs when the brain, cervical spine and extremities were scanned. Measurements for RF magnetic fields were collected at a distance of 1 m with an average scan duration of six minutes. Friedman’s test was used to compared exposure mean values from two 1.5 T scanners, and Wilcoxon test with Bonferroni adjustment was used to identify where the difference between exist. The Shapiro–Wilk test was also used to test for normality between exposure levels in 1.5 and 3.0 T scanners. The measured peak values for SMFs from the 3.0 T scanner at hospital A were 1300 milliTesla (mT) and 726 mT from 1.5 T scanner in hospital B. The difference in terms of SMFs exposure levels was observed between two 1.5 T scanners at a distance of 2 m. The difference between 1.5 T scanners at 1 m was also observed during repeated measurements when brain, cervical spine and extremities scans were performed. Scanners’ configurations, magnet type, clinical setting and location were identified as factors that could influence different propagation of SMFs between scanners of the same nominal B0. The RF pulse design, sequence setting flip-angle and scans performed influenced the measured RF magnetic fields. Three scanners were complaint with occupational exposure guidelines stipulated by the ICNIRP; however, peak levels that exist at 1 m could be managed through adoption of occupational health and safety programs.