Personal time-varying magnetic fields evaluation during activities in MRI sites

A person moving in and around the MRI site may experience strong time-varying magnetic fields. The physical consequence of the time variation of the magnetic flux density is the induction of currents in body parts. In this paper the time-varying gradient exposure associated with the magnetic flux densities is evaluated and measured. The acquired data, obtained from a personal magnetic dosimeter, represent the magnetic flux density function, B = B(t), related to the operator movement inside the MRI site. Such data have been processed to evaluate the corresponding dB/dt curves, that were estimated by calculating the time derivative. All the measurements were conducted on a 3.0 T MRI site, dedicated to research procedures, in two different conditions: at first during routine patient positioning, and secondly, simulating an emergency. In both the measurement conditions, two dosimeters, with different acquisition times, were simultaneously used. They were positioned the first time on the operator’s torso and the second one on his head. The analysis conducted, simulating both normal and emergency conditions, demonstrated that the dB/dt peak values strictly depended on human motion through strong static magnetic fields, and, sometimes, exceeded the recommended limit. This consequence highlighted the necessity of drawing up, as in case of ionizing radiation, behavioral rules to be followed by workers and patients. Therefore it will be necessary to assess risk conditions in a proper manner.