The effect of spatial averaging on dB/dt exposure values for implanted medical devices
Abstract
Magnetic resonance imaging (MRI) is a medical imaging modality that has seen continuous growth in the decades since its introduction. In conjunction with this increase in the use of MRI, there has also been a growth in the number of patients having implanted medical devices, such as pacemakers. These devices can have undesirable interactions with the MR system. The safety of these interactions must be guaranteed while ensuring that safety limits are not so conservative that they would preclude too many patients from benefiting from MRI. One factor that could make limits less restrictive is spatial averaging of the fields in MRI.
The objective of this investigation was to determine the effect that spatial averaging would have on the predicted time-varying magnetic field (dB/dt) values within realistic MRI gradient systems. ISO/TS 10974:2018(E) contains simulated data describing the peak dB/dt values that active implantable medical devices (AIMDs) could be exposed to when within varying volumes within the bore of the MRI scanner to account for the device location dependence of the dB/dt. For devices with realistic spatial extent (e.g. a 5 cm diameter component), the dB/dt relevant for testing would need to include information about the spatial average of the dB/dt over the device.
This investigation involved the development and validation of a numerical software system to simulate the fields produced by arbitrary gradient coil designs at any point within the gradient and for any device geometry placed within that environment. The software was shown to accurately predict physical measurements, and it was shown that the mean and peak dB/dt values differ between 2 and 17%. The difference between the peak and mean values increases monotonically with the distance from the central axis of the gradient coils.