Calibration and non-orthogonality correction of three-axis Hall sensors for the monitoring of MRI workers' exposure to static magnetic fields.

A Magnetic Resonance Imaging (MRI) scanner uses three different electromagnetic fields (EMF) to produce body images: a static permanent magnetic field (MF), several pulsed magnetic gradients, and a radiofrequency pulse. As a result, any occupation that includes an MRI exposes workers to a strong MF. The World Health Organization has now given the monitoring of occupational EMF exposure a high priority. One design for a low-cost, compact MF exposure monitor (« MR exposimeter ») uses a set of three orthogonally assembled Hall sensors. However, at such a strong EMF exposure intensity, the non-linearity and non-orthogonality (misalignment between the three Hall sensors) have an impact on the accuracy of EMF measurement. Therefore, a sensor characterization was performed in order to link Hall-effect output voltage to MF intensity. The sensor was then calibrated using an orthogonalization matrix and an offset vector. For each sensor configuration, the matrix and vector parameters were optimized with a calibration set generated by the movement of a three-axis sensor inside homogeneous MF areas. Once calibrated, the sensor was tested at different MF intensities and returned accuracy improvements. This calibration procedure was tested on synthetic data and performed on experimental data. The calibration parameters can be easily reused by the user, and their stability could be used as a quality control sensor. Finally, real-time monitoring test for static MF exposure was completed and validated on an MRI worker during a typical working day. Bioelectromagnetics. 39:108-119, 2018.