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

Abstract

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. © 2018 Wiley Periodicals, Inc.

Detection techniques of biological and chemical Hall sensors

“…Subsequently, Yamaguchi et al installed a Hall sensor (THM1176-HF, Metrolab) in the cap for the magnetic eld detection to avoid a strong magnetic eld in the laboratory as long-term use. 108,109 The hall voltage measured during the measurement is positively correlated with the local magnetic eld, and since the gradients of all switches during the imaging sequence are predetermined, the spatial position of the sensor can be calculated from the hall voltage measured. Occupational exposure to high static magnetic elds (SMF) during magnetic resonance imaging (MRI) has been shown to be hazardous to health.…”

Development of medical imaging sensors

“…By analyzing the acquired data, the MF intensity will change as the MRI worker changes location. 48 A new field-tolerant low criticaltemperature superconducting quantum interference device current sensor implemented. The sensor's low sensor noise and high magnetic field tolerance make it suitable for ULF MRI.…”

Evaluation of occupational exposure to static magnetic field in MRI sites based on body pose estimation and SMF analytical computation

“…4a) [Pasquier et al, 2013;Delmas et al, 2018] developed for this study. 4a) [Pasquier et al, 2013;Delmas et al, 2018] developed for this study.…”

“…In order to validate the proposed approach, magnetic field measurements were performed using a dosimeter (Fig. 4a) [Pasquier et al, 2013;Delmas et al, 2018] developed for this study. The B measurements were carried out in the 1.5 T MRI system (Siemens AERA 1.5 T).…”