Wooldridge, J; Bownds, D (2022) The Importance of Sensor Placement during the Measurement of RF Heating of Implants in MRI. IEEE Journal of Electromagnetics, RF and Microwaves in Medicine and Biology, 6 (3). pp. 399-405.

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Abstract

Purpose: This study investigates the importance of accurate sensor placement when assessing implant safety during MRI scans experimentally. The current standards may risk underestimation of temperature rise for some implants, and this study identifies the circumstances under which this might occur and suggests some mitigation strategies.

Methods: The study used a combination of measurement and finite element modelling to assess sensitivity of measurement to sensor placement. The measurements were carried out in accordance with ASTM F-2182 to obtain temperature rises in regions close to a set of calibration cylinders. The simulations used a coupled thermal-electromagnetic model created using COMSOL Multiphysics to replicate the measurement conditions virtually. The maximum and minimum temperature in spherical regions surrounding the tip of the cylinders was extracted from the models to provide an estimate of the range of temperatures that a misplaced sensor might experience.

Results: The sharp temperature gradients around the tip of the calibration cylinders that were a) close in length to the resonant wavelength for the gel material and b) narrow meant that the measured temperature in these cases was highly sensitive to sensor placement. This effect was observed in both measurements (where an asymmetry that is very likely to be caused by sensor misplacement was seen) and in model results.

Conclusion: Accurate measurement of the maximum temperature rise caused by biomedical implants during MRI scans requires careful placement of temperature sensors. Preliminary modelling of the relevant implant undergoing a scan can enable the experimentalist to understand this sensitivity and either mitigate against sensor misplacement or broaden the uncertainty estimate associated with the measurement to take this sensitivity into account, thus providing better confidence in the ensuing data.

Item Type:	Article
Keywords:	MRI, implant heating, SAR
Subjects:	Mathematics and Scientific Computing > Measurement Uncertainties
Divisions:	Data Science
Identification number/DOI:	10.1109/JERM.2022.3156428
Last Modified:	14 Oct 2022 13:27
URI:	https://eprintspublications.npl.co.uk/id/eprint/9519