Comparing Specific Absorption Rate (Tissue Heating) Management Methods for Parallel Transmit MRI at 7 Tesla

Williams, S. N. , Herrler, J., Liebig, P., McElhinney, P. , Allwood-Spiers, S., Foster, J. E., Gunamony, S. , Nagel, A. M. and Porter, D. A. (2021) Comparing Specific Absorption Rate (Tissue Heating) Management Methods for Parallel Transmit MRI at 7 Tesla. 13th Scottish Imaging Network: A Platform for Scientific Excellence Annual Scientific Meeting (SINAPSE), Virtual, 16-17 September 2021.

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Abstract

Parallel transmit (pTx) can reduce RF field inhomogeneity present at 7T, but needs additional consideration for the specific absorption rate (SAR) and subsequent tissue heating due to the superimposed electromagnetic fields. Virtual observation points (VOPs) provide a common method for evaluating SAR in parallel transmit (pTx) MRI. The VOP approach uses an Eigenvalue clustering method to group voxels in electromagnetic field (EMF) simulations of realistic human body models. The clustering amount is set by an overestimation factor with respect to the “worst case” SAR configuration of complex pTx channels. It has been suggested to include as many EMF body model simulations at as many positions as computationally feasible in the VOPs to account for potential local SAR hotpots. Nevertheless, commercial pTx coils for 7T are typically supplied without the EMF-based VOPs and instead apply a constant safety factor for local SAR supervision, which can be overly conservative. In this abstract we compare the use of EMF-based VOPs in a self-built pTx head coil to constant safety factor VOPs in a commercial pTx coil of similar design. We investigate the local SAR values with 7T MRI scans in healthy volunteers. Comparing the VOPs’ simulated and experimental performance in both coils, the commercial coil VOPs provide SAR estimates that are highly conservative compared to estimates from VOPs based on anatomical models, yet the image quality from both coils remains very similar (Figure 1). The self-built coil's EMF-based VOPs are closer to the modeled SAR deposition without clustering (Figure 2), thus allowing scanning within the IEC normal mode head limit of 10 W/kg, which wasn’t feasible with the commercial coil. In the future, larger scale EMF simulations are required to explore the effect of subject anatomy and to optimize the use of VOPs for wider application.

Item Type:Conference or Workshop Item
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Liebig, Patrick and Gunamony, Dr Shajan and Williams, Dr Sydney and Porter, Professor David and McElhinney, Dr Paul and Foster, Dr John and Allwood-Spiers, Sarah
Authors: Williams, S. N., Herrler, J., Liebig, P., McElhinney, P., Allwood-Spiers, S., Foster, J. E., Gunamony, S., Nagel, A. M., and Porter, D. A.
College/School:College of Medical Veterinary and Life Sciences
College of Medical Veterinary and Life Sciences > School of Medicine, Dentistry & Nursing
College of Medical Veterinary and Life Sciences > School of Psychology & Neuroscience
Research Group:Imaging Centre of Excellence
Copyright Holders:Copyright © The Author(s) 2021
Publisher Policy:Reproduced with the permission of the publisher
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