Using U‐Nets to predict the effects of head motion on simulated specific absorption rate for ultra‐high field magnetic resonance imaging with parallel transmission

Blanter, Katherine Anna, Plumley, Alix, Gungor, Alper, Malik, Shaihan and Kopanoglu, Emre ORCID: https://orcid.org/0000-0001-8982-4441 2026. Using U‐Nets to predict the effects of head motion on simulated specific absorption rate for ultra‐high field magnetic resonance imaging with parallel transmission. Magnetic Resonance in Medicine 96 (2) , pp. 650-665. 10.1002/mrm.70363

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Abstract

Purpose: Ultrahigh‐field MRI requires careful management of the specific absorption rate (SAR), which is subject and subject‐position dependent. Within‐scan subject motion may exacerbate local SAR exposure, necessitating large safety margins to prevent SAR underestimation, which hampers imaging performance. This study proposes a U‐Net architecture to adapt safety calculations to motion as it happens, to facilitate high‐performance scanning without compromising safety. Methods: Electromagnetic simulations were performed for five body models at multiple positions with an 8‐channel parallel‐transmit coil. Q‐matrices were transformed into real‐valued SAR distributions—to train U‐Nets to estimate motion‐induced effects on local SAR—which were then mapped back to Q‐matrices. Separate U‐Nets were trained for different types of body motion (rightward/leftward/anterior/posterior/yaw), which were then cascaded to predict the effect of composite (off‐axis) and larger displacements on SAR. Finally, network‐estimated local SAR distributions were compared with ground truth after‐motion local SAR for realistic parallel‐transmit pulses. Results: Subject motion had a statistically significant effect on local SAR, but network‐estimated safety models recovered a faithful representation of the ground truth after‐motion local SAR. For the investigated parallel‐transmit pulses, the proposed approach reduced the safety margin from 2.14‐fold to 1.3‐fold and ensured more than 68% of the imaging performance could be realized while a safety model that includes all simulated subject positions would have limited scanning performance to as low as 21% of the maximum. Conclusions: The proposed position‐aware SAR calculation approach allows smaller safety margins, which has the potential to enable higher‐performance UHF‐MRI scanning without compromising safety for subjects who are unable to remain still.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Psychology Research Institutes & Centres > Cardiff University Brain Research Imaging Centre (CUBRIC)
Additional Information:	License information from Publisher: LICENSE 1: URL: http://creativecommons.org/licenses/by/4.0/
Publisher:	Wiley
ISSN:	0740-3194
Funders:	Wellcome Trust, EPSRC
Projects:	204824/Z/16/Z, EP/T517951/1
Date of First Compliant Deposit:	7 April 2026
Date of Acceptance:	12 March 2026
Last Modified:	18 Jun 2026 14:27
URI:	https://orca.cardiff.ac.uk/id/eprint/186198

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