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A numerical and experimental study of RF shimming in the presence of hip prostheses using adaptive SAR at 3 T
Author(s) -
Destruel Aurelien,
Fuentes Miguel,
Weber Ewald,
O’Brien Kieran,
Jin Jin,
Liu Feng,
Barth Markus,
Crozier Stuart
Publication year - 2019
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.27688
Subject(s) - specific absorption rate , imaging phantom , electromagnetic coil , materials science , radio frequency , biomedical engineering , transmission (telecommunications) , thermal , image quality , acoustics , computer science , physics , optics , image (mathematics) , medicine , antenna (radio) , computer vision , telecommunications , quantum mechanics , meteorology
Purpose Parallel transmission techniques in MRI have the potential to improve the image quality near metal implants at 3 T. However, current testing of implants only evaluates the risk of radiofrequency (RF) heating in phantoms in circularly polarized mode. We investigate the influence of changing the transmission settings in a 2‐channel body coil on the peak temperature near 2 CoCrMo hip prostheses, using adaptive specific absorption rate (SAR) as an estimate of RF heating. Methods Adaptive SAR is a SAR averaging method that is optimized to correlate with thermal simulations and limit the temperature to 39°C near hip implants. The simulated peak temperature was compared when using whole‐body SAR, SAR 10g , and adaptive SAR as a constraint for the maximum allowed input power. Adaptive SAR was used as a fast estimate of temperature to evaluate the trade‐off between good image quality and low heating near the hip implants. Electromagnetic simulations were validated by simulating and measuring B 1 maps and electric fields in a phantom at 3 T. Results Simulations and measurements showed excellent agreement. Limiting whole‐body SAR to 2 W/kg and SAR 10g to 10 W/kg resulted in temperatures up to 49.3°C and 40.7°C near the hip implants after 30 minutes of RF exposure, respectively. Predictions based on adaptive SAR limited the temperature to 39°C, and allowed to improve the B 1 field distribution while preventing peak temperatures near the hip implants. Conclusion Significant RF heating can occur at 3 T near hip implants when parallel transmission is used. Adaptive SAR can be integrated in RF shimming algorithms to improve the uniformity and reduce heating.