Analysis of the Local Worst-Case SAR Exposure Caused by an MRI Multi-Transmit Body Coil in Anatomical Models of the Human Body

Esra Neufeld, Marie-Christine Gosselin, Manuel Murbach, Andreas Christ, Eugenia Cabot and Niels Kuster, Physics in Medicine and Biology, Volume 56, Number 15, pp. 4649–4659, August, 2011, online July 6

Multi-transmit coils can lead to an increase in whole-body and local SAR compared to conventional body coils excited in circular polarization. In this study, the maximum increase of SAR for three significantly different human anatomies was investigated numerically. The results demonstrate that the increase of SAR strongly depends on the anatomy. For the three models the wbSAR increased by a factor of up to 1.6 compared to conventional excitation and the psSAR by up to 13.4. For some locations the local averaged SAR goes up as much as 800 times. The ratio of the peak spatial SAR to the whole-body SAR increases by a factor of up to 47 and can reach values above 800. Due to the potentially much larger power deposition, additional, preferably patient-specific, considerations are necessary to avoid injuries by such systems.

The scientific and technical impact of the study can be summarized as:

• The future of MRI is in multi-transmit systems. We investigated if addtional safety considerations will be needed to avoid potential hazards to patients
• We documented worst-case increase of the overall peak spatial SAR as well as the local SAR enhancements
• The study demonstrated that the safety concepts for conventional MR systems are not sufficient for this emerging technology and has identified patient-specific evaluations as the most promising strategy 

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