Cem Murat Deniz1,2, Riccardo Lattanzi1, Yudong Zhu1, Graham Wiggins1, Daniel K. Sodickson1
1Center for Biomedical Imaging, Department of Radiology, NYU School of Medicine, New York, NY, USA; 2 Sackler Institute of Graduate Biomedical Sciences, NYU School of Medicine, New York, NY, USA
Minimizing SAR while maintaining a homogenous excitation is one of the principal challenges associated with the use of ultra high magnetic field strengths. We investigated the SAR behavior and the power requirements for parallel transmission as the gap between transmit elements and the surface of the object is increased. Two simulated geometrical arrangements of coil elements around a sphere were explored: one in which an increasing number of coils of fixed size were placed in a belt of increasing radius around the object, and another in which a fixed number of coils with increasing radius was arranged at increasing distance from the object. We found that global SAR and peak SAR during parallel excitation decreases with liftoff, approaching the lowest SAR allowed by electrodynamics (i.e. the ultimate intrinsic SAR). However, input power requirements to achieve the desired excitation increases rapidly with lift-off. Thus, for parallel transmission there are SAR benefits in moving coils away from the object, but RF power requirements may represent a practical limiting factor.