Radiofrequency energy deposition and radiofrequency power requirements in parallel transmission with increasing distance from the coil to the sample.
OBJECTIVE
We investigated global specific absorption rate (SAR) and radiofrequency (RF) power requirements in parallel transmission as the distance between the transmit coils and the sample was increased.


METHODS
We calculated ultimate intrinsic SAR (UISAR), which depends on object geometry and electrical properties but not on coil design, and we used it as the reference to compare the performance of various transmit arrays. We investigated the case of fixing coil size and increasing the number of coils while moving the array away from the sample, as well as the case of fixing coil number and scaling coil dimensions. We also investigated RF power requirements as a function of lift-off, and tracked local SAR distributions associated with global SAR optima.


RESULTS
In all cases, the target excitation profile was achieved and global SAR (as well as associated maximum local SAR) decreased with lift-off, approaching UISAR, which was constant for all lift-offs. We observed a lift-off value that optimizes the balance between global SAR and power losses in coil conductors. We showed that, using parallel transmission, global SAR can decrease at ultra high fields for finite arrays with a sufficient number of transmit elements.


CONCLUSIONS
For parallel transmission, the distance between coils and object can be optimized to reduce SAR and minimize RF power requirements associated with homogeneous excitation.
Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.|The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA.|NYU WIRELESS, New York University Polytechnic School of Engineering, Brooklyn, New York, USA.|RF Test Labs, Inc., New York, New York, USA.
Cem M
CM
Deniz
Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.|The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA.|NYU WIRELESS, New York University Polytechnic School of Engineering, Brooklyn, New York, USA.
Manushka V
MV
Vaidya
Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.|The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA.|NYU WIRELESS, New York University Polytechnic School of Engineering, Brooklyn, New York, USA.
Daniel K
DK
Sodickson
Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.|The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA.|NYU WIRELESS, New York University Polytechnic School of Engineering, Brooklyn, New York, USA.
Riccardo
R
Lattanzi
D065638:Absorption, Radiation; D003198:Computer Simulation; D004735:Energy Transfer; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D008954:Models, Biological; D011846:Radio Waves; D011874:Radiometry / Q000379:methods*; D014159:Transducers
parallel transmission; specific absorption rate; transmit coils; ultimate intrinsic SAR