Premium
Electrodynamic constraints on homogeneity and radiofrequency power deposition in multiple coil excitations
Author(s) -
Lattanzi Riccardo,
Sodickson Daniel K.,
Grant Aaron K.,
Zhu Yudong
Publication year - 2009
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.21782
Subject(s) - electromagnetic coil , specific absorption rate , excitation , homogeneity (statistics) , field strength , physics , computational physics , magnetic field , shim (computing) , computer science , electromagnetic field , scanner , nuclear magnetic resonance , parametric statistics , acoustics , optics , mathematics , telecommunications , quantum mechanics , statistics , medicine , machine learning , antenna (radio) , erectile dysfunction
The promise of increased signal‐to‐noise ratio and spatial/spectral resolution continues to drive MR technology toward higher magnetic field strengths. SAR management and B 1 inhomogeneity correction become critical issues at the high frequencies associated with high field MR. In recent years, multiple coil excitation techniques have been recognized as potentially powerful tools for controlling specific absorption rate (SAR) while simultaneously compensating for B 1 inhomogeneities. This work explores electrodynamic constraints on transmit homogeneity and SAR, for both fully parallel transmission and its time‐independent special case known as radiofrequency shimming. Ultimate intrinsic SAR—the lowest possible SAR consistent with electrodynamics for a particular excitation profile but independent of transmit coil design—is studied for different field strengths, object sizes, and pulse acceleration factors. The approach to the ultimate intrinsic limit with increasing numbers of finite transmit coils is also studied, and the tradeoff between homogeneity and SAR is explored for various excitation strategies. In the case of fully parallel transmission, ultimate intrinsic SAR shows flattening or slight reduction with increasing field strength, in contradiction to the traditionally cited quadratic dependency, but consistent with established electrodynamic principles. Magn Reson Med 61:315–334, 2009. © 2009 Wiley‐Liss, Inc.