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Specific absorption rate studies of the parallel transmission of inner‐volume excitations at 7T
Author(s) -
Zelinski Adam C.,
Angelone Leonardo M.,
Goyal Vivek K.,
Bonmassar Giorgio,
Adalsteinsson Elfar,
Wald Lawrence L.
Publication year - 2008
Publication title -
journal of magnetic resonance imaging
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.563
H-Index - 160
eISSN - 1522-2586
pISSN - 1053-1807
DOI - 10.1002/jmri.21548
Subject(s) - specific absorption rate , acceleration , excitation , finite difference time domain method , physics , absorption (acoustics) , root mean square , transmission (telecommunications) , computational physics , parallel communication , position (finance) , trajectory , function (biology) , optics , nuclear magnetic resonance , computer science , telecommunications , antenna (radio) , economics , biology , finance , classical mechanics , quantum mechanics , astronomy , evolutionary biology
Purpose To investigate the behavior of whole‐head and local specific absorption rate (SAR) as a function of trajectory acceleration factor and target excitation pattern due to the parallel transmission (pTX) of spatially tailored excitations at 7T. Materials and Methods Finite‐difference time domain (FDTD) simulations in a multitissue head model were used to obtain B 1 + and electric field maps of an eight‐channel transmit head array. Local and average SAR produced by 2D‐spiral‐trajectory excitations were examined as a function of trajectory acceleration factor, R , and a variety of target excitation parameters when pTX pulses are designed for constant root‐mean‐square excitation pattern error. Results Mean and local SAR grow quadratically with flip angle and more than quadratically with R , but the ratio of local to mean SAR is not monotonic with R . SAR varies greatly with target position, exhibiting different behaviors as a function of target shape and size for small and large R . For example, exciting large regions produces less SAR than exciting small ones for R ≥4, but the opposite trend occurs when R <4. Furthermore, smoother and symmetric patterns produce lower SAR. Conclusion Mean and local SAR vary by orders of magnitude depending on acceleration factor and excitation pattern, often exhibiting complex, nonintuitive behavior. To ensure safety compliance, it seems that model‐based validation of individual target patterns and corresponding pTX pulses is necessary. J. Magn. Reson. Imaging 2008;28:1005–1018. © 2008 Wiley‐Liss, Inc.