Specific absorption rate studies of the parallel transmission of inner-volume excitations at 7T.
OBJECTIVE
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.


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 >or=4, but the opposite trend occurs when R <4. Furthermore, smoother and symmetric patterns produce lower SAR.


CONCLUSIONS
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.
Zelinski
Adam C
AC
Massachusetts Institute of Technology, Research Laboratory of Electronics, Cambridge, Massachusetts, USA. zelinski@MIT.edu
Angelone
Leonardo M
LM
Goyal
Vivek K
VK
Bonmassar
Giorgio
G
Adalsteinsson
Elfar
E
Wald
Lawrence L
LL
D000042:Absorption; D000465:Algorithms; D016477:Artifacts; D004574:Electromagnetic Fields; D006257:Head; D006801:Humans; D007089:Image Enhancement / Q000295:instrumentation* / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods; D008962:Models, Theoretical; D019047:Phantoms, Imaging