• Yigitcan Eryaman, Bastien Guerin, Boris Keil, Azma Mareyam, Joaquin L Herraiz, Robert K Kosior, Adrian Martin, Angel Torrado-Carvajal, Norberto Malpica, Juan A Hernandez-Tamames, Emanuele Schiavi, Elfar Adalsteinsson, and Lawrence L Wald.
• Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA; Madrid-MIT M+ Vision Consortium, Madrid, Spain.
• Magn Reson Med. 2015 Apr 1; 73 (4): 1533-9.

PurposeLocal specific absorption rate (SAR) limits many applications of parallel transmit (pTx) in ultra high-field imaging. In this Note, we introduce the use of an array element, which is intentionally inefficient at generating spin excitation (a "dark mode") to attempt a partial cancellation of the electric field from those elements that do generate excitation. We show that adding dipole elements oriented orthogonal to their conventional orientation to a linear array of conventional loop elements can lower the local SAR hotspot in a C-spine array at 7 T.MethodsWe model electromagnetic fields in a head/torso model to calculate SAR and excitation B1 (+) patterns generated by conventional loop arrays and loop arrays with added electric dipole elements. We utilize the dark modes that are generated by the intentional and inefficient orientation of dipole elements in order to reduce peak 10g local SAR while maintaining excitation fidelity.ResultsFor B1 (+) shimming in the spine, the addition of dipole elements did not significantly alter the B1 (+) spatial pattern but reduced local SAR by 36%.ConclusionThe dipole elements provide a sufficiently complimentary B1 (+) and electric field pattern to the loop array that can be exploited by the radiofrequency shimming algorithm to reduce local SAR.© 2014 Wiley Periodicals, Inc.

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