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Effects of receive‐only inserts on specific absorption rate, B 1 + field, and Tx coil performance
Author(s) -
Krishnamurthy Narayanan,
Zhao Tiejun,
Ibrahim Tamer S.
Publication year - 2014
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.24152
Subject(s) - specific absorption rate , finite difference time domain method , electromagnetic coil , absorption (acoustics) , radiofrequency coil , coupling (piping) , head (geology) , materials science , nuclear magnetic resonance , optics , excitation , physics , waveform , acoustics , computational physics , computer science , telecommunications , composite material , quantum mechanics , geomorphology , voltage , antenna (radio) , geology
Purpose To evaluate the effect of different cylindrical and close‐conforming receive‐only array designs on spin excitation and specific absorption rate (SAR) of a 7 T transmit‐only head coil. Materials and Methods We developed finite difference time domain (FDTD) models of different receive‐only array geometries. Cylindrical and close‐fitting helmet arrays with varying copper trace widths, a TEM Tx coil model, and two head models were used in numerical simulations. Tx coil coupling was experimentally measured and validated with FDTD modeling. Results Changing copper trace width of loops in array models caused subtle changes in radiofrequency (RF) absorption (<5%). Changes in SAR distribution were observed in the head models with Rx‐only inserts. Peak SAR increased (−1 to +15%) in different tissues for a mean B 1 + in the brain of 2 μT. Total absorption in the head models for 1 W forward power increased (5%–21%) in the heads with Rx‐only inserts. Changes in RF absorption with different Rx inserts indicate a change in RF radiation of the Tx coil even when changes in B 1 + and coupling between ports of Tx coil were minimal. Conclusion Changes in local/global SAR and subtle changes in B 1 + field distributions were observed in the presence of Rx‐only inserts. Thus, incorporation of the receive‐only array effects are needed when evaluating SAR and designing RF transmit pulse waveform parameters for shimming and/or Tx‐SENSE for 7 T MRI. J. Magn. Reson. Imaging 2014;39:475–484 . © 2013 Wiley Periodicals, Inc .