Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements

Greg Metzger, Carl Snyder, Can Akgun, J. T Vaughan, Kamil Ugurbil, Pierre-Francois Van de Moortele

Research output: Contribution to journalArticlepeer-review

202 Scopus citations

Abstract

High-quality prostate images were obtained with transceiver arrays at 7T after performing subject-dependent local transmit B1 (B 1+) shimming to minimize B1+ losses resulting from destructive interferences. B1+ shimming was performed by altering the input phase of individual RF channels based on relative B1+ phase maps rapidly obtained in vivo for each channel of an eight-element stripline coil. The relative transmit phases needed to maximize B1+ coherence within a limited region around the prostate greatly differed from those dictated by coil geometry and were highly subject-dependent. A set of transmit phases determined by B 1+ shimming provided a gain in transmit efficiency of 4.2 ± 2.7 in the prostate when compared to the standard transmit phases determined by coil geometry. This increased efficiency resulted in large reductions in required RF power for a given flip angle in the prostate which, when accounted for in modeling studies, resulted in significant reductions of local specific absorption rates. Additionally, B1+ shimming decreased B1+ nonuniformity within the prostate from (24 ± 9%) to (5 ± 4%). This study demonstrates the tremendous impact of fast local B1+ phase shimming on ultrahigh magnetic field body imaging.

Original languageEnglish (US)
Pages (from-to)396-409
Number of pages14
JournalMagnetic resonance in medicine
Volume59
Issue number2
DOIs
StatePublished - Feb 2008

Keywords

  • 7T
  • B shimming
  • FDTD model
  • Prostate
  • SAR
  • Stripline array
  • Transceive array
  • Transmit array

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