Article

Metzger GJ, Snyder C, Akgun C, et al. Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements

Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, 2021 6th Street SE, Minneapolis, MN 55455, USA.
Magnetic Resonance in Medicine (Impact Factor: 3.4). 02/2008; 59(2):396-409. DOI: 10.1002/mrm.21476
Source: PubMed

ABSTRACT High-quality prostate images were obtained with transceiver arrays at 7T after performing subject-dependent local transmit B(1) (B(1) (+)) shimming to minimize B(1) (+) losses resulting from destructive interferences. B(1) (+) shimming was performed by altering the input phase of individual RF channels based on relative B(1) (+) phase maps rapidly obtained in vivo for each channel of an eight-element stripline coil. The relative transmit phases needed to maximize B(1) (+) 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, B(1) (+) shimming decreased B(1) (+) nonuniformity within the prostate from (24 +/- 9%) to (5 +/- 4%). This study demonstrates the tremendous impact of fast local B(1) (+) phase shimming on ultrahigh magnetic field body imaging.

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    • "It is possible that the change in phase of the RF field resulting from head motion may also affect the phase of the EPI signal. Although this dynamic RF phase change was not obvious in the data acquired here and to our knowledge has not been reported in the literature, it may become more significant at higher field strengths and could be investigated using B1 phase maps (e.g., [Metzger et al., 2008]). It is also important to note that for this study, phase and magnitude images were reconstructed from the raw data using a customized image reconstruction method. "
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    • "Possible local overheating due to the latter poses a serious safety concern at UHF. Consequently, when using a multiple element transmit coil which has been recognized as a powerful tool for inhomogeneity compensation and has been widely utilized at UHF [4]–[6], energy deposition in the body must be carefully controlled and kept under international safety guidelines . Although attempts to address this overheating concern have been made by constraining shimming or parallel transmission RF pulse design solutions with worst-case conditions derived from electromagnetic (EM) simulations [7]–[9], in some situations this may excessively limit the achievable SNR and contrast at UHF. "
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    • "However, increasing the number of RF coils invariably increases the electromagnetic coil coupling interactions, causing complex spatiotemporal RF field behavior and problems with the reconstruction process [14]. In addition, the number of coil channels increases , which further augments the complexity of the system and escalates fabrication costs. "
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