期刊
MAGNETIC RESONANCE IN MEDICINE
卷 54, 期 6, 页码 1503-1518出版社
WILEY
DOI: 10.1002/mrm.20708
关键词
MRI; ultrahigh magnetic field; RF coil array; B-1 field; human brain
资金
- NCRR NIH HHS [RR08079] Funding Source: Medline
- NIBIB NIH HHS [R01 EB000331, R01 EB000454] Funding Source: Medline
- NIMH NIH HHS [R01 MH55346] Funding Source: Medline
RF behavior in the human head becomes complex at ultrahigh magnetic fields. A bright center and a weak periphery are observed in images obtained with volume coils, while surface coils provide strong signal in the periphery. Intensity patterns reported with volume coils are often loosely referred to as di-electric resonances, while modeling studies ascribe them to superposition of traveling waves greatly dampened in lossy brain tissues, raising questions regarding the usage of this term. Here we address this question experimentally, taking full advantage of a transceiver coil array that was used in volume transmit mode, multiple receiver mode, or single transmit surface coil mode. We demonstrate with an appropriately conductive sphere phantom that destructive interferences are responsible for a weak B-1 in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B-1 proved remarkably similar for the different coil elements, although with opposite rotational direction. Additional simulation data closely matched our phantom results. In the human brain the phase patterns were more complex but still exhibited similarities between coil elements. Our results suggest that measuring spatial B-1 phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B-1 in user-defined areas of the brain.
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