TY - JOUR
T1 - B1 destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil
AU - Van de Moortele, Pierre-Francois
AU - Akgun, Can
AU - Adriany, Gregor
AU - Moeller, Steen
AU - Ritter, Johannes
AU - Collins, Christopher M.
AU - Smith, Michael B.
AU - Vaughan, J. T
AU - Ugurbil, Kamil
PY - 2005/12
Y1 - 2005/12
N2 - 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 "dielectric 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 B1 in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B1 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 B1 phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B1 in user-defined areas of the brain.
AB - 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 "dielectric 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 B1 in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B1 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 B1 phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B1 in user-defined areas of the brain.
KW - Human brain
KW - MRI
KW - RF coil array, B field
KW - Ultrahigh magnetic field
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U2 - 10.1002/mrm.20708
DO - 10.1002/mrm.20708
M3 - Article
C2 - 16270333
AN - SCOPUS:28444473098
SN - 0740-3194
VL - 54
SP - 1503
EP - 1518
JO - Magnetic resonance in medicine
JF - Magnetic resonance in medicine
IS - 6
ER -