<i>B</i>₁ destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil

TLDR

At ultrahigh magnetic fields, RF behavior in the human head becomes complex, with volume coils producing bright central and weak peripheral signals that are often called dielectric resonances but are actually due to damped traveling waves in lossy brain tissue, raising questions about the terminology. The study experimentally investigates this issue using a transceiver coil array operated in volume transmit, multi‑receiver, or single‑transmit surface coil modes. The experiments and simulations show that destructive interferences produce weak peripheral B1 without a standing wave pattern, that receive and transmit B1 phases are similar across elements (though rotated oppositely), and that mapping spatial B1 phase could enable in‑session RF shimming to achieve more homogeneous B1 in targeted brain regions.

Abstract

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 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.