Multi-channel helical-antenna inner-volume RF coils for ultra-high field MR scanners

RF coil design for human ultra-high field (7 T and higher) magnetic resonance (MR) imaging is an area of intense development, to overcome difficult challenges such as RF excitation spatial heterogeneity and low RF transfer efficiency into the spin system. This article proposes a novel category of multi-channel RF volume coil structures at both 7 T and 10.5 T based on a subject-loaded multifilar helical-antenna RF coil that aims at addressing these problems. In some prior applications of helix antennas as MR RF coils at 7 T, the imaged sample was positioned outside the helix. Here, we introduce a radically different approach, with the inner volume of a helix antenna being utilized to image a sample. The new coil uniquely combines traveling-wave behavior through the overall antenna wire structure and near-field RF interaction between the conducting elements and the imaged tissues. It thus benefits from the congruence of far- and near-field regimes. Design and analysis of the novel inner-volume coils are performed by numerical simulations using multiple computational electromagnetics techniques. The fabricated coil prototypes are tested, validated, and evaluated experimentally in 7-T and 10.5-T MR human wide bore (90-cm) MR scanners. Phantom data at 7 T show good consistency between numerical simulations and experimental results. Simulated B₁⁺ transmit efficiencies, in T/√W, are comparable to those of some of the conventional and state-of-the-art RF coil designs at 7 T. Experimental results at 10.5 T show the scalability of the helix coil design.