Magnetic resonance (MR) scanners operating at ultra-high magnetic field (UHF) (7T and above) provide substantial gains in signal-to-noise ratio and spatial resolution compared with 1.5–3T clinical scanners ¹. However, a fundamental issue at UHF, with increasing Larmor frequency (300 MHz at 7T), comes from the reduced RF wavelengths, down to the order of or smaller than the size of the imaged samples, and the main challenges include RF excitation spatial heterogeneity and low RF efficiency. RF coil design for human UHF scanners is an area of intense development, especially regarding the most challenging targets such as torso or body imaging. One example is twisting a birdcage coil toward a spiral shape ², for head RF excitation at 4T, thus with RF interactions still dominated by a near-field or quasi-static regime. Another example are helical antennas as traveling-wave sources at 7T with the imaged sample being positioned outside the helix ³.We use a novel category of multi-channel RF coil structures with volume coverage for UHF MR imaging (MRI) based on helix conducting elements, the helix coil, namely, subject-loaded multifilar helical-antenna RF volume coil, with its inner volume being utilized to image a sample, first proposed in ^4,5. The coil has been dramatically advanced since; the size is reduced by more than threefold, the power efficiency is very significantly increased, 8-channel coils were built (Fig. 1), designs were extended to 10.5T, and MRI experiments on prototypes at both 7T and 10.5T at the Center for Magnetic Resonance Research (CMRR), University of Minnesota, were performed (Fig. 2). Simulations are done using the higher-order method of moments ⁶. The helix coil allows for uniquely combining traveling-wave behavior through the overall coil wire structure while preserving near-field RF interaction between the inner side of the conducting elements and the imaged tissues. Single channel — or monofilar — helical-antenna coil is the simplest design ⁷. Taking advantage of the multi-channel RF technology available at CMRR at 7T and 10.5T, we have developed quadrifilar (4-channel) and octafilar (8-channel) helical-antenna volume coils, as shown in Figs. 1 and 2. The goal is to enable multi-channel RF methods (e.g., B₁ shimming) to further mitigate B₁⁺ field heterogeneity ⁸.Fig. 3 shows measurements at the CMRR and simulations at 7T of a 4-channel helical-antenna RF coil. Shown in Fig. 4 are CMRR-measured results for the 8-channel coil at 7T. Fig. 5 shows the results from CMRR experiments with the 4-channel helical-antenna RF coil prototype at 10.5T (note that 10.5T experiments are done without proper phasing of the four excitation ports of the coil, with which circular polarization would be further enhanced).We observe from Fig. 3 an excellent agreement between the measurements and simulations. Good circular polarization and B₁⁺ field strength and uniformity are observed, as well as a diverse interleaved field/phase pattern due to the four helices (four channels), and thus capability for parallel imaging. The power efficiency is good and the amount of power delivered to the imaged phantom is sufficiently high for all experiments and MRI processing. Data are obtained for absolute B₁ maps, g-factor, and GRAPPA X3 acceleration. Similar observations are made, from Fig. 4, for the 8-channel helical-antenna 7T coil, with a diverse interleaved pattern corresponding to eight channels and the resulting capability for parallel imaging and acceleration. Experimental and simulation results at 10.5T have demonstrated the scalability and versatility of the coil design. From Fig. 5, we observe that the received signal from the multiple channels presents an interleaved pattern of dominant channels, which indicates spatial encoding capability and is desirable for parallel imaging.The main goal of developing a subject-loaded multi-channel helical-antenna coil is to, when loaded with a subject, provide improved RF performance for UHF MRI while preserving the easiness of use of a volume coverage coil. This design benefits from the congruence of far- and near-field regimes. Multiple channels are utilized (4 and 8 in the prototype configuration) to enable all multi-transmit channel RF technology, which is expected to expand the capability to mitigate B₁⁺ heterogeneity, while also providing parallel imaging capability. The presented phantom data obtained at 7T show excellent consistency between numerical simulations and experimental results with 4- and 8-channel helix coils. The 10.5T machine at CMRR used for this work is the first (and only, as of today) operational human-size MR scanner reaching 10.5T. This work may also help decide which coil structure and parameters should be used as a starting point to maximize the chances for successful future studies at 10.5T.