Large improvement in RF magnetic fields and imaging SNR with whole-head high-permittivity slurry helmet for human-brain MRI applications at 7 T

Abstract

Purpose: To optimize the design and demonstrate the integration of a helmet-shaped container filled with a high-permittivity material (HPM) slurry with RF head coil arrays to improve RF coil sensitivity and SNR for human-brain proton MRI.

Methods: RF reception magnetic fields ( $B_1^{-}$ ) of a 32-channel receive-only coil array with various geometries and permittivity values of HPM slurry helmet are calculated with electromagnetic simulation at 7 T. A 16-channel transmit-only coil array, a 32-channel receive-only coil array, and a 2-piece HPM slurry helmet were constructed and assembled. RF transmission magnetic field ( $B_1^{+}$ ), $B_1^{-}$ , and MRI SNR maps from the entire human brain were measured and compared.

Results: Simulations showed that averaged $B_1^{-}$ improvement with the HPM slurry helmet increased from 57% to 87% as the relative permittivity (ε_r) of HPM slurry increased from 110 to 210. In vivo experiments showed that the average $B_1^{+}$ improvement over the human brain was 14.5% with the two-piece HPM slurry (ε_r ≈ 170) helmet, and the average $B_1^{-}$ and SNR were improved 63% and 34%, respectively, because the MRI noise level was increased by the lossy HPM.

Conclusion: The RF coil sensitivity and MRI SNR were largely improved with the two-piece HPM slurry helmet demonstrated by both electromagnetic simulations and in vivo human head experiments at 7 T. The findings demonstrate that incorporating an easily producible HPM slurry helmet into the RF coil array significantly enhances human-brain MRI SNR homogeneity and quality at ultrahigh field. Greater SNR improvement is anticipated using the less lossy HPM and optimal design.

Keywords: High‐permittivity slurry helmet; RF coil sensitivity; RF head array coil; human‐brain MRI; signal‐to‐noise; ultrahigh‐field MRI.