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

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 (er) of HPM slurry increased from110 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 (e(r) approximate to 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-brainMRI SNR homogeneity and quality at ultra-high field. Greater SNR improvement is anticipated using the less lossy HPM and optimal design.