A Flexible Graphite-Silicone Pad for Improved Fat Suppression in Cervical Spine MRI at 5 T via Passive B₀ Shimming

Susceptibility-induced $B_0$ inhomogeneity at the posterior neck degrades spectral fat suppression in cervical spine MRI at 5 T and above. We describe a flexible graphite-silicone pad for passive $B_0$ shimming and assess its effect on field homogeneity and fat-suppressed imaging alongside the scanner's active shim.

Pads were fabricated by dispersing high-purity graphite powder in a biocompatible silicone elastomer and casting the mixture in a 3D-printed mold shaped to the posterior neck and head-neck receive coil. Five graphite-to-silicone mass ratios (1.5:100 through 8:100) were screened in one volunteer on a 5 T scanner using dual-echo $B_0$ mapping and fat-suppressed $T_1$ - and $T_2$ -weighted FSE. The selected ratio (4.5:100) was evaluated in five healthy volunteers under volume-based active shimming, with and without the pad in randomized order.

In the screening volunteer under Volume shimming, the 4.5:100 pad reduced the posterior-neck field $\sigma$ from 0.740 to 0.458 ppm, a 38% reduction at matched shim mode. Across the five validation subjects, $\sigma$ decreased in every case under Volume shimming, from $0.911\pm 0.117$ to $0.455\pm 0.054$ ppm (individual reductions 41%-55%). Residual hyperintense fat seen with the standard pad was no longer present on sagittal $T_1$ - or $T_2$ -weighted images, and the improvement extended to the supraclavicular region on coronal $T_1$ -weighted images.

A flexible graphite-silicone pad complements active shimming and improves spectral fat suppression in cervical spine MRI at 5 T. Evaluation in patients and on EPI-based sequences remains to be done.