Multi-Contrast Human Brain CEST MRI at 11.7 T: First In Vivo Demonstration

To implement and evaluate chemical exchange saturation transfer (CEST) at ultra-high field on a whole-body 11.7 T scanner using parallel transmission (pTx).

Tailored pTx CEST saturation pulses were optimized to achieve a spatially uniform target B₁ ⁺ while satisfying SAR and power constraints either defined by conservatively defined radio-frequency (RF) power limits or by a virtual observation point (VOP)-based framework. The resulting pulse performance at 11.7 T was assessed through numerical simulations and compared against corresponding designs at 7 T. The proposed saturation pulses were integrated into a two-dimensional multi-slice EPI sequence. Experimental testing of the sequence was performed in vitro using metabolite-doped phantoms and in vivo in four healthy volunteers.

The pTx pulse designs optimized under VOP-based SAR constraints at 11.7 T achieved comparable performance to 7 T designs despite higher intrinsic B₁ ⁺ inhomogeneity. In vitro experiments validated the sequence implementation and confirmed detection of the targeted metabolites. In vivo experiments demonstrated acceptable saturation homogeneity (nRMSE ≤ 12.1%) in a slab of thickness 1 cm, with only minimal additional improvement achieved through B₁ ⁺ post-processing correction. Across the cohort, robust gray matter-white matter contrast was consistently observed for multiple CEST contrasts.

VOP-based SAR-constrained pTx CEST saturation pulse design enabled robust and reproducible contrast at 11.7 T by effectively mitigating B₁ ⁺ inhomogeneities while remaining within safety limits, facilitating translation to future in vivo clinical research applications.