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

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Abstract
Purpose: 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). Methods: Tailored pTx CEST saturation pulses were optimized to achieve a spatially uniform target B 1 + 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. Results: The pTx pulse designs optimized under VOP‐based SAR constraints at 11.7 T achieved comparable performance to 7 T designs despite higher intrinsic B 1 + 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 B 1 + post‐processing correction. Across the cohort, robust gray matter–white matter contrast was consistently observed for multiple CEST contrasts. Conclusion: VOP‐based SAR‐constrained pTx CEST saturation pulse design enabled robust and reproducible contrast at 11.7 T by effectively mitigating B 1 + inhomogeneities while remaining within safety limits, facilitating translation to future in vivo clinical research applications.
Funding Information
  • Agence Nationale de la Recherche (ANR‐19‐CE17‐0008)
  • Agence Nationale de la Recherche (ANR‐21‐ESRE‐0006)

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