Toward Cell-Specific Molecular MRI: Modulating Chemical Exchange Saturation Transfer Contrast With Manganese

Chemical exchange saturation transfer (CEST) MRI enables sensitive detection of low-concentration metabolites in vivo, but its biological interpretation is limited by the composite nature of the signal, which reflects contributions from multiple cell types. In the brain, these include intracellular metabolites in neurons and glial cells, as well as semisolid macromolecular pools. Here, we introduce MangaCEST, a strategy that uses manganese (Mn^{2 +})-induced intracellular relaxation changes to modulate the contributions of different cell populations to the CEST spectrum. Mn^{2 +} enters the cytoplasm via calcium-permeable channels and accumulates in excitable cells in an activity-dependent manner, shortening water T₁ and selectively reducing CEST contrast. Comparison of CEST Z-spectra with or without Mn^{2 +} enables MangaCEST to isolate Mn-sensitive signal components and differentially analyze intracellular contributions. To provide a cell-type-weighted Mn^{2 +} effect in a well-defined biological framework, we focused on the cerebellum, where preferential neuronal accumulation of Mn^{2 +} is well documented. In the developing chick embryo in vivo, Mn^{2 +} caused significant T₁ shortening (∼28%) and frequency-dependent CEST spectrum changes. Differential spectra showed attenuation at 3-4 ppm, consistent with contributions from neuronal-associated metabolite pools. These findings suggest that MangaCEST may represent a promising strategy for disentangling cellular contributions to molecular MRI signals in the brain.