Quantitative activity-induced manganese-dependent MRI for characterizing cortical layers in the primary somatosensory cortex of the rat

The ability of Mn²⁺ to follow Ca²⁺ pathways upon stimulation transform them into remarkable surrogate markers of neuronal activity using activity-induced manganese-dependent MRI (AIM–MRI). In the present study, a precise follow-up of physiological parameters during MnCl₂ and mannitol infusions improved the reproducibility of AIM–MRI allowing in-depth evaluation of the technique. Pixel-by-pixel T₁ data were investigated using histogram distributions in the barrel cortex (BC) and the thalamus before and after Mn²⁺ infusion, after blood brain barrier opening and after BC activation. Mean BC T₁ values dropped significantly upon trigeminal nerve (TGN) stimulation (?38 %, P = 0.02) in accordance with previous literature findings. T₁ histogram distributions showed that 34 % of T_1s in the range 600–1500 ms after Mn²⁺ + mannitol infusions shifted to 50–350 ms after TGN stimulation corresponding to a twofold increase of the percentage of pixels with the lowest T_1s in BC. Moreover, T₁ changes in response to stimulation increased significantly from superficial cortical layers (I–III) to deeper layers (V–VI). Cortical cytoarchitecture detection during a functional paradigm was performed extending the potential of AIM–MRI. Quantitative AIM–MRI could thus offer a means to interpret local neural activity across cortical layers while identification of the role of calcium dynamics in vivo during brain activation could play a key role in resolving neurovascular coupling mechanisms.