Biophysical models of diffusion in gray matter (GM) can provide unique information about microstructure of the human brain, in health and disease. Therefore, their compatibility with clinical settings is key. Neurite Exchange Imaging (NEXI) is a two-compartment model of GM microstructure that accounts for inter-compartment exchange, whose parameter estimation requires multi-shell multi-diffusion time data. In this work, we report the first estimates of NEXI in human cortex obtained on a clinical MRI scanner. To do that, we establish an acquisition protocol and fitting routine compatible with clinical scanners. The model signal equation can be expressed either in the narrow-pulse approximation, NEXI, or accounting for the actual width of the diffusion gradient pulses, SMEX. While NEXI enables a faster analytical fit and is a valid approximation for data acquired on high-performance gradient systems (preclinical and Connectom scanners), on which NEXI was first implemented, SMEX has significant relevance for data acquired on clinical scanners with longer gradient pulses. We establish that, in the context of broad pulses, SMEX estimates were more comparable to previous literature values. Furthermore, we evaluate the repeatability of NEXI estimates in the human cortex on a clinical MRI scanner and show intra-subject variability to be lower than inter-subject variability, which is promising for characterizing healthy and patient cohorts. Finally, we analyze the relationship of NEXI parameters on the cortical surface to the Myelin Water Fraction (MWF), estimated using an established multicomponent T 2 relaxation technique. Indeed, although it is present in small quantities in the cortex, myelin can be expected to decrease permeability. We confirm a strong correlation between the exchange time (t ex) estimates and the MWF, although the spatial correspondence between the two is brain-region specific and other drivers of t ex than myelin density are likely at play.

Neural tissue microstructure is dynamic during brain activity, presenting changes in cellular morphology and membrane permeability. The sensitivity of diffusion MRI (dMRI) to restrictions and hindrances in the form of cell membranes or subcellular structures enables the exploration of brain activity under a new paradigm, offering a more direct functional contrast than its blood-oxygenation-level-dependent (BOLD) counterpart. The current work aims at probing Mean Diffusivity (MD) and Mean Kurtosis (MK) changes and their time-dependence signature across various regions in the rat brain during somatosensory processing and integration, upon unilateral forepaw stimulation. We report a decrease in MD in the contralateral primary somatosensory cortex, forelimb region (S1FL), previously ascribed to cellular swelling and increased tortuosity in the extracellular space, paralleled by a positive BOLD response. For the first time, we also report a paired decrease in MK during stimulation in S1FL, suggesting increased membrane permeability. This observation was further supported by the reduction in exchange time estimated from the kurtosis time-dependence analyses. Conversely, the secondary somatosensory cortex and subcortical areas, formerly reported as responsive to sensory stimulation in rodents (thalamus, striatum, hippocampal subfields), displayed a marked MD and MK increase, paralleled by a weak-to-absent BOLD response. Overall, MD and MK uncovered functional-induced changes with higher sensitivity than BOLD. Although the exact origin of the MD and MK increase is yet to be unraveled, the potential of dMRI to provide complementary functional insights, even below the BOLD detection threshold, has been showcased.

Biophysical models of diffusion tailored to quantify gray matter microstructure are gathering increasing interest. The two-compartment Neurite EXchange Imaging (NEXI) model has been proposed recently to account for neurites, extra-cellular space, and exchange across the cell membrane. NEXI parameter estimation requires multi-shell multi-diffusion time data and has so far only been implemented experimentally on animal data collected on a preclinical magnetic resonance imaging (MRI) set-up. In this work, the translation of NEXI to the human cortex in vivo was achieved using a 3 T Connectom MRI system with 300 mT/m gradients, that enables the acquisition of a broad range of b-values (0 – 7.5 ms/µm²) with a window covering short to intermediate diffusion times (20 – 49 ms) suitable for the characteristic exchange times (10 – 50 ms). Microstructure estimates of four model variants: NEXI, NEXI dot (its extension with the addition of a dot compartment), and their respective versions that correct for the Rician noise floor (NEXI RM and NEXI dot,RM ) that particularly impacts high b-value signal, were compared. The reliability of estimates in each model variant was evaluated in synthetic and human in vivo data. In the latter, the intra-subject (scan-rescan) versus between-subjects variability of microstructure estimates was compared in the cortex. The better performance of NEXI RM highlights the importance of correcting for Rician bias in the NEXI model to obtain accurate estimates of microstructure parameters in the human cortex, and the sensitivity of the NEXI framework to individual differences in cortical microstructure. This application of NEXI in humans represents a significant step, unlocking new avenues for studying neurodevelopment, aging, and various neurodegenerative disorders.