Neurite Exchange Imaging (NEXI): A minimal model of diffusion in gray matter with inter-compartment water exchange

Abstract

Biophysical models of diffusion in white matter are based on what is now commonly referred to as the "Standard Model" (SM) of non-exchanging anisotropic Gaussian compartments. In this work, we focus on diffusion MRI in gray matter, which requires rethinking basic microstructure modeling blocks. In particular, at least three contributions beyond the SM need to be considered: water exchange across the cell membrane - between neurites and the extracellular space; non-Gaussian diffusion along neuronal and glial processes - resulting from structural disorder; and signal contribution from soma. For the first contribution, we propose Neurite Exchange Imaging (NEXI) as an extension of the SM of diffusion, which builds on the anisotropic Kärger model of two exchanging compartments. Using datasets acquired at multiple diffusion weightings (b) and diffusion times (t) in the rat brain in vivo, we show that for the investigated diffusion time window (~10-45 ms) there is minimal diffusivity time-dependence and more pronounced kurtosis decay with time in gray matter, which is well fit by the exchange model. Moreover, we observe lower signal for longer diffusion times at high b. In light of these observations, we identify exchange as the mechanism that best explains these signal signatures in both low-b and high-b regime, and thereby propose NEXI as the minimal model for gray matter microstructure mapping. We finally highlight multi-b multi-t acquisitions protocols as being best suited to estimate NEXI model parameters reliably. Using this approach, we estimate the inter-compartment water exchange time to be 15 - 60 ms in the rat cortex and hippocampus in vivo, which is of the same order or shorter than the diffusion time in typical diffusion MRI acquisitions. This suggests water exchange as an essential component for interpreting diffusion MRI measurements in gray matter.

References in corpus (3)

• Diffusion time dependence, power-law scaling, and exchange in gray matter
• The Kärger vs bi-exponential model: theoretical insights and experimental validations
• Parameter estimation for WMTI-Watson model of white matter using encoder-decoder recurrent neural network

Cited by in corpus (8)

• Diffusion time dependence, power-law scaling, and exchange in gray matter
• Quantifying human gray matter microstructure using NEXI and 300 mT/m gradients
• Cellular EXchange Imaging (CEXI): Evaluation of a diffusion model including water exchange in cells using numerical phantoms of permeable spheres
• Parameter estimation for WMTI-Watson model of white matter using encoder-decoder recurrent neural network
• Restriction-induced time-dependent transcytolemmal water exchange: Revisiting the Kärger exchange model
• Robust, fast and accurate mapping of diffusional mean kurtosis
• Comprehensive characterization of tumor therapeutic response with simultaneous mapping cell size, density, and transcytolemmal water exchange
• Beyond directions: Symmetry-aware rotation sets for triaxial diffusion encoding by geometric filter optimization