Journal Article

Measuring non-parametric distributions of intravoxel mean diffusivities using a clinical MRI scanner

Basser, Peter J., Avram, Alexandru V., Sarlls, Joelle E.

2019

Overview

We measure spectra of water mobilities (i.e., mean diffusivities) from intravoxel pools in brain tissues of healthy subjects with a non-parametric approach. Using a single-shot isotropic diffusion encoding (IDE) preparation, we eliminate signal confounds caused by anisotropic diffusion, including microscopic anisotropy, and acquire in vivo diffusion-weighted images (DWIs) over a wide range of diffusion sensitizations. We analyze the measured IDE signal decays using a regularized inverse laplace transform (ILT) to derive a probability distribution of mean diffusivities of tissue water in each voxel. Based on numerical simulations we assess the sensitivity and accuracy of our ILT analysis and optimize an experimental protocol for use with clinical MRI scanners. In vivo spectra of intravoxel mean diffusivities measured in healthy subjects generally show single-peak distributions throughout the brain parenchyma, with small differences in peak location and shape among white matter, cortical and subcortical gray matter, and cerebrospinal fluid. Mean diffusivity distributions (MDDs) with multiple peaks are observed primarily in voxels at tissue interfaces and are likely due to partial volume contributions. To quantify tissue-specific MDDs with improved statistical power, we average voxel-wise normalized MDDs in corresponding regions-of-interest (ROIs). This non-parametric, rotation-invariant assessment of isotropic diffusivities of tissue water may reflect important microstructural information, such as cell packing and cell size, and active physiological processes, such as water transport and exchange, which may enhance biological specificity in the clinical diagnosis and characterization of ischemic stroke, cancer, neuroinflammation, and neurodegenerative disorders and diseases. •We optimize a clinical protocol to measure isotropic diffusion encoded brain images.•We derive rotation-invariant distributions of intravoxel mean diffusivities in vivo.•We assess the uncertainty of measuring intravoxel mean diffusivity distributions.•Mean diffusivities distributions show mostly single peaks in healthy brain voxels.

• Publisher: Elsevier Inc,Elsevier Limited
• Subject: Algorithms / Anisotropy / Brain Mapping - methods / Brain research / Cancer / Cell size / Cerebrospinal fluid / Cortex / Design / Diffusion / Diffusion Magnetic Resonance Imaging - methods / Humans / Image Interpretation, Computer-Assisted - methods / Inflammation / Interfaces / Ischemia / Isotropic diffusion encoding (IDE) / Isotropic diffusivity / Magnetic resonance imaging / Mean apparent diffusion coefficient (mADC) / Mean diffusivity (MD) / Mean diffusivity distribution (MDD) / Models, Neurological / Monte Carlo Method / MR diffusometry / MR relaxometry / Neurodegenerative diseases / Neurological disorders / Nonparametric statistics / Parenchyma / Physiology / Rotation-invariant / Scanners / Substantia alba / Substantia grisea / Tissue microstructure