title: Neurite density imaging versus imaging of microscopic anisotropy in diffusion MRI: A model comparison using spherical tensor encoding.
abstract: In diffusion MRI (dMRI), microscopic diffusion anisotropy can be obscured by orientation dispersion. Separation of these properties is of high importance, since it could allow dMRI to non-invasively probe elongated structures such as neurites (axons and dendrites). However, conventional dMRI, based on single diffusion encoding (SDE), entangles microscopic anisotropy and orientation dispersion with intra-voxel variance in isotropic diffusivity. SDE-based methods for estimating microscopic anisotropy, such as the neurite orientation dispersion and density imaging (NODDI) method, must thus rely on model assumptions to disentangle these features. An alternative approach is to directly quantify microscopic anisotropy by the use of variable shape of the b-tensor. Along those lines, we here present the 'constrained diffusional variance decomposition' (CODIVIDE) method, which jointly analyzes data acquired with diffusion encoding applied in a single direction at a time (linear tensor encoding, LTE) and in all directions (spherical tensor encoding, STE). We then contrast the two approaches by comparing neurite density estimated using NODDI with microscopic anisotropy estimated using CODIVIDE. Data were acquired in healthy volunteers and in glioma patients. NODDI and CODIVIDE differed the most in gray matter and in gliomas, where NODDI detected a neurite fraction higher than expected from the level of microscopic diffusion anisotropy found with CODIVIDE. The discrepancies could be explained by the NODDI tortuosity assumption, which enforces a connection between the neurite density and the mean diffusivity of tissue. Our results suggest that this assumption is invalid, which leads to a NODDI neurite density that is inconsistent between LTE and STE data. Using simulations, we demonstrate that the NODDI assumptions result in parameter bias that precludes the use of NODDI to map neurite density. With CODIVIDE, we found high levels of microscopic anisotropy in white matter, intermediate levels in structures such as the thalamus and the putamen, and low levels in the cortex and in gliomas. We conclude that accurate mapping of microscopic anisotropy requires data acquired with variable shape of the b-tensor.
authors.affiliation: Clinical Sciences Lund, Medical Radiation Physics, Lund University, Lund, Sweden. Electronic address: bjorn.lampinen@med.lu.se.
authors.forename: Björn
authors.initials: B
authors.lastname: Lampinen
authors.affiliation: Clinical Sciences Lund, Medical Radiation Physics, Lund University, Lund, Sweden.
authors.forename: Filip
authors.initials: F
authors.lastname: Szczepankiewicz
authors.affiliation: Department of Psychology, Faculty of Social Science, Lund University, Lund, Sweden.
authors.forename: Johan
authors.initials: J
authors.lastname: Mårtensson
authors.affiliation: Clinical Sciences Lund, Radiology, Lund University, Lund, Sweden.
authors.forename: Danielle
authors.initials: D
authors.lastname: van Westen
authors.affiliation: Clinical Sciences Lund, Radiology, Lund University, Lund, Sweden.
authors.forename: Pia C
authors.initials: PC
authors.lastname: Sundgren
authors.affiliation: Clinical Sciences Lund, Radiology, Lund University, Lund, Sweden.
authors.forename: Markus
authors.initials: M
authors.lastname: Nilsson
mesh_terms: D000328:Adult; D016880:Anisotropy; D001921:Brain / Q000000981:diagnostic imaging*; D001932:Brain Neoplasms / Q000000981:diagnostic imaging*; D002540:Cerebral Cortex / Q000000981:diagnostic imaging; D003198:Computer Simulation; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D056324:Diffusion Tensor Imaging / Q000379:methods*; D005910:Glioma / Q000000981:diagnostic imaging*; D066128:Gray Matter / Q000000981:diagnostic imaging; D006801:Humans; D008962:Models, Theoretical; D016501:Neurites; D011699:Putamen / Q000000981:diagnostic imaging; D013788:Thalamus / Q000000981:diagnostic imaging; D066127:White Matter / Q000000981:diagnostic imaging
keywords: Diffusion MRI; Microscopic anisotropy; Spherical tensor encoding