A Method for In-Vivo Mapping of Axonal Diameter Distributions in the Human Brain Using Diffusion-Based Axonal Spectrum Imaging (AxSI)

Abstract

In this paper we demonstrate a generalized and simplified pipeline called axonal spectrum imaging (AxSI) for in-vivo estimation of axonal characteristics in the human brain. Whole-brain estimation of the axon diameter, in-vivo and non-invasively, across all fiber systems will allow exploring uncharted aspects of brain structure and function relations with emphasis on connectivity and connectome analysis. While axon diameter mapping is important in and of itself, its correlation with conduction velocity will allow, for the first time, the explorations of information transfer mechanisms within the brain. We demonstrate various well-known aspects of axonal morphometry (e.g., the corpus callosum axon diameter variation) as well as other aspects that are less explored (e.g., axon diameter-based separation of the superior longitudinal fasciculus into segments). Moreover, we have created an MNI based mean axon diameter map over the entire brain for a large cohort of subjects providing the reference basis for future studies exploring relation between axon properties, its connectome representation, and other functional and behavioral aspects of the brain.

Weighting the structural connectome: Exploring its impact on network properties and predicting cognitive performance in the human brain

“…However, there is currently no consensus on the most effective method for assessing the strength of these connections, mainly due to continuous fail in demonstrating high functional or behavioral correlations, and hence to accept one weighting method as preferred. In this study, a new approach was presented that involves weighting connections based on their mean axon diameter using the AxSI method ( Gast et al, 2023 ), in addition to commonly used methods such as the NOS reconstructed from fiber tracking analysis or the FA index from DTI method.…”

“…The weight was calculated as the mean FA value for all reconstructed streamlines that connect each pair of brain areas. ADD SC: Using AxSI analysis ( Gast et al, 2023 ), the average ADD per voxel was calculated. Then, estimated mean axon diameter (eMAD) per streamline was calculated.…”

“…ADD SC: Using AxSI analysis ( Gast et al, 2023 ), the average ADD per voxel was calculated. Then, estimated mean axon diameter (eMAD) per streamline was calculated.…”

“…Recently, a novel method called axonal spectrum imaging (AxSI) has been proposed to estimate the axon diameter distribution (ADD) of streamlines in the human brain ( Gast et al, 2023 ). This method utilizes a multishell dMRI framework to estimate the average ADD index for each reconstructed streamline.…”

“…Nevertheless, it showcases various significant benefits. Notably, its robust and simple modeling, which increases its applicability and make it compatible with a variety of scan protocols, including some existing scans like the HCP database ( Gast et al, 2023 ). Since axon diameter is closely related to properties such as conduction speed, further development of such methods in conjunction with graph analysis tools could provide a physiologically meaningful framework for estimating the strength and conduction delay of anatomical connectivity between regions.…”