Axon diameter mapping in the living human brain with ultra-high gradient diffusion MRI using 500 mT/m gradient strength

Motivation: Noninvasive quantification of axon diameter in the living human brain offers valuable insights into the mesoscopic organization of white matter. Current methods for mapping axon diameter using diffusion MRI are limited by gradient strength. Goal(s): To evaluate the sensitivity of axon diameter mapping to small diameter axons on the Connectome 2.0 scanner (Gmax=500mT/m) compared to the original Connectome scanner (Gmax=300mT/m). Approach: The AxCaliber-SMT model was fitted to diffusion MRI data in 10 healthy subjects scanned on Connectome 2.0 and Connectome 1.0. Results: Median axon diameter in the corticospinal tract was 2.63um on Connectome 2.0 and 4.00um on Connectome 1.0. Impact: Connectome 2.0 pushes the resolution limit and signal-to-noise ratio of axonal diameter mapping, allowing for greater sensitivity toward small diameter axons at the individual level for a variety of neuroscientific and clinical applications.