Universal power-law scaling of water diffusion in human brain defines what we see with MRI

TL;DR

This paper reveals a universal power-law scaling in water diffusion signals in the human brain, enabling MRI to infer cellular-level structures beyond its traditional resolution limits, thus advancing neuroimaging capabilities.

Contribution

It demonstrates that biophysical modeling of water diffusion can surpass MRI resolution limits by identifying a universal power-law scaling in diffusion signals, validating models of neuronal tissue.

Findings

Universal power-law scaling of water diffusion in brain tissue

dMRI can quantify intra-axonal properties below MRI resolution

Validation of diffusion models in neuronal tissue

Abstract

Development of successful therapies for neurological disorders depends on our ability to diagnose and monitor the progression of underlying pathologies at the cellular level. Physics and physiology limit the resolution of human MRI to millimeters, three orders of magnitude coarser than the cell dimensions of microns. A promising way to access cellular structure is provided by diffusion-weighted MRI (dMRI), a modality which exploits the sensitivity of the MRI signal to micron-level Brownian motion of water molecules strongly hindered by cell walls. By analyzing diffusion of water molecules in human subjects, here we demonstrate that biophysical modeling has the potential to break the intrinsic MRI resolution limits. The observation of a universal power-law scaling of the dMRI signal identifies the contribution from water specifically confined inside narrow impermeable axons, validating the overarching assumption behind models of diffusion in neuronal tissue. This scaling behavior establishes dMRI as an in vivo instrument able to quantify intra-axonal properties orders of magnitude below the nominal MRI resolution, spurring our understanding of brain anatomy and function.