Mapping orientational and microstructural metrics of neuronal integrity with in vivo diffusion MRI
Diagnosis of brain disorders is hindered by the lack of an imaging technique that reveals the architecture of neuronal tissue at the cellular level, where the associated pathological processes develop. Accessing tissue integrity at the micrometer scale, three orders of magnitude below MRI resolution, would be akin to super resolution in microscopy. For MRI, bridging the resolution gap relies on biophysical modeling of water diffusion hindered by cell walls. Here we develop a general framework for estimating orientational and microstructural parameters of neurites (axons and dendrites) By employing a set of rotational invariants, we analytically reveal the nontrivial topology of parameter estimation landscape, showing that multiple branches of parameters describe the measurement almost equally well, with only one of them corresponding to the biophysical reality. A comprehensive acquisition shows that the branch choice differs for white and for gray matter. Our framework reveals hidden degeneracies in MRI parameter estimation for neuronal tissue, provides microstructural and orientational maps in the whole brain without constraints or priors, and connects modern biophysical modeling with clinical MRI.