Higher b-values improve the correlation between diffusion MRI and the cortical microarchitecture.

Purpose: In diffusion MRI (dMRI), it remains unclear to know how much increase of b-value is conveying additional biological meaning. We tested the correlations between cortical microarchitecture and diffusion metrics computed from standard (1000 s/mm), high (3000 s/mm), to very high (5000 s/mm) b-value dMRI.

Methods: Healthy volunteers were scanned with a dMRI pulse sequence that was first optimized together with a T1-WI and T2-WI. Averaged cortical surface map of estimated myelin (T1-WI/T2-WI) was compared with surface maps of mean diffusivity (MD) computed from each b-value (MD, MD, and MD) and to surface map of mean kurtosis (MK computed from the 0-, 1000-, to 3000-s/mm shells) in 360 cortical parcels using Spearman correlations, multiple linear regressions, and Akaike information criteria (AIC).

Results: Surface map from MD showed variations not related to myelin but the MD and MD maps inversely mirrored estimated myelin map; lower MD values being observed in more myelinated cortical areas. MK mirrored myelinated cortical areas. Quantitatively, Spearman correlations between myelin and MD became more and more negative as long as b-values increased while the correlation was positive between myelin and MK. Multiple regression models confirmed negative associations between myelin and MD that were significantly better from MD to MD and MD (R = 0.33, p < 0.001; R = 0.43, p < 0.001; and R = 0.50, p < 0.001) and positive association between myelin and MK (R = 0.53, p < 0.001). Comparisons of the 3 statistical models showed the best performances with MK and MD (AIC < AIC < AIC < AIC).

Conclusion: Higher b-values are more closely related to subtle cellular variations of the cortical microarchitecture.