Different MRI biomarkers for fiber and myelin density have been proposed for MR g-ratio mapping, leaving open the question which biomarker is optimal. Here, we compare four different MRI biomarkers for fiber density using standard Stejeskal-Tanner diffusion encoding to the microscopic diffusion anisotropy (MA) measured by double-diffusion encoding. Thereby, we hypothesize that a better measure of the microscopic environment shows higher (and more significant) correlations to the MA metric. Our preliminary results showed that the marker by Kaden et al. (2016) shows higher correlation to MA than NODDI, suggesting it to be a better biomarker for fiber density.