Magnetic resonance imaging (MRI) studies show that the majority of the T1-weighted contrast stems from myeloarchitecture, but cytoarchitecture still has an impact on the signal. Many cortical MRI markers are thought of as being driven mostly by myelin. Here, we compare the spatial organization and age effects of cortical thickness (CT) to 3 measures of cortical microstructure (gray-white matter contrast [GWC], the boundary sharpness coefficient [BSC], gray matter [GM] T1w/T2w ratio, and superficial white matter [SWM] T1w/T2w ratio), and probe their specificity to cellular organization using the histological reconstruction of BigBrain.
For each marker and vertex, the mean value across the 127 healthy subjects (aged 18-81, 76F, 51M) was computed and a linear model with age and sex as predictors was fitted. The markers were also generated on the BigBrain volume. The correlation of the surface maps was assessed and significance tested via spin tests.
Overall, results showed higher correlations between spatial distributions than between age trajectories. The GWC and BSC showed significant positive spatial correspondence with their homologous measures derived using BigBrain, while T1w/T2w ratio measures did not.
Our finding of spatial correlations being higher for mean values than for age effect indicates that some measures tend to covary at the cortex-wide level, but age trajectories are likely influenced by different interactions of microstructural changes. Our BigBrain results indicate a general trend of GM T1w signal and myelin being inversely related to the density of cells, although that relationship is weak to moderate.