Visualizing the entire cortical myelination pattern in marmosets with magnetic resonance imaging.

Myeloarchitecture, the pattern of myelin density across the cerebral cortex, has long been visualized in histological sections to identify distinct anatomical areas of the cortex. In humans, two-dimensional (2D) magnetic resonance imaging (MRI) has been used to visualize myeloarchitecture in select areas of the cortex, such as the stripe of Gennari in the primary visual cortex and Heschl's gyrus in the primary auditory cortex. Here, we investigated the use of MRI contrast based on longitudinal relaxation time (T(1)) to visualize myeloarchitecture in vivo over the entire cortex of the common marmoset (Callithrix jacchus), a small non-human primate that is becoming increasingly important in neuroscience and neurobiology research. Using quantitative T(1) mapping, we found that T(1) at 7T in a cortical region with a high myelin content was 15% shorter than T(1) in a region with a low myelin content. To maximize this T(1) contrast for imaging cortical myelination patterns, we optimized a magnetization-prepared rapidly acquired gradient echo (MP-RAGE) sequence. In whole-brain, 3D T(1)-weighted images made in vivo with the sequence, we identified six major cortical areas with high myelination and confirmed the results with histological sections stained for myelin. We also identified several subtle features of myeloarchitecture, showing the sensitivity of our technique. The ability to image myeloarchitecture over the entire cortex may prove useful in studies of longitudinal changes of the topography of the cortex associated with development and neuronal plasticity, as well as for guiding and confirming the location of functional measurements.