Home Computer Science Computational Diffusion MRI: MICCAI Workshop, Athens, Greece, October 2016
The most salient feature of the parcellation is the emergence of distinct and spatially coherent clusters along the anterior (white cluster) and posterior (red cluster) banks of the central sulcus. These have been provisionally labelled 3b and 4 due to their consistency with the location and extent of Brodmann areas 3b and 4. These cortical areas correspond to part of primary somatosensory cortex (S-I) and to primary motor cortex (M-I), respectively. Comparison with the myelin map (left) indicates that the white cluster ID correlates with an area of high myelination, whereas, the red cluster correlates with a drop in myelination. Figure 3 confirms that both the myelin and diffusion measurements discriminate these regions. However, the independently derived diffusion based parcellation produces a more spatially coherent area along the posterior bank of the central sulcus, when compared to the variability of the myelin map in Fig. 2.
There are several factors that may have lead to the group average parcellation exhibiting the closest agreement to the Brodmann map within the central sulcus. The boundary between S-I and M-I represents one of the most prominent transitions within the cortex [20, 29]. The input layers of S-I possess many small cell bodies giving it a granular appearance. In contrast, long cortico-spinal projections in M-I result in large pyramidal cell bodies known (in the foot representation) as Betz Cells, giving an agranular appearance. S-I also exhibits highly myelinated tangential bands of Baillarger, which are less prominent in M-I. Furthermore, in vivo studies at 7T support the hypothesis that differences in the laminar composition between these two regions are manifested in dMRI signal .
Another factor which may have improved the detection of these areas is that they demonstrate relatively low intersubject variability. S-I occupies the posterior bank of the central sulcus and extends back into the postcentral gyrus and M-I occupies anterior bank of the central sulcus, extending forwards into the precentral gyrus, with their transition consistently located at the fundus of the central sulcus [5, 29] near the location of area 3a, which receives predominant input from muscle receptors. This is consistent with the location and extent of the red and white clusters in the parcellation. Therefore, it is likely that architectural changes in these regions, as characterised by their feature vectors, are reinforced by averaging across multiple subjects.
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