When the white matter region growing has been performed, the subsequent steps Dilate white matter and Smooth white matter are performed to improve the quality of the segmented white / grey matter boundary.
If a surface would be reconstructed directly on the basis of the region growing result, it would contain a rather noisy boundary that would also contain a high number of topological "errors" like holes or handles. Under the a priori knowledge that the cortical sheet is a smoothly varying surface, morphological operations help to create a more smooth segmentation result. The white matter dilation step (Dilate white matter option) expands the segmentation into the grey matter for one voxel. The expansion is carried out without introducing connections between the banks of sulci. The boundary of the resulting segmented volume runs roughly through the middle of the cortical sheet. This dilation step produces a much more smooth boundary and usually also removes several holes in the surface. The subsequent white matter smoothing step (Smooth white matter option) further smoothes the obtained boundary. You can control the number of smoothing steps by using the Iterations spin box. A value of "5" (default) normally produces good results.
The snapshot above shows the results of the white / grey matter segmentation after performing the dilation and smoothing step.
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