Figure 6.

A volume adjustment factor is determined based on the exterior surface of the skull. This surface is automatically extracted from the T1-weighted MR image of each subject. A given subject's skull surface image is affinely matched with the skull surface image of the reference. The volume scaling that is defined by the affine transformation yields the adjustment factor. A, Example of a skull surface superimposed to an original MR image. The skull surface around the forebrain is found quite robustly. However, below the orbits and tentorium of the cerebellum, it becomes more difficult to define the skull reliably. For this reason, only the skull above the orbits is used for calculation of the adjustment factor. B, Image of a reference brain, onto which the extracted surface of its skull is superimposed (gray contour), and the affinely matched skull image of a subject (white contour). C, The scatterplot of all subjects' volume adjustment factors against their age shows no significant linear relationship (a linear regression yielded p = 0.814).