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62Figure 3-2 Analyzing cortical data. Theschematic shows a sequence of imageprocessing steps that can be used to maphow aging affects the cortex. The stepsinclude aligning MRI data to a standardspace, tissue classification, cortical patternmatching, as well as averaging andcomparing local measures of corticalgray matter volumes across subjects. Tohelp compare cortical features from subjectswhose anatomy differs, individualgyral patterns are flattened and alignedwith a group average gyral pattern (a tof). Group variability (g) and corticalasymmetry can also be computed. Correlationscan be mapped between agerelatedgray matter deficits and geneticrisk factors. Maps may also be generatedvisualizing linkages between deficitsand clinical symptoms, cognitive scores,and medication effects. The only stepshere that are currently not automated arethe tracing of sulci on the cortex. Somemanual editing may also be required toassist algorithms that delete dura andscalp from images, especially if there isvery little CSF in the subdural space. Seecolor insert. (Thompson et al., 2003.)