Deformable Brain Atlases

6.1 Anatomic Variability

The use of spatial normalization schemes based upon deep white matter features (the AC and PC), as outlined previously, will never completely accommodate the most variable of brain structures, the cortex. The cortex is also the site of interest for most functional activation studies. Considerable normal variations in sulcal geometry are well documented in primary motor, somatosensory, and auditory cortex [69, 82], primary and association visual cortex [99], frontal and prefrontal areas [83], and lateral perisylvian cortex [44,71,97,98]. More recent 3D analyses of anatomic variability, based on post mortem and normal and diseased populations in vivo, have found a highly heterogeneous pattern of anatomic variation [107,112,115], Fig. 4).

Given this complex structural variability between normal individuals, and particularly between different populations (healthy and diseased), a fixed brain atlas may fail to serve as a faithful representation of every brain [63,84]. Since no two brains are the same, this presents a challenge for attempts to create standardized atlases. Even in the absence of any pathology, brain structures vary between individuals in every metric: shape, size, position, and orientation relative to each other. Such normal variations have complicated the goals of comparing functional and anatomic data from many subjects [82,84].

Numerous studies have measured how severe the inter-subject variations in anatomy are, even after transforming individual anatomic data into the Talairach stereotaxic system (Fig. 4). Clearly, direct averaging of digital brain maps, after

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