Asymmetry of the brain surface from deformation field analysis

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Abstract
The detection of asymmetry of exposed brain surfaces is examined, and a new method, deformation‐based asymmetry (DBA), is introduced. DBA is based on analysis of two high‐resolution magnetic resonance brain images, each with features representative of the subject group from which they were derived. Warping of individual brain images to their group representative image using octree spatial normalization provides sets of displacement vectors that are used in estimating deformation variance. For DBA group‐representative left and right hemisphere images are compared. Representative hemisphere images are warped to each other and asymmetry analyzed using standardized d‐values calculated as the ratio of displacement vector magnitude to the estimated component of variance in the direction of the displacement vector for each surface voxel. D‐values were calculated within hemispheres by dividing subjects into two equal groups and comparing left‐to‐left and right‐to‐right. D‐values from this ipsilateral hemisphere grouping were pooled. D‐values from contralateral hemispheres were compared with the pooled ipsilateral hemisphere data. The proportion of d‐values above a fixed level was used to test for difference between the two groups. High‐resolution magnetic resonance (MR) images from 20 young, right‐handed males were studied using DBA. No significant differences were seen between sub‐grouped ipsilateral d‐values (P > 0.10). Highly significant asymmetries (P < 0.0001) were found between hemispheres, and in each lobe. Common right frontal and left occipital petalias were seen. The DBA method can theoretically be applied to any two groups of globally similar structures where analysis of dissimilarity of regional features is sought. Hum. Brain Mapping 19:79–89, 2003.