Shape analysis of brain ventricles using SPHARM

Enlarged ventricular size and/or asymmetry have been found markers for psychiatric illness, including schizophrenia. However, this morphometric feature is non-specific and occurs in many other brain diseases, and its variability in healthy controls is not sufficiently understood. We studied ventricular size and shape in 3D MRI (N=20) of monozygotic (N=5) and dizygotic (N=5) twin pairs. Left and right lateral, third and fourth ventricles were segmented from high-resolution T1w SPGR MRI using supervised classification and 3D connectivity. Surfaces of binary segmentations of left and right lateral ventricles were parametrized and described by a series expansion using spherical harmonics. Objects were aligned using the intrinsic coordinate system of the ellipsoid described by the first order expansion. The metric for pairwise shape similarity was the mean squared distance (MSD) between object surfaces. Without normalization for size, MZ twin pairs only showed a trend to have more similar lateral ventricles than DZ twins. After scaling by individual volumes, however, the pairwise shape difference between right lateral ventricles of MZ twins became very small with small group variance, differing significantly from DZ twin pairs. This finding suggests that there is new information in shape not represented by size, a property that might improve understanding of neurodevelopmental and neurodegenerative changes of brain objects and of heritability of size and shape of brain structures. The findings further suggest that alignment and normalization of objects are key issues in statistical shape analysis which need further exploration.