Hippocampal surface analysis using spherical harmonic function applied to surface conformal mapping

Using spherical harmonics of an inverse conformal map, we compared hippocampal surfaces of sixteen Alzheimer (AD) and fourteen control subjects. Hippocampal surfaces were conformally mapped to a sphere. Maps were regularly sampled and exact, high-degree spherical harmonic transforms of the inverse maps were computed. Using the transforms shape descriptors corresponding to the degree of the harmonics and invariant to translation, rotation, and scale were obtained and normalized against sample mean. Two-dimensional visualizations of the shape descriptors were indicative of global as well as local shape features of hippocampal surfaces. These descriptors are potentially useful for visual detection of global patterns and creation of population-based, probabilistic, disease-specific digital atlases, especially for comparison of global shape features.