Summary: Abstract--In earlier work, we have shown the importance of
including 3D shape characteristics when analyzing regions of
interest (ROIs) in magnetic resonance imaging (MRI) data.
Spherical harmonics (SPHARM) based ROI shape descriptors
were proposed and shown to provide important complementary
information to traditionally used simple volumetric ROI
measures. In this paper we extend our SPHARM shape
parameterization technique by using functions defined on
concentric spherical shells. We then propose the use of a novel
radial transform to obtain unique features even under
independent rotations of the constituting shells. These enhanced
features enable the analysis of 3D ROIs with complex topologies
including those with possible disconnections (e.g. ventricles).
We validate the proposed 3D shape descriptors on synthetic
data and demonstrate their sensitivity to subtle shape changes
in the presence of inter-subject variability. We also apply our
approach to real MRI data and detect significant shape changes
in the left and right thalamus in Parkinson's disease (PD)
patients when compared against normal volunteers,
complementing the observed volumetric changes.