Cerebellar Isolation, Parcellation, and Conformal Surface Mapping

The topographic organization of motor, sensory and cognitive functions in the human cerebellum is poorly understood, and, owing to its anatomical organization, most of the folial surface is hidden from view. In order to facilitate surface-based analysis of functional activation within the cerebellar cortex, we constructed a "pipeline" for extracting, labelling and conformally mapping the cortical surface. Methods The pipeline utilizes a cerebellar template volume isolated from a high-resolution, high-contrast-to-noise T1-weighted MRI brain volume [1] and parcellated according to Schmahmann et al. [2]. See Figure 1. Isolation of a cerebellar subvolume was landmark driven; the posterior commissure, obex and apex of the fourth ventricle defined a standard orientation [3]. A template-to-source warp facilitiated removal of the cerebrum. The brainstem was then removed from the cerebellum by a computerized cut-plane that stepped though the volume until it reached the lingula, sparing wrap-around cerebellar tissue. The final perimeter of the cut surface served as the boundary for flat-mapping to a plane. Parcellation was effected by warping the template cerebellum to a source subvolume, applying the resulting transform to the template parcellation, and editing the labels to correct errors due to poorly resolved fissures and/or variable fissuration. Marching Cubes was used to extract an isovalue surface from individual cerebellar subvolumes; however, these surfaces commonly exhibited undesirable local topological defects (including handles, "pinched" vertices and fins), which had to be manually corrected. PET activation volumes were aligned to their corresponding anatomical MRI volumes, and activated voxels were projected onto the nearest vertex of the surface mesh. Surfaces bearing activation labels were conformally flattened using CirclePack software [5] and visualized in the Euclidean and hyperbolic planes and on a sphere. ResultsRendered views and hyperbolic flat maps of the cerebellar cortex from two subjects whoperformed a static force experiment [4] are illustrated in Figure 2. Suprathreshold voxelswithin 5 mm of the cortical surface were projected onto the surface mesh and labelledusing a hot-metal colorscale. The cerebella are displayed in our standard orientation (not co-registered); the flat maps sharetwo landmarks but are not in a common space. References1. Holmes CJ, et al. NeuroImage. 3(3):S28, 1996.2. Schmahmann JD, et al. MRI Atlas of the Human Cerebellum. Academic Press, San Diego, 2000.3. Rehm K, et al. NeuroImage. 11(5):S536, 2000.4. Muley SA, et al. NeuroImage. In press, 2001.5. Bowers PL, Stephenson K. Memoirs of the American Mathematical Society. Submitted, 2001. This work was supported in part by NIH grant MH57180.Figure 1. Cerebellar templatewith fissure labels. Top,posteriorview (L,R indicate object leftand right); Bottom,anterior view. Figure 2. Cerebellar surfaceswith PET activation data andfissure labels.