Inter-subject spatial registration of the human cerebellum

Kelly Rehm*, Jon Anderson†, Roger Woods‡, David Rottenberg*†

*University of Minnesota Department of Radiology
†University of Minnesota Department of Neurology
‡UCLA Department of Neurology

Modeling & Analysis

Abstract

We and others have discussed the benefits that may accrue from registering the cerebellum and the cerebrum independently [1]. Thus, we compared linear transformation methods and nonlinear (warping) approaches for registering high-resolution T1-weighted MR volumes with respect to landmark localization and intensity characteristics.

Methods

Sixteen T1-weighted MRI scans of normal subjects were acquired during an fMRI experiment using a static force protocol [2]. Voxel dimensions were 0.86 x 0.86 x 1mm. The Montreal Neurological Institute 27-scan-average T1 MRI volume [3] served as our template and was manually stripped and divided into three spatial subcompartments (left/right cerebrum, brainstem plus cerebellum). Seven cerebellar landmarks were selected based on the Schmahmann atlas [4] for comparing alignment methods: apex of V4, tips of the lingula and nodulus, and floors of the primary, preculminate, prepyramidal and secondary fissures in the midline. A cerebellum-brainstem subcompartment was generated for each subject by computing a warp transformation of the template volume onto the subject volume, and the compartment boundary was automatically adjusted according to local intensity characteristics and used to compute a smooth "membrane" enclosing the cerebellum and brainstem.

Five registration methods were evaluated: (1) Rigid-body translation and rotation to a standard pose in which the posterior commissure, obex and apex of the fourth ventricle define three orthogonal planes [5], (2) Method 1 plus 12-part linear scaling according to Grodd, et al. [1], (3) Grodd's method, which differs from Method 1 in the definition of rotations, (4) A fourth-order warp transformation [6] of subject to template without compartmentalization, (5) A fourth-order warp transformation computed from the cerebellar subcompartment. Alignment quality was assessed using the following metrics: (1) landmark cluster radius, (2) landmark displacement vs. the template volume, (3) intersection volume, and (4) correlation of group average and template intensities. Cluster radius was quantified by the standard deviation of the distances of subjects' landmarks from the cluster centroid.

Results

For all methods, the average cluster radius was approximately one voxel (see Table 1). As expected, displacement was reduced by warping, and warping increased the volume of intersection. The slight degradation in landmark metrics for Method 5 compared to Method 4 may reflect errors in landmark placement that are "corrected" by improved alignment -- suggested by increased intensity correlation. Decreased blurring rom left to right in Figures 1 and 2 indicates improved alignment. For intersubject registration of normal subjects, we conclude that the cerebellum should be treated as a separate compartment, and non-linear registration is preferable to piecewise linear scaling for this application.

Table 1. Alignment metrics.

Method	1	2	3	4	5
cluster radius	1.15 mm	0.97 mm	1.01 mm	1.01 mm	1.03 mm
displacement	3.26 mm	2.36 mm	2.57 mm	1.75 mm	1.94 mm
intersection volume	144.6 cc	168.4 cc	163.9 cc	202.0 cc	202.1 cc
correlation	0.38	0.49	0.52	0.75	0.81

References

1. Grodd W, et al Human Brain Mapping 13:55-73,2001.
2. Muley, et al, Neuroimage. 13:185-195.
3. Holmes CJ, et al NeuroImage. 3(3):S28, 1996.
4. Schmahmann JD, et al MRI Atlas of the Human Cerebellum. Academic Press, San Diego, 2000.
5. Rehm K, et al NeuroImage. 11(5):S536, 2000.
6. Woods RP, et al Journal of Computer Assisted Tomography 22:153-165, 1998.

Acknowledgments

This work was supported in part by NIH grant MH57180.