Description of Registration Method for Ashburner

The coregistration is a three step procedure:

1) Simultaneously estimate 12 parameter affine transformations that register the two images to template images of the same modalities and contrasts. A Newton-like least squares method can be used since the images and templates are of the same modalities. The affine registrations are constrained so that only parameters reflecting rigid body displacements are allowed to differ between the pair of affine transformations.

2) The method assumes that the brains are composed only of three tissue types: gray matter, white matter and CSF. Gray and white matter partitions are extracted from the images using a modified mixture model cluster analysis method. The model assumes a spatially varying a priori probability of each voxel being a particular tissue type. The prior probabilities are described by images conforming to the space of the template images described in step (1). Since the mappings between the images and templates is known, then the a priori images can simply be ovelayed on to the images to be partitioned. This partitioning is an iterative procedure, whereby parameters describing the distribution of voxel intensities within each partition are computed. Using these parameters and the a priori probability images, a probability is determined for each voxel belonging to each of the partitions. The loop continues by recomputing the parameters of the distributions using the probabilities, and using the probabilities to recompute the parameters.

3) The affine transformation parameters obtained from step (1) are used as starting estimates for the final step, which involves registering the image segments extracted using step (2). Rigid body transformations are estimated that simultaneously match both the gray and the white matter partitions from the pair of images, by minimizing the residual squared difference between them.

Method 1 uses only the first step. Method 2 uses all three steps.

References

1. "Multimodal Image Coregistration and Partitioning - a Unified Framework.", J. Ashburner & K.J. Friston, 1997, NeuroImage, Vol. 6, Issue 3, pp. 209-217.

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