An alternative to using pre-segmented surfaces is to use distinctive surface features defined by their local geometry. Using the tools of differential geometry it is possible to define two principal curvatures of a surface in 3D space. A crest line is an indication of a ridge in the surface and is defined as the loci of points where the value of the largest curvature is locally maximal in its principal direction . Registration between two images proceeds by applying the ICP algorithm to these crest lines. Gueziec et al.  have proposed using hash tables of geometric invariants for each curve together with the Hough transform and a modified ICP algorithm. These methods, while useful for intramodality registration of high-resolution images, are less applicable to lower-resolution PET images.
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