Via journal club with this paper:
@article{macdonald2003msi,
title={{Multiple surface identification and matching in magnetic resonance images}},
author={MacDonald, D. and Avis, D. and Evans, A.C.},
journal={Proceedings of SPIE},
volume={2359},
pages={160},
year={2003},
publisher={SPIE}
}Go to http://dx.doi.org/10.1006/nimg.1999.0534 or CorticalSurfaceMacdonald (Cambridge permissions)
See also there a pdf version of MacDonald's thesis A Method for Identifying Geometrically Simple Surfaces from Three Dimensional Images or go to his personal pages at http://www.bic.mni.mcgill.ca/users/david/