Simple methods (Thresholding, Region-Growing)
Classification + Clustering (based on machine learning and pattern recognition techniques)
Deformable models / level sets (shape prior, but not very rigorous, solely based on some geometric properties like curvature)
Active shape models (very strong shape prior, based on actual geometry of the structure to be segmented à model-based method)
Random Fields/graph-cuts (one of the most recent methods)