ABSTRACT
Rigid and affine image registration, although adequate for many registration tasks mostly involving the brain, is insufficient for the registration problems that pertain to the extracranial anatomy. Soft-tissue organs of the abdomen and the thorax, for example, often deform leading to variations of shape and size from factors such as respiration, beating of the heart, and gravity. The registration of images of such organs or organ systems requires nonrigid image registration.
