Deformations incorporating rigid structures

Medical image registration algorithms invariably assume that the objects in the images can be treated as a single rigid body. In practice, some parts of a patient, usually bony structures, may move as rigid bodies, while others may deform. To address this, we have developed a new technique that allows identified objects in the image to move as rigid bodies, while the remainder smoothly deforms. The transformation is based on a radial basis function solution with a basis function modified by use of distance transforms based on rigid structures within the image. These distance transforms are also used to form an underlying transformation which ensures an interpolating solution. The resulting deformation technique is valid in any dimension, subject to the choice of the basis function. We demonstrate this algorithm in two dimensions on illustrative images as well as on sagittal magnetic resonance images collected from a volunteer. (C) 1997 Academic Press.