Iso-shaping rigid bodies for estimating their motion from image sequences

In many medical imaging applications, due to the limited field of view of imaging devices, often, acquired images include only a part of a structure. In such situations, it is impossible to guarantee that the images will contain exactly the same physical extent of the structure at different scans, which leads to difficulties in registration and in many other tasks, such as the analysis of the morphology, architecture, and kinematics. To facilitate such analysis, we developed a general method, referred to as isoshaping, that generates structures of the same shape from segmented images. The basis for this method is to automatically find a set of key points, called shape centers, in the segmented partial anatomic structure such that these points are present in all images and that they represent the same physical location in the object, and then trim the structure using these points as reference. The application area considered here is the analysis of the morphology, architecture, and kinematics of the foot joints from MR images acquired at different joint positions, load conditions, and longitudinal time instances. The accuracy of the method is studied and it is quantitatively demonstrated that isoshaping improves the results of registration.