1. A system for performing a model-based segmentation of an anatomical structure in a medical image, the system comprising: an input that obtains image data of the medical image; a data storage comprising model data defining a deformable model for segmenting a type of anatomical structure, the deformable model comprising parts to be fitted to corresponding parts of the anatomical structure, wherein the data storage further comprises definition data defining a geometric relation between a first part and a second part of the deformable model; and a processor that: performs a model-based segmentation of the anatomical structure by applying the deformable model to the image data to produce a deformed model with corresponding first and second parts; locates an image feature in the image data that matches the geometric relation with the first part of the deformed model based on an intensity profile orthogonally to the first part of the deformed model; adjusts a fit of the second part of the deformed model based on a location of the image feature so as to better fit the second part of the deformed model to the second part of the anatomical structure; and displays the anatomic structure based on the deformed model.