1. A system for performing a model-based segmentation of an anatomical structure in a medical image, the system comprising: an input for obtaining 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; and a processor configured for performing a model-based segmentation of the anatomical structure by applying the deformable model to the image data; wherein the data storage further comprises definition data defining a geometric relation between a first part and a second part of the deformable model of which a corresponding first part of the anatomical structure is presumed to be better visible in the image data than a corresponding second part of the anatomical structure; wherein the processor is further configured for, after said applying of the deformable model to the image data: i) using the definition data to locate an image feature in the image data which matches the geometric relation with the first part of the deformable model; ii) adjusting a fit of the second part of the deformable model based on a location of the image feature so as to better fit the second part of the deformable model to the second part of the anatomical structure; iii) locating the image feature by searching for the image feature in an intensity profile of the image data using the geometric relation as search parameter; and iv) establishing the intensity profile orthogonally to the first part of the deformable model.