1. A method of performing computerized simulations of image-guided procedures, the method comprising: producing a digital image-based model of an anatomical structure in a patient based on medical image data obtained from the patient, the digital image-based model including a first 3D polygonal mesh representing the medical image data; producing, based on the image-based model and data representing anatomical structures not included in the medical image data, an extended model that represents the anatomical structure in the patient and adjacent anatomical regions not included in the medical image data, wherein producing the extended model comprises: automatically selecting from a set of pre-designed generic 3D polygonal meshes a second 3D polygonal mesh representing anatomical structures not included in the medical image data, and positioning the second 3D polygonal mesh in alignment with a boundary of the first 3D polygonal mesh; displaying a graphical representation of the extended model; and performing a computerized simulation of an image-guided procedure using the extended model.