A new system for completely MR-based computer assisted orthopaedic surgery

We develop a new reference system for computer assisted orthopaedic surgery comple- tely based on magnetic resonance (MR) images without any use of computer tomogra- phy (CT) images. In contrast to existing CT-based systems two major problems have to be solved. Firstly, the MR-images contain geometrical distortions. These distortions are caused by the fact that the body itself and the surronding air have different magnetic properties. These difference leads to a distortion in the static magnetic field (B0) of the MR scanner. Thus the position encoding in the MR imaging process is disturbed. Second, the preoperative planning in the MR must be registered to intra-operative flu- oroscopic images. We have found that approaches previously developed for CT-based registration must be modified substantially. The main problems are: