Navigation systems for image-guided therapy. A review.

Navigation is visually interactive targeting based on the simultaneous disp lay of instrument position and of the corresponding two- or three-dimension al image data sets. In this way it unifies anatomic information and therape utic action. Medical navigation systems (MNS) can simulate realtime image g uidance and thereby reduce radiation exposure as well as provide the full r ange of digital image processing during an intervention. Navigation is base d on the tracking of medical instruments in space and the transformation of image, patient, and instrument coordinates into a common reference system. If the patient coordinate system is used as the common base, the process i s called patient-based navigation (PBN). If, however, the imaging modality is present in the interventional suite and its reference system is used, mo dality-based navigation (MBN) results. MBN does not need pre-interventional registration and inherently provides intra-operative imaging. In neurosurg ery MNS's have been well established since years. They are in use for frame less biopsies and for minimizing the access morbidity in deeply situated pa thologies. Currently there is a fast expansion of navigation into other sur gical disciplines, e.g., orthopaedic surgery. The clinical accuracy of an M NS is hard to determine since an independent method of measurement is mostl y not available during surgery. Normally, a deviation of below one up to ab out 5 mm between the display of the MNS and the actual position of an anato mic structure is reported. So far there have been only very few prospective randomized clinical trials between conventional and navigated intervention s [1 - 3].