Embodiments of the invention introduce the concept of a virtual fiducial marker (VFM). This marker will not be placed prior to preoperative imaging. It will therefore not initially appear in the preoperative 3D image. Instead, the marker will be virtually placed in the 3D data set prior to surgery, and as such the marker does not even need to be attached to the patient, if the patient is not expected to move during surgery. The idea behind such a VFM comes from the observation that there is a big variation in the difficulty of an image based registration depending on the quality of the images and their field of view. However, provided two initial registrations to high quality wide field of view images can be carried out, it then becomes possible to use these registrations to insert a virtual fiducial marker within the 3D preoperative image. Once inserted it may then be used to obtain a starting position for a registration procedure, and particularly to obtain in-plane translations. It may also be used at the end of a registration to provide a measure of registration accuracy, to verify the registration and detect mis-registrations.
