CT imaging is commonly used by physicians to monitor the development of pathological conditions, e.g. brain tumours. Accurate monitoring of a unique point in the brain over consecutive CT scans in between which a patient's head may have moved requires determination of the relative position and orientation of the skull coordinate system with respect to the imager coordinate system, known as registration. A configuration of fiducial markers can be attached to the skull to provide means for registration by fitting of its known geometry with image data obtained from CT scanning. Typical requirements on fiducial based registration systems are related to fiducial and target registration errors and the extent to which the surrounding image is unaffected.
In [1], six 0.80 mm tantalum spherical fiducials were placed in phantom bones to register data from CT and roentgen stereogrammetric analysis, achieving a root mean square fiducial registration error of 0.152...
