A method and apparatus for defining the location of a medical instrument relative to features of a medical workspace including a patient's body region are described. Pairs of two-dimensional images are obtained, preferably by means of two video cameras making images of the workspace along different sightlines...http://www.google.ca/patents/US6165181?utm_source=gb-gplus-sharePatent US6165181 - Apparatus and method for photogrammetric surgical localization
