Once the acquisition and 3-D reconstruction data have been loaded into the Viewing Wand workstation, further preparation must be performed before the system can actually be used to guide the surgeon. First, when the anesthetized patient is on the operating table, the FARO arm must be firmly fixed to the Mayfield clamp (i.e., the device that holds the head of the patient in place) to ensure that, if the head has to be moved during the surgery, the arm will keep the same position relative to it.
Secondly, to allow the position of the tip of the probe to be computed by the Viewing Wand, the probe must be calibrated. This is performed by inserting it into a hole at the base of the arm and sampling a few points with the probe at different angles, keeping always its tip in the bottom of the hole. Optionally, a ``ball-bar'' test can be performed, allowing any failure of the mechanical arm to be readily detected.
Finally, the registration, allowing the patient and image spaces to be correlated, must be performed. This is typically accomplished by homologous point matching, which consists of identifying at least three points on both the patient and the image, or by surface matching, which involves sampling a large number of points on the patient's skin with the probe. Registration will be described further in the following chapters.
Once all these steps have been completed, the surgeon can see a representation of the probe on the computer screen, at the proper location on the image of the patient, and the system is ready to provided guidance during the surgical procedure.