For image-guided stereotactic surgery, in the first part of the procedure, a CT scan or MRI (or occasionally, angiogram) is performed. For increased precision, “fiducial” markers or a stereotactic frame is attached to the patient’s head during this image acquisition phase. Acceptable accuracy for biopsy can often be obtained using high resolution thin cut imaging slices (usually with a 0 angle of the gantry), and then surface matching algorithms in the guidance system will match the pre-op CT/MRI to the patient’s head. This is not accurate enough for lesion generation or electrode placement. The second part of the procedure usually takes place in an operating room. The patient is “registered” with the pre-op images, and then tracking cameras follow the movement of instruments with appropriate attachments to show in “real-time” the location of the instrument with respect to the pre-op image. An important limitation to be aware of is the fact that the pre-op images are “historical” and are not updated as the surgical procedure alters the anatomy of the patient. Even the administration of mannitol can cause brain shifts that may cause the target of the surgery to move away from its pre-op location by several millimeters 1).