Augmentation of reality using an operating microscope for otolaryngology and neurosurgical guidance.

The operating microscope is an integral part of many neurosurgery and otolaryngology procedures; the surgeon often uses the microscopic view for a large portion of the operation. Information from preoperative radiological images is often viewed only on X-ray films. The surgeon then has the difficult task of relating this information to the appearance of the surgical view. Image guidance techniques attempt to relate these two sets of information by registering the patient in the operating room to preoperative images using locating devices. Conventionally, image data are presented on a computer monitor, which requires the surgeon to look away from the operative scene. We describe a guidance system, for procedures in which the operating microscope is used, which super-imposes image-derived data upon the operative scene. We create a model of relevant structures (e.g., tumor volume, blood vessels, and nerves) from multimodality preoperative images. By calibrating microscope optics, registering the patient to image coordinates, and tracking the microscope and patient intraoperatively, we can generate stereo projections of the three-dimensional model and project them into the microscope eyepieces, allowing critical structures to be overlaid on the operative scene in the correct position. Measurements with a head phantom gave a root mean square (RMS) error of 1.08 mm, and the estimated error for a human volunteer is between 2 and 3 mm. Initial evaluation in the operating room was very promising.