A frameless stereotaxic operating microscope for neurosurgery is discussed. The microscope projects computed tomography (CT) information into the microscope, superposing a tumor outline or guidance to predetermined targets in the surgical field in the correct scale, position, and orientation. The transposition of data from CT to OR (operating-room) space is accomplished with a set of fiducial points which are localized in both CT and OR space, eliminating the need for conventional mechanical stereotaxic frame attached to the head of the patient. Once this transformation matrix has been determined, the operating microscope's position is monitored and information derived from the CT scan is projected into the operating microscope. A nonimaging ultrasonic rangefinder, consisting of microphones and sound emitters, enables the precise location of the sound emitters in OR space to be determined. Under computer control, one sound emitter is fired, and the distance from it to each of the microphones is determined. The operating microscope has three sound emitters attached to it, so its position and orientation can be uniquely determined. Sound emitters are placed on the fiducial points on the patient's scalp, and the CT-OR transformation is determined. In this way, an open surgical field is maintained, yet stereotaxic information and guidance is available.
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