Intraoperative Image-Guidance Technology

Since first described in 1993, intraoperative computer-assisted or image-guided surgery has been advocated to improve the efficacy of endoscopic sinus surgery and reduce complications. Despite this, it is not clear whether the current published literature supports these claims. To date, the vast majority of publications have dealt predominantly with the accuracy of a number of commercially available imageguidance systems and accessories. It is unknown what evidence-based research exists on the efficacy of image-guided technology. The ideal image-guidance device should correlate well with the actual anatomy that is being operated on. To do this, it must be accurate within 2 mm or less to minimize the risk of complications from the device itself. In addition, intraoperative image guidance should not be affected by head movement, should not require a second computed tomographic (CT) scan or magnetic resonance imaging (MRI) scan, should require minimum training for its use and calibration, should be cost-effective (current price around $140 000), and should be easily operated by the surgeon. Ease of operation eliminates the need for an operating room technician or circulating nurse, which may increase the cost. Ideally, image-guidance technology should also provide a real-time image to minimize the chance of intraoperative distortion of tissue by the surgeon. This is especially important for surgeons performing more advanced procedures of the orbit, skull base, or brain. Currently, there are 4 types of tracking technologies: sonic, electromechanical, optical, and electromagnetic. With appropriate calibration, the mean error for most of these products is around 2 mm. Only 3 of these (electromechanical, optical, and electromagnetic) are commercially available in the United States. Sonic digitizers determine position by measuring the time needed for sound from an emitter to travel to a microphone array of known geometry. Unfortunately, the speed of sound varies with temperature. Differences in temperature between the probe and the array may result in accuracy errors. Sonic digitizers have been described but not widely used commercially in this country. Electromechanical digitizers (ie, InstaTrak; ISG Technologies Inc, Mississauga, Ontario) rely on detectors located within the joints of a mechanical arm. Because the geometry of each arm segment and the length of the operating probe are known, motion of the arm transmitted by the joint sensor and the exact position of the probe can be calculated. Significant disadvantages of this system include the need for a second preoperative CT scan, timeconsuming placement of skin fiducials, reregistration every time the head moves intraoperatively, and a cumbersome apparatus. In addition, with the fiducial method, the skin markers may move or come off when the patient is anesthetized and facial muscles relax. Currently, ISG Technologies Inc has replaced its mechanical viewing wand with the OptoTrak infrared system. Optical digitizers (ie, Flashpoint 5000 3-D Optical Localizer; Image Guided Technologies Inc, Boulder, Colo; and the OptoTrak System; Northern Digital Inc, Waterloo, Ontario) rely on infrared-emitting diodes placed on the operating probe or instrument in a known geometric pattern. A channel array near the operating table detects the position of the infrared-emitting diodes. This obviates the need to immobilize the head during surgery or for reregistration for intraoperative head movement. Electromagnetic digitizers (ie, InstaTrak System; Visualization Technology Inc, Woburn, Mass) superimpose a magnetic field around the area of interest. The position can be determined by using a probe that can actually detect gradients in the magnetic field. Ferromagnetic substances, aluminum, and electromagnetic radiation all distort the magnetic fields and impair the accuracy of localization. However, many of these initial problems have been resolved. The advantage with the electromagnetic systems is that the CT scan can be performed at any time before surgery rather than on the previous or same day, as necessitated when skin fiducials are used. From the Department of Otolaryngology, University of Miami School of Medicine, Miami, Fla. Roy R. Casiano, MD CLINICAL CHALLENGES IN OTOLARYNGOLOGY