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Fig. 3. The drill tip has entered the carotid alert zone as shown in the virtual and cross-sectional views. An auditory alert reminiscent of an arterial Doppler trace is triggered when the drill is positioned in this zone to provide structure-specific proxim- ity information.

not helpful. Most suggested being able to turn off the alarms once landmarks and structures were clearly identified.

of the contours was off-putting. The surface mesh on the virtual view was earmarked as a potential area for fur- ther development, with some finding it confusing or dis- tracting; others would have preferred a more lifelike semitransparent surface rendering. Auditory Alerts A number of auditory features for critical structure proximity alerts were upgraded during the trial in response to suggestions. Subjects were observed to instinctively shift their gaze toward the source of the alarm. To reduce this interruption, speakers were placed directly next to the endoscopic monitor. The first three participants found the abstract sound alerts (beeps) dis- tracting and often unhelpful. It was difficult to distin- guish acoustically which anatomical structure was close and how far away it was. Auditory icons are sounds that in some way relate to the reason for alarm. They should be intuitive and easily learnt. Auditory icons were devel- oped for the dura and carotid arteries to provide more information to the surgeon. For example, the sound cho- sen to represent proximity to a major vessel (carotid artery) was reminiscent of an arterial Doppler trace. Structure- and distance-specific alarms then gave the operator navigational data without additional visual stimuli or having to look away from the main monitor. Most of the remaining subjects appreciated this new feedback medium. Alert zones were manually contoured and generally around 2 to 3 mm. There was agreement that this is an appropriate distance. All subjects thought the alarms required some form of customization, especially after a structure had been safely identified. For example, once they had confirmed the position of the carotid, a con- stant alarm while they were drilling adjacent to it was

Applications There was strong consensus as to the potential clin- ical applications for this technology. The combination of high spatial demand, reduced or absent surface land- marks, and proximity to critical structures is where image guidance was thought to be particularly useful. Any procedure where there is a significant amount of time spent on a task where these conditions exist would benefit from live navigational feedback and could ulti- mately reduce operating time, according to our subjects. The main tasks identified during this exercise were dril- ling adjacent to the carotid, particularly near the clival and petrous portions, and approaching the dura through thick bone (Fig. 3). Recommendations All participants thought that the technology was ready for clinical trials, but some improvements were suggested. The ability to customize the settings, particu- larly the alarm zones, was advised. This included being able to turn individual structure alerts off once the structure was successfully identified. The labor required for anatomical contouring on preoperative imaging was frequently recognized as a barrier to implementation, as it would need to be created, or at least verified, by the operating surgeon.

DISCUSSION Development of advanced navigational systems is occurring in many centers worldwide for an ever-expanding

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