Validation of automated ultrasound-CT registration of vertebrae.

PURPOSE: Image-guided spine surgery requires registration of the patient anatomy and preoperative computed tomography (CT) images. A technique for intraoperative ultrasound image registration to preoperative CT scans was developed and tested. Validation of the ultrasound-CT registration technique was performed using porcine cadavers.
METHODS: An ultrasound-CT registration technique was evaluated using 18 thoracic and lumbar vertebrae of 3 porcine cadavers with 10 different sweep patterns for ultrasound acquisition. For each sweep pattern at each vertebra, 100 randomly simulated initial misalignments were introduced. Each misalignment was registered. The resulting registration transformations were compared to gold standard registrations based on implanted fiducials to assess accuracy and robustness of the technique.
RESULTS: The orthogonal-sweep acquisition was found to perform best and yielded a registration accuracy of 1.65 mm across all vertebrae on all porcine cadavers, where 82.5% of the registrations resulted in target registration errors below the 2 mm threshold recommended by a joint report from the experts in the field. In addition, we found that registration accuracy varies by the sweep pattern and vertebral level, but neighboring vertebrae tend to result in statistically similar accuracy. Ultrasound-CT registration took an average of 2.5 min to run, and the total registration time per vertebra (also including time for ultrasound acquisition and reconstruction) is approximately 8 min.
CONCLUSIONS: A previously described ultrasound-CT registration technique yields clinically acceptable accuracy and robustness on multiple vertebrae across multiple porcine cadavers. The total registration time is shorter than that of surface point-based manual registration.