Towards accurate, robust and practical ultrasound-CT registration of vertebrae for image-guided spine surgery.

PURPOSE: Accurate registration of patient anatomy and preoperative computed tomography (CT) images is key to successful image-guided spine surgery. Current manual landmark and surface-based techniques are time-consuming and not always accurate. Intraoperative ultrasound imaging of the vertebrae, combined with automated registration, could improve surgery by improving accuracy, reducing operative time, and decreasing invasiveness.
METHODS: We present a simple ultrasound-CT registration technique that is automated, accurate, and robust. Registration is achieved by aligning the posterior vertebral surface, extracted from both CT and ultrasound images, using a forward and a backward scan line tracing method, respectively. The registration technique is validated using a simple plastic phantom in a water bath and a more realistic porcine cadaver in a simulation of open back surgery.
RESULTS: Clinically relevant accuracy was estimated by comparing automated registrations with gold standard imaging fiducial-based reference transformations, which yielded target registration errors of under 1 mm for the plastic phantom and under 1.6 mm for the porcine cadaver.
CONCLUSIONS: Our registration technique demonstrates good accuracy and robustness under clinically realistic conditions and thus warrants further studies on its surgical application.