Navigated 3-Dimensional Intraoperative Ultrasound for Spine Surgery.

Objective: To integrate 3-dimensional (3D) intraoperative ultrasound (iUS) data in spinal navigation.
Methods: In 11 patients with intradural spinal tumors, 3D-iUS was performed before and after tumor resection. Intraoperative computed tomography (iCT) was used for automatic patient registration for spinal navigation; fiducial-based registration was performed in 1 case. The outlines of the vertebra were defined in preoperative image data by automatic mapping; risk and target structures were segmented manually; all these data were rigidly and if necessary non-rigidly registered with iCT. For 3D-iUS acquisition, tracked convex-shaped transducers (contact surface: 29 x 10 mm; scanning frequency: 10-3.8 MHz or 13-5 MHz) were used.
Results: Navigated 3D-iUS was successfully implemented in all cases; 3D-iUS datasets were acquired and could be used as 3D image data for further navigation after iUS scanning. The 3D objects defined in preoperative image data, outlining the vertebra, target and risk structures, could be visualized in the 3D-iUS data. Navigated 3D-iUS allowed to reliably evaluate the extent of resection in all cases and updating of navigation, ensuring high navigational accuracy. The target registration error applying iCT-based automatic registration was 0.78 ± 0.23 mm. The effective dose for iCT was 0.11 ± 0.077 mSv for cervical and 1.75 ± 0.72 mSv for thoracic scans.
Conclusions: Using 3D-iUS can be successfully integrated in spinal navigation. Automatic registration applying low-dose iCT and non-linear image registration offers displaying preoperative images in the same orientation as the 3D-iUS scan, as well as visualizing segmented structures in the navigated 3D-iUS data. This greatly facilitates image interpretation. Navigated 3D-iUS provides a possibility for navigation updating and immediate online quality control.
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