Technical note: Validation of 3D ultrasound for image registration during oncological liver surgery

Purpose: Registration of pre- and intraoperative images is a crucial step of surgical liver navigation, where rigid registration of vessel centerlines is currently commonly used. When using 3D ultrasound (US), accuracy during navigation might be influenced by the size of the intraoperative US volume, yet the relationship between registration accuracy and US volume size is understudied. In this study, we specify an optimal 3D US volume size for registration using varying volumes of liver vasculature. While previous studies measured accuracy at registered fiducials, in this work, accuracy is determined at the target lesion which is clinically the most relevant structure. Methods: Three-dimensional US volumes were acquired in 14 patients after laparotomy and liver mobilization. Manual segmentation of vasculature and centerline extraction was performed. Intraoperative and preoperative vasculature centerlines were registered with coherent point drift, using different sub-volumes (sphere with radius r = 30, 40, …, 120 mm). Accuracy was measured by fiducial registration error (FRE) between vessel centerlines and target registration error (TRE) at the center of the target lesion. Results: The lowest FRE for vessel registration was reached with r = 50 mm (6.5 ± 2.5 mm), the highest with r = 120 mm (7.1 ± 2.1 mm). Clinical accuracy at the target lesion, resulted most accurate (TRE = 8.8 ± 5.0 mm) in sub-volumes with a radius of 50 mm. Smaller US sub-volumes resulted in lower average TREs when compared to larger US sub-volumes (Pearson's correlation coefficient R = 0.91, p < 0.001). Conclusion: Our results indicate that there is a linear correlation between US volume size and registration accuracy at the tumor. Volumes with radii of 50 mm around the target lesion yield higher accuracy (p < 0.05) (Trial number IRBd18032, 11 September 2018).