Robot-assisted ultrasound probe calibration for image-guided interventions.

Background: Trackable ultrasound probes facilitate ultrasound-guided procedures, allowing real-time fusion of augmented ultrasound images and live video streams. The integration aids surgeons in accurately locating lesions within organs, and this could only be achieved through a precise registration between the ultrasound probe and the ultrasound image. Currently, calibration and registration processes are often manual, labor-intensive, time-consuming, and suboptimal. Technologists manually manipulate a stylus, moving it through various poses within the ultrasound probe's imaging plane to detect its tip in the ultrasound image. This paper addresses this challenge by proposing a novel automated calibration approach for trackable ultrasound probes.

Methods: We utilized a robotic manipulator (KUKA LBR iiwa 7) to execute stylus movements, eliminating the cumbersome manual positioning of the probe. We incorporated a 6-degree-of-freedom electromagnetic tracker into the ultrasound probe to enable real-time pose and orientation tracking. Also, we developed a feature detection algorithm to effectively identify in plane stylus tip coordinates from recorded ultrasound feeds, facilitating automatic selection of calibration correspondences.

Results: The proposed system performed comparably to manual ultrasound feature segmentation, yielding a mean re-projection error of 0.38 mm compared to a manual landmark selection error of 0.34 mm. We also achieved an image plane reconstruction of 0.80 deg with manual segmentation and 0.20 deg with automatic segmentation.

Conclusion: The proposed system allowed for fully automated calibration while maintaining the same level of accuracy as the state-of-the-art methods. It streamlines the process of using a trackable US probe by simplifying recalibration after sterilization when the electromagnetic tracker is externally attached and is required to be disassembled for cleaning and sterilization, or as a part of out-of-factory calibration of US probe with embedded trackers where probes are in mass production.