The effect of imaging and tracking parameters on ultrasound probe calibration robustness

Ultrasound-guided interventions are progressively incorporating additional augmented reality (AR) components to improve navigation. A fundamental requirement to integrate ultrasound (US) images into AR environments is US probe calibration, which places images from a tracked US probe in the context of the tracker. To improve the incompatibility of common US probe calibration methods with clinical environments, Chen et al. developed an US calibration method (GUScal) with a focus for intraoperative application. To understand the effect of image quality and tracking accuracy on this calibration method, novice users were recruited to perform the calibration under three different conditions: (1) free-hand with one focal depth, (2) free-hand with three focal depths, and (3) using a mechanical arm to fix the needle with one focal depth. An expert user repeated this process 15 times per condition. The resultant transformation matrices and associated times were recorded for all calibrations. Numerical and visual analysis was conducted to compare the users calibrations results to each other and to the expert results. Based on our results we concluded including multiple focal depths produced the most precise calibrations for novice users. The expert user results showed that stabilizing the needle through a mechanical arm improved the calibration with practice. We recommend the inclusion of additional focal depths and a method to stabilize the needle to produce an accurate calibration in approximately 5 minutes.