Accurate Localization of 2D Ultrasound Probes Using Outside-In Tracking For Scanning the Neonatal Hip

Developmental Dysplasia of the Hip (DDH) is a common neonatal condition in which delayed diagnosis can lead to lifelong morbidity. If untreated, DDH can cause limping, leg-length discrepancy, pain, disability, and osteoarthritis. 3D ultrasound screening can reduce measurement variability compared to 2D imaging by providing volumetric context and standardized assessment planes. Despite this, 3D ultrasound systems remain expensive and are not widely accessible in most clinical settings. To address this accessibility gap, the Surgical Technologies Lab developed Tracked 2D Ultrasound, which enables clinicians to synthesize 3D volumes from standard 2D ultrasound probes using optical tracking. Although the Lab has demonstrated that this approach achieves sub-millimetric and sub-degree accuracy in position estimation, it introduces practical limitations in clinical workflow. To overcome these constraints, we implemented an Outside-In tracking approach using two markers within the camera’s field of view: one affixed to the ultrasound probe and another to the patient. This is expected to improve probe ergonomics by lowering its weight and lowering its center of mass, to enable more versatile probe use, and to allow the sonographer to concentrate on the patient while being notified of patient motion during the scan via the second marker. The tracking system employs HydraMarkers, flexible fiducial markers that can be conformed to the probe geometry and remain detectable under partial occlusion, enabling robust pose estimation while accommodating curved surfaces. The system was validated through experimental testing under clinically representative conditions. Performance was assessed across all six degrees of freedom in terms of translational and rotational accuracy, repeatability, and robustness. The Outside-In approach achieved sub-millimetric translational and sub-degree rotational accuracy, supporting reliable 3D reconstruction for DDH assessment.