Automatic Force-Based Probe Positioning for Precise Robotic Ultrasound Acquisition

The correct orientation of an ultrasound (US) probe is one of the main parameters governing the US image quality. With the rise of robotic ultrasound systems (RUSS), methods that can automatically compute the orientation promise repeatable, automatic acquisition from predefined angles resulting in high-quality US imaging. In this article, we propose a method to automatically position a US probe orthogonally to the tissue surface, thereby improving sound propagation and enabling RUSS to reach predefined orientations relatively to the surface normal at the contact point. The method relies on the derivation of the underlying mechanical model. Two rotations around orthogonal axes are carried out, while the contact force is being recorded. Then, the force data are fed into the model to estimate the normal direction. Accordingly, the probe orientation can be computed without requiring visual features. The method is applicable to the convex and linear probes. It has been evaluated on a phantom with varying tilt angles and on multiple human tissues (forearm, upper arm, lower back, and leg). As a result, it has outperformed existing methods in terms of accuracy. The mean (±SD) absolute angular difference on the in-vivo tissues averaged over all anatomies and probe types is 2.9±1.6° and 2.2±1.5° on the phantom.