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.

I. Introduction

The importance of orthopedic surgery amplifies with an increase in life expectancy, as elderly people are more prone to bone-related injuries [1]. For example, 18% of elderly people and 25% of children in the USA suffer from distal radius fracture [1], [2]. The ultrasound (US) image is widely used in scanning bones and many other clinical applications owing to its low cost, high accessibility, and absence of ionizing radiation. For example, in 2017, over 9.2 million US scans were performed in England, which is twice and three times larger than the number of computer tomography (CT) scans and magnetic resonance imaging (MRI), respectively, during the same period [3].