Volumetric ultrasound and photoacoustic imaging of inflammatory arthritis in human finger joints via robotic arm powered arc scans.

Ultrasound (US) imaging is a standard clinical tool for assessment of soft tissue inflammation, particularly for imaging the peripheral joints of the hands and feet, which are usually the first to be affected by rheumatoid arthritis. Robotic US has recently gained attention as a means of delivering repeatable and operator-independent imaging. Previously developed robotic US for 3D volumetric imaging of human finger joints relies on the linear scan of the target joint to collect a series of 2D B-mode US images. Although effective in many contexts and simple in control and image reconstruction, the linear scan is suboptimal for imaging cylindrical tissue structures such as human fingers, often exhibiting degraded image quality in peripheral regions due to poor angular alignment with the tissue surface. To address this limitation, we developed a robotic US system incorporating an arc-shaped scanning trajectory designed to maintain a consistently perpendicular orientation to the curved surface of the finger throughout the scan. In an experiment on a spherical phantom, arc scan yielded improved boundary sharpness and image contrast compared to linear scan. In a clinical study involving both a healthy volunteer and an arthritis patient, the arc scan produced better B-mode US image quality in imaging finger joints compared to the linear scan, as reflected in more consistent representations of phalangeal and soft tissue structures along different radial directions. Functional Doppler US and photoacoustic (PA) imaging of finger joints were also conducted. Arc scan and linear scan achieved comparable detection of vascular signals. These findings demonstrate that arc scan facilitated by the robotic arm can achieve improved B-mode US imaging of tissue anatomy in human finger joints while preserving the functional imaging capability of Doppler US and PA imaging.