Ultrasound Acoustic Phase Analysis Enables Robotic Visual-Servoing of Magnetic Microrobots

Microrobots (MRs) have attracted growing interest for their potentialities in diagnosis and noninvasive intervention in hard-to-reach body areas. The safe operation of biomedical MRs requires fine control capabilities, which strongly depend on precise and robust feedback about their position over time. Ultrasound acoustic phase analysis (US-APA) may allow for a reliable feedback strategy for MR imaging and tracking in tissue. In this article, we combine task-specific magnetic actuation and related US-APA motion tracking to achieve closed-loop navigation of a magnetic MR, rolling on the boundary of a lumen in a tissue-mimicking phantom. A C-arm system attached to a robotic platform is used to precisely position the magnetic actuation source and US-APA detection unit within the workspace, thus enabling MR visual-servoing. In the first place, the proposed approach allows to perform supervised localization of the MR without any a-priori knowledge of its position. After localization, a robust real-time tracking enables closed-loop MR teleoperation in the phantom lumina over a travel distance of 80 mm (145 body lengths), both in static and counter flow, thus achieving an average position tracking error of 368 micron (0.67 body lengths). For the first time, our results validate US-APA as a reliable feedback strategy for visual-servoing control of MRs in simulated in-body environment.