Optical Flow and Acoustic Phase Analysis comparison in Ultrasound-based microrobot tracking

Micrometer-sized devices hold the potential to revolutionize medicine by improving diagnostic abilities, increasing therapeutic efficiency and reducing adverse effects. To safely operate microrobots (MRs) inside the human body, accurate localization strategies based on medical imaging should be adopted. Ultrasound (US) imaging has recently gained attention for medical robotics and microrobotics thanks to its noninvasiveness and real-time performances. In this letter we compare US Acoustic Phase Analysis (US-APA), a state-ofthe-art Phase tracking method, with US Optical Flow (US-OF), a computer vision Speckle tracking method. To this aim, different MRs dimensions, different locomotion strategies and working environments were considered. In particular, cylindrical magnetic MRs with diameters spanning from 1.20 down to 0.25 mm were fabricated. They were actuated in tissue-mimicking phantoms through a permanent magnet mounted on a robotic arm to reproduce intravascular rolling and in place vibration in tissue. US-OF proved to be comparable to US-APA in localizing vibrating MRs, achieving a tracking error comparable to one body length. It slightly improved localization during rolling, by consistently tracking the MR with errors lower than 0.6 body length. Additionally, US-OF outperformed US-APA in temporal performances, reaching an output rate forty times higher (about 40 Hz in USOF and about 1 Hz in US-APA). We believe that these results demonstrate the possible effective use of Optical Flow for MRs tracking, with a relevant advantage in terms of portability and intelligibility.