Contrast-Enhanced Robotic Capsule Tracking in Ultrasound Using a Dynamic Acoustic Retroreflector

Ingestible robotic capsules are a minimallyinvasive option for diagnosing and treating conditions of the gastrointestinal tract. Ultrasound has the potential to provide the accurate and real-time image guidance that is necessary for site-specific procedures; however, existing ultrasound-based methods have limited clinical utility due to factors such as low capsule contrast, high background noise, unsuitable capsule motion requirements, low frame rates, and reliance on raw ultrasound radiofrequency data, which is rarely accessible in clinical systems. This work presents a high-contrast ultrasound tracking target that can be equipped on a robotic capsule, and that emits a periodic flashing signal in clinically-available B mode images. The tracking target is a shape-changing acoustic retroreflector whose retroreflection can be constructed and destroyed by changing the device's configuration under magnetic actuation. Subsequent spatial localization of the periodic intensity signal in B -mode is accomplished with an efficient Fourier-based network. Tracking of a stationary mock capsule was evaluated in an ex-vivo porcine stomach-agar phantom using a stationary and moving ultrasound probe. The mean tracking error was 2.2 mm with a stationary probe and 3.0 mm with a moving probe. Tracking update rates reached up to 120 Hz. Our method is compatible with B-mode, stationary capsules, and a moving probe, demonstrating potential for clinical use in the localization of robotic capsules.