Hybrid position/force control system of the ultrasonic treatment device by parallel-link robot

Microbubbles are widely used as contrast agents in ultrasound diagnosis. Microbubbles may also has therapeutic uses in the heat amplification of high-intensity focused ultrasound ablation or as carriers of acoustic targeted drug gene delivery therapy. However, microbubbles injected into a blood vessel are diffused throughout the whole body ; therefore, their efficiency is still limited. If microbubbles could be controlled in vivo, their efficiency and efficacy would be significantly improved. To address this issue, we have proposed a technique that controls microbubble behavior in blood vessels using ultrasound emitted from the body surface. To apply the technique in vivo, robotic ultrasound transducer positioning on body surface is required. For this purpose, we have developed a robotic system and confirmed that microbubble can be manipulated by the system. In more practical condition, focal length of an ultrasound transducer has to be considered. To address the issue, we propose a control system considering the focal length in this study. The system consists of a parallel-link robot for ultrasound transducer positioning, a robot controller, and an optical tracking device. The robot has three arms, and a transducer holder, and a six-axis force sensor. The robot controller generates ultrasound emission plans using body surface position measured by the tracking device, and manipulate the robot. As for validation of the system, we performed following experiments ; 1) positioning accuracy evaluation without contact, 2) evaluation of contact forces control, and 3) in vitro ultrasound emission tests. From the first experiment, positioning accuracy was less than 1 mm. As for the contact force control validation, the system could keep required reaction force for ultrasound emission on a phantom surface within 1.5 mm errors. In the third experiment, the errors in the perpendicular direction of the ultrasound axis and the direction of the axis were 0.71 mm and 5.52 mm, respectively. From the results, we confirmed that the system could emit ultrasound to a target by using a hydrophone in a poly(ethylene glycol) monomethacrylate (PEGMA) phantom. Consequently, the results demonstrated that the proposed system could generate appropriate plan and manipulate an ultrasound transducer on body surface considering contact condition with body surface.