A novel comanipulation device for assisting needle placement in ultrasound guided prostate biopsies

A novel robotic device, aimed at assisting a urologist in performing prostate biopsies guided by an endorectal ultrasound probe, is described. The paper describes the robot kinematics and the actuation system. The actuation system combines electromagnetic brakes, balancing springs, and electrical motors with cable transmissions. The robot supports two modes of operation: the free mode, where the entire control of the probe movements is left to the urologist, and the blocked mode, where the robot precisely maintains the probe at a given position and orientation with respect to the prostate. For the blocked mode, the set of specifications is antagonistic: firstly, a security constraint requires a low robot stiffness to allow to compliantly adapt to potential movements from the patient; secondly, a precision constraint requires a high robot stiffness in order to maintain the position and orientation of the probe in the presence of unmodeled external forces, when the robot is switched from the free mode to the blocked mode. A control strategy is developed to obtain this behavior. It combines an inner impedance controller with a relatively low stiffness and an outer intelligent position integrator that operates only during a limited period of time, when switching from the free mode to the blocked mode. Both in vitro and in cadavero experimental results show the efficiency of this approach.