A robot system for use in surgical procedures has two movable arms, each arm having a six of degrees of freedom of movement and an end effector which can be rolled about its axis and an actuator which can slide along the axis for operating different tools adapted to be supported by the end effector. Each end effector including optical force sensors for detecting forces applied to the tool by engagement with the part of the patient. The robot is MRI compatible and can be configured to operate within a closed magnet bore. The arms are driven about vertical and horizontal axes by piezoelectric motors.