Novel design of an MR-safe pneumatic stepper motor for MRI-guided robotic interventions

Magnetic resonance imaging (MRI) superiority is wellknown by providing non-ionizing radiation, noninvasive and high-contrast imaging in particular for soft tissues [1]. These advantages have prompted MRI for the various application in surgical interventions ranging from neurosurgery, cardiac ablation to prostate biopsies. However, MR-safe mechatronics is still confronted by the fundamental challenge, namely to maintain zero interference of its imaging operation during the MRI navigation. Currently, four types of MRI actuations have been explored at different MR-safety conditions [2]: 1) electric actuators, e.g. piezoelectric motors and ultrasonic motors; 2) fluid-power motors; 3) MRpowered actuators. In terms of adaptability in general hospital setup, image quality and MR-safety, pneumatic actuators are advantageous in both material and energetics considerations. The material of pneumatic actuators could be non-magnetic and non-conducting, minimizing the effects on inhomogeneity of magnetic field. Pressured clean air as power supply is commonly available in MRI scanner rooms. This ensures zero image artifacts caused by the electromagnetic (EM) waves of electricity. Such pneumatic stepper actuators with the capability of generating accurate stepwise motion have been introduced recently. Stoianovici et al. [3] invented the first MR-safe pneumatic stepper motor that comprises three actuated diaphragms driving a hoop gear. Several pneumatic stepper motors [4-6] have been sequentially developed and their performances (e.g. torque-speed) have been demonstrated. However, many technical challenges still have not been addressed in these motors, e.g. typically large in motor size, high cost for the complicated fabrication and sterilization, dissatisfactory signal-to-noise ratio (SNR), as well as image distortion induced by the proximal electronics and valves of motor drivers. In this paper, we propose a novel MR-safe pneumatic stepper motor, whose design could be relatively compact, flexibly customized for various actuation requirements. Such a motor can be made of a homogeneous material for ease of minimization and reconfiguration. One set of design parameters are selected for the experimental evaluation. Self-locking and high speed (up to 160RPM) is achieved in both rotation directions. Steady torque within a wide range of speed can also be preserved. Low imaging interference has been experimentally demonstrated while operating the motor inside the MRI scanner. Regarding these specifications, this motor is potential to be incorporated into an MRI-compatible robot for needle manipulation during intra-operative procedures, e.g. prostate surgery.