An Mri Coil-mounted Multi-probe Robotic Positioner for Cryoablation

Cryoablation is a percutaneous procedure for treating solid tumors using needle-like instruments. This paper presents an interventional guidance device for faster and more accurate alignment and insertion of multiple probes during cryoablation performed in closed bore magnetic resonance (MR) imaging systems. The device is compact and is intended to be mounted onto a Siemens 110 mm MR loop coil. A cable-driven twodegrees-of-freedom spherical mechanism mimics the wrist motion as it orients the intervention probes about a remote center of motion located 15 mm above the skin. A carriage interfaces with the probes via a thumbscrew-fastened latch to passively release the probes from their tracks, enabling them to be inserted sequentially and freeing them to move with respiration. Small actuator modules containing piezoelectric encoder-based motors are designed to be snap-fit into the device for ease of replacement and sterilization. The robot MRI compatibility was validated with standard cryoablation imaging sequences in 3T MR environment, yielding a maximum of 4% signal to noise ratio during actuator motion. Bench-level device characterization demonstrated a maximum error of 0.78° in the carriage movement. Needle-tip placement experiments for multiple targets in gelatin were performed using our image-guided navigation software, measuring an average targeting error of 2.0 mm. INTRODUCTION Cryoablation, a minimally invasive procedure, treats soft tissue cancer found in the lung, liver, breast, kidney and prostate through the precise placement of liquid nitrogen or pressurized argon gas filled probes [1]. Recent interventional cryoablation studies reported almost 100% efficacy for the treatment of small renal tumors (≤4 cm) [2, 3]. This method is less painful, has lower risk of developing metastatic disease and requires fewer retreatments than radiofrequency ablation [4-6].