A Decoupled Parallelogram-Flexure Mechanism for Remote Center of Motion in Magnetic Resonance Imaging-Guided Abdominal Interventions

Abstract This article presents the design, analysis, and experimental validation of a magnetic resonance imaging (MRI)-conditional robotic system for abdominal intervention featuring a mechanically stationary remote center of motion (RCM). The proposed architecture employs a decoupled dual-parallelogram mechanism with flexure-based compliant joints, achieving compactness and wear-free operation. The actuation layout of the flexures was optimized using freedom and constraint topology to minimize parasitic deflection and improve transmission efficiency. An inchworm-type pneumatic insertion module with flexure-integrated gripping enables long-stroke needle advancement within the constrained MRI bore. The fabricated prototype, measuring 296 × 230 × 190 mm3, provides three active degrees-of-freedom (2R1T) driven entirely by nonmagnetic pneumatic actuators. Experimental characterization demonstrates a yaw motion range of ±20deg, pitch range of −40deg to +25deg, and translational range of 50.8 mm. The compliant virtual pivot achieves a ±24deg rotation range with stresses maintained within 80% of yield strength, confirming structural safety. RCM accuracy tests using an OptiTrack motion capture system revealed root mean square errors of 0.65 mm (pitch) and 1.04 mm (yaw) without stabilizing flexures, validating precise geometric constraint through the flexure-based joints. The pneumatic collet gripper generated a maximum gripping force of 13 N at 100 psi, exceeding clinical puncture-force requirements. These results confirm that the decoupled parallelogram-flexure architecture achieves the desired balance of MRI compatibility, geometric precision, and mechanical robustness. Compared with existing MRI-guided abdominal robotic systems, the proposed mechanically stationary RCM minimizes physical interference with the patient’s abdomen while preserving a compact footprint within the MRI bore. The resulting kinematic structure admits an intuitive spherical mapping between joint space and needle trajectory, simplifying trajectory planning and intra-procedural adjustment under image guidance. Moreover, the integrated inchworm-type insertion mechanism enables continuous, long-range needle advancement without repeated manual re-positioning or scanner exit, thereby streamlining clinical workflow and enhancing procedural efficiency.