MR SAM: Magnetic Resonance Compatible Surgical Manipulator

This paper discusses the optimal design of MR SAM a robot for tool placement during magnetic resonance image-guided surgery. The robot is required to operate in a magnetic environment, therefore a unique set of constraints was placed on the design. The prime constraint was that the final design must be MR safe. This requires the actuators to be placed away from the centre of the imaging area, which led to the decision to investigate a parallel robot architecture. A work cell model was developed along with requirements for the tool motion. For detailed analysis we chose a PSUx6 kinematic structure. The robot uses parallel linear actuators to generate the motion of the tool. Investigations into the effect of geometric parameters on manipulability were performed. It was found that manipulability is maximised when the travelling plate radius is maximised and leg length is minimised. It was also found that if the actuators are arranged in a semi-circular fashion, a vertical shift in the workspace occurred, which is beneficial for the application. It was shown that the optimal base geometry uses three pairs of actuators at 120° intervals around the base and that short (< 60 cm) legs were needed to prevent interference between the scanner and the robot. The length and resolution requirements for the actuators were also found. The robot is predicted to have a spatial resolution of <0.8 mm and an angular resolution of <0.5°. The maximal workspace volume was calculated to be 0.13 m. The actuators should be >50 cm in length and have a sensing resolution of <3.0 x 10 mm. This information is currently used for the detailed design and construction of a prototype.