Experimental Validation of an Elastically Averaged Binary Manipulator for MRI-Guided Prostate Cancer Interventions

Polymer-based binary robots and mechatronics devices can lead to simple, robust, and cost effective solutions for Magnetic Resonnace Image-guided (MRI) medical procedures. A binary manipulator using 12 elastically averaged air muscles has been proposed for MRI-guided biopsies and brachytherapies procedures used for prostate cancer diagnostic and treatment. In this design, radially-distributed air muscles position a needle guide relatively to the MRI table. The system constitutes an active compliant mechanism where the compliance relieves the over-constraint imposed by the redundant parallel architecture. This paper presents experimental results for repeatability, accuracy, and stiffness of a fully functional manipulator prototype. Results show an experimental repeatability of 0.1 mm for point-to-point manipulation on a workspace diameter of 80 mm. Manipulator average accuracy is 4.7 mm when based on the nominal (uncalibrated) model and improves to 2.1 mm when using a calibrated model. The estimated stiffness at the end-effector is ∼0.95 N/mm and is sufficient to withstand the needle insertion forces without major deflection. Needle trajectories during state change appear to be primarily driven by the system’s elastic energy gradient. The study shows the manipulator prototype to meet its design criteria and to have the potential of becoming an effective and low-cost manipulator for MRI-guided prostate cancer treatment.