Accuracy study of a robotic system for MRI-guided prostate needle placement

Background: Accurate needle placement is the first concern in percutaneous MRI-guided prostate interventions. In this phantom study, different sources contributing to the overall needle placement error of a MRI-guided robot for prostate biopsy have been identified, quantified and minimized to the possible extent. Methods: The overall needle placement error of the system was evaluated in a prostate phantom. This error was broken into two parts: the error associated with the robotic system (called 'before-insertion error') and the error associated with needle-tissue interaction (called 'due-to-insertion error'). Before-insertion error was measured directly in a soft phantom and different sources contributing into this part were identified and quantified. A calibration methodology was developed to minimize the 4-DOF manipulator's error. The due-to-insertion error was indirectly approximated by comparing the overall error and the before-insertion error. The effect of sterilization on the manipulator's accuracy and repeatability was also studied. Results: The average overall system error in the phantom study was 2.5mm (STD=1.1mm). The average robotic system error in the Super Soft plastic phantom was 1.3mm (STD=0.7mm). Assuming orthogonal error components, the needle-tissue interaction error was found to be approximately 2.13mm, thus making a larger contribution to the overall error. The average susceptibility artifact shift was 0.2mm. The manipulator's targeting accuracy was 0.71mm (STD=0.21mm) after robot calibration. The robot's repeatability was 0.13mm. Sterilization had no noticeable influence on the robot's accuracy and repeatability. Conclusions: The experimental methodology presented in this paper may help researchers to identify, quantify and minimize different sources contributing into the overall needle placement error of an MRI-guided robotic system for prostate needle placement. In the robotic system analysed here, the overall error of the studied system remained within the acceptable range.