Comparison of Two Real Time Tracking Methods for a Robotic Assistance System

Introduction Percutaneous interventions are difficult to perform in closed-bore MRI systems, because access to the target organ is restricted due to the limited space in the magnet. This limitation was overcome with a commercial robotic assistance system (RAS), which can precisely position and orient a medical instrument (e.g., a needle) with a robotic arm. To co-register the robot position in the magnet with the MR coordinate system, passive markers filled with contrast agent are attached to distal end of the arm. Compared to localization techniques with active MR marker coils [1], passive markers do not require any additional rf hardware and, thus, do not lead to potentially dangerous device heating at conducting structures [2]. To localize passive markers with sub-pixel precision, recently a fully automatic cross-correlation algorithm has been developed [3, 4]. In this work a real-time pulse sequence with integrated cross-correlation algorithm is developed to track the medical instrument during the intervention. The precision of this marker localization sequence is compared to a tracking method that uses marker coordinates measured by the RAS.