© Springer International Publishing Switzerland 2016 657 E. Kyriacou et al. (eds.), XIV Mediterranean Conference on Medical and Biological Engineering and Computing 2016, IFMBE Proceedings 57, DOI: 10.1007/978-3-319-32703-7_127 Tracking of MRI Interventional Devices with Computer-Controlled Detunable Markers Junmo An 1 , Xin Liu 1 , Mahmut Unan 1 , Eftychios G. Christoforou 2 , Andrew G. Webb 3 , and Nikolaos V. Tsekos 1 1 Medical Robotics Laboratory, University of Houston, Houston, TX, USA 2 KIOS Research Center, University of Cyprus, Nicosia, Cyprus 3 C.J.Gorter Center for High Field MRI, Leiden University Medical Center, Leiden, Netherlands Abstract— Accurate localization and fast tracking of interventional tools is an area of paramount importance in procedures with real-time MR guidance. Optically detunable radiofrequency (RF) coils are a robust method for the identifi- cation of individual MR markers for MR-compatible robotics system. In this study, we describe a technique for localizing and tracking four RF coils. Each coil consists of a 3 mm diame- ter inductively coupled solenoid coil with a photoresistor that is optically tuned and detuned. In particular, by selecting only one marker to be MR visible per data collection repetition, this particular marker point of the manipulator is unambiguously distinguished. This technique allows simplification of both the data acquisition and the post processing algorithms. A user- programmable microcontroller times the ON/OFF state of light sources and triggers the MR scanner so that a particular image collection depicts a specific RF coil or combination. Phantom studies at 1.5T demonstrated the technique illustrat- ing the selective observation of one or multiple markers. The described technique can use imaging to track multiple points on an interventional device, such as the shaft of steerable cath- eters and the end-effectors of MR-compatible manipulators. Keywords— Manipulator-driven tracking, optically detunable, RF coils, MR-compatible manipulator, MRI. I. INTRODUCTION Magnetic resonance imaging (MRI) is a reliable medical imaging technique that not only provides excellent visualization of soft-tissue contrast but also excludes use of ionizing radiation. MR markers are an important aspect of localizing and tracking MR-compatible interventional tools. Previous studies of diverse types of MR markers have reported to track and localize interventional devices. Such MR markers can be typically made by passive signal sources [1, 2], active radiofrequency (RF) coils [3] or semi- active inductively coupled RF coils [4-6]. Multiple MR markers are needed e.g., at least two mark- ers are needed to determine the position and localization of a straight interventional tool such as straight needles. More complex and articulated instruments may require a larger number of MR markers for tracking. Interventional manipu- lators, in this case, may require an appropriate number of markers and marker positioning on their articulated end-effectors to ensure adequate monitoring and tracking [7, 8]. To identify the markers on MR image, optical detuning method [9, 10] has been proposed to make one marker or all markers together observable per data acquisition cycle. The purpose of this study is to develop a technique that multiple optically detunable MR markers can be selectively tuned and detuned by the motion of the maneuvering portion of the manipulator. In other words, only one or several mark- ers can be individually active per MR data acquisition to unambiguously identify the markers for tracking manipulator maneuvering. This technique can increase the speed of MR data acquisition and marker identification on MR images. II. MATERIALS AND METHODS A. Design of Controlled Optically Detunable Markers Figure 1(left) illustrates the schematic of a MR-visible marker based on an optically detunable RLC resonant cir- cuit in parallel and Fig. 1(right) shows the photograph of an MR marker. It consists of a small solenoid coil with seven turns of 26 American Wire Gauge (AWG) copper magnet wire (Belden Inc., Richmond, IN) wound around a biocom- patible concentric tube (2.0 mm inner and 3.0 mm outer diameter) and a variable non-magnetic capacitor (8PF - 50PF, Johanson Manufacturing Co., Boonton, NJ) and a photoresistor (GL4548, Nanyang Senba Optical Electronic Co., Shen zhen, China). The tube was filled with hydrogel material (MM3005, IZI Medical Products, Baltimore, MD), which served as the passive MR signal source. Its internal signal source is less evaporative and easier to use than a Gadolinium-based contrast agent. We used a variable capac- itor for tuning because it is even easier to adjust accurately capacitance than coil inductance. The resonant circuit was tuned to 63.7 MHz, the Larmor frequency of our 1.5T MRI scanner (MAGNETOM Avanto, Siemens AG, Medical Solutions, Erlangen, Germany) and can also be tuned to the frequency (127.71 MHz) of 3.0T scanner without recon- structing the circuit by manually trimming the variable ca- pacitor. Its capacitor was placed as close to the solenoid coil as possible for a higher Q factor in an RF coil. For optically detuning via an optical fiber (Eska GH4001, Mitsubishi Rayon Co., Japan), a photoresistor is placed in parallel with