Towards MRI-Guided Vascular Intervention with an Electromagnetic Tracking System and 3D Navigation Software

D. F. Kacher, S. P. DiMaio, E. Samset, B. Fetics, E. Nevo, F. A. Jolesz Radiology, Brigham and Women's Hospital, Boston, MA, United States, Radiology, Brigham and Women's Hospital, Boston, M, United States, Radiology, Brigham and Women's Hospital, Boston, MA, United Arab Emirates, Robin Medical, Baltimore, MD, United States MRI-guidance has increasing potential to augment or replace conventional X-ray fluoroscopy in cardiac, peripheral-, and neurovascular interventions [1]. Catheter tracking with MRI, using frequency encoding, to localize the NMR signal near small RF coils has been proven feasible [2]. Recently, an electromagnetic (EM) tracking system that utilizes the scanner’s gradients has emerged [3]. Coupling such tracking systems with near real-time imaging [4] and 3D visualization of previously acquired data sets [5] yields a powerful tool for the interventionalist. EM tracking has long been used in the cath lab setting for catheter tracking. The advantage of EM tracking over MRI tracking, is with a second transmitter, it is foreseeable to seemlessly track the sensor inside and outside the bore. Methods: Tracking System. The EndoScout system (Robin Medical Inc, Baltimore MD USA) employs a set of three orthogonal micro-coils to determine the position and orientation of a sensor. The system is calibrated during installation by mapping the gradient coil currents (Gx, Gy and Gz) to the induced gradient fields (Bx, By and Bz), as detected by the sensor. The micro-coil signals are amplified and digitized with dedicated hardware. The induced voltage in the micro-coils can uniquely localize the sensor with any pulse sequence, without the need to toggle modes between tracking and imaging. Tracking data is acquired in the same frame of reference as the MR scans, obviating a need for registration. Moreover, coil dimensions can be minimized since they do not require an imbedded NMR signal source. The mean dynamic errors reported for this system in open air are 0.25mm (σ = 0.29 mm) and 0.07 mm (σ = 0.35) for motion in the XY and XZ planes, respectively [6]. Experimental Setup: A 15 mm ID section of silicon tubing was positioned inside the head coil in a Signa SP 0.5T open MRI scanner (GE Healthcare Milwaukee, WI). SPGR images of the entire volume were acquired, loaded into our in-house software, 3D Slicer [6], and segmented. A 9.4mm cubic sensor containing three 7.4mm diameter orthogonal coils [Fig 1] was advanced through the water-filled tubing. SPGR images centered at the sensor location were acquired every 5 seconds, using the “realtime” feature of the GE workstation in communication with the scanner (precursor of I-drive), and superimposed on the rendering of the tubing within the 3D Slicer visualization interface [Fig 2]. Scan planes were either orthogonal or parallel to the axis of the sensor. A visual icon representing the sensor can be turned on to indicate the position and orientation of the sensor.