Real-Time Active MR-Tracking of Metallic Stylets in MR-Guided Radiation Therapy

PurposeTo develop an active MR-tracking system to guide placement of metallic devices for radiation therapy. MethodsAn actively tracked metallic stylet for brachytherapy was constructed by adding printed-circuit micro-coils to a commercial stylet. The coil design was optimized by electromagnetic simulation, and has a radio-frequency lobe pattern extending approximate to 5 mm beyond the strong B-0 inhomogeneity region near the metal surface. An MR-tracking sequence with phase-field dithering was used to overcome residual effects of B-0 and B-1 inhomogeneities caused by the metal, as well as from inductive coupling to surrounding metallic stylets. The tracking system was integrated with a graphical workstation for real-time visualization. The 3 Tesla MRI catheter-insertion procedures were tested in phantoms and ex vivo animal tissue, and then performed in three patients during interstitial brachytherapy. ResultsThe tracking system provided high-resolution (0.6 x 0.6 x 0.6 mm(3)) and rapid (16 to 40 frames per second, with three to one phase-field dithering directions) catheter localization in phantoms, animals, and three gynecologic cancer patients. ConclusionThis is the first demonstration of active tracking of the shaft of metallic stylet in MR-guided brachytherapy. It holds the promise of assisting physicians to achieve better targeting and improving outcomes in interstitial brachytherapy. Magn Reson Med 73:1803-1811, 2015. (c) 2014 Wiley Periodicals, Inc.