title: Real-time active MR-tracking of metallic stylets in MR-guided radiation therapy.
abstract: OBJECTIVE
  To develop an active MR-tracking system to guide placement of metallic devices for radiation therapy.
  
  
  METHODS
  An 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 ∼5 mm beyond the strong B0 inhomogeneity region near the metal surface. An MR-tracking sequence with phase-field dithering was used to overcome residual effects of B0 and B1 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.
  
  
  RESULTS
  The tracking system provided high-resolution (0.6 × 0.6 × 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.
  
  
  CONCLUSIONS
  This 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.
authors.affiliation: Radiology, Brigham and Women's Hospital, Boston, Massachusetts, USA; Radiation Oncology, Brigham and Women's Hospital, Boston, Massachusetts, USA.
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.forename: Charles L
authors.initials: CL
authors.lastname: Dumoulin
authors.forename: Akila N
authors.initials: AN
authors.lastname: Viswanathan
authors.forename: Zion T H
authors.initials: ZT
authors.lastname: Tse
authors.forename: Alireza
authors.initials: A
authors.lastname: Mehrtash
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Loew
authors.forename: Isaiah
authors.initials: I
authors.lastname: Norton
authors.forename: Junichi
authors.initials: J
authors.lastname: Tokuda
authors.forename: Ravi T
authors.initials: RT
authors.lastname: Seethamraju
authors.forename: Tina
authors.initials: T
authors.lastname: Kapur
authors.forename: Antonio L
authors.initials: AL
authors.lastname: Damato
authors.forename: Robert A
authors.initials: RA
authors.lastname: Cormack
authors.forename: Ehud J
authors.initials: EJ
authors.lastname: Schmidt
mesh_terms: D000818:Animals; D016477:Artifacts; D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods*; D002645:Chickens; D003196:Computer Graphics; D003198:Computer Simulation; D004574:Electromagnetic Fields; D004867:Equipment Design; D005260:Female; D057918:Fiducial Markers; D005833:Genital Neoplasms, Female / Q000532:radiotherapy*; D007089:Image Enhancement / Q000295:instrumentation / Q000379:methods; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000295:instrumentation* / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D008670:Metals; D019047:Phantoms, Imaging; D011881:Radiotherapy, Computer-Assisted / Q000295:instrumentation* / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation* / Q000379:methods*; D012984:Software
chemical_list: D008670:Metals
keywords: active MR-tracking; metallic device; phase-field dithering; radiation therapy