title: Thermal magnetic resonance: physics considerations and electromagnetic field simulations up to 23.5 Tesla (1GHz).
abstract: BACKGROUND
  Glioblastoma multiforme is the most common and most aggressive malign brain tumor. The 5-year survival rate after tumor resection and adjuvant chemoradiation is only 10 %, with almost all recurrences occurring in the initially treated site. Attempts to improve local control using a higher radiation dose were not successful so that alternative additive treatments are urgently needed. Given the strong rationale for hyperthermia as part of a multimodal treatment for patients with glioblastoma, non-invasive radio frequency (RF) hyperthermia might significantly improve treatment results.
  
  
  METHODS
  A non-invasive applicator was constructed utilizing the magnetic resonance (MR) spin excitation frequency for controlled RF hyperthermia and MR imaging in an integrated system, which we refer to as thermal MR. Applicator designs at RF frequencies 300 MHz, 500 MHz and 1GHz were investigated and examined for absolute applicable thermal dose and temperature hotspot size. Electromagnetic field (EMF) and temperature simulations were performed in human voxel models. RF heating experiments were conducted at 300 MHz and 500 MHz to characterize the applicator performance and validate the simulations.
  
  
  RESULTS
  The feasibility of thermal MR was demonstrated at 7.0 T. The temperature could be increased by ~11 °C in 3 min in the center of a head sized phantom. Modification of the RF phases allowed steering of a temperature hotspot to a deliberately selected location. RF heating was monitored using the integrated system for MR thermometry and high spatial resolution MRI. EMF and thermal simulations demonstrated that local RF hyperthermia using the integrated system is feasible to reach a maximum temperature in the center of the human brain of 46.8 °C after 3 min of RF heating while surface temperatures stayed below 41 °C. Using higher RF frequencies reduces the size of the temperature hotspot significantly.
  
  
  CONCLUSIONS
  The opportunities and capabilities of thermal magnetic resonance for RF hyperthermia interventions of intracranial lesions are intriguing. Employing such systems as an alternative additive treatment for glioblastoma multiforme might be able to improve local control by "fighting fire with fire". Interventions are not limited to the human brain and might include temperature driven targeted drug and MR contrast agent delivery and help to understand temperature dependent bio- and physiological processes in-vivo.
authors.affiliation: Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine, Berlin, Germany. Lukas.Winter@mdc-berlin.de.
authors.forename: Lukas
authors.initials: L
authors.lastname: Winter
authors.affiliation: Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine, Berlin, Germany.
authors.forename: Celal
authors.initials: C
authors.lastname: Oezerdem
authors.affiliation: Physikalisch Technische Bundesanstalt (PTB), Berlin, Germany.
authors.forename: Werner
authors.initials: W
authors.lastname: Hoffmann
authors.affiliation: Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine, Berlin, Germany.
authors.forename: Tessa
authors.initials: T
authors.lastname: van de Lindt
authors.affiliation: Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine, Berlin, Germany.
authors.forename: Joao
authors.initials: J
authors.lastname: Periquito
authors.affiliation: Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine, Berlin, Germany.
authors.forename: Yiyi
authors.initials: Y
authors.lastname: Ji
authors.affiliation: Department of Radiation Oncology, Charité Universitätsmedizin Berlin, Berlin, Germany.
authors.forename: Pirus
authors.initials: P
authors.lastname: Ghadjar
authors.affiliation: Department of Radiation Oncology, Charité Universitätsmedizin Berlin, Berlin, Germany.
authors.forename: Volker
authors.initials: V
authors.lastname: Budach
authors.affiliation: Department of Radiation Oncology, Charité Universitätsmedizin Berlin, Berlin, Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Wust
authors.affiliation: Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine, Berlin, Germany.|Experimental and Clinical Research Center (ECRC), a joint cooperation between the Charité Medical Faculty and the Max-Delbrück Center for Molecular Medicine, Berlin, Germany.|MRI.TOOLS GmbH, Berlin, Germany.
authors.forename: Thoralf
authors.initials: T
authors.lastname: Niendorf
mesh_terms: D004574:Electromagnetic Fields; D006801:Humans; D006979:Hyperthermia, Induced / Q000379:methods*; D008279:Magnetic Resonance Imaging; D009682:Magnetic Resonance Spectroscopy / Q000295:instrumentation / Q000379:methods* / Q000627:therapeutic use; D008962:Models, Theoretical; D010825:Physics; D000078702:Radiofrequency Therapy