title: Technical Note: Experimental results from a prototype high-field inline MRI-linac.
abstract: OBJECTIVE
  The pursuit of real-time image guided radiotherapy using optimal tissue contrast has seen the development of several hybrid magnetic resonance imaging (MRI)-treatment systems, high field and low field, and inline and perpendicular configurations. As part of a new MRI-linac program, an MRI scanner was integrated with a linear accelerator to enable investigations of a coupled inline MRI-linac system. This work describes results from a prototype experimental system to demonstrate the feasibility of a high field inline MR-linac.
  
  
  METHODS
  The magnet is a 1.5 T MRI system (Sonata, Siemens Healthcare) was located in a purpose built radiofrequency (RF) cage enabling shielding from and close proximity to a linear accelerator with inline (and future perpendicular) orientation. A portable linear accelerator (Linatron, Varian) was installed together with a multileaf collimator (Millennium, Varian) to provide dynamic field collimation and the whole assembly built onto a stainless-steel rail system. A series of MRI-linac experiments was performed to investigate (1) image quality with beam on measured using a macropodine (kangaroo) ex vivo phantom; (2) the noise as a function of beam state measured using a 6-channel surface coil array; and (3) electron contamination effects measured using Gafchromic film and an electronic portal imaging device (EPID).
  
  
  RESULTS
  (1) Image quality was unaffected by the radiation beam with the macropodine phantom image with the beam on being almost identical to the image with the beam off. (2) Noise measured with a surface RF coil produced a 25% elevation of background intensity when the radiation beam was on. (3) Film and EPID measurements demonstrated electron focusing occurring along the centerline of the magnet axis.
  
  
  CONCLUSIONS
  A proof-of-concept high-field MRI-linac has been built and experimentally characterized. This system has allowed us to establish the efficacy of a high field inline MRI-linac and study a number of the technical challenges and solutions.
authors: Department of Medical Physics, Ingham Institute for Applied Medical Research, Liverpool NSW 2170, Australia; Radiation Physics, Liverpool Cancer Therapy Centre, Liverpool NSW 2170, Australia; School of Medicine, University of New South Wales, Sydney NSW 2170, Australia; and Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.
authors: G P
authors: GP
authors: Liney
authors: Department of Medical Physics, Ingham Institute for Applied Medical Research, Liverpool NSW 2170, Australia.
authors: B
authors: B
authors: Dong
authors: Radiation Physics, Liverpool Cancer Therapy Centre, Liverpool NSW 2170, Australia.
authors: J
authors: J
authors: Begg
authors: Radiation Physics & Liverpool Cancer Therapy Centre, Liverpool, NSW 2170, Australia and Radiation Physics Laboratory, Sydney Medical School, University of Sydney, NSW 2006, Australia.
authors: P
authors: P
authors: Vial
authors: Department of Medical Physics, Ingham Institute for Applied Medical Research, Liverpool NSW 2170, Australia.
authors: K
authors: K
authors: Zhang
authors: Radiation Physics Laboratory, Sydney Medical School, University of Sydney, NSW 2006, Australia.
authors: F
authors: F
authors: Lee
authors: Medical Physics, Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia and Radiation Physics, Liverpool Cancer Therapy Centre, Liverpool NSW 2170, Australia.
authors: A
authors: A
authors: Walker
authors: Medical Physics, Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia and Radiation Physics, Liverpool Cancer Therapy Centre, Liverpool NSW 2170, Australia.
authors: R
authors: R
authors: Rai
authors: Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.
authors: T
authors: T
authors: Causer
authors: Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.
authors: S J
authors: SJ
authors: Alnaghy
authors: Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia and Illawarra Cancer Care Centre, Wollongong Hospital, NSW 2500, Australia.
authors: B M
authors: BM
authors: Oborn
authors: Department of Medical Physics, Ingham Institute for Applied Medical Research, Liverpool NSW 2170, Australia; Radiation Physics, Liverpool Cancer Therapy Centre, Liverpool NSW 2170, Australia; School of Medicine, University of New South Wales, Sydney NSW 2170, Australia; Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia; and Radiation Physics Laboratory, Sydney Medical School, University of Sydney, NSW 2006, Australia.
authors: L
authors: L
authors: Holloway
authors: Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.
authors: P
authors: P
authors: Metcalfe
authors: Department of Medical Physics, Ingham Institute for Applied Medical Research, Liverpool NSW 2170, Australia and School of Medicine, University of New South Wales, Sydney NSW 2170, Australia.
authors: M
authors: M
authors: Barton
authors: School of Information Technology & Electrical Engineering, University of Queensland, Brisbane, QLD 4072, Australia.
authors: S
authors: S
authors: Crozier
authors: Radiation Physics Laboratory, Sydney Medical School, University of Sydney, Sydney NSW 2170, Australia.
authors: P
authors: P
authors: Keall
mesh_terms: D004867:Equipment Design; D005240:Feasibility Studies; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D010315:Particle Accelerators; D061089:Radiotherapy, Image-Guided