Purpose: 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.