Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Purpose/Objective
Although proton therapy is expected to greatly benefit from integration with magnetic resonance (MR) imaging for on-line image guidance, to date such integration has not been realized. Both the MR scanner’s static (B0) and gradient magnetic fields may compromise beam quality. The aim of our study was 1) to align the field-of-view (FOV) of an MR scanner with a horizontal fixed proton beam line and 2) to assess the effects of the scanner’s B0 and gradient fields on the beam.

Material/methods
Beam alignment: An open MR scanner (MRJ2200, Paramed) featuring a 0.22 T vertical magnetic field was mounted on a trolley and RF-shielded by a compact Faraday cage (Fig. 1). To ensure that the beam traverses the scanner’s magnetic isocentre for beam energies between 70 and 230 MeV, the Lorentz-force induced beam deflection was predicted by Monte Carlo (Geant4) simulations based on Hall probe (HHP-VU, Arepoc) based mapping of the scanner's B0 field. The magnetic isocentre of the scanner was marked by the overlapping gradient fields being visible as dark crosses in 3 orthogonal slices using an MR imaging phantom (ACR Small Phantom). The proton beam was collimated to Ø10 mm and localized in the FOV by radiochromic film (Gafchromic EBT3, Ashland) affixed vertically to the phantom’s front.
Beam quality assessment: With Faraday cage removed, beam profiles were acquired with and without MR scanner for 72, 125 and 219 MeV beams using a pixelated scintillation detector (Lynx, IBA Dosimetry) positioned at 220 cm from the beam exit window. These measurements were repeated while performing spin echo and gradient echo sequences (gradient up to 5.7 mT/m). Planar dose distributions of 72 and 125 MeV beams were measured at the scanner’s FOV with films placed horizontally between two PMMA slabs.

Results
Beam alignment: As a mean lateral deflection of 2 cm was predicted at the magnetic isocenter, the scanner was laterally displaced by 2 cm from the beam’s central axis. The dose distribution on the vertically oriented film confirmed a proper alignment of the beam and the FOV. Thus, the scanner's position was fixed and a cylindrical beam guide was installed into the Faraday cage at beam entrance.
Beam quality assessment: On the scintillation detector, the beam showed a horizontal deflection of 22, 16 and 11 cm for 72, 125 and 219 MeV, respectively, and a vertical deflection below 0.6 mm. The horizontal deflection was taken into account for installing a beam stopper, while vertical deflection was considered negligible. The beam profiles were not affected by the gradient fields of the sequences. Planar film measurements showed curved beam paths with a lateral Bragg peak displacement of 2 and 5 mm for 72 and 125 MeV, respectively (Fig. 2).

Conclusion
Alignment of an open MR scanner’s FOV with a horizontal fixed proton beam has been realized taking into account the scanner’s B0 field induced beam deflection. Sequence-dependent gradient fields do not affect the beam profile.