Characterization of in-beam MR imaging performance during proton beam irradiation

Characterization of in-beam MR imaging performance during proton beam irradiation

Hoffmann, A.; Gantz, S.; Grossinger, P.; Karsch, L.; Pawelke, J.; Serra, A.; Smeets, J.; Schellhammer, S.

Given the sensitivity of proton therapy (PT) to anatomical changes, it could greatly benefit from integration with magnetic resonance (MR) imaging. Hence, there is growing interest to investigate the technical feasibility of MR-integrated proton therapy (MRiPT). The aim was to operate an MRI system in the beam of a PT facility and to characterize the MR imaging performance during simultaneous irradiation.

Material and Methods
A 0.22 T open MR scanner (MrJ2200, Paramed Medical Systems) was installed in a compact Faraday cage at the fixed horizontal beamline of our PT facility. A beam guide in the wall of the cage allows beam transmission to the field-of-view (FOV) of the scanner. The scanner’s magnetic isocenter was aligned, such that a 10 mm diameter collimated proton beam of 125 MeV was stopped in the most distal image slice of the ACR Small Phantom, which was centrally positioned in the FOV inside a dedicated knee coil. Prior to irradiation, the magnet was shimmed and the magnetic field homogeneity (MFH) was mapped over a 22 cm diameter spherical volume by a magnetic field camera (MFC3045, Metrolab). To assess the effect of magnetic fringe fields of the nearby beam line magnets, the MFH measurements were repeated while these magnets were energized for beam energies between 70-220 MeV. During irradiation, the phantom was imaged using T1 and T2-weighted spin echo (SE) sequences with parameter settings according to the phantom test guidance from the ACR. Additionally, two gradient echo (GRES and GREL) scans were performed with a short repetition time (TR) and long echo time (TE): TR = 30 and 80 ms, and TE = of 8 and 30 ms, respectively, a flip angle of 20 deg and acquired voxel size of 0.63x0.79x5.00 mm3. A validated software tool (MATLAB) was used to extract the ACR imaging parameters and to estimate a geometric transformation from image pairs with and without beam.

After shimming, the peak-to-peak MFH was 88 ppm, which is within the scanner’s operating specifications. The MFH measurements with and without energized beam line magnets show no significant differences, but the baseline resonance frequency was increased by 70-110 Hz depending on beam energy. The SE and GRE image quality was sufficient for analysis. Differences in ACR parameters due to operating the beam line magnets or the beam were within measurement uncertainties. A sequence-dependent translation of 0.5-3 mm in frequency encoding direction was observed in the images due to empowering of the beam line magnets, with GREL being the most sensitive sequence.

No degradation of the performance of the in-beam MR system was found during simultaneous operation with the PT system. Although MR imaging during irradiation does not deteriorate the ACR parameters, there is a sequence-dependent off-resonance image displacement when the beam line magnets are energized. This proof-of-concept justifies further research towards the development of a first prototype for MRiPT.

Keywords: MR imaging; proton therapy; image quality

Publ.-Id: 26205