Image Performance Characterization of an In-Beam Low-Field Magnetic Resonance Imaging Systemm During Static Proton Beam Irradiation

Image guidance using in-beam real-time magnetic resonance (MR) imaging is expected to improve the targeting accuracy of proton therapy for moving tumors, by reducing treatment margins, detecting inter- and intrafractional anatomical changes and enabling beam gating. The aim of this study was to quantitatively characterize the static magnetic field and image quality of a 0.22 T open MR scanner that has been integrated with a static proton research beamline. The magnetic field and image quality studies were performed using high-precision magnetometry and standardized diagnostic image quality assessment protocols, respectively. The magnetic field homogeneity was found to be typical of the scanner used (98 ppm). Operation of the beamline magnets changed the central resonance frequency and magnetic field homogeneity by a maximum of 16 Hz and 3 ppm, respectively. It was shown that the in-beam MR scanner features sufficient image quality and influences of simultaneous irradiation on the images are restricted to a small sequencedependent image translation and a minor reduction in signalto-noise ratio. Nevertheless, specific measures have to be taken to minimize these effects in order to achieve accurate and reproducible imaging which is required for a future clinical application of MR integrated proton therapy.