The following relates generally to use of magnetic resonance (MR) imaging as guidance in radiation therapy, and more specifically to use of MR imaging as guidance in proton therapy. In some embodiments, a cryogenic dewar is provided with multiple channels allowing a proton beam from a proton beam source to pass through. The proton beam may first be aligned with a first channel, and the dewar may then be rotated along with the proton source. The dewar may then be rotated to align a second channel with the proton beam.