LAUSR

Previous studies have shown that in‐beam magnetic resonance imaging (MRI) can be used to visualize a proton beam during the irradiation of liquid‐filled phantoms. The beam energy‐ and current‐dependent local image contrast observed in water was identified to be predominantly caused by beam‐induced buoyant convection and associated flow effects. Besides this flow dependency, the MR signal change was found to be characterized by a change in the T1${\rm T}_{1}$ relaxation time of water, hinting at a radiochemical contribution, which was hypothesized to lie in oxygen depletion‐evoked T1${\rm T}_{1}$ relaxation time lengthening. The elucidation of the underlying contrast mechanism is required to enable the further assessment of the application potential of MRI‐based proton beam visualization in tissue.