MRI magnitude signal-based proton beam visualisation in water phantoms reflects composite effects of beam-induced buoyant convection and radiation chemistry

Objective. Local magnetic resonance (MR) signal loss was previously observed during proton beam
irradiation of free-floating water phantoms at ambient temperature using a research prototype in-
beam magnetic resonance imaging (MRI) scanner. The emergence of this MR signal loss was
hypothesised to be dependent on beam-induced convection. The aim of this study was therefore to
unravel whether physical conditions allowing the development of convection must prevail for the
beam-induced MRI signatures to emerge. Approach. The convection dependence of MRI magnitude
signal-based proton beam visualisation was investigated in combined irradiation and imaging
experiments using a gradient echo (GE)-based time-of-flight (ToF) angiography pulse sequence,
which was first tested for its suitability for proton beam visualisation in free-floating water phantoms
at ambient temperature. Subsequently, buoyant convection was selectively suppressed in water
phantoms using either mechanical barriers or temperature control of water expansivity. The
underlying contrast mechanism was further assessed using sagittal imaging and variation of T1
relaxation time-weighting. Main results. In the absence of convection-driven water flow, weak beam-
induced MR signal changes occurred, whereas strong changes did occur when convection was not
mechanically or thermally inhibited. Moreover, the degree of signal loss was found to change with the
variation of T1-weighting. Consequently, beam-induced MR signal loss in free-floating water
phantoms at ambient temperature does not exclusively originate from buoyant convection, but is
caused by local composite effects of beam-induced motion and radiation chemistry resulting in a local
change in the water T1 relaxation time. Significance. The identification of ToF angiography sequence-
based proton beam visualisation in water phantoms to result from composite effects of beam-induced
motion and radiation chemistry represents the starting point for the future elucidation of the currently
unexplained motion-based MRI contrast mechanism and the identification of the proton beam-
induced material change causing T1 relaxation time lengthening.