Investigation of contrast mechanisms for MRI phase signal-based proton beam visualization in water phantoms

Purpose: The low sensitivity and limitation to water phantoms of convection-dependent MRI
magnitude signal-based proton beam visualization hinder its in-vivo applicability in MR-integrated
proton beam therapy. The purpose of the present study was therefore to assess possible contrast
mechanisms for MRI phase signal-based proton beam visualization that can potentially be exploited
to enhance the sensitivity of the method and extend its applicability to tissue materials.
Methods: To assess whether proton beam-induced magnetic field perturbations, changes in material
susceptibility or convection result in detectable changes in the MRI phase signal, water phantom
characteristics, experiment timing and imaging parameters were varied in combined irradiation and
imaging experiments using a time-of-flight angiography pulse sequence on a prototype in-beam MRI
scanner. Velocity encoding was used to further probe and quantify beam-induced convection.
Results: MRI phase signal-based proton beam visualization proved feasible. The observed phase
difference contrast was evoked by beam-induced buoyant convection with flow velocities in the mm/s
range. Proton beam-induced magnetic field perturbations or changes in magnetic susceptibility did not
influence the MRI phase signal. Velocity encoding was identified as a means to further enhance the
detection sensitivity.
Conclusion: Because the MRI phase difference contrast observed during proton beam irradiation of water phantoms is caused by beam-induced convection, this method will unlikely be transferable to tightly compartmentalized tissue wherein flow effects are restricted. Strongly velocity encoded pulse
sequences, however, were identified as promising candidates for the future development of MRI-
based methods for water phantom-based geometric quality assurance in MR-integrated proton beam
therapy.