Proton beam visualisation for in-beam MR imaging

Introduction
In-beam MRI is expected to improve the targeting accuracy of proton therapy for moving target volumes providing real-time anatomical images and allowing the simultaneous visualisation of the therapeutic proton beam in liquid-filled phantoms [1,2]. The aim of this contribution is to provide an overview of our previous work on MRI-based proton beam visualisation.

Materials & Methods
A 0.22 T open MR scanner was positioned at a fixed horizontal proton research beamline in a clinical proton therapy facility. Water, ethanol, petroleum and mayonnaise phantoms were irradiated with nominal proton beam energies between 190 - 225 MeV at beam currents of 1 - 64 nA. A range of pulse sequences was used for the acquisition of a horizontal slice within the beam volume. Material, sequence, beam current and energy dependence of the beam signal were evaluated.

Results
The proton beam induces a beam current and energy dependent MRI signal in liquids of low viscosity. For fixed beam current setting, the beam range in water extracted from the MR images matches the expected residual range within a few millimetres. Gradient echo-based pulse sequences appear more sensitive to the beam-induced effect than spin echo-based sequences.

Summary
The method holds potential for on-line quality assurance for MR-integrated proton therapy. The underlying image contrast mechanism requires elucidation to enable the development of specifically tailored sequences with increased sensitivity for the beam-induced effect.

Appendix 1

Figure 1: Beam current dependence of the 207 MeV beam signal in water acquired using a Time-of-Flight-Angiography sequence.


Figure 2: Inversion Recovery-Gradient Echo images of water under irradiation at a beam current of 9 nA. The dotted lines indicate the expected proton ranges.

References
[1] Schellhammer SM. Technical feasibility of MR-integrated proton therapy: Beam deflection
and image quality. Doctoral thesis, Technische Universität Dresden, 2019.
[2] Gantz S. Investigation of the physical and technical feasibility of MRI integrated proton
therapy using a horizontally scanning beam. Doctoral thesis, Technische Universität Dresden,
2021.