Experimental investigation of a stopping proton beam in liquid water using MR imaging

Introduction
To date, proton therapy is hampered by the lack of reliable in-vivo real-time feedback on the beam range, profile and energy deposition. So far, no technique enables the determination of beam effects on images also showing anatomical information in 2D/3D with high temporal and spatial resolution. The aim of this study is to demonstrate the possibility of visualizing a stopping proton beam in water using MR imaging.
Materials & methods
An open 0.22 T MR scanner was combined with a static proton research beamline to acquire MR images during simultaneous proton beam irradiation. Proton beams with an energy of 190−225 MeV and current of 3−64 nA impinged centrally on a 20 cm PMMA range modulator and were stopped in a water-filled phantom placed inside a dedicated knee MR receiver coil. A variety of different MR pulse sequences including T1- and T2-weighted Spin Echo (SE), Turbo Spin Echo, spoiled and unspoiled T1-weighted Gradient Echo (GE), inversion recovery gradient echo (IRGE), FLASH, Scout and time-of-flight (TOF) Angio were used. For each sequence, coronal images were acquired both with and without irradiation.
Results
The unspoiled GE sequence exhibited a hyper-intense central line artefact that showed a beam energy and current dependent twist under irradiation. The spoiled GE, IRGE, FLASH, Scout and TOF Angio sequences showed hyper- or hypo-intense signatures in the images that varied with the expected range and mimicked the shape of a 2D dose profile. The intensity of the effects depends on the beam current. The beam range determined from the MR images agrees to the expected range within a few millimeters. No beam induced signal changes were observed in the SE sequences.
Conclusion
A stopping proton beam in liquid water can be visualized with MRI. The observed signatures are beam energy and range as well as beam current and dose dependent. The underlying physical principles and the transferability to non-liquid materials needs further investigation.