Accuracy verification of protoacoustic measurements in a heterogeneous phantom by an optical hydrophone.

BACKGROUND: Protoacoustics has emerged as a promising real-time range measurement method for proton therapy. Optical hydrophones (OHs) are considered suitable to detect protoacoustic waves owing to their ultracompact size and high sensitivity. In our previous research, we demonstrated that the time-of-arrival (TOA) measured by an OH showed good agreement with the simulated ground truth in a homogeneous medium. PURPOSE: The purpose of the study was to experimentally evaluate the accuracy of the TOA and compression peak pressures detected by the OH. Protoacoustic waves that undergo the typical distortions occurring in the human body were investigated. In such cases, the use of small detectors such as OHs is desirable to minimize the effects of detector size and directivity. METHODS: A 100-MeV proton pencil beam emitted from a fixed-field alternating gradient accelerator was irradiated onto a homogeneous water phantom and a water phantom with a half- or full-sized silicone plate downstream of the Bragg peak (BP) or a bone plate that covered half of the beam cross-section in the beam path. The OH was shifted 70 mm laterally across the beam axis downstream of the BP to measure the protoacoustic waves. The k-WAVE acoustic wave transport simulation was employed as the ground truth. The TOA and the first compression peak pressures were compared between the simulation and experiment. RESULTS: The TOA deviation against the ground truth was primarily attributed to alignment errors of the measurement devices and phantoms, with deviations of < 1 mm. The peak pressure distribution closely resembled the ground truth, with FWHM differences of 0.0%-3.0% for the tested geometries. CONCLUSION: The OH was able to determine the TOA and peak pressures with sufficient accuracy in heterogeneous phantoms, even without considering the effect of the size of the detector or directivity on the measurements.