Reconstruction of thermoacoustic emission sources from proton irradiation using numerical time reversal

Abstract

Dose delivery in proton beam therapy for cancer treatment can be mapped by analyzing thermoacoustic emissions measured by ultrasound arrays. Here, a method is presented for spatial mapping of thermoacoustic sources using numerical time reversal, simulating physical re-transmission of measured emissions into the medium. The spatial distribution of acoustic sources is shown to be approximated by the amplitude envelope of the time-reversed field, evaluated at the time of emission. Given calibration of the array sensitivity and knowledge of tissue properties, this approach approximately reconstructs the induced acoustic pressure, equal to the product of radiation dose, density, and Grueneisen parameter. Numerical time reversal is implemented using two models for array elements, as either ideal line sources or diffracting rectangular radiators. Demonstrated reconstructions employ previously reported measurements of thermoacoustic emissions from proton energy deposition in tissue-mimicking phantoms. For a phantom incorporating a bone layer, reconstructions account for the higher sound speed in bone. Spatial resolution of reconstructions, assessed by widths of reconstructed Bragg peaks, is improved in the array direction by incorporation of diffraction effects. In comparisons with corresponding Monte Carlo simulations, source distributions correspond well with simulated proton dose, while source localization with respect to room coordinates is improved by incorporating sound speed inhomogeneities.