Cancer is the second leading cause of death in the United States. Approximately half of all cancer patients receive radiation therapy, in which linear accelerators are used to deliver high doses of x-ray radiation to tumors, inducing cell death. X-ray energy deposition causes pressure changes that produce acoustic signals due to the photoacoustic effect. Here, clinical x-ray beams were directed at test objects made of antimonial lead and other metallic materials within a water tank. Photoacoustic signals were measured using a calibrated broadband hydrophone and validated using simulations in k-Wave. Linear and two-dimensional synthetic apertures were formed by mechanically scanning the x-ray source and test object within a single plane. Tomographic images of test objects, reconstructed from measured photoacoustic signals, show good agreement with object geometry. X-ray doses incurred by the test objects are mapped based on the reconstructed acoustic pressure sources and Grüneisen parameter of the material employed. Potential applications to in vivo dosimetry for x-ray and proton therapy, potentially enabling safer and more effective treatments, are discussed.

Objective. Mapping of dose delivery in proton beam therapy can potentially be performed by analyzing thermoacoustic emissions measured by ultrasound arrays. Here, a method is derived and demonstrated for spatial mapping of thermoacoustic sources using numerical time reversal, simulating re-transmission of measured emissions into the medium. Approach. Spatial distributions of thermoacoustic emission sources are shown to be approximated by the analytic-signal form of the time-reversed acoustic field, evaluated at the time of the initial proton pulse. Given calibration of the array sensitivity and knowledge of tissue properties, this approach approximately reconstructs the acoustic source amplitude, equal to the product of the time derivative of the radiation dose rate, mass density, and Grüneisen parameter. This approach was implemented using two models for acoustic fields of the array elements, one modeling elements as line sources and the other as rectangular radiators. Thermoacoustic source reconstructions employed previously reported measurements of emissions from proton energy deposition in tissue-mimicking phantoms. For a phantom incorporating a bone layer, reconstructions accounted for the higher sound speed in bone. Dependence of reconstruction quality on array aperture size and signal-to-noise ratio was consistent with previous acoustic simulation studies. Main Results. Thermoacoustic source distributions were successfully reconstructed from acoustic emissions measured by a linear ultrasound array. Spatial resolution of reconstructions was significantly improved in the azimuthal (array) direction by incorporation of array element diffraction. Source localization agreed well with Monte Carlo simulations of energy deposition, and was improved by incorporating effects of inhomogeneous sound speed. Significance. The presented numerical time reversal approach reconstructs thermoacoustic sources from proton beam radiation, based on straightforward processing of acoustic emissions measured by ultrasound arrays. This approach may be useful for ranging and dosimetry of clinical proton beams, if acoustic emissions of sufficient amplitude and bandwidth can be generated by therapeutic proton sources.

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.