Proton dosimetry using superheated droplets: Quantification with ultrasound

Abstract

Superheated nanodroplets were proposed for dosimetry in proton therapy. Proton energy deposition within the droplet core leads to droplet vaporization. Vaporizations can be localized through acoustic pulse-echo, and used to measure the proton stopping distribution. Since protons can only lead to vaporizations as they stop, the probability of vaporization is estimated as the probability of a proton stopping within a droplet. Assuming that the number of vaporizations follows a binomial distribution, conversion from vaporizations to number of protons requires knowledge of: droplet size distribution and concentration, and acoustic FoV volume. Here we address the latter. We propose an experimental and numerical method to characterize the 2-way acoustic elevational width of a linear array. First, the impulse response and 1-way field of the acoustic array was measured using a hydrophone. Then, the 1-way field was fitted to field-ii simulations using a least-squares algorithm to estimate the optimal parameters. Finally, the 2-way acoustic field was simulated. The effective elevational width depends on the scattering cross section of the resulting microbubbles, and was determined from the experimental range of backscattering intensities. The method allowed to predict the vaporization counts of experimental measurements where three different size-sorted droplet populations were used.