Characterising focused ultrasound via high speed shadowgraphic imaging at 10 million frames per second

In this paper, ultra-high speed shadowgraphic imaging at 10 × 10⁶ frames-per-second with an effective temporal resolution of 10ns per frame, is used to characterise aspects of the acoustic field, the medium of propagation, and phase characterization of a needle hydrophone. Specifically, at the third harmonic of the driving transducer, constructive and destructive interference is observed in close proximity to the needle hydrophone, due to reflections of the propagating field from the needle hydrophone. At the fundamental, speed of sound estimation from cross-correlating temporal changes in gray scale value of two individual pixels along propagating axis. Furthermore, phase characterisation of needle hydrophone from 0.7MHz to 1MHz and at 1.2MHz, using deconvolved needle hydrophone data at the fundamental driving frequency, 1.1MHz, to estimate a propagation time delay between pixel sample position and deconvolved waveform. Most reliable results for both speed of sound estimation and phase characterisation of the needle hydrophone are achieved using optical data close to the acoustic focus, where both measurements are in agreement with theory and calibration data.