Measure for measure: Diffraction correction for consistent quantification of bubble-related acoustic emissions

To characterize and improve ultrasound-mediated therapies, system-independent measures of cavitation activity are needed. Here, an approach is described to quantify acoustic emissions from ensembles of cavitating bubbles, as measured by passive cavitation detection or imaging (PCD or PCI). An analytic diffraction-correction factor relates measured frequency-dependent pressure to cavitation-radiated power or energy per unit area or volume of a region of interest (ROI). This approach is illustrated via previous experiments modeling two therapeutic ultrasound scenarios. For diffraction-corrected PCD measurements during sonophoresis of ex-vivo porcine skin [Rich et al., JASA 144, 3563–3574 (2018)], radiated subharmonic acoustic power per unit ROI area is highly correlated with decreases in skin electrical resistance (r = 0.823), across sonication frequencies (0.41 and 2.0 MHz), sonication amplitudes, and degassing conditions. For thermal bulk ablation of liver tissue by unfocused ultrasound [Karunakaran et al., UMB 47, 2360–2376 (2021)], measured thermal lesion dimensions are related to PCD-measured radiated powers and energies of subharmonic and broadband acoustic emissions per unit ROI volume across two sonication conditions (3.1 MHz, 20 min; 4.8 MHz, 10 min) and three overpressure conditions modulating bubble activity. Implications are discussed for defining appropriate cavitation doses based on the therapeutic application and PCD or PCI configuration.