Measurement of flow-dependent decorrelation times of scattering samples using acousto-optic light

Acousto-optic imaging (AOI) combines light and ultrasound (US) to probe the optical properties inside a multiple scattering medium, such as biological tissue, with a resolution set by the ultrasound. However, in dynamic media like tissue, the motion of scatterers decorrelates the US-tagged optical field, broadening its spectral width and reducing the efficiency of holography-based detection schemes. Speckle decorrelation is usually considered a limitation, as it degrades the signal-to-noise ratio. Here, we show that, despite this, it can be exploited as a source of contrast. Using four-phase digital holography, we measured the spectral broadening of US-tagged light propagating through a scattering fluid flowing at controlled speeds through a tube embedded in an agar phantom. By sweeping a frequency shift of our reference wave over a 0 kHz-700 kHz range, we determined spectral width going from 222 kHz for a static fluid to 581 kHz for a flow speed of 3 ms^(-1). This corresponds to a decorrelation time of the tagged field going from 0.72 µs to 0.27 µs. This decrease demonstrates that the spectral width of the acousto-optic signal carries quantitative information on local flow dynamics within the acoustic focus. These results suggest that, beyond the need to compensate for the associated loss of signal, decorrelation could be used as an additional, depth-selective contrast mechanism for the non-invasive characterization of flow dynamics in scattering media such as biological tissue.