Photoacoustic speckle tracking for motion estimation and flow analysis.

This study explores photoacoustic (PA) speckle tracking to characterize flow as an alternative to ultrasound (US) speckle tracking or current PA flow imaging methods. In cases where tracking of submicrometer particles is required, the US signal-to-noise ratio and contrast might be low due to limited reflectivity of subwavelength size targets at low concentrations. However, it may be possible to perform more accurate velocimetry using PAs due to different contrast mechanisms utilized in PA imaging. Here, we introduce a PA-based speckle tracking method that overcomes the directional dependence of Doppler imaging and the limited field of view of current correlation-based methods used in PA flow imaging. The feasibility of this method is demonstrated in a potential application-minimally invasive diagnosis of ventricular shunt malfunction, where the velocity of optically absorbing particles was estimated in a shunt catheter using block matching of PA and US signals. Overall, our study demonstrates the potential of the PA-based motion tracking method under various flow rates where US imaging cannot be effectively used for specking tracking because of its low contrast and low signal-to-noise ratio.