Vector Flow Imaging in Layered Models With a High Speed of Sound Contrast Using Pulse-Echo Ultrasound and Photoacoustics.

In this study, we develop vector flow imaging (VFI) techniques for multilayered models with a high wavespeed contrast using photoacoustic (PA) and ultrasonic imaging. Refraction-corrected delay-and-sum (RC-DAS) image reconstruction enforces Snell's law to accurately calculate time delays within each layer. We compare RC-DAS against conventional delay-and-sum (DAS) for VFI in benchtop phantoms made of transparent polymethyl methacrylate (PMMA) in a water bath. The flow of a slow-moving suspension of carbon microspheres (~4 mm/s) beneath a PMMA layer is studied using two phantoms, where the PMMA layer has different shapes and thicknesses. We acquire data using interleaved PA and multiangle plane wave ultrasound (US) acquisitions measured with a 7.6-MHz linear US array. PA waves are generated by a 1064-nm wavelength nanosecond-pulsed laser at 50 Hz, and multiangle plane wave US data are acquired at 100 Hz for 11 steering angles between ±10°. RCDAS improves the flow speed accuracy, reducing the mean absolute error (MAE) by 0.56-0.70 mm/s compared to the theoretical parabolic flow profile. The error in direction estimates improves when we use RC-DAS, with the interdecile range (IDR) reducing by up to 17°. This work emphasizes the importance of refraction correction for accurate flow measurements in layered media with PA and ultrasonic imaging. While both imaging modalities can quantify flow in these multilayered models, the modality best-suited for a specific application will depend on the imaging target and flow dynamics. These techniques show promise for biomedical applications such as intraosseous and transcranial blood flow quantification, and in nondestructive testing (NDT) to monitor fluid motion.