Quantitative Photoacoustic Vascular and Hemodynamic Imaging: Complementary Assessment with Reference Modalities

Photoacoustic imaging (PAI) provides label-free visualization of vascular structure and tissue oxygenation by combining optical absorption contrast with acoustic detection, offering a favorable depth-to-resolution trade-off compared with conventional optical techniques. However, quantitative interpretation of PAI-derived vascular and hemodynamic biomarkers remains challenging because of heterogeneous optical fluence, motion, sequential wavelength acquisition, and limited standardization across imaging platforms and analysis methods. This review summarizes studies that assess PAI-derived vascular and hemodynamic parameters alongside established reference modalities, including near-infrared spectroscopy, photoplethysmography, laser speckle contrast imaging, hyperspectral imaging, Doppler and ultrafast ultrasound, ultrasound localization microscopy, and magnetic resonance imaging. Particular emphasis is placed on the inter-modality relevance of vascular and hemodynamic biomarkers, categorized as direct, indirect, or complementary assessment, across measurements of vascular morphology, perfusion, blood-flow dynamics, hemoglobin-related changes, and oxygenation. Across these studies, reference modalities provide structural or physiological context for interpreting PAI measurements, while PAI offers spatially and depth-resolved assessment of vascular and oxygenation heterogeneity. A better understanding of these inter-modality relationships, combined with standardized quantitative analysis, will be important for improving the reliability, physiological interpretation, and clinical translation of PAI-derived vascular and hemodynamic biomarkers.