Imaging light fluence in blood vessels by combining photoacoustic fluctuation imaging and ultrasound power Doppler.

Numerous optical biomedical imaging or therapeutic modalities suffer from unknown light fluence distribution at depths. Photoacoustic (PA) imaging, which enables imaging blood vessels at the acoustic resolution, probes the product between the fluence and effective optical absorption that depends on the size or density of blood vessels. In the case of unresolved vessels, fluence and absorption can not be decoupled using PA imaging alone without the use of inverse problems. Thus, we propose combining two modalities that are sensitive to blood vessels to directly image fluence maps within vascularized areas, including in unresolved vessels.To achieve fluence imaging, the combination of photoacoustic fluctuation (PAFI) and Ultrasound Power Doppler (UPD) images is considered. After exposing a new theoretical expression of the UPD image, we establish a fluence imaging method giving quantitative fluence in blood vessels. Fluence imaging involves resolution compensation with a PSF filter that is compared to alternative simpler corrections.This method universally applies to arbitrary hematocrit and multi-scale vessel imaging. Using a spherical sparse array, we demonstrate 3D fluence imaging within blood vessels in simulation and experiments which is not possible with PAFI alone.Overall, we show that combining PAFI and UPD has the potential for real-time light dosimetry or could enhance quantitative inverse problems in PA imaging.