Photoacoustic imaging for assessing flow-mediated oxygenation for peripheral arterial disease

Peripheral arterial disease (PAD) is a common cardiovascular disease that afflicts a large population worldwide. Reduced blood flow to the limbs leads to tissue ischemia and tissue hypoxia. Oxygen-dependent light absorption properties of hemoglobin enable photoacoustics to image tissue oxygenation. The goal of this study is to develop a photoacoustic imaging (PAD technique for diagnosis and monitoring of PAD. The studies were performed in a mouse model where each animal was scanned bilaterally with a pulsed laser at optical wavelengths of 750 nm and 850 nm. The photoacoustic signals generated by the two-wavelength irradiation was unmixed to generate a parametric map of estimated oxygen saturation. The parametric map was superimposed in color on grayscale ultrasound images. Transient limb ischemia was induced by tying off the hind limb between the external iliac and femoral arteries for five minutes. PAI was performed before, during, and after tie-off. Oxygen saturation (sO2) of femoral muscles was measured before, during, and post occlusion. Marked reduction in oxygenation during occlusion was observed followed by hyper-oxygenation which exceeded the pre-occlusion baseline value during post-occlusion. The excess oxygenation above the baseline was transient, gradually returning to the baseline over time as the muscle returned to its normal hemodynamic state. In conclusion, this study reports a novel approach using photoacoustic imaging to observe tissue oxygenation modulation in response to stress-induced ischemia. The observation of hyper-oxygenation following stress release due to endothelium-mediated dilation of the microvasculature has not been previously reported from using photoacoustics, which could serve as a novel approach for assessing endothelial dysfunction noninvasively in PAD patients.