Abstract
Small animals are the preferred laboratory models for studying various diseases. Small animal imaging provides the opportunity to evaluate pathologic progression in a much-compressed time frame while it requires much higher resolution and image contrast than clinical medical imaging. Moreover, there is a significant need to develop technologies for visualizing micro-vascular growth and remodeling in pathogenic process. In this study, we developed a non-invasive, high frequency dark-field confocal photoacoustic microscope (PAM) system. PAM, a hybrid biophotonic imaging technique detecting absorbed photons ultrasonically through the photoacoustic effect, has the potential to address these needs mentioned above. With strong endogenous blood optical absorption contrast and high frequency ultrasound detection, PAM can provide high contrast and high spatial resolution for micro-vasculature imaging of small animals. Currently, we have developed 25-MHz and 50-MHz dark-field confocal PAM systems. Our PAM system provides two imaging modes – B-scan (2D imaging) and C-scan (3D imaging). When using a 25-MHz ultrasonic transducer, its maximum achievable resolution is 68 um in axial and 171 um in lateral, and the penetration depth can reach at least 6 mm. With a 50-MHz transducer, higher resolution can be achieved – 36 um in axial and 65 um in lateral while compromising with penetration depth – at least 3 mm. The performance of our PAM is further verified with in vivo small animal experiments – high resolution and high contrast subcutaneous and tumor micro-vasculature imaging of mice can be obtained. We also tried to explore new biomedical applications of our PAM, applying it to image micro-vascular changes in injured Achilles tendons of mice. In addition, by taking advantage of the strong near-infrared absorption of gold nanorods (AuNRs) and their extravasation tendency from blood-brain barrier (BBB) opening foci due to their nano-scale sizes, we employed our PAM system along with AuNRs as photoacoustic contrast agents to monitor focused-ultrasound induced BBB opening in small animal models in vivo, verifying the feasibility. Future work will focus on optimization of our PAM system and in vivo small animal experimental procedures for the tendon and BBB-opening imaging. Our preliminary studies show promising of photoacoustic imaging as a useful aid of sports medicine and BBB opening monitoring.