In vivo photoacoustic micro-imaging of microvascular changes in achilles tendon of mice

Achilles tendon injuries are common clinical problems for athletes, especially in runners. Several studies indicate that Achilles tendinopathy is accompanied with neovascularization or hypervascularity; thus, assessments of vascularity are important when assessing inflammation changes in tendon injuries. Photoacoustic imaging, taking the advantages of good ultrasonic resolution and high optical absorption contrast, has been shown a promising tool for vascular imaging. In this study, we demonstrate the potential of photoacoustic microscopy in monitoring the microvascular and morphological changes in tendon injuries in a mouse model in vivo. A 25-MHz dark-field confocal PAM was used to image microvascular changes in Achilles tendon of mice before and during collagenase-induced tendinitis. A wavelength of 532 nm was applied for photoacoustic excitation because blood vessels own strong optical absorption at this wavelength, guaranteeing that detected photoacoustic signals mainly come from vessels. Co-registration of PAM B-mode images with 25-MHz ultrasound (USM) ones was also performed. Morphological changs in Achilles tendons due to inflammation and edema were revealed by the PAM and USM images. Proliferation of new blood vessels within the tendons was also observed. Observed microvascular changes during tendinitis were similar to the findings in the literatures. Future work includes time-course assessments of microcirculation changes in tendonitis before and after treatment. More cases and histological verifications will be used to obtain statistically significant data.