Photoacoustic imaging of lymphatic pumping

The lymphatic system is crucial for maintaining fluid balance in tissues and for immune cell trafficking; however, there are only a few methods for imaging lymphatic function in vivo. Lymph node biopsies are a common approach to diagnose illness; however, lymphedema is a very common morbid side-effect of lymph node biopsies. There is a need to better understand lymphatic pumping and perhaps even to provide imaging techniques to guide surgical interventions with minimal impact to lymphatic function. Although intensified CCD imaging of fluorescence from indocyanine green (ICG) has offered exciting new tools for studying lymphatic pumping in deep tissue, the spatial resolution is poor due to the scattering of light in tissues. Photoacoustic imaging is a powerful technique that combines optical absorption contrast with acoustic resolution allowing for higher resolution imaging at depth compared to optical imaging. Photoacoustic imaging has proven useful for imaging microvascular networks and contrast agents, studying angiogenesis, and estimating oxygen saturation [1- 4]. Photoacoustic imaging has been used to visualize sentinel lymph nodes [5] and a high-resolution system was demonstrated to image the superficial lymphatic network [6]. We demonstrate photoacoustic imaging of lymphatic pumping using the VisualSonics Vevo LAZR photoacoustic imaging platform. Either ICG or porphyrin nanodroplets were injected in the footpad of the hindlimb of athymic nude mice. The contrast agents migrated to the popliteal lymph node in the fat pad of the hindlimb. Background-subtracted imaging revealed pulsatile spikes in photoacoustic signal intensity within an afferent lymphatic vessel believed to be due to the lymphatic pumping of the photoacoustic contrast agent. Lateral spatial resolution of ∼180 micrometers is significantly higher than competing fluorescence imaging techniques which are limit by light scattering.