In vivo multi-scale photoacoustic microscopy of human skin

Scalability is a key feature of photoacoustic microscopy (PAM). Reports have shown that PAM systems can be designed to possess sub-micron resolution at shallow depths or penetrate centimeters deep at the expense of resolution while the number of resolved pixels in the depth direction remains high. This capability to readily tune the imaging parameters while maintaining the same inherent contrast could be extremely useful for a variety of biomedical applications. Human skin, with its layered vascular structure whose dimensions scale with depth, provides an ideal imaging target to illustrate this advantage. Here, we present results from in vivo human skin imaging experiments using two different PAM systems, an approach which enables better characterization of the cutaneous microvasculature throughout the imaging depth. Specifically, we show images from several distinct areas of skin: the palm and the forearm. For each region, the same area was imaged with both an optical-resolution PAM (OR-PAM) and an acoustic-resolution PAM (AR-PAM), and the subsequent images were combined into composite images. The OR-PAM provides less than 5 μm lateral resolution, capable of imaging the smallest capillary vessels, while the AR-PAM enables imaging at depths of several millimeters. Several structures are identifiable in the OR-PAM images which cannot be differentiated in AR-PAM images, namely thin epidermal and stratum corneum layers, undulations in the dermal papillae, and capillary loops. However, the AR-PAM provides images of larger vessels, deeper than the OR-PAM can penetrate. These results demonstrate how PAM's scalability can be utilized to more fully characterize cutaneous vasculature, potentially impacting the assessment of numerous cardiovascular related and cutaneous diseases.