Photoacoustic imaging through a cortical bone replica with anisotropic elasticity

Photoacoustic (PA) imaging is an emerging modality, which combines the high optical absorption contrast of biological chromophores with centimeter imaging depths and sub-millimeter resolution of ultrasonic (US) waves. However, PA imaging through cortical bone remains an unmet challenge. Cortical bone is an anisotropic medium, which is not accurately modeled using existing PA image reconstruction methods. In this Letter, we address the PA source localization problem for imaging through a cortical bone-mimicking layer. Our approach accounts for both refraction and elastic anisotropy to accurately reconstruct US and PA images in the presence of a cortical bone replica. We demonstrate our technique using a PA and US experiment, where we image a 700 mu m diameter target beneath a cortical bone-mimicking plate. Pulse-echo US experiment is used to estimate the wavespeed in each layer and create an anatomical image of the bone replica, and the PA source is reconstructed using the wavespeed model defined with US. We compute the thickness of the plate with less than 1% error, whereas isotropic assumptions overestimate the thickness by 20% or more. Incorporating both refraction and anisotropy accurately localizes the target with PA and US at the true depth, whereas isotropic assumptions blur the lateral dimension and mislocate the target depth by 1.5-4mm.