Non-ionizing Label-free Photoacoustic Imaging of Bones

X-ray based radiography, the main modality for diagnostic imaging of bone structures and fractures, provides sensitive images, but it inherently involves potentially harmful X-ray exposure. As non-ionizing alternatives, various optical imaging methods have been explored. Here, we demonstrate non-ionizing, label-free, multispectral photoacoustic (PA) imaging of bones in small animals in vivo, in situ, and ex vivo. Using near-infrared light excitation and acoustic detection, the spine and ribs were successfully visualized in high-resolution PA images. PA 3D volume images of the spine and ribs were clearly visualized together with blood vessels and several organs including the spleen, liver, and cecum, without using any exogenous contrast agent nor ionizing radiation. Quantification results of multispectral PA signals from blood vessels and bones were in good agreement with their absorption coefficients. Further, a rib fracture was photoacoustically imaged. Our results demonstrate PA imaging's potential as a non-ionizing and label-free technique for imaging bone tissues.