Photoacoustic imaging in nanomedicine

During the last few decades, molecular imaging has been explosively investigated as a critical diagnostic tool for early cancer detection to improve patient survival rates and treatment outcomes. However, the current clinical standard, which is tissue biopsy, is invasive and suffers from a high false-negative rate due to sampling errors at the early stage. Most cancers develop over several years and are characterized by molecular and architectural changes before invasion and metastasis. A high-resolution molecular imaging modality that can provide diagnostic information in vivo and in a noninvasive fashion could greatly enhance a clinician's ability to detect early cancers. Photoacoustic (PA) imaging is an emerging biomedical imaging technique that can perform real-time, cross-sectional imaging of tissue microanatomy in vivo, without the need for tissue removal. The contrast is based on the absorption of light and its conversion into heat to generate PA signals. PA imaging produces a map of the initial acoustic pressure distribution, or absorbed energy density, in tissue that is irradiated by a short laser pulse. As such, it enables the technique to quantify the concentrations of specific contrast agents that are attached to the pathological site. Therefore, the PA contrast agents, typically nanoplatforms, have been significantly explored to boost the detection sensitivity and targeting specificity of molecular PA imaging. In this chapter, nanoscale PA labels and their potential applications will be discussed.