Organic Nanostructures for Photoacoustic Imaging

Photoacoustic imaging (PAI) has gained significant attention over the last two decades as a noninvasive, non-ionizing, real-time, and inexpensive medical imaging modality. By exploiting the low acoustic scattering compared to optical scattering in biological tissues, PAI can achieve high resolution while maintaining superior optical contrast in deep tissues. PAI can probe many endogenous chromophores based on their strong optical absorption contrast, including oxygenated hemoglobin, deoxygenated hemoglobin, melanin, water, and lipid. However, these intrinsic absorbers are typically activated by visible (e.g., wavelength 400 <= lambda <= 700 nm) or infrared (lambda > 1000 nm) light, at which wavelengths light penetration is severely limited. Because the optimal spectral region is the near-infrared (NIR, 700 <= lambda <= 1000 nm) window for deep-tissue imaging, additional contrast mechanisms for PAI should be found. So far, various exogenous contrast agents have been developed for this purpose including small monomers, inorganic nanoparticles (NPs), organic NPs, etc. For successful and fast clinical translation, the proposed contrast agents must be safe and degradable in vivo, and the organic nanostructures may be good candidates for this purpose. In this article, we review recent progress in developing organic nanoformulations as PAI contrast agents.