Contrast Media & Molecular Imaging

Photoacoustic tomography (PAT) has become one of the fastest growing fields in biomedical imaging. Because of its high scalability and unique optical absorption contrast, PAT is capable of performing anatomical, functional, molecular and fluid-dynamic imaging at various system levels, and is playing an increasingly important role in fundamental biological research and clinical practice. This Review discusses recent technical progress in PAT since the second half of year 2009 and presents corresponding applications and prospects.

We summarize the applications of carbon nanotubes and their hybrid nanoparticles for imaging, sensing and theranostics applications. Particular emphasis is given to photoacoustic and photothermal effects and their respective applications in imaging, sensing and theranostics applications. We review the safety aspects, physical basis for contrast enhancement and provide concrete list of experiments needed for the validation of new photoacoustic imaging agents.

Gold nanocages characterized by hollow interiors, ultrathin and porous walls are demonstrated as a new class of contrast agents for in vivo photoacoustic imaging of different components in biological tissues, such as cerebral cortex, lymph nodes and melanoma tumors, showing great potential for cancer diagnosis.

In this review we summarize our effort to advance molecular photoacoustic imaging with colloidal gold nano beacons (GNB). GNB represent a robust nanoparticle platform for various molecular imaging applications with photoacoustic imaging. For angiogenesis, an essential microanatomical biomarker of tumor and cardiovascular disease progression, integrin targeted GNBs allowed visualization of numerous angiogenic sprouts and bridges that were otherwise undetectable from inherent blood signal alone, offering sensitive and specific discrimination and quantification of angiogenesis in vivo.

Gold nanorods (AuNR) exhibit intense absorption peaks for light at longer wavelengths, owing to the excitation of longitudinal plasmons. Light absorption and the subsequent temperature rise causes strong photoacoustic signals to be produced. This feature combined with the relative inertness and favorable surface chemistry of gold, has seen AuNR attracting attention as photoacoustic contrast agents. In this article we provide a short overview of the current status of the use of AuNR in molecular imaging using photoacoustics.

Multifunctional microbubbles (MBs) and nanobubbles (NBs) are contrast agents integrating multiple disease-targeting, imaging and therapeutic functions. Highly absorbing optical contrast agents, such as gold nanoparticles, India ink and Indocyanine Green, can be encapsulated in polymer MBs and NBs for contrast-enhanced photoacoustic imaging. Potential applications of MB- and NB-enhanced photoacsoutic imaging include disease targeting, drug delivery and therapeutic assessment.