Photoacoustic imaging.

Photoacoustic imaging, which is based on the photoacoustic effect, has developed extensively over the last decade. Possessing many attractive characteristics such as the use of nonionizing electromagnetic waves, good resolution and contrast, portable instrumention, and the ability to partially quantitate the signal, photoacoustic techniques have been applied to the imaging of cancer, wound healing, disorders in the brain, and gene expression, among others. As a promising structural, functional, and molecular imaging modality for a wide range of biomedical applications, photoacoustic imaging can be categorized into two types of systems: photoacoustic tomography (PAT), which is the focus of this article, and photoacoustic microscopy (PAM). We first briefly describe the endogenous (e.g., hemoglobin and melanin) and the exogenous (e.g., indocyanine green [ICG], various gold nanoparticles, single-walled carbon nanotubes [SWNTs], quantum dots [QDs], and fluorescent proteins) contrast agents for photoacoustic imaging. Next, we discuss in detail the applications of nontargeted photoacoustic imaging. Recently, molecular photoacoustic (MPA) imaging has gained significant interest, and a few proof-of-principle studies have been reported. We summarize the current state of the art of MPA imaging, including the imaging of gene expression and the combination of photoacoustic imaging with other imaging modalities. Last, we point out obstacles facing photoacoustic imaging. Although photoacoustic imaging will likely continue to be a highly vibrant research field for years to come, the key question of whether MPA imaging could provide significant advantages over nontargeted photoacoustic imaging remains to be answered in the future.