Contrast Agents–Based Molecular Photoacoustic Tomography
In photoacoustic tomography (PAT), the interaction of visible and near-infrared (NIR) light with tissue is dominated by absorbing chromophores including HbR, HbO2, and H2O and scattering particles including the cell nuclei and membranes. The sensitivity and specificity of PAT to visualize a pathological disorder are governed by its contrast: the ability of the disease to differentially absorb or scatter light compared with the surrounding healthy tissue. This native or endogenous contrast may not be sufficient, and in most cases, the interactions of light with tissue are not disease specific. Therefore, there is a role for exogenously administered contrastenhancing agents that have affinity for the disease site through biochemical interactions, providing not only sensitive but also disease-specific signals. The application of contrast agents is able to increase the sensitivity and specificity of PAT and render it a powerful molecular imaging tool. Both organic dyes such as indocyanine green (ICG) (see Section 6.2) and inorganic nanoparticles are good candidates as photoacoustic contrast agents. In this chapter, we focus our discussion on several types of nanoparticles as PAT contrast agents.