In Vivo Photoacoustic Tomography of Chemicals: High-Resolution Functional and Molecular Optical Imaging at New Depths

Abstract

High-resolution volumetric optical imaging modalities, \nsuch as confocal microscopy, two-photon microscopy, and \noptical coherence tomography, have become increasingly \nimportant in the biomedical imaging field. However, due to \nstrong light scattering, the penetration depths of these \nimaging modalities are limited to the optical transport mean \nfree path in biological tissues, for example, ∼1 mm in the \nskin. Photoacoustic tomography (PAT), an emerging hybrid \nimaging modality that can provide strong endogenous and \nexogenous optical absorption contrasts with high ultrasonic \nspatial resolution using the photoacoustic (PA) effect, has \novercome the fundamental depth limitation. The image \nresolution is scalable with the ultrasonic frequency. The \nimaging depth is limited to the reach of photons and up to \na few centimeters deep in biological tissues. This Review \nwill focus on the following aspects of PAT described in \nworks published from 2003 to 2009: (1) multiscale PAT \nsystems, (2) morphological and functional PAT using \nintrinsic contrasts (hemoglobin or melanin), and (3) functional \nand molecular PAT using exogenous contrast agents \n(organic dyes, nanoparticles, reporter genes, or fluorescence \nproteins).