In vivo photoacoustic tomography

Conventional high-resolution volumetric optical imaging modalities, such as confocal microscopy, two-photon microscopy, and optical coherence tomography, have revolutionized preclinical and clinical biomedical researches. However, due to inevitable light scattering (e.g., the one optical transport mean free path: ~ 1 mm) in biological tissues, keeping high spatial resolution beyond this depth using pure optical methods is greatly challenging. By measuring diffusive photons and estimating photon trajectories with mathematical algorithm, diffuse optical tomography can provide both real-time optical absorption and scattering information in deep tissues. However, the spatial resolution of this technique is poor. Photoacoustic tomography (PAT, also referred to as optoacoustic tomography) is an emerging hybrid imaging modality which breaks these fundamental limitations of existing optical imaging modalities. The image contrast of PAT relies on strong optical absorption in light excitation phase, whereas the spatial resolution depends on ultrasonic parameters in the ultrasonic emission phase. The imaging depth can reach up to several centimeters in tissues with a resolution of submillimeters. In this chapter, the following topics of PAT will be discussed; (1) various PAT modalities, (2) morphological, functional, and molecular PAT using both intrinsic and extrinsic contrasts, and (3) current and potential clinical applications of PAT.