Photoacoustic tomography has been developed for in vivo functional, metabolic, molecular, and histologic imaging by physically combining optical and ultrasonic waves. Broad applications include early-cancer detection and brain imaging. Highresolution optical imaging-such as confocal microscopy, two-photon microscopy, and optical coherence tomography-is limited to superficial imaging within the optical diffusion limit (~1 mm in the skin) of the surface of scattering tissue. By synergistically

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... bining light and sound, photoacoustic tomography provides deep penetration at high ultrasonic resolution and high optical contrast. Electronic Supplementary Material The online version of this chapter (https://doi. org/10.1007/978-981-13-7908-6_1) contains supplementary material, which is available to authorized users.