Fundamentals of Photoacoustic Tomography
Photoacoustic tomography (PAT), or optoacoustic tomography (OAT), is based on the photoacoustic (PA) effect discovered by Alexander Graham Bell (1880), who observed that audible sound is produced when chopped sunlight is incident on optically absorbing materials. Since then, techniques based on PA spectroscopy and microscopy have been developed and used primarily for the studies of gases and solids (Tam 1986). Methods for tomographic PA imaging or PAT did not appear until 1990s first for nonbiological medium (Gusev and Karabutov 1993) and then for biological tissue (Oraevsky et al. 1994, Kruger and Liu 1994). PAT is basically concerned with an inverse problem where a single short-pulsed light beam illuminates an object and the photoacoustic waves excited by thermoelastic expansion are measured using wideband ultrasound transducers in multiple locations around the object. The geometry of the object and spatial distribution of the optical absorption are obtained from the measured scattered fields using a reconstruction algorithm or formed directly by time-resolved photoacoustic signals. In this chapter, we present the basic PA theory, several image reconstruction methods, and essential hardware components needed for implementing a PAT system.