Photoacoustic imaging (PAI) is also known as optoacoustic imaging. It combines optical imaging with high specificity and deep penetration of ultrasonic imaging and hence is a hybrid imaging modality. Photoacoustic imaging has proven its capability in multiscale noninvasive imaging with an optical contrast mechanism, thus bridging the microscopic and macroscopic domains in biomedical research.

PAI mainly includes photoacoustic microscopy (PAM) and photoacoustic tomography (PAT). PAM systems are based on the coaxial and confocal configuration of the acoustic detection and optical excitation which could achieve high-resolution imaging with up to a resolution of sub-micron. PAT systems can exhibit superior volumetric imaging beyond the optical ballistic limit with ultrasound spatial resolution. Hence, a deep penetration can be achieved up to a few centimeters.

In the past two decades, many attempts have been made to advance the capability of PAI, as well as extend its application areas. In recent years, new PAI techniques were invented to improve the resolution, speed, and contrast mechanism. The topics of this methods collection includes new methods in PAM and PAT.

The topics of the PAM could be including the various practical application of the PAM systems (such as functional imaging, photoacoustic histology), high speed PAM with fast multi-wavelength laser source, various scanning mechanisms for PAM, new acoustic detection in PAM applications (such as parallel acoustic detection and encoded acoustic detection). The topics of the PAT could be including the PAT detection geometries, methods for reconstructing the PAT images, various practical application of the PAT systems, and new ultrasound transducers for PAT applications.