Optical-resolution photoacoustic microscopy: auscultation of biological systems at the cellular level.

Photoacoustic microscopy (PAM) offers unprecedented sensitivity to optical absorption and opens a new window to study biological systems at multiple length- and timescales. In particular, optical-resolution PAM (OR-PAM) has pushed the technical envelope to submicron length scales and millisecond timescales. Here, we review the state of the art of OR-PAM in biophysical research. With properly chosen optical wavelengths, OR-PAM can spectrally differentiate a variety of endogenous and exogenous chromophores, unveiling the anatomical, functional, metabolic, and molecular information of biological systems. Newly uncovered contrast mechanisms of linear dichroism and Förster resonance energy transfer further distinguish OR-PAM. Integrating multiple contrasts and advanced scanning mechanisms has capacitated OR-PAM to comprehensively interrogate biological systems at the cellular level in real time. Two future directions are discussed, where OR-PAM holds the potential to translate basic biophysical research into clinical healthcare.