Acoustic resolution photoacoustic microscopy

Even if conventional optical imaging systems such as multiphoton microscopy (MPM), confocal microscopy (CM), fluorescence microscopy (FM), and optical coherence tomography (OCT) are regarded as revolutionary microscopic imaging modalities to reveal the inner information of biological tissues with very high spatial resolution, it is inherently restricted to image deep tissues due to strong optical scatting in biological tissues. Photoacoustic imaging (PAI) is a hybrid imaging modality to combine strong optical contrast and high ultrasonic resolution in deep tissues. In a microscopic imaging perspective, photoaocustic microscopy (PAM) can be implemented in two forms: optical-resolution (OR) and acoustic-resolution (AR) PAM. In OR-PAM, the lateral spatial resolution is determined by tight optical focusing, but the penetration depth is limited to one optical transport mean free path. In AR-PAM, the lateral spatial resolution is determined by loose acoustic focusing, but the penetration depth can be much enhanced and reach to several centimeters. Therefore, AR-PAM gains great attention for both preclinical and clinical applications. This review explains the principle, implementation, and applications of AR-PAM.