Endoscopy has been widely used in biomedical imaging and integrated with various optical and acoustic imaging modalities. Photoacoustic imaging (PAI), one of the fastest growing biomedical imaging modalities, is a noninvasive and nonionizing method that owns rich optical contrast, deep acoustic penetration depth, multiscale and multiparametric imaging capability. Hence, it is preferred to miniaturize the volume of PAI and develop an emerged endoscopic imaging modality referred to as photoacoustic endoscopy (PAE). It has been developed for more than one decade since the first report of PAE. Unfortunately, until now, there is no mature photoacoustic endoscopic technique recognized in clinic due to various technical limitations. To address this concern, recent development of new scanning mechanisms, adoption of novel optical/acoustic devices, utilization of superior computation methods and exploration of multimodality strategies have significantly promoted the progress of PAE toward clinic. In this review, we comprehensively reviewed recent progresses in single-and multimodality PAE with new physics, mechanisms and strategies to achieve practical devices for potential applicable scenarios including esophageal, gastrointestinal, urogenital and intravascular imaging. We ended this review with challenges and prospects for future development of PAE. K E Y W O R D S multimodality, photoacoustic endoscopy, potential clinical applications, scanning mechanism 1 | INTRODUCTION Endoscopy is an emerging imaging technology that involves optics, acoustics, precision machinery and data processing. [1,2] Owning to the urgent need of imaging deep-located biological tissues/organs, biomedical endoscopy has been extensively applied in various fundamental and clinical studies. [3-5] Comparing with clinical imaging modalities such as X-ray, computed tomography (CT), magnetic resonance imaging (MRI), nuclear emission CT, and ultrasonography (US) which are able to visualize internal biological tissues/organs, endoscopic modalities Heng Guo and Ying Li contributed equally.
