Autofocusing optical-resolution photoacoustic endoscopy.

Photoacoustic (PA) endoscopy has the potential to diagnose early diseases in the gastrointestinal tract. For the first time, to our knowledge, we developed an autofocusing PA endoscope (AF-PAE) for the usually irregular gastrointestinal tract imaging to solve the deterioration of transverse resolution caused by the defocus scanning of the probe. The 9-mm-diameter AF-PAE probe integrated a 6-mm aspheric lens and 6-mm liquid lens to automatically adjust the optical focal length, and an unfocused ultrasonic transducer with a center frequency of 15 MHz is coaxially set for detecting PA signals. With this probe, the AF-PAE achieved a focus-shifting range from approximately 2 to 10 mm with high transverse resolution and image contrast in a 360° field of view. Phantom experiment and vasculature distribution of a resected rabbit rectum have been performed to demonstrate the imaging ability of the AF-PAE for potential clinical applications in colorectal vessel imaging and subsequent diagnosis.