Photoacoustic endoscopy shows promise in the detection of gastrointestinal cancer, inflammation, and other lesions. High-resolution endoscopic imaging of the hemodynamic response necessitates a small-sized, high-sensitivity ultrasound sensor. Here, we utilize a laser ultrasound sensor to develop a miniaturized, optical-resolution photoacoustic endoscope. The sensor can boost the acoustic response by a gain factor of ωo/Ω (the frequency ratio of the signal light and measured ultrasound) by measuring the acoustically induced optical phase change. As a result, we achieve a noise-equivalent pressure density (NEPD) below 1.5 mPa·Hz−1/2 over the measured range of 5 to 25 MHz. The heterodyne phase detection using dual-frequency laser beams of the sensor can offer resistance to thermal drift and vibrational perturbations. The endoscope is used to in vivo image a rat rectum and visualize the oxygen saturation changes during acute inflammation, which can hardly be observed with other imaging modalities.

Photoacoustic endoscopy shows promise in the detection of gastrointestinal cancer, inflammation, and other lesions. However, imaging the hemodynamic response in disease at high spatial resolution is hindered by the ultrasound detection capability. Here we developed a miniaturized, optical-resolution photoacoustic endoscope to image an inflamed rat rectum and the hemodynamic response in terms of oxygen-saturation change, which can hardly be seen with other imaging modalities. This was achieved by detecting the optically-induced ultrasound waves with a dual-frequency laser sensor, which boosts the sensitivity by a gain factor ω0/Ω (the frequency ratio of the laser light and measured ultrasound) to the upper limit of a frequency measurement system. A noise-equivalent pressure density (NEPD) below 1.5 mPa·Hz-1/2 over the measured range of 5 to 25 MHz was achieved, comparable to that of the most sensitive state-of-the-art optical sensors. The heterodyne detection stabilizes the sensor output using the dual-frequency laser beams, which offers resistance to thermal drift and gastrointestinal peristalsis.