Optical-resolution functional gastrointestinal photoacoustic endoscopy based on optical heterodyne detection of ultrasound

Abstract 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.