Acousto-optic Volumetric Gating for Reflection-Mode Deep Optical Imaging within a Scattering Medium

Abstract

Various external gating approaches, based on position, time, and polarization, have proven to be effective in selectively rejecting multiply scattered waves, thereby extending the imaging depth of deep-tissue optical microscopy. However, in a highly scattering medium, a significant portion of multiply scattered waves can bypass these gating operations because of the dissociation between the wave properties inside and outside the scattering medium. Here, we propose a method, termed volumetric gating, that introduces ultrasound focus to confocal reflectance imaging to directly suppress the multiply scattered waves traveling outside the imaging volume. The volumetric gating axially rejects the multiply scattered wave traveling to a depth shallower than the object plane while simultaneously suppressing the deeper penetrating portion extended beyond the transverse area of the ultrasonic focus of 30 x 90 mu m(2). These joint gating actions along the axial and lateral directions attenuate the multiply scattered waves by a factor of 1/1000 or smaller, thereby extending the imaging depth to 12.1 times the scattering mean free path with a diffraction-limited resolution of 1.5 mu m. We showed that volumetric gating enables noninvasive imaging of the internal microscopic structures inside tubular organs such as the mouse colon and small intestine. We further developed theoretical and experimental frameworks to characterize the axial distribution of optical energy within scattering media. The volumetric gating will serve as an important addition to deep-tissue imaging modalities and a useful tool for studying wave propagation in scattering media.