AO DIVER: Development of a three-dimensional adaptive optics system to advance the depth limits of multiphoton imaging

Abstract

Multiphoton microscopy (MPM) can non-invasively measure the dynamic biochemical properties deep in scattering biological samples and has the potential to accelerate clinical research with advances in deep tissue imaging. However, in most samples, the imaging depth of MPM is limited to fractions of a millimeter due to blurring caused by refractive index mismatching throughout tissue and background fluorescence, overshadowing the signal in conventional MPM. To overcome these challenges, we developed a novel 3D adaptive optics (AO) system that uses an interpolated network of endogenous guide stars to focus laser light more efficiently into highly scattering samples. The synergistic combination of our AO system with DIVER detection technology enables millimeter-scale imaging with diffraction-limited resolution with optimization times between 15 s and 65 s. We characterized the algorithm and wavefront interpolation performance in a flat 2D sample and in 3D using fluorescent beads embedded in gels of various optical heterogeneity. We also tested the system in biological tissue, improving image brightness by a factor of 5 at depths of ∼0.4 mm in the fresh green fluorescent protein-tagged mouse skin and ∼2 mm in a formalin-fixed yellow fluorescent protein-tagged mouse brain. By collecting forward and back-scattered fluorescence light to optimize the excitation wavefront, AO DIVER allows imaging of the tissue architecture at depths that are inaccessible to conventional multiphoton microscopes.