Deep Volumetric Super-Resolution Imaging in Thick Biological Specimens With Sparse Scanning SIM.

High-resolution fluorescence microscopy deep inside thick, live specimens remains a major challenge due to scattering and limited penetration depth. Structured illumination microscopy (SIM) is widely used for fast super-resolution imaging, yet its sensitivity to scattering strongly restricts volumetric applications. Here we present sparse scanning SIM (SS-SIM), a variant for deep imaging that integrates rapid laser beam scanning with pixel-addressed intensity modulation and sCMOS detection. SS-SIM operates with either one- or two-photon excitation and achieves a spatial resolution of ∼150 nm, and maintains a 1.6/1.7-fold (lateral/axial) resolution enhancement over wide-field microscopy throughout the available imaging depth limited by the working distances of our water-immersion objectives (300/600 µm), enabling volumetric visualization of dense biological samples. By combining simplicity, robustness, and super-resolution performance, SS-SIM expands the applicability of SIM to thick tissues, organoids, and small organisms, providing a practical route to high-contrast volumetric imaging in complex biological environments.