Wide-field two-photon random illumination microscopy (2P-RIM)

Biological and biomedical samples are routinely examined using focused two-photon (2P) fluorescence microscopy due to its intrinsic axial sectioning and reduced out-of-focus bleaching. However, 2P imaging often requires excitation intensities that can damage samples through ionization and radical formation. In addition, the lateral resolution of 2P microscopy is lower compared to linear one-photon (1P) fluorescence microscopy. Wide-field 2P microscopy, using cameras, holds promise for reducing phototoxicity while maintaining high image acquisition rates. Wide-field imaging trades the high power and short integration times of sequential single point scanning for the low power and extended integration times of parallel detection across millions of pixels. However, generating effective axial sectioning over arbitrarily large fields of view (FOVs) has remained a challenge. In this work, we introduce 2P random illumination microscopy (2P-RIM), an easy-to-implement 2P wide-field technique that achieves low photo-damage imaging, micrometric axial sectioning, and enhanced lateral resolution for arbitrarily large FOVs. By using wide-field speckled illuminations in conjunction with an image standard deviation matching algorithm, 2P-RIM demonstrated multicolor imaging over FOVs greater than 200 μm, lateral resolution of 220 nm, axial sectioning of 2 μm, and peak excitation powers about 10 times lower than those used in focused laser scanning microscopy.