Extended field-of-view imaging in miniature two-photon microscopes through aberration corrected GRIN lenses

Miniature two-photon microscopes, such as MINI2P, are powerful tools to monitor the activity of brain cells in freely moving animals. To image deep in the brain, the MINI2P can be coupled to gradient index (GRIN) lenses, which are implanted in the tissue and relay the imaging beam to the target region of interest. However, GRIN lenses have intrinsic optical aberrations, which severely degrade imaging spatial resolution especially in lateral portions of the field-of-view (FOV) leading to restricted imaging FOV. Here, we tested the hypothesis that coupling MINI2P with aberration corrected GRIN lenses results in improved spatial resolution and extended FOV during two-photon fluorescence imaging. We coupled the MINI2P with a cylindrical GRIN lens (GRIN length, 4.07 mm; GRIN cross section, 0.5 mm), in which we corrected aberrations using a micro-fabricated polymer lens. We found that the axial dimension of both the on-axis and off-axis point-spread-function was significantly improved in MINI2P coupled with corrected GRIN lenses compared to MINI2P coupled with uncorrected GRIN lenses. Moreover, using corrected GRIN lenses enabled MINI2P imaging with > 3-fold larger FOV radius compared to using uncorrected GRIN lenses. We confirmed extended FOV imaging when MINI2P was coupled to corrected GRIN lenses by performing two-photon imaging of thalamic neurons expressing a fluorescent indicator in fixed brain tissue.