Repairing spherical aberration during in vivo two-photon imaging

In vivo two photon microscopy generally requires a cranial window implant to aid in stabilization of the
brain. These windows introduce aberration into the optical excitation, due to the use of glass coverslips
and other media with different refractive index than that for which the microscope objective is designed.
These mismatches introduce spherical aberration which can have dramatic effects on the axial pointspread
function psf. We have developed a framework for computational simulations of typically used
configurations including up to five media interfaces, as a means to understand the impact of spherical
aberration, and to search for experimental solutions to correct for it. Our simulations suggest that
spherical aberration even from a single glass coverslip ~170 microns can introduce a substantial loss of
z-axis resolution, and blurring of signals from neighboring neural elements. We propose the use
of opposite refractive index mismatch to correct for spherical aberration. Our simulations suggest that
an appropriate magnitude of opposite refractive index can almost fully recover the ideal psf, and our
experimental tests support these simulations.