Maximized field-of-view deep-brain calcium imaging through gradient-index lenses

Advances in genetically encoded fluorescent indicators have enabled increasingly sensitive optical recordings of neural activity. However, light scattering in the mammalian brain tissue restricts optical access to deeper regions. To address this limitation, researchers often employ implanted gradient-index (GRIN) lenses to reach deep brain areas. Nevertheless, the severe optical aberrations of GRIN lenses significantly reduce the effective field of view (FOV). In this work, we present a simple and robust imaging approach that combines low-NA telecentric scanning (LNTS) of laser excitation with high-NA fluorescence collection to increase the FOV. This configuration effectively eliminates common aberrations such as astigmatism and field curvature, resulting in a FOV [~]100% as large as the GRIN lens facet area -- corresponding to a [~]400% increase in imaging area compared with conventional approaches. We validate this method through both structural and functional in vivo imaging. The highly consistent imaging performance, fully maximized imaging FOV, and the very simple optical design make this method well-suited for broad dissemination in neuroscience research.