Two-photon microscopy has revolutionized the ability to image neural activity in the brain of a living animal. Compared to single-photon microscopy, this technique allows for targeted excitation of small volumes in deep tissue. Moreover, the two-photon technique provides tissue penetration up to 800-1000 μm.

As powerful as it is, the imaging depth of a stand-alone two-photon microscope does not allow for deep brain region imaging. One technique used to image deeper is to couple the two-photon microscope with a gradient index (GRIN) lens. Implanted in the tissue, the GRIN lens allows the user to record from deep areas through long working distance objectives.

Even though two-photon GRIN lens-coupled imaging allows for optical sectioning and 3D imaging, this technique has limitations regarding axial scanning speed and contrast. By adding a tunable acoustic GRIN (TAG) lens to their setup, Chien et al. designed a new system allowing in vivo imaging of neurons in deep mouse brain areas with high-contrast and high-temporal resolution. The publication is available on bioRxiv following this link https://www.biorxiv.org/content/10.1101/2020.09.19.304675v1.

In vivo functional imaging of neuronal activity from SCN of a head-fixed anesthetized mouse - Chien et al. 2020