Multiphoton GRIN‐Lens Microendoscopy for Living Brains

ABSTRACT Exploration of neural activity at high spatiotemporal resolution in live animals is essential for advancing the understanding of brain function. Multiphoton microscopy has emerged over the past three decades as a powerful tool for in vivo neuroimaging, providing 3D subcellular spatial resolution and sub‐second temporal resolution. However, its imaging depth is fundamentally limited to approximately 2 mm due to light scattering, leaving most subcortical brain regions inaccessible in mammals. Gradient refractive index (GRIN) lens‐based multiphoton microendoscopy offers a minimally invasive approach that extends imaging depth up to 10 cm while maintaining 3D µm resolution. The technique, however, remains constrained by intrinsic optical aberrations of GRIN lenses, which degrade image quality and limit both the field of view and the imaging volume. Recent advances, including adaptive optics, aspheric correctors, and geometric transformation techniques, provide state‐of‐the‐art aberration correction and expand the volume of view to the cubic millimeter scale. Applications of GRIN multiphoton microendoscopy in functional neuroimaging demonstrate its potential for high‐throughput volumetric imaging with enhanced spatiotemporal resolution. These innovations enable longitudinal studies of large‐scale neural dynamics and support the development of next‐generation photonic systems for deep brain connectome mapping.