Fig. 1. Light attenuation coefficient in mouse skull bone, skin, and brain cortex (fresh tissues)^[13]

Fig. 2. Implantable fluorescence endoscopic microscopy. (a) Imaging depth of the microscopy used in in vivo mouse brain imaging; (b) the fluorescence endoscopic microscopy using a GRIN lens (left) or single multimode fiber (right) as implantation medium

Fig. 3. In vivo long term endoscopic brain imaging after GRIN lens implantation. (a) GRIN lens cannula design and imaging of neurons in the lateral hypothalamus and striatum^[20], scale bar: 20 μm; (b) GRIN lens cannula design for brain injury model and neurons imaging results^[25], scale bar: 10 μm

Fig. 4. Compensation methods of aberration in GRIN lens-based endoscopic imaging systems and their imaging results. (a) Processing an aspheric microlens on a coverslip^[38]; (b) HiLo-AO-based wavefront correction^[42]

Fig. 5. In vivo endoscopic brain imaging with a two-photon fluorescence microscope and a GRIN lens. (a) SLM regulates the light field to achieve multi focus or pattern illumination^[44], scale bar: 100 μm; (b) fast volume imaging of neurons with high resolution based on Bessel beam^[45], scale bar: 20 μm; (c) cone neuron imaging based on lookup table and direct wavefront detection for AO correction^[46], scale bar: 5 μm

Fig. 6. In vivo endoscopic neuron functional imaging using a miniaturized head-mounted microscope combined a GRIN lens^[56]

Fig. 7. MATRIEX technology used for in vivo multiarea endoscopic brain imaging^[61]

Fig. 8. The beam was refocused after passing through the scattering medium^[72]

Fig. 9. Several methods for providing dynamic calibration capabilities to multimode fiber-based fluorescence microscopy. (a) Introducing a virtual beacon^[74]; (b) processing a part reflector on the fiber’s distal end^[75]; (c) machining metasurface structures^[76]; (d) introducing a guide star^[77]; (e) using a CNN model to help extract speckle information^[81]

Fig. 10. Single multimode fiber endoscopic brain imaging using SLM^[85]. (a) Schematic diagram of imaging device; (b) imaging brain regions and scanning diagrams; (c) in vivo imaging results of neurons in the dorsal striatum, scale bar: 10 μm; (d) implantation path of fiber in the cerebral cortex is shown by the white dashed line; (e) dynamic characterization of Ca²⁺ signals in neurons of isolated brain slices; (f) fluctuations in Ca²⁺ signals of auditory neurons in the thalamic region when stimulated by sound (single pixel)

Fig. 11. DMD-based endoscopic brain imaging with single multimode fiber^[86]. (a) Schematic diagram of the imaging device, the lower right corner is the optical microscopy image of the experimental fiber. (b) Upper: primary visual cortex neuron soma; middle: inhibitory neuronal synaptic nodes in the dentate gyrus; down: blood cells flow after vascular rupture. The time interval from left to right is 0.57 s

Fig. 12. Application of sideview multimode fiber for in vivo brain imaging^[89]. (a) Schematic diagram of imaging device and sideview multimode fiber; (b) mouse brain imaging results in vivo, green represents the GFP labeled neurons, red represents the blood vessels marked with dye, scale bar: 20 μm; (c) dynamic characterization results of Ca²⁺ signals in neurons and hemodynamics

Fig. 13. Application of endoscopic brain imaging technology in clinical brain tumor diagnosis. (a) Wide-field and confocal endoscopic imaging of ICG-labeled meningioma tissues and H&E staining result of postoperative tissue section^[99]; (b) label-free FLIM endoscopic imaging in the identification of necrotic brain tissue^[111]