Fig. 1. Typical scanning of two-photon endomicroscopy (TPEM) system^[31]. (a) Fiber bundle based proximal scanning; (b) MEMS-based distal scanning; (c) piezoelectric ceramic based fiber distal scanning

Fig. 2. Schematic of PZT scanning TPEM^[40]

Fig. 3. Raster scanning TPEM and its imaging^[70]. (a) System schematic; (b)－(d) two-photon fluorescence (TPF) images of mouse lung; (e)－(g) TPF images of mouse colon

Fig. 4. PZT spiral scanning TPEM^[71]. (a) System schematic; (b) structure of DC-PCF inner core; (c) structure of DC-PCF cladding; (d) TPF images of mouse kidney at different depths

Fig. 5. Structure of achromatic micro-objective and TPEM imaging of different mouse tissues^[73]. (a) Structure of achromatic micro-objective; (b) focal shift test results; (c) images of mouse liver; (d) image of mouse small intestine; (e) image of 　mouse cervix tissue

Fig. 6. Structure and appearance of TPEM probe based on Lissajous scanning and TPF images of mouse tissues captured by TPEM^[75]. (a)－(b) Structure and appearance of probe; (c) structure of Lissajous PZT fiber scanner; (d) time-lapse images for Lissajous scanning captured for 2.5, 10, 20, 50, and 100 ms; (e) TPF image of mouse liver slice; (f) TPF 　image of mouse small intestine; (g) TPF image of mouse ear

Fig. 7. Structure and imaging of multimodal endomicroscopy^[77]. (a) System structure; (b) structure of double-core pure silica double-clad fiber (DCF); (c) structure of achromatic objective; (d) coherent anti-Stokes Raman scattering (CARS) image of human epithelial tissue; (e) TPF image of human epithelial tissue

Fig. 8. Probe structure and imaging of TPEM^[78]. (a) Schematic of four-plate piezoelectric TPEM; (b) structure and photograph of integrated TPEM probe; (c)－(d) images of mouse colon and lung

Fig. 9. PZT spiral scanning TPEM and its imaging^[79-80]. (a) Appearance of miniature micro-objective; (b) structure of DC-ARF; (c) schematic of PZT spiral scanning TPEM; (d) appearance of integrated TPEM probe; (e) inverted microscope captured stomach wall slice image; (f)－(g) TPF images of stomach wall slice

Fig. 10. Tissues imaging. (a)－(i) TPF redox images of mouse kidney ischemia-reperfusion model in vivo^[69]; (j)－(r) fluorescence intensity, lifetime and HE staining images for kidney, liver and tumor^[86]

Fig. 11. Miniature two-photon microscopy for brain imaging in freely behaving mice. (a) Appearance of miniature two-photon microscopy probe on a fingertip and mounted to mouse head^[89]; (b) image of neuronal somata in GCaMP6f-expressing mouse^[89]; (c) appearance of enlarged field-of-view, three-dimensional miniature two-photon microscopy probe mounted to mouse head^[90]; (d) three-dimensional image of neurons in GCaMP6s-expressing mouse^[90]

Fig. 12. Probe and imaging of PZT-scanning two-photon microscopy^[91]. (a) Appearance of probe attached to the head of a freely behaving mouse; (b)－(d) images of GCaMP6m-expressing mouse dendritic spines