With the rapid development of life sciences,
there is an increasing demand for intravital fluorescence
imaging of small animals. However, large dimensions and
limited working distances of objective lenses in traditional
fluorescence microscopes have limited their imaging
applications mostly to superficial tissues. To overcome
these disadvantages, researchers have developed the
graded-index (GRIN) probes with small diameters for
imaging internal organs of small animals in a minimally
invasive fashion. However, dynamic imaging based on
GRIN lens has not been studied extensively. Here, this
paper presented a fluorescence endoscopic imaging system
based on GRIN lenses using one-photon and two-photon
excitation. GRIN lenses with 1.15 mm diameter and 7.65
mm length were used in the system. The images were
acquired by a compact laser scanning imaging system with
a resonant galvo-mirror system to scan the laser beam and
a photomultiplier tube (PMT) to detect fluorescence
signals. Experimental results showed that this system
using two-photon excitation could implement dynamic
fluorescence microendoscopic imaging and monitor the
movement of blood flow beneath the skin in anesthetized
mice while producing images with higher contrast and
signal to noise ratio (SNR) than those using one photon
excitation. It would be a useful tool for studying biological
processes of small animals or plants in vivo

• Fluorescence microendoscopy imaging based on GRIN lenses with one- and two-photon excitation modes