Figure 1 shows our experimental setup. Fluorophores are excited near their absorption maximum using a laser diode module (Cube 445-40C, 40 mW, Coherent, Santa Clara, California) at a wavelength of .26 The projection optic produces a small laser spot size which is further reduced by the many projection fibers acting like projection pinholes. Confocality is ensured by a detection pinhole of in diameter. Laser power as measured at the tip of the probe is 0.3 mW. Excitation light is coupled into the excitation–emission path by a dichroic mirror. A resonant scanner (CRS, GSI Group, Bedford, Massachusetts) and a galvanometer (VM500, GSI Group, Bedford, Massachusetts) scan the excitation point source over the fibers of an image guide (FIGH-30-650S, Myriad Fiber Imaging, Massachusetts). The image guide is composed of 30,000 individual fibers and light is coupled into the fibers using a multi-immersion microscope objective lens (Nikon, , Tokyo, Japan). Index matching oil of refractive index 1.492 (Cargille Laboratories, Cedar Grove, New Jersey) is used between the image guide and objective lens to closely match the refractive index of the fiber cores. The image guide carries the excitation signal to the imaging site and a chromatically corrected lens assembly at the distal end of the image guide projects the excitation and detection light to and from the tissue. The distal objective lens has a magnification of thereby producing an imaging field of view of in diameter with lateral and axial resolutions of 1.3 and , respectively. The lateral resolution is limited by the fiber spacing of the image guide ( between neighboring fiber cores), whereas the NA of the imaging guide (0.35) limits the numerical aperture at the tissue to 1.05.