Tissue scattering has a significant effect on the image resolution and light collection efficiency in confocal microscopy.
The dual-axes (DA) confocal architecture has many advantages including high axial resolution with low numerical
aperture lenses and long working distance for use in vivo as a microendoscope. In addition, less scattered light along the
illumination path may be collected and introduced as noise. In this paper, we use Monte Carlo tissue scattering
simulations to compare the dual-axes and conventional single-axis (SA) configurations. Simulation results show that the
axial response for the dual axes configuration varies with pinhole size and optical thickness of scattering media in a way
that differs from the single axis architecture. The DA configuration is able to filter out efficiently multiply-scattered
photons and out-of-focus light, thus allowing imaging with greater tissue penetration depths to provide vertical crosssectional
images, which has significant implications for in vivo imaging.
Here we describe a simple optical design for a MEMS-based dual-axes fiber optic confocal scanning microscope that has
been miniaturized for handheld imaging of tissues, and which is capable of being further scaled to smaller dimensions
for endoscope compatibility while preserving its field-of-view (FOV), working distance, and resolution. Based on the
principle of parallel beams that are focused by a single parabolic mirror to a common point, the design allows the use of
replicated optical components mounted and aligned within a rugged cylindrical housing that is designed for use as a
handheld tissue microscope. A MEMS scanner is used for high speed scanning in the X-Y plane below the tissue
surface. An additional design feature is a mechanism for controlling a variable working distance, thus producing a scan
in the Z direction and allowing capture of 3-D volumetric images of tissue. The design parameters that affect the
resolution, FOV, and working distance are analyzed using ASAPTM optical modeling software and verified by
experimental results. Other features of this design include use of a solid immersion lens (SIL), which enhances both
resolution and FOV, and also provides index matching between the optics and the tissue.