Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 282

a fiber-optic bundle in 1993, demonstrating its optical sectioning capabilities with
conventional microscope objectives used for focusing at the distal (tissue) end of the
fiber [ 73 ]. Later, Richards-Kortum et al. developed a fiber-bundle-based confocal
microscope which imaged reflected (backscattered) light. This system used a fiber
bundle with 7-m spacing between neighboring cores, with a custom designed
8-element, 7-mm-diameter, and miniature 3 /NA 1.0 objective lens at the distal
tip (see Sect. 8.2 for more details). The lateral resolution was approximately 2
microns with a 250-m diameter FOV [ 59 , 74 ]. This fiber-optic scanning confocal
reflectance system was evaluated in pilot clinical studies for the cervix [ 75 ] and oral
cavity [ 13 ], where in both cases, alterations in nuclear morphology were apparent
under confocal imaging which correlated with the grade of neoplasia on pathology.
A similar confocal system operating in fluorescence mode was commercialized
by Mauna Kea Technologies, with early systems focused on 488-nm excitation
for fluorescein-stained tissue [ 26 ] and later versions using multiple excitation
sources [ 76 ]. The Mauna Kea system is designed to be compatible with a range
of fiber-optic probes, each offering different imaging parameters including FOV,
resolution, and working distance, depending on the properties of the fiber bundle
and the optical element(s) at the distal tip. For example, one probe designed for
gastrointestinal imaging has 3:5-m lateral resolution over a 600-m FOV, while
a high-resolution version is available with 1-m resolution over a 240-mFOV
[ 77 ]. The Mauna Kea fiber-bundle-based fluorescence confocal platform has been
evaluated in several clinical areas, including in vivo clinical studies in the lung
and gastrointestinal tract [ 78 , 79 ]. Independent research groups have also designed
and built fiber-bundle-based point-scanning fluorescence confocal systems, such as
that described by Lane et al., which uses a 561-nm excitation source to enable
imaging of cresyl violet-stained tissue [ 80 ]. These authors applied this system to
the bronchial epithelium, with a clinical focus on evaluating tissue to study the
efficacy of chemopreventive agents without biopsy removal, which itself can induce
spontaneous lesion regression.
These point-scanning confocal systems require a focused beam to be raster
scanned across the sample along one axis at several kHz, which is usually
accomplished with a resonant scanner or polygon mirror. Scanning along the “slow”
axis is performed with a conventional galvo mirror at a much slower rate of 10 s of
Hz. The frame rate of the system is often limited by the speed of the fast scanner, but
this component can actually be eliminated by scanning a focused line along only one
axis, with a single row of CCD camera pixels, or a linear array detector for detection.
Gmitro et al. developed a confocal fluorescence microendoscope which uses a fiber-
optic bundle and galvo-based line scanning at the proximal end. A custom designed
6-element, 3-mm-diameter, NA 0.46 miniature objective was also used at the distal
tip [ 48 , 81 ]. Gmitro's group has carried out extensive development of this platform,
which has also incorporated spectral detection capabilities [ 82 ] and recently entered
into in vivo pilot clinical studies [ 83 ].
The second distinct approach for beam scanning at the distal tip of a microendo-
scope involves directly translating the tip of the single fiber used for light delivery
and collection. No separate scanning mirrors are involved and the beam-focusing