Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 286

As even more advanced modes of nonlinear microscopy such as coherent
anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS)
are demonstrated in benchtop platforms, there are research groups working on
endoscopic implementations to facilitate clinical applications. With these, and other
coherent Raman scattering (CRS) techniques, some elements of previous endomi-
croscopy systems including scanners and micro-optics are directly compatible, but
other unique challenges remain. CRS uses two tunable laser beams to illuminate
a sample at “pump” and “Stokes” frequencies; when the frequency difference is
adjusted to match the vibrational mode of an intrinsic biological component, non-
linear interactions can generate light at new frequencies or transfer intensity between
source frequencies. A significant feature of CRS is its use of picosecond, rather than
femtosecond excitation pulses which incur far less broadening on delivery through
conventional optical fiber than the femtosecond pulses used for multiphoton and
harmonic imaging [ 96 ]. However, four-wave mixing in the fiber itself can produce
an unwanted background contribution to the measured signal, which can be removed
through use of separate fibers for illumination and collection [ 97 ]. A second key
issue in developing CRS endomicroscopy is the need to direct both illuminating
beams onto the sample with tightly overlapping foci. GRIN lenses tend to exhibit
relatively large axial chromatic aberrations which can reduce the efficiency of
nonlinear signal generation. Murugkar et al. designed and assembled a miniature
lens system for CARS microscopy which comprised multiple lenses, each 1.8 mm
in diameter, to compensate for chromatic aberration at the source wavelengths
(800 and 1,040 nm) [ 98 ]. Saar et al. also designed a custom miniature objective
for CRS imaging, this time by using two separate GRIN lenses separated by a
diffractive optic, to create an imaging system with zero axial chromatic aberration
at the illuminating pulse wavelengths [ 99 ]. This system also used the piezoelectric
tube scanners used for other nonlinear endomicroscopy studies (described above) to
translate the delivery fiber in a spiral scan pattern. In order to increase the collection
efficiency of the signal photons, these authors used a photodiode positioned around
the illumination optics, directly at the probe's tip. Numerical simulations indicated
that this approach can more efficiently collect multiple-scattered CRS light from
within thick samples, compared to methods which attempt to couple signal light
back into an optical fiber.

8.3.5 Optical Coherence Tomography

Originally demonstrated in the early 1990s, optical coherence tomography (OCT)
imaging generates cross-sectional images of tissue microstructure using interfer-
ometric detection of backscattered light. Analogous to ultrasound, OCT typically
achieves spatial resolution of around 2-15 microns. After initial application to oph-
thalmology and dermatology, the development of fiber-optic probes and catheters
in the mid-1990s led to new applications in cardiology and gastroenterology [ 100 ].
OCT probes are almost exclusively based on conventional single-mode fiber, with