Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 256

resolution over the depth of imaging. With spatial resolution around 10 microns,
OCT systems are capable of endoscopic imaging at frame rates (10s of frames per
second) which permits imaging over large areas in a relatively short time [ 10 ].
Full-field contact imaging techniques involve placing either a fiber-optic bundle
or lens system directly in contact with the tissue surface, with image relayed onto a
sensor. In these cases, the FOV is defined by the area of the sensor which is projected
onto the tissue via the fiber bundle or lens system. For a bare fiber bundle, this area
corresponds simply to the diameter of the fiber itself. The spatial resolution is on
the other hand related to fiber spacing. With the addition of lenses to the fiber tip,
the FOV and resolution are decreased and increased, respectively, by a factor equal
to the magnification of the lens system.
A summary of typical operational parameters for currently implemented endomi-
croscopy systems is presented in Table 8.1 .Tables 8.2 and 8.3 present examples of
images obtained with these different modalities.

8.2.1.2 Optical Sectioning

High-resolution imaging within thick tissue relies on an ability to preferentially
collect in-focus light over out-of-focus background. Sometimes termed “optical
sectioning,” both confocal microscopy and nonlinear microscopy can selectively
image tissue at a single plane beneath the surface. Confocal imaging achieves this
by placement of a pinhole in front of the detector at a conjugate plane to the
tissue depth being imaged. Light emerging from above or below this location in the
tissue is preferentially blocked by the pinhole, while light from the imaged plane
passes through. Following [ 11 ], confocal microscope can provide lateral resolution
defined by:

0:4
NA

d xy ; (8.1)

while axial resolution is

1:4n
NA 2

d z : (8.2)

This is for pinhole diameter matching the full width at half maximum (FWHM) of
the Airy disk intensity and allows 75 % of irradiance to pass through the pinhole.
Nonlinear imaging achieves optical sectioning by an alternative mechanism;
due to the nonlinear dependence of emitted signal on incident light intensity,
negligible signal is generated at planes above or below the focal spot in tissue.
Nonlinear microscopy therefore does not require a pinhole to block out of focus
light. Regardless of mechanism used to achieve optical sectioning, the working
distance of the miniature endoscope objective must be sufficient to enable the
focal plane to reach the desired imaging depth beneath the tissue surface. Both
confocal and nonlinear techniques require miniature objective lenses with high NA