Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 255

tissues and in vivo animal models being reported. For more details on the current
development and implementation of these endomicroscopy modes, see Sect. 8.3 .

8.2 Design Requirements and System Components

8.2.1 Key Operational Parameters

8.2.1.1 Spatial Resolution and Field of View

The key distinction between conventional endoscopy and endomicroscopy is spatial
resolution, the ability to provide the endoscopist with images of cellular or near-
cellular level detail. With increased resolution, there is, of course, a concurrent
reduction in FOV, just as one experiences when switching between different
objectives on a benchtop microscope. In the field of endomicroscopy, however,
systems usually require numerical aperture (NA) values above 0.4 and even up
to 1.0. This requirement arises from the need to optimize the collection efficiency
for weak optical signals, especially for nonlinear processes and for systems which
aim to image deep beneath the tissue surface. This ability to selectively image
a single subsurface tissue plane or “optical section” can extend to depths of
around 200-400 m with confocal microscopy [ 7 ]andover500 m for multiphoton
imaging [ 8 ].
The lateral FOV should be maximized to make system more practical to use,
but at high NA is usually in the range of 250-500 m in endomicroscopy systems.
This is dictated by three main reasons: (a) fundamental principle of Lagrange
invariant FOV, NA, working distance, and optics diameter are strongly interrelated
and have to be balanced for practical solution; (b) overall ability to fabricate
and assemble miniature high-quality optics; and (c) the diameter of available
coherent fiber bundles or the maximum scanning angle generated by miniaturized
actuators or scanners. While simultaneously achieving high NA and FOV for small
optics is challenging, the task can be somewhat mitigated by use of the shorter
working distances (the distance between the imaging lens surface and the tissue).
Another approach to overcome the small FOV limitation in endomicroscopy is to
continuously stitch together images as the imaged field is swept across the tissue,
building up a larger “macro” image for display on the monitor screen [ 9 ].
As mentioned above, optical coherence tomography generally operates in a
regime where spatial resolution is lower than confocal or nonlinear microscopies,
but the FOV is correspondingly larger. In most instances, the optical beam is scanned
(linearly or circumferentially) to determine the lateral extent of the FOV, while the
axial (depth) extent is determined by the external hardware configuration (either the
physical scan range of the interferometer's reference path or the spectral resolution
of the detection apparatus). The distal probe tip optics is typically configured with
relatively low NA and large Rayleigh range to provide relatively uniform lateral