Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 287

most incorporating fixed optical elements at the distal tip and a proximal mechanism
to scan the entire length of the fiber, either linearly or circumferentially [ 101 - 103 ].
Beam focusing at the distal tip is usually achieved with a spacer and GRIN lens
bonded to the fiber tip, followed by a microprism to deflect the converging beam
through 90 degrees, exiting the probe perpendicular to the axis of the fiber. Early
studies established the characteristic image features of normal and neoplastic tissue
in the esophagus [ 104 , 105 ], the appearance of atherosclerotic plaques, and vascular
response to stenting in the coronary arteries [ 106 , 107 ].
The shift from time-domain OCT to the much faster Fourier-domain platforms
in the mid-2000s revolutionized the field and also presented new challenges
for endoscopic implementations. The ability to acquire and process several tens
of thousands of A-lines per second required the development of probes which
could scan much faster than those used in time-domain platforms [ 24 ]. Optical
frequency-domain imaging (OFDI) has become the predominant form of OCT
for endoscopic and catheter-based imaging; the use of rotary probes spinning at
over 100 revolutions per second with linear pullback at several millimeters per
second enables several-centimeter-long segments to be mapped at microscopic
resolution (7-m-axial, 30-m-transverse). Tearney et al. demonstrated the ability
of high-speed OFDI with a 2.6-F catheter to image a range of features, including
thin-capped fibroatheromas, calcium, stent struts, and intimal hyperplasia within
the coronary arteries of living patients [ 25 ]. Cardiovascular imaging requires a
brief saline purge to displace blood from the imaging field, but the high speed
of OFDI still enables several-centimeter segments to be imaged per flush. The
same research group implemented OFDI in the gastrointestinal tract of human
patients, where due to the much larger lumen compared the coronary vessels, the
imaging probe was contained within an inflatable balloon catheter [ 108 ]. Once
in position, the balloon was inflated to a diameter of 25 mm, which served
to dilate the esophageal lumen and center the rotating probe. When rotated
and translated to create a spiral scan path, the probe visualized tissue within a
6.5-cm-long window in the balloon. This enabled the characteristic image features
of normal squamous mucosa, cardia, specialized intestinal metaplasia (SIM), and
high-grade dysplasia/intramucosal carcinoma. Similar high-speed OCT systems are
also under investigation in the esophagus for monitoring response to therapy, with
the subsurface imaging capability of OCT potentially used to detect residual or
missed areas of concern, even when hidden beneath normal epithelium [ 109 ].

8.4 Summary

Endomicroscopy - a microscopy at the tip of the endoscope is a reach and emerging
area encompassing several imaging modalities starting from wide-field imaging,
confocal imaging, nonlinear imaging, optical coherence tomography, and others.
It serves as a complement to the macroscopic views obtained either by classical
endoscopy or visual observation. To accomplish its goal it applies several modern