Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 281

on conventional endoscopy. Current limitations include the inability to see beyond
the most superficial tissue layer (a few 10s of micrometers of tissue) and a lack of
established criteria with which to make a diagnosis.

8.3.2 Fiber-Optic Endomicroscopy

The simplest implementation of fiber-optic-based endomicroscopy uses a length
of image guide fiber (also known as a coherent fiber bundle) to relay an image
of the tissue located in contact with the distal tip to an image sensor (CCD,
CMOS, film, or the human eye). This approach has been demonstrated in wide-field
epifluorescence and reflectance modes. Epifluorescent imaging has used exogenous
contrast agents to generate contrast; Dromard et al. used topical fluorescein to enable
contact imaging of the skin [ 64 ]. Muldoon et al. carried out pilot clinical studies
which established the relevant image features observable in this configuration
which can be used to distinguish normal from neoplastic tissue in the oral cavity
and esophagus [ 65 , 66 ]. A portable, battery-powered version of this system was
described by Pierce et al. [ 67 ], which was subsequently used during screening
endoscopy in the esophagus and colon of patients [ 68 ]. Zhong et al. reported a
quantitative study of tumor detection and photodynamic therapy treatment with a
similar system configured for imaging indocyanine green in the near infrared [ 69 ].
This simplicity and relatively low cost of this platform has led to its use in several
applications, including imaging of tuberculosis bacilli [ 70 ] and for detecting lymph
node metastases in breast cancer patients [ 71 ]. A fiber bundle can be combined with
a GRIN lens to effectively improve the spatial resolution of the system by directly
bonding the low NA side of the GRIN assembly to the distal face of the bundle.
The spatial resolution of the system will be increased beyond that dictated by the
core spacing of the bundle alone by a factor equal to the GRIN lens magnification;
however, the FOV is reduced by the same factor. Flusberg et al. demonstrated an
elegant miniaturized epifluorescence imaging setup with a fiber bundle and GRIN
lens assembly, weighing only 1.1 g, sufficiently light to attach directly to a living
mouse to study cellular processes (blood flow and calcium dynamics) as the animal
was free to move [ 72 ].

8.3.3 Scanning Fiber-Optic Endomicroscopy

While conventional epifluorescence imaging can be performed through a fiber
bundle, wide-field imaging remains vulnerable to image contrast degradation due
to generation and collection of out-of-focus light. As with the approaches described
previously using only a GRIN lens assembly, optical sectioning can be achieved by
either performing beam scanning proximal to a coherent fiber bundle. Gmitro and
Aziz described a commercial scanning fluorescence confocal microscope coupled to