concentrations of naturally occurring fluorophore biomolecules, such as collagen, to
generate wide-area images of internal tissues without exogenous contrast agents.
Fluorescence excitation is provided by illumination with a high-intensity blue light
on the endoscope; emission is visualized (via false color video) with an optical
camera configured to capture light at various emission wavelengths. AFE imagery
can provide information about abnormalities that may not be easily detectable with
WLE, such as differences in tissue density, epithelial thickness, or blood flow rate
(Fig. 1c). Though AFE systems can address some of the limitations of standard
WLE, they are constrained by many of the same drawbacks. Furthermore, although
AFE has been shown to improve detection rates of lesions, it also typically has a
higher rate of false positives, such as in detection of early gastric neoplasia [11],
necessitating more frequent follow-up biopsy analyses.
Analogous to white light chromoendoscopy, induced fluorescence endoscopy
(IFE) relies upon administration of exogenous fluorescent contrast agents and
illumination with excitatory wavelength light to visualize specific internal anatomy.
IFE can provide enhancement of visual contrast beyond what is possible with WLE,
chromoendoscopy, NBI, or AFI. Furthermore, the use of far-red or near-infrared
fluorophores with IFE can increase the penetration depth of imaging and enable
detection of non-superficial structures and lesions, overcoming a key limitation of
other optical endoscopic technologies. However, this newer technology has not yet
seen widespread clinical adoption, due mainly to the limited number of fluorescent
contrast agents available to clinicians. Frequently used contrast agents in clinical IFE
include 5-aminolevulinic acid (5-ALA) and hexaminolevulinate (for cancer detection in upper gastroendoscopy and bladder cystoscopy, respectively) and the nearinfrared fluorophore indocyanine green (ICG, for enhanced fluorescent labeling of
vasculature) (Fig. 1d). Another near-infrared fluorochrome, methylene blue, is FDAand EMA-approved as a chromogenic dye; because it is also fluorescent, its use with
IFE has been investigated to help surgeons locate and avoid injury to the ureters of
colorectal surgery patients [12]. Additionally, IFE holds promise for endoscopic
visualization of targeted molecular probes; however, this application has not yet
been exploited clinically, due to the lack of targeted fluorescent contrast agents with
regulatory approval.
Previously described visible-spectrum imaging modalities are applicable to
either wide-area imaging (e.g., during open or laparoscopic surgery) or magnification endoscopy. However, with respect to fluorescence imaging, confocal laser
endomicroscopy (CLEM) is the preeminent technology for fluorescent endoscopic
examination under magnification. CLEM combines blue laser fluorescence imaging
with intravital confocal microscopy, enabling live confocal plane imaging of
endoluminar microanatomy at cellular or subcellular resolutions. This enables superior image quality, increased penetration depth, and improved anatomical differentiation when compared to conventional magnification endoscopy. The development
of clinical CLEM technology confers two primary benefits to endoscopic diagnosis:
first, in some instances, it may permit completely optical percutaneous biopsy of
patients, negating the need for potentially traumatic tissue sample collection and
outside histopathological analyses; second, in cases where traditional tissue biopsies
The Present and Future of Optical Imaging Technologies in the Clinic:. . .
207
generate wide-area images of internal tissues without exogenous contrast agents.
Fluorescence excitation is provided by illumination with a high-intensity blue light
on the endoscope; emission is visualized (via false color video) with an optical
camera configured to capture light at various emission wavelengths. AFE imagery
can provide information about abnormalities that may not be easily detectable with
WLE, such as differences in tissue density, epithelial thickness, or blood flow rate
(Fig. 1c). Though AFE systems can address some of the limitations of standard
WLE, they are constrained by many of the same drawbacks. Furthermore, although
AFE has been shown to improve detection rates of lesions, it also typically has a
higher rate of false positives, such as in detection of early gastric neoplasia [11],
necessitating more frequent follow-up biopsy analyses.
Analogous to white light chromoendoscopy, induced fluorescence endoscopy
(IFE) relies upon administration of exogenous fluorescent contrast agents and
illumination with excitatory wavelength light to visualize specific internal anatomy.
IFE can provide enhancement of visual contrast beyond what is possible with WLE,
chromoendoscopy, NBI, or AFI. Furthermore, the use of far-red or near-infrared
fluorophores with IFE can increase the penetration depth of imaging and enable
detection of non-superficial structures and lesions, overcoming a key limitation of
other optical endoscopic technologies. However, this newer technology has not yet
seen widespread clinical adoption, due mainly to the limited number of fluorescent
contrast agents available to clinicians. Frequently used contrast agents in clinical IFE
include 5-aminolevulinic acid (5-ALA) and hexaminolevulinate (for cancer detection in upper gastroendoscopy and bladder cystoscopy, respectively) and the nearinfrared fluorophore indocyanine green (ICG, for enhanced fluorescent labeling of
vasculature) (Fig. 1d). Another near-infrared fluorochrome, methylene blue, is FDAand EMA-approved as a chromogenic dye; because it is also fluorescent, its use with
IFE has been investigated to help surgeons locate and avoid injury to the ureters of
colorectal surgery patients [12]. Additionally, IFE holds promise for endoscopic
visualization of targeted molecular probes; however, this application has not yet
been exploited clinically, due to the lack of targeted fluorescent contrast agents with
regulatory approval.
Previously described visible-spectrum imaging modalities are applicable to
either wide-area imaging (e.g., during open or laparoscopic surgery) or magnification endoscopy. However, with respect to fluorescence imaging, confocal laser
endomicroscopy (CLEM) is the preeminent technology for fluorescent endoscopic
examination under magnification. CLEM combines blue laser fluorescence imaging
with intravital confocal microscopy, enabling live confocal plane imaging of
endoluminar microanatomy at cellular or subcellular resolutions. This enables superior image quality, increased penetration depth, and improved anatomical differentiation when compared to conventional magnification endoscopy. The development
of clinical CLEM technology confers two primary benefits to endoscopic diagnosis:
first, in some instances, it may permit completely optical percutaneous biopsy of
patients, negating the need for potentially traumatic tissue sample collection and
outside histopathological analyses; second, in cases where traditional tissue biopsies
The Present and Future of Optical Imaging Technologies in the Clinic:. . .
207