are still required, it can enable more accurate identification of relevant biopsy sites. A
variety of contrast agents are used in CLEM, though all are currently used off-label
for this application. Fluorescein is administered intravenously to provide fluorescent
contrast to blood vasculature. Other contrast agents can be applied topically at the
imaging site, which may or may not require prior removal of mucin from the surface.
Topical imaging agents can include fluorescent stains to highlight intracellular
regions and lamina propria (fluorescein sodium) or cell nuclei (cresyl violet or
acriflavine). Additional fluorescent contrast agents with longer-wavelength excitation, such as methylene blue, have been demonstrated in preclinical research and
could be used in future clinical work as well; however, currently, only scopes
equipped with blue light lasers (488 nm) are approved for clinical use. The microscopic field of view of CLEM necessitates wide-area image-guided placement of the
CLEM probe prior to image acquisition. Existing CLEM endoscopes usually rely on
simultaneous wide-area WLE for correct placement of the microscopic probe at the
lesion of interest. Therefore, the CLEM examination site selection is subject to the
same limitations inherent to macroscopic WLE. However, other probe guidance
methods have been used with CLEM, such as ultrasonography or X-ray fluoroscopy
[13, 14].
Near-Infrared Imaging Modalities Beyond fluorescence imaging in the clinic,
a new technique, optical coherence tomography (OCT), utilizes interferometry
with near-infrared (NIR) light backscattering to construct a high-resolution image
of three-dimensional tissues from a series of two-dimensional tomographic crosssectional images. From an application point of view, OCT is similar in function
to high-frequency ultrasonography; however, OCT relies on light rather than sound
for generation of an image. Furthermore, the emitted near-infrared light is able to
traverse air-filled spaces; unlike HF ultrasonography, it does not require direct probe
contact (nor a water interface) with the tissue areas of interest. This makes it a useful
imaging modality for mapping the surface and subsurface topology of large or
expansive endoluminal cavities and therefore highly suited for endoscopic imaging.
OCT scans can provide information on cutaneous and subcutaneous endoluminal
surface density and morphology, enabling rapid wide-area identification of subsurface abnormalities. OCT endoscopes provide excellent spatial resolution, with
sub-20 μm resolutions possible. OCT systems also provide temporal resolution by
scanning multiple times per second, thus allowing acquisition and observation of
images in real time. The primary benefit of OCT to clinical endoscopy comes in
the ability to rapidly map the volumetric microstructure of large and complex
endoluminal cavity surfaces (Fig. 1e). Though NIR provides improved light penetration into tissue compared to shorter wavelengths, the depth of OCT scanning is
limited to a few millimeters due to excessive light scattering at greater depths.
Endoscopic OCT has been clinically approved for upper GI endoscopic procedures
and for intravascular endoscopy. It is also being evaluated for a variety of new
indications, including diagnosis of urothelial carcinoma in the bladder [15],
and imaging of bronchial airway remodeling in patients with chronic obstructive
pulmonary disease [16]. A compact OCT endoscope housed within a small,
208
E. P. Stater et al.
variety of contrast agents are used in CLEM, though all are currently used off-label
for this application. Fluorescein is administered intravenously to provide fluorescent
contrast to blood vasculature. Other contrast agents can be applied topically at the
imaging site, which may or may not require prior removal of mucin from the surface.
Topical imaging agents can include fluorescent stains to highlight intracellular
regions and lamina propria (fluorescein sodium) or cell nuclei (cresyl violet or
acriflavine). Additional fluorescent contrast agents with longer-wavelength excitation, such as methylene blue, have been demonstrated in preclinical research and
could be used in future clinical work as well; however, currently, only scopes
equipped with blue light lasers (488 nm) are approved for clinical use. The microscopic field of view of CLEM necessitates wide-area image-guided placement of the
CLEM probe prior to image acquisition. Existing CLEM endoscopes usually rely on
simultaneous wide-area WLE for correct placement of the microscopic probe at the
lesion of interest. Therefore, the CLEM examination site selection is subject to the
same limitations inherent to macroscopic WLE. However, other probe guidance
methods have been used with CLEM, such as ultrasonography or X-ray fluoroscopy
[13, 14].
Near-Infrared Imaging Modalities Beyond fluorescence imaging in the clinic,
a new technique, optical coherence tomography (OCT), utilizes interferometry
with near-infrared (NIR) light backscattering to construct a high-resolution image
of three-dimensional tissues from a series of two-dimensional tomographic crosssectional images. From an application point of view, OCT is similar in function
to high-frequency ultrasonography; however, OCT relies on light rather than sound
for generation of an image. Furthermore, the emitted near-infrared light is able to
traverse air-filled spaces; unlike HF ultrasonography, it does not require direct probe
contact (nor a water interface) with the tissue areas of interest. This makes it a useful
imaging modality for mapping the surface and subsurface topology of large or
expansive endoluminal cavities and therefore highly suited for endoscopic imaging.
OCT scans can provide information on cutaneous and subcutaneous endoluminal
surface density and morphology, enabling rapid wide-area identification of subsurface abnormalities. OCT endoscopes provide excellent spatial resolution, with
sub-20 μm resolutions possible. OCT systems also provide temporal resolution by
scanning multiple times per second, thus allowing acquisition and observation of
images in real time. The primary benefit of OCT to clinical endoscopy comes in
the ability to rapidly map the volumetric microstructure of large and complex
endoluminal cavity surfaces (Fig. 1e). Though NIR provides improved light penetration into tissue compared to shorter wavelengths, the depth of OCT scanning is
limited to a few millimeters due to excessive light scattering at greater depths.
Endoscopic OCT has been clinically approved for upper GI endoscopic procedures
and for intravascular endoscopy. It is also being evaluated for a variety of new
indications, including diagnosis of urothelial carcinoma in the bladder [15],
and imaging of bronchial airway remodeling in patients with chronic obstructive
pulmonary disease [16]. A compact OCT endoscope housed within a small,
208
E. P. Stater et al.