Molecular imaging endoscopy requires high resolution to observe the large surface area of the GI mucosa and subsequently localize molecular changes in tumors. Optical spectroscopic and/or imaging techniques offer the potential for detecting the very earliest mucosal changes at the microstructural, biochemical and molecular levels. Several optical techniques currently under investigation for the endoscopic detection of precancerous GI lesions includes fluorescence spectroscopy and imaging, Raman spectroscopy, light-scattering spectroscopy (LSS), optical coherence tomography (OCT), and confocal fluorescence endomicroscopy[17
AFI visualizes lesions including neoplasms not detectable by conventional white-light endoscopy due to differences in tissue fluorescence intensity. During AFI, normal tissue is pseudocolored green and blood vessels are dark green, whereas the hypertrophic fundic mucosa of the stomach and dysplastic or neoplastic areas appear magenta[8
]. New AFI systems have a xenon light source (XCLV-260HP; Olympus, Tokyo, Japan) with a rotary red/green/blue band-pass filter. With this light source, the mucosa is sequentially illuminated with red, green, and blue light at a frequency of 20 cycles/s. The high-resolution videoendoscope (XCF-Q240FAI, Olympus) has two separate monochromatic charge-coupled devices (CCD), one for white-light endoscopy and one for AFI. The white-light mode can be switched to the autofluorescence mode by pressing a small button on the control head, and the switch is completed in 3 s[18
].In the AFI mode, blue-spectrum light (395-475 nm) is delivered to excite AF, together with light in the green (540-560 nm) and red (600-620 nm) spectra. The AFI-CCD has a barrier filter that allows detection of all light with wavelengths from 490 nm to 625 nm, thereby eliminating blue excitation light. The sequentially detected images from AF along with the green reflectance, and red reflectance are integrated by the imaging processor into one AF image. AFI does not require the administration of fluorescence probes. Thus, it can be applied for cancer screening tests. The sensitivity for premalignant GI lesions increases when AFI is combined with high definition white-light imaging and narrow-band imaging to provide endoscopic trimodal imaging[19
]. Endoscopic trimodal imaging has been proposed as an alternative to overcome the problems of AFI. Endoscopes with a widefield of view that can detect induced fluorescence during targeted endoscopic imaging have not yet been evaluated in larger clinical trials.
Raman spectroscopy is a form of image enhancement based on the principle that incident light (with wavelengths in the near-infrared region of the spectrum) can induce tissue biomolecules to vibrate and rotate. When light interacts with tissue molecules, it can be absorbed or scattered. Almost all of the scattered light is of the same wavelength as the incident light (elastic scattering)[20
]. However, a small fraction of light undergoes so-called Raman (inelastic) scattering, in which slight shifts in energy and wavelength relative to the incident light occur because of energy exchange within a molecular structure. Raman spectroscopy can detect tissue changes at the molecular level, yielding unique “spectral fingerprints” of tissues as they become abnormal. Molckovsky et al[21
] reported the first in vivo
study using a fiber-optic probe via
the accessory channel of the colonoscope. This study resulted in impressive accuracy of diagnosing hyperplastic (n
= 9) and adenomatous (n
= 10) polyps (100% sensitivity, 89% specificity, 95% overall accuracy).
LSS is based on white-light (400 nm to 700 nm) reflectance, whereby photons incident on tissue are backscattered without a change in their wavelength, providing microstructural information about the tissue. LSS measurements are performed with fiber-optic probes placed on the tissue surface via
the accessory channel of the endoscope. Analysis of the intensity and wavelength of light reflected from the tissue surface provides an estimate of the size and degree of crowding of epithelial cell nuclei[17
]. Recent preliminary work has suggested that LSS can be useful to identify even earlier subcellular changes associated with cancer progression[22
]. In this study, a new generation of light scattering technology has detected submicron-size architectural changes in an endoscopically normal rectum. These changes were associated with the presence of neoplasia located elsewhere in the colon.
Confocal microscopy is based on tissue illumination with a low-power laser. The reflected light from the tissue is refocused onto the detector by the same lens, meaning that only returning light refocused through a pinhole is detected[23
]. This process provides high-resolution images from a thin section within otherwise optically thick tissue. With technical developments, a miniaturized confocal laser scanner has been integrated into the distal tip of a flexible white-light endoscope for clinical use. Confocal endomicroscopy (Pentax EC-3870 CIFK; Pentax, Tokyo, Japan) enables confocal microscopy in addition to standard videoendoscopy[24
]. The diameters of both the distal tip and the insertion tube are 12.8 mm. The distal tip contains an air- and water-jet nozzle, two light guides, a water-jet channel used to apply contrast agent, and a 2.8 mm working channel. The system uses a 488-nm excitation wavelength laser and enables the detection of 205 nm to 585 nm wavelength fluorescence.Confocal images are collected at a scan rate of approximately one frame/s, ata maximum resolution of 1024 × 1024 pixels. The optical slice thickness is 7 μm (axial resolution), and the lateral resolution is 0.7 μm.The range of the z
-axis is 0 to 250 μm below the surface layer. Screen images approximate a 1000-fold magnification of the tissue in vivo
. Confocal images can be generated simultaneously with endoscopic images. A slightly different approach is used for flexible probe-based confocal microscopy. Probe-based confocal laser endomicroscopy (pCLE; Cellvizio-GI; Mauna Kea Technologies, Paris, France) has been developed recently and has the advantage that a miniprobe can be easily passed through the working channel of a standard endoscope[25
]. Probes generate dynamic (12 frames/s) images with a scanning field of 30 000 pixels. In addition to faster acquisition, the advantages include greater versatility of pCLE probes, which can be used in conjunction with virtually any endoscope,cholangioscope, bronchoscope, or ureteroscope, and for ad hoc
usage, such as when a lesion is detected with a normal endoscope[26
]. However, pCLE has a slightly lower resolution (approximately 1 μm compared with 0.7 μm for the Pentax confocal endomicroscope) and a smaller field of view (240-600 μm).
In the past few years, newly developed procedures and technologies have improved endoscopic recognition of GI neoplasms. Narrow band imaging (NBI) (with which Olympus scopes are equipped), the contrast enhancement system (i-scan) (associated with Pentax scopes) and multiband imaging (MBI) (with which theFujinon scope is equipped) are used in combination with magnification and high resolution endoscopy[27
]. These imaging techniques can improve visualization of the vascular network and surface texture of the mucosa and can facilitate endoscopic diagnoses. NBI uses rotating filters in front of light sources to narrow the bandwidth of the projected light, and increases the blue spectrum intensity of the light used. This shorter wavelength is more readily absorbed by hemoglobin and has shallow penetration into only the superficial layer, thereby enhancing the visualization of superficial capillaries. The advantages of NBI include enhanced mucosal contrast at the push of a button and ease of neoplastic and non-neoplastic lesion differentiation. However, NBI results in poorer illumination of the background and a learning curve effect is observed, even for experienced endoscopists[28
To date, AFI, NBI and CLE have been compared separately with conventional endoscopy. Trials should be extended to investigate different patient groups, as the optimal endoscopic modality may vary. AFI or NBI may be the examination of choice for general screening and CLE may be used for ulcerative colitis surveillance. Further large randomized controlled trials are needed to determine which modality would be the most suitable for various patient subpopulations.