Advanced Endoscopic Imaging: Magnification Endoscopy, Confocal Laser Endomicrosopy, and Optical Coherence Tomography

Helmut Neumann, MD, PhD, FASGE and Peter R. Galle, MD, PhD

Introduction

Since the introduction of the first flexible endoscope in 1961, endoscopy has rapidly evolved from a pure diagnostic technique to a therapeutic platform. Within recent years, various new technologies have been developed to increase detection rates throughout the whole luminal gastrointestinal (GI) tract.¹ With increasing detection rates the debate of an adequate characterization became more and more important as benign lesions normally do not require subsequent endoscopic therapy.² Moreover, with the advent of advanced endoscopic therapeutic techniques, which now even allow for therapy of early cancerous lesions, new requirements were raised for real-time optical diagnosis. To date, well-trained endoscopists can already predict, based on endoscopic imaging, whether a lesion is nonneoplastic or neoplastic (ie, resect and discard strategy) or whether a malignant lesion is amendable to an endoscopic intervention or requires a more aggressive surgical approach (ie, imaging guiding therapy).³ Therefore, the adoption of modern diagnostic imaging modalities in endoscopy allows for more precise diagnosis and patient-centered medicine. Accordingly, personalized medicine based on endoscopic imaging becomes feasible with the advantages of being more accurate and cost effective.

In this chapter we will focus on the most recent developments in the field of advanced endoscopic imaging, discussing the latest innovations in magnifying endoscopy, confocal laser endomicroscopy (CLE), and optical coherence tomography (OCT) with the aim of providing a comprehensive overview of the different techniques and their potential application in the clinical practice.

Technical Aspects of Magnification Endoscopy

The development of optical magnification endoscopy (also called zoom endoscopy) goes back to 1978.4 However, application of magnifying endoscopy was mostly limited to expert centers in Japan as stabilization of magnified images was extremely awkward during the early times. Nowadays, with the advent of built-in computer-based stabilization systems (eg, prefreeze, freeze scan, or antiblur), magnification endoscopy is relatively easy to perform and therefore increasingly used throughout the world. Optical magnification endoscopes are equipped with a movable lens in the distal tip of the endoscope controlling the focal distance therefore providing magnified images. Various systems are on the market providing optical magnification of up to 136-fold. As images are normally missing contrast because of the high level of magnification, optical magnification endoscopy is typically combined with chromoendoscopy (CE) methods. In this context various techniques are used to enhance the surface pattern (eg, CE with indigo carmine or crystal violet) or the mucosal vascular pattern morphology (eg, Narrow Band Imaging [NBI, Olympus], Blue Light Imaging [BLI, Fujifilm], or optical-enhancement i-Scan [Pentax]).⁵ These CE techniques are discussed elsewhere in this book.

Technical Aspects of Confocal Laser Endomicroscopy

CLE is based on the application of fluorescence agents. These can be applied systemically (eg, fluorescein sodium) or topically (eg, fluorescein sodium, acriflavine, crystal violet). Afterward, a blue laser with a wavelength of 488 nm is engaged. The term confocal refers to the alignment both of illumination and collection systems in one focal plane. The laser light is reflected from the fluorescence agent within the tissue, thereby enabling confocal imaging in focus at a magnification of up to 1000-fold.⁶ To date, various CLE systems are available. One system is integrated into a standard high-resolution endoscope (iCLE), and one offers a through-the-scope, probe-based solution (pCLE) with differing diameters for various indications throughout the GI tract.

Technological details of the technique have been described elsewhere; however, the major difference between iCLE and pCLE is the resolution (higher for iCLE) and the image playback (single images for iCLE and video sequences for pCLE).⁶

Technical Aspects of Optical Coherence Tomography

OCT is based on the principle of interferometry. Therefore, the technique is similar to ultrasound but instead of an acoustic signal, OCT uses light to measure the path length between the instrument and the target tissue.⁷ The term coherence describes a property of light that is inversely proportional to its wavelength bandwidth and quadratically proportional to its central wavelength. Modern OCT instruments offer a resolution to 10 µm and imaging down to a depth of 3 mm. Scanning over a length of 6 cm is performed within a period of 90 seconds. Application of OCT does not require any prestaining of the mucosa, and the system can operate with an air or water interface.

Magnification Endoscopy in the Upper Gastrointestinal Tract

Detection of early cancerous lesions is of paramount importance as it allows for not only early diagnosis but also for endoscopic therapy. Nevertheless, detection of early cancerous lesions is often difficult as they typically show only subtle signs of mucosal disintegration, and more advanced signs, like ulcers or bleeding, are often missing. Therefore, the advantage of optical magnification endoscopy is the potential to highlight subtle mucosal surface and vascular pattern details to facilitate early diagnosis for subsequent endoscopic therapy.

Esophagus—Squamous Cell Cancer

The esophageal mucosa is characterized by a fine vascular network extending from the submucosal vessels. The intrapapillary capillaries are visualized as single-loop vessels and are therefore named intrapapillary capillary loops (IPCLs). This fine and regular reticular network is not maintained in pathologic conditions affecting the esophageal mucosa and is therefore used for detection of squamous dysplasia and cancer.8 The IPCL pattern classification is used to describe tissue atypia of the squamous epithelium.⁹ Type I and Type II represent normal and inflammatory changes; Type III is a feature of atrophic mucosa or low-grade dysplasia (less vascular proliferation is seen), while Type IV is seen in noninvasive, high-grade dysplasia (increased vascular proliferation). IPCL Type V1 indicates carcinoma in situ and is characterized by dilation of the IPCL, a meandering path, caliber change, and nonuniformity.

Based on this classification subsequent therapy can be recommended. For Type I and Type II, proton pump inhibitor therapy might be the matter of choice; a Type III lesion should be surveyed every year, and Type IV and Type V lesions require endoscopic therapy with endoscopic mucosal resection or endoscopic submucosal dissection.

Esophagus—Barrett’s

Barrett’s esophagus is considered a precancerous condition, and various classifications have been proposed for its endoscopic assessment and grading. The main classification systems (ie, Kansas, Amsterdam, and Nottingham) are all based on mucosal surface and vascular pattern morphology, which are generally divided into regular or irregular phenotypes.^10–12 Although initial data for these classifications were excellent, in the clinical-like situation they are still showing limitations in accuracy and interobserver agreement. Therefore, biopsies are still recommended for correct evaluation of Barrett’s mucosa although magnifying imaging allows for a more targeted approach.¹³

Stomach

Magnified observation of the microvasculature is helpful for characterizing flat cancerous lesions, the degree of differentiation, and the lateral extent of the disease. Magnification imaging is based both on assessment of the surface and vascular pattern morphology. Most recently, a classification has been proposed by an international group to expedite diagnosis of gastric cancerous lesions.¹⁴ First, lesions are divided according to the Paris classification into protruded (0-IIa), flat (0-IIb), or depressed (0-IIc) types. After examination with white light and CE, magnifying endoscopy is recommended to assess the microsurface of the lesion to identify whether mucosal changes are predictive of gastritis, intestinal metaplasia, or cancer. Finally, optical CE (eg, NBI, BLI, optical-enhancement i-Scan) is used concomitant to magnification to assess the microvasculature. Changes indicating cancer are an irregular vascular network, meandering path, deranged shape, heterogenic appearance, and altered density. Further characterization of the microvasculature network allows distinction into differentiated carcinoma (fine network vessels) or poorly differentiated carcinoma (corkscrew vessels).

Magnification Endoscopy in the Lower Gastrointestinal Tract

Differentiation of colorectal polyps is of interest as most diminutive lesions located in the rectosigmoid are hyperplastic lesions not warranting any endoscopic therapy. Differentiation of nonneoplastic and neoplastic lesions can be performed with the most advanced endoscopic imaging techniques although the interobserver agreement is mostly not sufficient for nonexperts.15 In contrast, differentiation of neoplastic lesions in the colorectum requires extensive work with magnifying endoscopy focusing both on the mucosal surface and vascular pattern morphology.¹⁶ Kudo et al proposed the pit pattern classification in 1996 differentiating colorectal lesions in 5 subtypes.¹⁷ The classification allows differentiation of nonneoplastic and neoplastic lesions but is also used for assessment of invasion depth of colorectal neoplasms. In short, Type I shows roundish pits; Type II stellar or papillary pits; Type III S small, roundish pits; Type III L roundish and tubular pits; Type IV branch-like or gyrus-like pits; Type V I irregular and distorted pits; and Type V N nonstructural and absence of pits. Types I and II are typically considered nonneoplastic lesions although recent data have highlighted that even serrated lesions in the colorectum could show Types I and II pits. For these lesions, additional parameters are currently under debate to differentiate serrated lesions from hyperplastic ones (eg, mucus cap, occurrence in the right colon).¹⁸ Types III and IV lesions are generally considered benign adenomas, only exceptionally including high-grade dysplasia or low-grade carcinoma. In contrast, the Type V I pit pattern suggests adenoma with high-grade dysplasia or cancer. The Type V N pit pattern is suggestive of deep submucosal invasive carcinoma.

A recent meta-analysis of 20 studies consisting of 5111 colorectal lesions in 3418 patients showed that the classification is an accurate diagnostic method for the differentiation of neoplastic colorectal lesions with sensitivity for magnifying CE of 93% and a specificity of 87%, respectively.¹⁹

Confocal Laser Endomicroscopy

Since its introduction in 2003, multiple studies have evaluated the effectiveness of confocal imaging for diagnosis of various GI diseases. Confocal imaging has a limited field of view between 240 and 600 µm and a fixed-image plane depth varying between 55 and 130 µm depending on the system used. In addition, the most widely used fluorescence agent is fluorescein sodium, which highlights only architectural details of the tissue.⁶ Therefore, confocal imaging alone normally does not allow for identification of low-grade dysplasia. In addition, differentiation of neoplasia and inflammation is difficult as no direct cellular details are visible. In the early days confocal imaging was shown to be effective for diagnosis of Barrett’s esophagus, esophageal squamous cell cancer, and colorectal polyps.⁶ However, given the more recent developments in the field of advanced endoscopic imaging, including magnifying endoscopy, NBI, BLI and others, the limited availability of confocal imaging, the high costs of the technique, and the need for specific operator training, there is no longer any clinical indication for the usage of CLE for characterization of GI lesions.

Increasing knowledge of the importance of functional parameters for the development of various diseases, however, has led to a new indication for confocal imaging. Early studies in this field have shown that confocal imaging allows detecting translocation of commensal bacteria into the inflamed mucosa in patients with inflammatory bowel disease (IBD), suggesting that confocal imaging may be used for assessment of mucosal integrity.20 In this context, confocal imaging is increasingly studied in patients with IBD.²¹ More recent data have shown that endomicroscopy can identify reproducible microscopic changes in the terminal ileum that are risk factors for relapse in patients with otherwise inactive Crohn’s disease (eg, fluorescein leakage).²² Confocal imaging has also been studied for in vivo functional imaging of mucosal barrier defects in patients with intestinal metaplasia.²³ The paracellular permeability was significantly increased both in Helicobacter pylori-negative and H pylori-positive samples, showing that gastric intestinal metaplasia is associated with impaired paracellular barrier function. Of note, the role of confocal imaging for assessment of functional imaging has also been evaluated in patients with irritable bowel syndrome. Diluted food antigens were administered directly to the duodenal mucosa through the working channel of an endoscope. Based on confocal analysis of irritable bowel syndrome patients with suspected food intolerance, exposure to candidate food antigens caused immediate breaks, increased intervillous spaces, and increased intraepithelial lymphocytes in the intestinal mucosa. These changes were also associated with patient responses to exclusion diets.²⁴ Modern miniaturized confocal probes can also be passed through a 22-gauge puncture needle (so-called “nCLE”) or can be used to evaluate the pancreatic and hepatic duct system. Data have shown the effectiveness of nCLE in differentiating mucinous vs nonmucinous pancreatic cystic lesions and solid pancreatic masses.^25,26 In the evaluation of indeterminate pancreatobiliary strictures, CLE with endoscopic retrograde cholangiopancreatography (ERCP) compared to ERCP alone could increase the detection of cancerous strictures with a sensitivity of 98% vs 45% and had a negative predictive value of 97% vs 69%. However, the specificity and the positive predictive value decreased (67% vs 100% and 71% vs 100%, respectively) when compared to index pathology.²⁷ Given the difficulty of correctly discriminating inflammatory and malignant lesions with confocal imaging, however, there is no recommendation so far for its use outside clinical studies. Recently, increasing efforts have been made to perform molecular imaging with CLE. Molecular imaging is aiming at highlighting specific receptors by their molecular signature. Potentially, molecular imaging might therefore allow for identification of disease-specific alterations and offers therefore the potential of individualized, molecularly targeted diagnosis and therapy.⁶ Early data have already shown the potential of molecular imaging in highlighting specific epitopes that are often overexpressed in oncological diseases (eg, vascular endothelial growth factor, epidermal growth factor receptor).⁶ In addition, molecular imaging has shown potential in identifying single bacteria and might therefore become of interest for microbiome research.²⁸ More recent in vivo trials have also shown the potential of molecular imaging in identifying dysplastic colonocytes and for prediction of therapeutic response to anti–tumor necrosis factor antibody therapies in patients suffering from IBD.^29,30 Most recently, molecular imaging with a specific dye accumulating in c-Met-expressing tumors has also been identified as a potential tool to improve detection rates of colorectal polyps during colonoscopy.³¹ These early results suggest a strong role of in vivo immunohistochemistry for an individualized patient diagnosis and therapy based on endoscopic imaging.

Optical Coherence Tomography

To date, OCT is still limited to specific centers focusing mainly on diagnosis and treatment of esophageal diseases. The principle of OCT lies in its potential for obtaining an optical biopsy during the endoscopic procedure by generating high-resolution cross-sectional visualization of esophageal wall layers. One recently introduced systematic review focused on performance characteristics of OCT in assessment of Barrett’s esophagus and esophageal cancer.³² Accuracy of OCT at identifying intestinal metaplasia was 81% to 97%, and specificity was 57% to 92%. For identifying dysplasia and early cancer, sensitivity was 68% to 83%, and specificity was 75% to 82%. In addition to the results of prospective trials, observational studies described significant variability in the ability of OCT to accurately identify subsquamous intestinal metaplasia. Two prospective studies compared the accuracy of OCT at staging early squamous cell carcinoma to histologic resection specimens and reported an accuracy of > 90%. Of note, although OCT may identify intestinal metaplasia and dysplasia, its accuracy may not yet meet recommended thresholds to replace 4-quadrant biopsies in clinical practice.32 Only limited data from pilot studies are available for the application of OCT in the lower GI tract showing the potential application of OCT for distinguishing normal from pathologic colorectal tissue.³³ Intraductal OCT during ERCP has also been studied for the investigation of biliary strictures.³⁴ Combining brushings/biopsy with the observation of at least one OCT criterion resulted in the diagnosis of malignancy in 84% of patients. Similarly, OCT has been evaluated in a pilot study including 12 patients for investigating main pancreatic duct strictures showing that OCT is feasible during ERCP in cases of main pancreatic duct segmental stricture and is superior to brush cytology in distinguishing nonneoplastic from neoplastic lesions.³⁵

Summary

Although early results of OCT are encouraging, prospective trials are highly warranted to confirm the potential application of the technology. Thus far, convincing results are available only for Barrett’s esophagus, and high cost is one of the major limitations to adoption of OCT in clinical practice.

Confocal imaging is currently revealing a renaissance with potential applications in the field of functional and molecular imaging. With the development of highly specific molecular markers, the technology might enable guidance of pharmaceutical therapies in the future.

Of the techniques discussed in this chapter, magnifying endoscopy is already ready for prime time and extensively being used even in centers outside Japan. New computer-assisted technologies will further strengthen its role in diagnosis and therapy of luminal GI diseases.

References

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