At present, the exact cause of the disorganized architecture and altered light-scattering associated with dysplastic tissue by OCT imaging is unknown. A number of factors have been suggested, including subcellular morphological changes, altered fibrovascular stroma and abnormal mucin content associated with neoplastic tissue change, proliferation of cells leading to a loss of epithelial and stromal orientation, and altered cytological features such as an increased nuclear-to-cytoplasm ratio that may alter infrared light back-scattering[9,29
In its current form and resolution, OCT will likely localize areas displaying architectural distortion to guide biopsy.
Most of the so far published studies used OCT imaging to detect dysplasia and early cancer within Barrett’s epithelium. Since the penetration depth of OCT does not exceed 1-2 mm, the technique could be useful, not only in detecting dysplasia, but also in staging superficial cancers that are difficult to stage accurately with ultrasound endoscopy. The technique appears, therefore, of crucial importance in the management of the disease.
Barrett’s epithelium is characterized by the presence of specialized intestinal metaplasia within the esophageal mucosa. The hallmark histologic feature of specialized intestinal metaplasia is the presence of goblet cells. Identification of Barrett’s epithelium is of clinical relevance since the lesion requires endoscopic follow-up, being a recognized precancerous condition. In clinical practice Barrett’s epithelium is generally identified by performing multiple biopsies within the areas of gastric metaplasia, either in a random manner or after a previous vital staining with methylene blue (MB).
Although the inter-subject variability of OCT imaging of normal squamous epithelium and gastric mucosa appears to be low, the OCT imaging of Barrett's epithelium demonstrated a greater variability in the previous published studies. OCT features predictive for the presence of intestinal metaplasia are: (1) the absence of the layered structure of the normal squamous epithelium and the presence of the vertical crypt-and-pit morphology of normal gastric mucosa; (2) a disorganized architecture with inhomogeneous back-scattering of the signal and an irregular mucosal surface; and (3) the presence of submucosal glands characterized at the OCT imaging as pockets of low reflectance below the epithelial surface[30-32
] (Figure ). When these OCT criteria were applied to images acquired prospectively, the criteria were found to be 97% sensitive and 92% specific for specialized intestinal metaplasia, with a PPV of 84%. The presence of the crypt-and-pit architecture may render difficult to discriminate between intestinal metaplasia and normal or inflamed gastric mucosa[9
Figure 6 Barrett’s esophagus (arrow). OCT features predictive for the presence of intestinal metaplasia are: The absence of the layered structure of the normal squamous epithelium and the presence of the vertical crypt-and-pit morphology of normal gastric (more ...)
Unfortunately, up to now, attempts to identify OCT patterns characteristic for dysplasia, mainly the high-grade type, have been substantially disappointing. The increased nuclear-to-cytoplasmic ratio occurring in dysplasia may alter the light reflection characteristics, giving a more inhomogeneous back-scattering of the signal (Figure ). Because the degree of reflectivity depends upon nuclear size, a markedly homogenous and hypo-reflective back-scattering of the signal should indicate the presence of high-grade dysplasia; moreover, it is possible that by quantitating the OCT signal as a function of depth, OCT would be able to characterize high-grade dysplasia within intestinal metaplasia tissue. Poneros et al[17
], by using two parameters of tissue reflectivity as an indicator of dysplasia, retrospectively diagnosed high-grade dysplasia with 100% sensitivity and 85% specificity. Such an accurate analysis of the degree of signal reflectivity requires to avoid areas with incorrect artifact signal properties: This may be obtained by the identification of a precisely defined area with homogeneous signal reflectance, an adequate catheter-tissue contact, and a reduction of motion artifacts. More recently, the morphological appearance of the OCT images, rather than the quantitative analysis of the OCT signal in the image, were used for the diagnosis and grading of dysplasia; for this purpose an endoscope fitted with an EMR standard cap was used, to stabilize the mucosal surface and avoid movement from esophageal peristalsis and transmitted cardiac and respiratory motion. In this study sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy for dysplasia were respectively, 68%, 82%, 53%, 89%, and 78%[33
Figure 7 OCT image showing high-grade dysplasia (A) compared to histology (B). Because the degree of reflectivity depends upon nuclear size, a markedly inhomogenous and hypo-reflective back-scattering of the signal should indicate the presence of high-grade dysplasia. (more ...)
However, with the current available OCT devices, the recognition of dysplasia within intestinal metaplasia and mainly the differentiation between low- and high-grade dysplasia appears difficult[34
] (Figure ).
OCT image showing normal oesophageal mucosa (a) and Barrett’s epithelium (b) with focal high-grade dysplasia (arrow).
OCT features characteristic for adenocarcinoma arising from Barrett’s epithelium are the lack of the regular esophageal wall layered morphology and a markedly heterogeneous back-reflectance of the signal[35,36
] (Figure ). These features permit to clearly identify the lesion and differentiate between the neoplastic and non-neoplastic tissue in advanced disease.
Figure 9 Magnified OCT imaging of early stage adenocarcinoma raised within Barrett's epithelium. The lack of the regular esophageal wall layered morphology and a markedly heterogeneous back-reflectance of the signal characterize the neoplastic lesion, which is (more ...)
Figure shows and compares OCT findings of normal esophageal mucosa, Barrett's epithelium, dysplasia, and adenocarcinoma.
Comparison of OCT findings of normal esophageal mucosa (A), Barrett's epithelium (B), dysplasia (C), and adenocarcinoma (D).
Despite some studies were conducted about the use of the OCT in the stomach and small intestine, no data are actually available about its use in detecting dysplasia.
Only few data are present in literature concerning the use of the OCT to detect dysplasia or cancer in the colon[37-39
]. In a study by Pfau et al[39
] on 24 patients, 30 dysplastic adenomas and 14 hyperplastic polyps were studied; the real-time OCT investigation showed that adenomas were more disorganized than the hyperplastic polyps, with a significantly more disorganized structure (P
= 0.0005). Moreover, the infrared-light back-scattering of the adenomatous polyps appeared more hypo-reflective than hyperplastic (P
= 0.0007). By using a computer-generated method to quantify the degree of scattering of individual pixels within a specified area in each image (60 × 60 pixels), it was found that the mean differences in light scattering were significantly greater between adenomatous and normal tissue (mean difference = 45.81), than between hyperplastic polyps and normal tissue (mean difference = 14.86). The real-time OCT infra-red light back-scattering score of polyps was also demonstrated to be a significant predictor of an adenomatous status. However, differently from the dysplasia occurring within Barrett’s epithelium, in the study done by these authors defined OCT parameters histologically proven to detect colonic dysplasia were not found.
Pancreatico-biliary ductal system
Pathological pancreatic ductal system has been investigated by our group in humans in two ex vivo
] performed on multiple surgical pancreatic specimens obtained from patients with pancreatic head adenocarcinoma.
In chronic inflammatory changes involving the main pancreatic duct, OCT still showed conserved three-layer architecture. However, the inner, hypo-reflective layer appeared slightly larger than normal and the intermediate layer appeared more hyper-reflective than in normal tissue; this is probably because of the dense mononuclear cell infiltrate. The back-scattered signal was heterogeneous with marked hypo- or hyper-reflectance in some sections. The agreement between OCT and histology in the definition of MPD chronic inflammatory changes was poor (27.7%).
The OCT pattern in presence of dysplasia of the main pancreatic duct epithelium was characterized by an inner layer markedly thickened, strongly hypo-reflective and heterogeneous; this OCT finding is probably due to the initial structural disorganization (increased mitosis and altered nucleus/cytoplasm ratio). The surface between the inner and intermediate layers appeared irregular. As in chronic inflammatory tissue, dysplasia too gave strong hyper-reflectance of the intermediate layer, particularly in the part closest to the inner layer. The outer layer did not differ from other non-malignant conditions and appeared homogeneously hypo-reflective (Figure ). However, in chronic pancreatitis and dysplasia, only 62% of cases OCT and histology were concordant. The K statistic used to assess agreement between the two procedures was equal to 0.059 for non-neoplastic MPD wall appearance.
Figure 11 Magnified OCT images from section of main pancreatic duct showing low-grade dysplasia. The surface between the inner and intermediate layers appeared irregular. Dysplasia presents strong hyper-reflectance of the intermediate layer, particularly in the (more ...)
Overall, normal wall structure and chronic inflammatory or low-grade dysplastic changes cannot be distinguished in 38% of the sections because the architecture of the layers and surface light reflection did not show a characteristic OCT pattern.
In all sections with histologically proven adenocarcinoma, OCT showed a totally subverted MPD wall architecture. The three layers of the ductal wall and their linear, regular surface, normally giving a homogeneous back-scattered signal, were not recognizable. The margins between the connective-fibro-muscular layer and acinar tissue were unidentifiable. The back-scattering of the signal appeared strongly heterogeneous, with minute, multiple, non-reflective areas in the disorganized pancreatic microstructure. Of sections with adenocarcinoma, OCT and histology were 100% concordant.
Figure shows magnified OCT images from sections of main pancreatic duct with normal tissue, chronic pancreatitis, low-grade dysplasia, and adenocarcinoma.
Magnified OCT images from sections with either normal (A), tumor-associated chronic inflammation (B), low-grade dysplasia (C), and adenocarcinoma (D) tissue.
Totally subverted wall architecture was also observed by OCT in presence of neoplastic tissue within the common bile duct[40
] (Figure ).
Magnified OCT images of neoplastic tissue within the intrapancreatic portion of the common bile duct.
Studies in vivo
were performed in animals[25
] and humans[27
]. We evaluated the diagnostic accuracy of OCT for the diagnosis of carcinoma, during ERCP, in a series of patients with MPD strictures of unknown etiology. In this study, the accuracy of OCT for detection of neoplastic tissue was 100%, compared with 66.7 % for intraductal brush cytology. The study showed that OCT is feasible during an ERCP procedure and was superior to brush cytology in distinguishing non-neoplastic from neoplastic lesions[26
In conclusion, OCT appears a promising technique for real-time, high-resolution, cross-sectional imaging of the inner layer of the wall of the GI and pancreato-biliary tract, during the routine endoscopy. The technique recognizes with high definition the mucosa, muscularis mucosae and submucosa, and seems particularly useful in the study of the esophageal mucosa; given its superior resolution compared with other imaging modalities such as endoscopic ultrasonography (EUS) or catheter-probe EUS (CPEUS), OCT has great potential as a powerful adjunct to standard endoscopy in identification and surveillance of Barrett’s epithelium, in order to detect high-grade dysplasia and adenocarcinoma at early stage and identify cases in whom mucosectomy becomes a curative procedure.
In the pancreatico-biliary ductal system, OCT can be used to discriminate between non-neoplastic and neoplastic tissue when strictures of unknown etiology are identified during an ERCP procedure, being its diagnostic accuracy higher than reported for intraductal brush cytology.
However, despite the promising studies reported in literature, with the current available OCT devices the recognition of dysplasia within intestinal metaplasia and mainly the differentiation between low- and high-grade dysplasia appears difficult.
On the other hand, since OCT has a penetration depth that does not exceed the 2 mm, it has a greater capability of diagnosing adenocarcinoma confined within mucosa and submucosa and could, therefore, be useful in staging superficial cancers that are difficult to stage accurately by EUS.
Features characteristic for adenocarcinoma within Barrett’s epithelium are the lack of the regular layered morphology of the esophageal wall and a markedly heterogeneous back-reflectance of the signal. However, further studies are needed to evaluate whether OCT can identify and stage the lesion at an early stage.
OCT appears more promising in the differential diagnosis between non-neoplastic and neoplastic lesions arising within the pancreatico-biliary ductal system, since the ductal wall layered structure can be recognized easier and clearer.
At present, it seems to be fairly premature to affirm that OCT plays a role in the real-time diagnosis of dysplasia in vivo. However, improvements in both axial and lateral resolutions to the subcellular level (< 5 μm) together with the development of better light sources and optics, may allow dysplastic cells to be better identified in the future. Doppler OCT could also offer a unique ability to provide detailed subsurface imaging of mucosal microvascular networks.