Narrow-band imaging (NBI) is a method of image enhancement that modifies white light (WL) into narrowed bands of light with a center wavelength of 415 (blue) and 540 nm (green), which are absorbed by hemoglobin [1-3]. Diagnosis based on angiogenesis might be ideal for early detection and diagnosis of neoplastic lesions, because angiogenesis is essential for the transition of a premalignant lesion in a hyperproliferative state to a malignant lesion [4-6]. NBI becomes more powerful when adapted to a magnifying endoscope providing a range of low to high optical magnification (×80 maximum) by means of a simple, one-touch operation [7]. NBI providing enhanced images of vessels and surface patterns of lesions has greatly contributed to detection and diagnosis of colorectal tumors, and changed the role of ordinary diagnostic methods such as WL imaging and chromoendoscopy.

This review highlights the application of NBI colonoscopy with or without optical zoom magnification for the detection and diagnosis of colorectal lesions, and discusses the education and training of practitioners, and future perspectives.

So far, screening colonoscopy for detection of colorectal polyps has been performed using WL. With the development and dissemination of NBI, a number of randomized controlled trials for evaluating the efficacy of NBI in screening colonoscopy have been conducted, and meta-analysis of these trials has concluded that high-definition (HD) NBI colonoscopy is better than standard-definition WL colonoscopy and equal to HD-WL colonoscopy for detection of colorectal adenomas and polyps [8,9]. Therefore, at present, screening colonoscopy is performed using HD-WL, and the NBI technique is employed mainly for diagnosis of any colorectal polyps that are detected.

The brightness and resolution of colonoscopic views obtained using NBI have been improved markedly since the introduction of the Olympus 290 series (EVIS LUCERA ELITE; Olympus, Tokyo, Japan) and 190 series (EVIS EXERA III) endoscopy systems, in comparison with the previous 260 and 180 series, respectively (Figs. 1, 2). In a prospective multicenter randomized controlled trial using the 290 series, Horimatsu et al. [10] compared the mean number of colorectal polyps detected per patient between HD-NBI and HD-WL colonoscopy, and found that the number was significantly higher for the former than for the latter (2.01 vs. 1.56, p=0.032). Therefore, they concluded that HD-NBI colonoscopy using the 290 series endoscopy system was superior to HD-WL colonoscopy for detection of colorectal polyps. Moreover, they found that significantly more sessile serrated adenomas/polyps, which have recently attracted much attention as possible precursor lesions of colorectal cancers with microsatellite instability, were detected using NBI than by WL (0.05 vs. 0.01 per patient, p=0.036). Leung et al. [11] conducted a prospective randomized controlled trial to compare the adenoma and polyp detection rates between HD-NBI and HD-WL colonoscopy using the novel 190 series endoscopy system, and found that both rates were significantly higher for NBI than for WL (adenoma: 48.3% vs. 34.4%, p=0.01; polyps: 61.1% vs. 48.3%, p=0.02). Therefore, they concluded that HD-NBI colonoscopy using the 190 series was superior to HD-WL colonoscopy for detection of colorectal adenomas and polyps.

Previous studies have demonstrated the usefulness of NBI in screening endoscopy for early detection of cancers in the head and neck region and esophagus [12]. Although further studies are required to confirm the usefulness of NBI in screening colonoscopy for detection of colorectal polyps, the available evidence suggests that screening endoscopy using NBI with these newly developed endoscopy systems may become a standard tool, not only for the head and neck region and esophagus but also for the colon and rectum (Fig. 3).

As mentioned above, although NBI with magnifying colonoscopy can replace magnifying chromoendoscopy for prediction of histology, it is not superior to magnifying chromoendoscopy for estimation of cancer invasion depth. Based on the available evidence, we have proposed a three-step strategy for management of colorectal lesions, which is based on NBI colonoscopy (Fig. 5) [23]. Chromoendoscopy is necessary when we suspect that a lesion has invaded deeply into the submucosal layer, a situation observed in only 5% of all neoplastic lesions.

Diagnosis of colorectal lesions by experts using NBI has been established, and the next challenge is to focus on training system for non-experts.

Recently, many articles have described the learning curve of NBI-based diagnosis using various training modules for differentiating between neoplasic and non-neoplastic lesion. Some in vitro studies have demonstrated that non-experts can achieve a high accuracy (81% to 90.8%) and good interobserver agreement with a short and simple training course or a computer-based program [24-27]. However, two prospective in vivo studies have demonstrated inferior accuracy (75.6% to 81%) after a video-based training program [28,29]. These differences suggest that assessment of endoscopic images in real-time requires expertise. Endoscopic imaging in real time can be influenced by the location of a lesion, and the presence of mucus or residual fecal material on a lesion. It is necessary to learn how to obtain good evaluable images and to evaluate lesions in vivo. Dai et al. [30] reported that a short interactive training program in combination with continual feedback involving 30 cases in vivo allowed less experienced endoscopists to improve their diagnostic accuracy for non-target lesions (70.0% to 96.7%) and target lesions (76.7% to 95.0%). Continuous feedback in vivo may be a key factor for improvement of accuracy when attempting to predict polyp histology in real time. Further investigations will be necessary to establish adequate in vivo training programs.

At present, all polyps are basically removed and submitted for formal pathological examination because conventional WL colonoscopy has limited accuracy (59% to 84%) for differentiating between neoplastic and non-neoplastic lesions [13,31-35]. If it were possible to accurately assess colorectal polyp pathology using endoscopy alone, then tiny rectosigmoid hyperplastic polyps would be left in situ, thus reducing the risks associated with polypectomy, whereas diminutive adenomas could be resected and discarded, thus saving the cost of histological evaluation. Today many endoscopic modalities, such as NBI, autofluence imaging, Fuji Intelligent Chromo Endoscopy, and i-scan are available. Which modality should be used for assessment of colorectal polyp pathology endoscopically? The American Society for Gastrointestinal Endoscopy (ASGE) has suggested that the necessary thresholds for assessment of histology using endoscopic technology are >90% agreement for determining post-polypectomy surveillance intervals with a ≥90% negative predictive value (when used with high confidence) for rectosigmoid polyps with an adenomatous histology [36]. A systematic review and meta-analysis by the ASGE has confirmed that the thresholds for real-time endoscopic assessment of the histology of diminutive polyps have been met, at least for NBI performed by an expert when assessments are made with high confidence [37]. Iwatate et al. [38] have reported that high-magnification endoscopy significantly improved the rate of high confidence for differentiating between neoplastic and non-neoplastic colorectal lesions. In other words, magnifying endoscopy with NBI could enhance the application of real-time optical diagnosis in routine clinical practice.

In future, NBI will become a vital tool for detection, characterization and optical biopsy of colorectal polyps, especially when used with magnifying endoscopy.