# Application of OCT in the Gastrointestinal Tract
## 3 OCT Applications in the GI Tract  
### Esophagus  
Barrett's Esophagus (BE) is the main precursor to esophageal adenocarcinoma (EAC). EAC is the eighth most common cancer type in the world, and despite falling incidence rates of squamous cell carcinoma, the incidence of EAC continues to rise globally, especially in western countries (-including the United States) [53, 54]. Alarmingly, its incidence increased 6.5-fold from 1975-2009 according to SEER data [55], making the incidence for EAC one of the fastest rising of all cancers in the US, particularly in the white male population [56]. EAC is thought to arise stepwise from specialized intestinal metaplasia (SIM) to low grade dysplasia (LGD), high grade dysplasia (HGD) and then intramucosal carcinoma (IMC) [57, 58]. Guidance for surveillance of BE neoplastic progression has been provided by the Seattle Protocol, which advocates for targeted biopsies or resection of suspicious esophageal areas followed by random four-quadrant biopsies at set intervals [59]. This sampling limits itself to less than 5% of the esophageal surface area [12]. Pinch biopsies often do not acquire deeper structures and one study showed they miss the lamina propria more than 60% of the time [60, 61], raising concerns regarding under-sampling for the often focal and small areas of dysplasia. Additionally, after ablation, residual BE tissues may become buried under the neosquamous epithelium as subsquamous intestinal metaplasia (SSIM) and elude detection [62]. Given these needs and the relative simplicity of a tubular structure for OCT imaging, OCT in the GI tract has been applied largely to the esophagus. OCT, with its volumetric, subsurface and near-microscopic imaging, is positioned to help the diagnostic yield of biopsies or even obviate biopsies if it can demonstrate sufficient sensitivity and specificity according the Preservation and Incorporation of Valuable Endoscopic Innovations (PIVI) standards from the American Society for GastrointestinalEndoscopy. In order to eliminate random mucosal biopsies in BE patients, PIVI recommends that an imaging technology with targeted biopsies should have a per-patient sensitivity of at least 90% and a negative predictive value (NPV) of at least 98% for detecting high-grade dysplasia (HGD) or early esophageal adenocarcinoma (EAC) [12; 63]. In the esophagus, [PERSON] et al. showed that OCT could differentiate IMC and HGD from LGD, intermediate-grade dysplasia (IGD) and SIM without dysplasia at the squamocolumnar junction (SCJ) ([PERSON] 2006) [64]. The endoscopic balloon-centering catheter [34] laid the foundation for the commercialization of a GI-focused OCT system (called volumetric laser endomicroscopy, or VLE; NvisionVLE@Imaging System, Ninepoint Medical) [65]. VLE provides real-time volumetric scans with \(\sim\)3 mm imaging depth and 7 \(\upmu\)um axial resolution over a 6 cm stretch of the esophageal inner wall (Figure 3). As will be discussed below, VLE can be used with diagnostic algorithms to detect dysplasia [66] and laser-marking for more precise targeting and co-registry of images with biopsies and better delineation of areas for ablation [67; 68]. Similar promising OCT imaging systems are emerging for GI applications including LuminScan (tm) (Micro-Tech, Nanjing, CN). On the other hand, imaging probes introduced without balloons may be helpful in real-time imaging during biopsies and may help delineate lateral margins during endoscopic resections [69]. Given the relevance of buried glands and SIM in BE recurrence and WLE's inability to survey beneath the mucosal surface, OCT can be used as a uniquely-suited tool after radiofrequency ablation (RFA) to evaluate tissue recovery and detect buried or residual glands [12; 60; 70]. Recently, [PERSON] et al. demonstrated 3D-OCT's ability to detect SSIM in BE patients undergoing RFA as a potential predictor for the number of RFA sessions needed for complete eradication of intestinal metaplasia [60; 70; 71].