Although Crohn's disease and ulcerative colitis are the main subtypes of inflammatory bowel disease, they differ substantially in disease behavior, prognosis, and treatment paradigm. However, making an accurate diagnosis of Crohn's disease versus ulcerative colitis and assessing disease activity beyond the level of mucosal inflammation remain challenging with contemporary modalities. The objective of the study is to determine the novel role of endoscopic ultrasound in A) differentiating Crohn's colitis versus ulcerative colitis and B) monitoring disease activity in these patients.

Crohn's disease (CD) and ulcerative colitis (UC) are two principal subtypes of inflammatory bowel disease (IBD). Disease behavior, prognosis, and therapy differ substantially between these two subtypes. However, up to 15% of patients may have misclassification of their IBD subtypes leading to significant delay in appropriate management and prognostication. The misclassification of CD and UC is due to limitations in contemporary modalities used to diagnose these diseases. While CD involves transmural inflammation and UC is limited to mucosal inflammation, the combination of endoscopy, histology, and cross-sectional imaging typically used to establish the diagnosis do not reliably distinguish mucosal from submucosal inflammation. Consequently, disease reclassification often occurs at the time of surgery when transmural inflammation can be determined from the surgical specimen. At that time, medical therapy has already failed. The optimal time for accurate IBD classification would be at the initial diagnosis, allowing for appropriate targeted therapy to achieve optimal disease outcomes.

Endoscopic ultrasound (EUS) can provide detailed information about luminal wall layers. To date, the use of endoscopic ultrasound (EUS) for colorectal disease has been limited to staging of subepithelial lesions and examining CD-related perianal complications. The ultrasound miniprobe device (UM-2/3R, Olympus) is a thin ultrasound catheter that can be passed through the colonoscope's accessory channel to perform detailed ultrasonic assessment of any colon wall segments. Prior studies have demonstrated that the submucosal layer is significant thicker in active CD compared to active UC, while active UC has thicker mucosal layer comparatively. This study tests the hypothesis that the addition of the miniprobe ultrasound catheter at the time of colonoscopy will help to differentiate active CD with colonic involvement and UC from non-IBD controls by comparing the differential thickness in the mucosal and submucosal layer among these groups of patients. Furthermore, the investigators hypothesize that the thickness of differential wall layers in CD and UC patients will correlate to clinical and endoscopic disease activity. The significance of these findings will help establish an accurate diagnosis of IBD subtypes early in the disease course and provide for a reliable method to monitor disease activity not only at the mucosal layer but also in deeper luminal wall layers.

Inclusion Criteria:

• Adults patients ≥ 18 years of age with CD with at least colonic involvement, UC, or non-IBD controls who have been referred for colonoscopy for clinical reasons. The clinical reasons may include colorectal cancer screening, surveillance, diagnostic for CD or UC flare, or gastrointestinal symptoms.

Exclusion Criteria:

• Pregnant patients.
• Patients with known current colorectal cancer, infectious colitis, diverticulitis, or microscopic colitis.
• Patients who have undergone surgery involving the cecum or rectum.