Gel Immersion Improves Visualization in Underwater Endoscopic Mucosal Resection for Colorectal Lesions: A Prospective Crossover Trial

Objective Underwater endoscopic mucosal resection (UEMR) is widely used for colorectal polyp resection, but visualization may be impaired with poor bowel preparation. Gel‐immersion endoscopy has been proposed to enhance image clarity. In this study, digital image analysis was applied to quantitatively compare lesion visibility during UEMR with water or gel immersion. Methods In this prospective, randomized crossover trial, patients with non‐pedunculated colorectal adenomatous lesions measuring 6–19 mm were sequentially examined with water and gel immersion using white light imaging (WLI), blue laser imaging (BLI), and linked color imaging (LCI). The primary endpoint was the color difference (ΔE) between the lesion and surrounding mucosa, calculated using the Commission Internationale d'Eclairage (CIE) L*a*b* color space. Visibility scores were assessed by blinded expert endoscopists (jRCTs062220033). Results In total, 72 lesions (median size, 8 mm) were analyzed. ΔE values were significantly greater with gel than water immersion in both WLI ( p = 0.03) and BLI ( p = 0.03), but not in LCI ( p = 0.10). The improvement in ΔE values with gel immersion was more pronounced under poor bowel preparation. Visibility scores were significantly higher with gel immersion in both WLI and BLI overall and under poor bowel preparation, whereas no significant differences were observed under good bowel preparation. Conclusion Gel immersion significantly improves colorectal lesion visibility during UEMR, particularly under poor bowel preparation and with WLI or BLI. This technique may serve as a valuable adjunct to enhance diagnostic and therapeutic precision in challenging endoscopic conditions.