Comparision of the border of colorectal adenomatous lesions between underwater and under-gel

Interventions: Endoscopic observation should be performed in Underwater and Under-gel according to the preliminary
allocation.
After imaging, aspirate saline or Viscoclear as much as possible, and perform the next endoscopic
observation with Underwater and Under-gel according to the prior allocation.
Afterwards, mark two points on the right and left sides of the lesion border, and then perform UEMR
or Under-gel EMR to resect the lesion.;Colonoscopy;Under-gel EMR
Primary outcome(s): the visibility of tumor borders in underwater and under-gel
Study Design: randomized controlled trial, open(masking not used), active control, crossover assignment, diagnostic purpose