Indocyanine Green Fluorescent Molecular Imaging of the Gastrointestinal Tract

* Participants will be scheduled for their regular lower endoscopy. On the day of the procedure, participants will come to the endoscopy unit at Massachusetts General Hospital. After arriving, participants will receive an intravenous line and meet with the research doctor to go over the procedure. The participant will then be transferred to the endoscopy procedure room and be administered sedatives until they are comfortable to proceed with the procedure. * The research doctor will pass the flexible study sigmoidoscope into the lower part of the rectum. At this time, a dose of the ICG contrast agent will be administered through the intravenous line. Any areas that are suspicious will be photographed using the camera in the scope. We will also biopsy/remove any suspicious areas and send it to the pathologist as we normally do during endoscopy. * Over the course of the study procedure, small doses of ICG will be administered to help find any precancerous areas. Altogether, the study exam should not take more than 10 to 15 minutes.