A colonoscopy is a procedure in which a doctor uses a long, thin tube with a light and camera on the end to look at a person's large intestine (colon) and rectum. They may also do a biopsy, which is when they take a small sample of tissue to be analyzed in a laboratory. Injections of large bowel usually refer to a procedure in which a doctor injects a contrast solution into a person's large intestine in order to better see it on an x-ray.