A method for generating colonography images for colorectal screening of a patient. The patient is administered an amount of radiopaque stool marker which will enable stool present in the patient's colon to be distinguished from soft tissue. The patient's colon is imaged after the stool marker has been administered to generate colonography images. Preparation of the colon is simulated by processing the colonography images to remove marked stool before the images are observed during a diagnosis session.