CECT Abdomen should not be used as a screening test as CECT is individualised to specific organ by giving contrast and acquisition at specific prefixed times. Abdomen (belly) organs are usually screened using Ultrasound and then CECT as per the findings. Colonoscopy being an invasive procedure is used on case to case basis as per advise of physician.

Imaging like CT abdomen , should be done according to your symptoms and signs. CT is being done to clinch the diagnosis and not being used as a screening tool when you are asymptomatic.