Is a colonoscopy necessary? if so, will a colon ct suffice?

Not the same: These are not the same. Colonoscopy is the introduction of a probe through the anal orifice to visualize the colon walls and obtain samples of suspicious lesions. The colon ct is the image reconstruction of the colon. It means that you need to be exposed to big amounts of radiation to obtain the image (less defined), but if you have a lesion you won't have a biopsy: you will need a colonoscopy.

Sometimes: There are clear indications for a colonoscopy. Age, blood from below, change in bowel habits, stool caliber, abdominal pain, etc. A virtual colonoscopy (ct colo) still requires you to take the bowel prep, which most argue is the worst part of the colonoscopy. You are asleep during a colonoscopy, so you won't even know what's going on. But with a real one, they can take biopsies. Can't with ct.