Can CT detect an aaa, or do I need an ultrasound?

Yes. Ct scans of the abdomen and pelvis will definitely show an abdominal aortic aneurysm. We actually order a ct angiogram (a fine cut ct scan with IV contrast) before deciding what the best approach is to repair the aneurysm.
Yes. Ultrasound is a good screening tool because is relatively inexpensive and very safe. Ct actually preferable to eval specifically since it will show what other arteries involved and determine best treatment approach. There are many ct protocols, each with different uses, though i expect most that image the abdomen could detect its presence.