Contrast & Thickness. Ct angiograms are ct scans with IV contrast. The most unique difference is the thickness of the "cuts". Ct angiogram cuts are typically 2.5mm or thinner. With this resolution 3d reconstructions can be created of the arteries.
Timing. Cta performed with thinner cuts and timing to catch the contrast in the arteries. Otherwise they are the same. The data is processed by a computer afterward to highlight the vessels with cta.