This technique uses a summation of mri (magnetic resonance imaging) slices which are reconstructed in a way in which the arteries are seen.  They can also be reconstructed to see the veins.  This procedure is a good screening method for aneurysms, although it will not pick up very small aneurysms.  Also, it cannot be used in patients who have aneurysm clips, as some of the older ones are not MRI compatible, and these may potentially twist in the scanner.