A magnetic resonance angiogram (MRA) is acquired using a contrast enhancement method in which a series of NMR images are rapidly acquired during a time resolved phase of the examination in which the contrast bolus makes a first pass through the arteries and veins. Arterial and venous voxels are automatically identified in the images using either of two disclosed methods. The signals from identified arterial voxels are used to produce an arterial contrast enhancement reference curve that is used to segment arterial voxels by a correlation process. Venous voxels are segmented in the same manner using a calculated venous contrast enhancement reference curve.