1. A method for performing a scan with a CT imaging system using a contrast agent which is injected into a subject to enhance image contrast, the steps comprising: performing a baseline scan of the subject with the CT imaging system; displaying a baseline image reconstructed with data acquired in the baseline scan; selecting a region in the baseline image to be monitored; performing a monitoring operation after injection of the contrast agent into the subject which includes: a) performing a monitor scan of the subject; b) displaying a monitor image reconstructed from data acquired in the monitor scan; c) calculating a contrast enhancement amount using attenuation values in the monitor image from the selected region; d) displaying the contrast enhancement amount; and e) repeating steps a) through d) until an indication that the image scan should begin is received; and performing an image scan and reconstructing the contrast enhanced image.