MRI uses a magnetic field and radio waves to produce images of body tissues and organs. For this procedure, the subject lies on a table that can slide in and out of the scanner (a metal cylinder), wearing earplugs to muffle loud knocking noises that occur during the scanning process. The procedure lasts about 90 minutes; the patient is asked to lie still for up to 25 minutes at a time. The subject can communicate with the MRI staff at all times during the scan. During part of the scan a contrast agent is administered through a catheter (plastic tube) placed in an arm vein to enhance the images.