MRI uses a magnetic field and radio waves to produce images of body tissues and organs. The patient lies on a table that can slide in and out of the scanner (a narrow metal cylinder), wearing earplugs to muffle loud knocking and thumping sounds that occur during the scanning. Most scans last between 45 and 90 minutes. Subjects may be asked to lie still for up to 30 minutes at a time, and can communicate with the MRI staff at all times during the procedure.