MRI is a diagnostic test that uses a strong magnetic field and radio waves to show structural and chemical changes in tissue. For the procedure, the patient lies on a table that slides into the scanner-a metal cylinder surrounded by a magnetic field. Earplugs are worn to muffle the loud knocking sounds that occur when the pictures are being taken. The scan, taken before surgery, will last about 20 minutes; the two after surgery take about 10 minutes to perform.