- Magnetic resonance imaging (MRI) to examine the heart muscle and blood flow to the heart. MRI uses a magnetic field and radio waves to produce images of body tissues and organs. The patient lies on a table that is moved into the doughnut-shaped MRI scanner, wearing earplugs to muffle loud knocking and thumping sounds that occur during the scanning process. A catheter (plastic tube) is placed in a vein in the child's arm so that a chemical called gadolinium can be injected during the MRI study. Gadolinium brightens areas of the heart, improving the ability to see the heart and blood flow.