3. Magnetic resonance imaging (MRI) - Images of the carotid arteries are taken while the volunteer lies on a table in a narrow cylinder containing a magnetic field. A padded sensor called an MRI coil is placed over the neck and earplugs are placed in the ears to muffle the loud noise of the machine during scanning. During the second half of the exam, gadolinium is injected through a catheter (thin, flexible tube) inserted into a vein. Gadolinium is a contrast agent that is used to brighten the scan images.