CT involves the use of specialized x-rays to obtain images of the brain. The patient lies still in the scanner for a short time while the X-ray images are formed. MRI uses a strong magnetic field and radio waves to demonstrate structural and chemical changes in tissue. MRI is more sensitive than x-ray in evaluating acute stroke. The patient lies on a table in a metal cylinder (the scanner) while the pictures are being taken. During part of the MRI, a medicine called gadolinium contrast is injected in a vein. This medicine brightens the images, creating better pictures of the blood flow.