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During the test, the doctor inserts a long, thin, flexible tube called a catheter into a blood vessel in your arm, upper thigh or neck. The tube is then threaded into your coronary arteries, and the dye is injected into your bloodstream. Special X-rays taken while the dye is flowing through the coronary arteries let your doctor study the flow of blood through your heart and blood vessels, and help your doctor find any blockages that can cause a heart attack.