Considering a patient's age, sex, risk factors, and symptoms helps doctors to judge the likelihood of significant CAD. Several tests can help in the evaluation. An electrocardiogram uses electrodes to examine the heart's electrical activity. An exercise stress test examines the heart's electrical activity while the patient is exercising on a treadmill or exercise bike. Other stress tests, called imaging stress tests, use special dyes or ultrasonography to take pictures of the heart before, during, and after exercise. Pharmacologic stress tests use drugs to simulate the effects of exercise on the heart and are options for patients who cannot exercise. Cardiac catheterization is the most definitive, but most risky, test for CAD. Cardiac catheterization involves the injection of dye into the bloodstream, so that doctors can take pictures of the arteries.