You will begin the study by walking on a treadmill while a cardiologist supervises the test. For those who are unable to walk on a treadmill, an injection of a "pharmacologic stressing" agent will be given. Near the end of the exercise phase, the radiotracer will be injected. Soon following, a set of images of the heart will be taken. It is always necessary to take an additional set of images of your heart at rest. These images may be taken prior to your exercise test or a few hours after the first set of images is completed.