Cardiac SPECT or single photon emission computed tomography also known as myocardial perfusion imaging, is a non-invasive nuclear imaging study used to measure blood flow to the heart at rest and under stress. A radionuclide, such as Technetium is injected into the body, and images are taken with a Gamma camera and computers, which is similar to many other nuclear imaging studies. The blood flow to the heart is measured at rest and after exercise. For exercise, the patient may walk on a treadmill, which is known as a nuclear exercise stress test. If patients are unable to walk along a treadmill, then stress may be induced by a pharmacologic compound, such as Persantine or Adenosine, which causes a simulated response to stress caused by exercise within the body. This is known as a pharmacological nuclear stress test.