Elements and molecules that emit ionizing radiation are known as radionuclides. This radioactivity is in the form of subatomic particles or gamma rays. Depending on the type of radioactive emission and other characteristics of the compound, different agents can be used for different purposes. In scintigraphy, radiation from radionuclides introduced into a patient are detected by special sensors called gamma cameras. The data then are processed to create two-dimensional images. In SPECT, sensors are arranged in a variety of positions around the patient and data are processed into 3 dimensional images.
In positron emission tomography (PET), the radionuclide is a compound that the body uses -which is to say a metabolite- that has been altered to include an atom that emits positrons. A positron is a subatomic particle that has the same mass as an electron, but is positively charged. It is the antimatter counterpart of an electron. When a positron is emitted and meets up with electrons from other sources, annihilation of both particles takes place and they are converted into gamma rays according to Einstein’s equation E = MC^2.
Cardiac radionuclide imaging uses radionuclides along with gamma cameras to obtain information on the function of the heart. It can be used clinically to diagnose and assess various medical conditions. In cardiac imaging, radionuclides are introduced into the body by injection. Gamma radiation, produced directly by the radionuclides or indirectly as a result of their presence, is then detected by special sensors. Data are then processed to produce an image.
Radionuclides used in cardiac imaging include technetium-99 and thallium-201. The number following the name of the element represents the total of the number of protons and neutrons in the nucleus of the element's atoms. Radionuclides can be useful in assessing various aspects of cardiac function. The flow of blood through the heart muscle (myocardium) can be tracked as the sensors, known as gamma cameras (cameras that detect gamma rays), track radiation from the radionuclide flowing in myocardial blood vessels. Coronary artery disease can be evaluated as well. In this case the flow of radionuclides, and therefore the blood, is tracked through the coronary arteries, the main blood vessels that nourish the heart. Similarly, the extent of damage to the heart following a myocardial infarction (interruption of blood flow to an area of the heart muscle) can be evaluated, as can the improvement of blood flow following coronary artery bypass surgery (CABG). Together with electrocardiography (ECG) radionuclides also can be used to assess how well the heart muscle moves.