Thursday, May 21, 2015
Cardiac PET-A Valuable Tool for Cardiac Diagnosis
By: Donald G. Gordon, MD, FACC with Cardiovascular Associates of the Southeast
Introduced in the 1980s, Myocardial Profusion Imaging (MPI) has had a major impact in the diagnosis of coronary artery disease. MPI is a well-established cardiac diagnostic process in which a radioactive tracer (radionuclide) is injected intravenously under the separate conditions of rest and stress. The tracer is taken up by the heart in relation to blood flow.
The most common method to accomplish MPI is called Singular Photon Emission Computerized Tomography (SPECT). As its name implies, the radioactive tracers in the myocardium emit single photons (gamma rays) that are detected by a standard nuclear camera and computer software generating scans which can be read as tomographic slices. However, in certain instances, the Positron Emission Tomography (PET) method may be more beneficial.
What is a Cardiac PET?
A Cardiac MPI PET uses a radionuclide technique combined with a low dose CT machine to produce a series of high quality images of the heart. Computer graphics can be used to create a 3-dimensional image of the heart.
Rubidium-82, the radionuclide used in PET is quite remarkable. It has a very short half-life (read minutes, not hours) and emits a positron which is a tiny bit of anti-matter that combines with an ordinary tissue electron. They annihilate each other, converting their masses to energy producing TWO highly energetic photons (gamma rays) that travel outward at 180 degrees. This geometry and energy production contributes to the high image quality.
Despite the high photon energies, the absorbed radiation dose to the patient during a PET is the lowest of all current cardiac nuclear procedures (due to the short half-life of Rb-82).
What patients might benefit from a Cardiac PET?
Patients who might benefit from a Cardiac PET include:
1. Patients in which there is a high index of suspicion for coronary artery disease but available objective evidence is inconclusive
2. Patients in which there is (or could be) a low count density with SPECT due to obesity or low cardiac output.
3. Patients in which there are attenuation problems due to significant size and/or density of breast tissue in women, or elevation to the left hemidiaphragm , as well as other possible extra cardiac attenuators such as plural effusions
Why Cardiac PET?
Cardiac MPI PET scans rarely produce "false positives" (test results indicating heart disease where none exists) or “false negatives “(“normal” test results when in fact significant coronary heart disease exists). Because of their high accuracy, Cardiac MPI PET scans are often used to confirm other tests when a false positive or false negative is suspected. This non-invasive, highly accurate diagnostic tool may benefit patients in need of cardiac imaging.
Donald G. Gordon, MD, FACC is a cardiologist at Cardiovascular Associates of the Southeast in Birmingham, Alabama. He is board certified in both cardiology and nuclear medicine.