Positron Emission Tomography (PET) is a sophisticated, non-invasive medical imaging technique used in nuclear medicine. It utilizes a small amount of a short-lived radioactive substance, known as a radiotracer, to create detailed images of the body’s function. When applied to the heart, cardiac PET precisely assesses the organ’s metabolism and blood flow. The test provides a functional map, offering insights into how well the heart muscle is working at a cellular level.
The Science Behind Cardiac PET
The cardiac PET scan relies on the physics of radioactive decay and detection. A specialized radioactive tracer, such as Rubidium-82 for blood flow studies or \(\text{F-18}\) fluorodeoxyglucose (\(\text{FDG}\)) for metabolic studies, is introduced into the patient’s bloodstream via an intravenous line. The tracer accumulates in the heart muscle according to blood flow or metabolic activity.
The radioisotope within the tracer is unstable and undergoes beta-plus decay, emitting a positively charged particle known as a positron. This positron travels a short distance before colliding with an electron in the surrounding tissue, resulting in the annihilation of both particles into pure energy. This annihilation event produces a pair of high-energy gamma rays that travel away from the source in opposite directions.
The PET scanner, a large ring of detectors, registers these simultaneous pairs of photons. A computer uses the timing and location of these detections to triangulate the precise point where the annihilation occurred. By collecting millions of these events, the computer reconstructs a detailed, color-coded, three-dimensional image of the heart. Areas with greater blood flow or metabolic activity show a higher concentration of the tracer and appear brighter on the final image.
Primary Diagnostic Uses
The high-resolution images produced by cardiac PET scans provide two distinct types of information for heart assessment.
Myocardial Perfusion
The first major application is the precise assessment of myocardial perfusion, which measures the blood flow to the heart muscle. This is often performed as a stress test, capturing images both at rest and after the heart rate is safely increased using a pharmacological agent. By comparing the resting images to the stress images, the physician identifies areas of the heart muscle that are not receiving adequate blood supply during increased demand. This process is highly accurate for diagnosing and characterizing the severity of coronary artery disease (CAD), pinpointing blockages in the coronary arteries. The ability to quantify the absolute blood flow makes this test more specific than other perfusion imaging methods.
Myocardial Viability
The second primary use is the evaluation of myocardial viability, which determines the health of heart tissue damaged by a prior heart attack. The goal is to distinguish between permanently scarred tissue, which will not recover function, and “hibernating” myocardium, which is still alive but metabolically sluggish due to chronic poor blood flow. If the tissue is viable, a procedure like bypass surgery or stenting may restore its function. Viability assessment typically uses a glucose-analog tracer like \(\text{FDG}\) to map the heart muscle’s metabolic activity. If a segment of heart muscle shows poor blood flow but high metabolic activity, it is considered viable and potentially salvageable. Cardiac PET is also uniquely suited to diagnose inflammatory conditions, such as cardiac sarcoidosis, by detecting areas of abnormal immune cell activity.
Preparing for the Scan and the Procedure
Patient preparation for a cardiac PET scan is crucial to ensure the accuracy of the images, especially when using metabolic tracers. Patients are instructed to fast for four to six hours before the procedure to regulate blood sugar levels. They must also strictly avoid all sources of caffeine, including coffee, tea, soda, and certain medications, for at least 12 to 24 hours prior to the test because caffeine can interfere with the pharmacological stress agents used during the scan.
The procedure itself usually takes between one and three hours to complete, with the patient lying still on a narrow table that slides into the PET scanner. An intravenous line is placed in the arm for the administration of the radiotracer and, if necessary, the stress agent. The test involves two main imaging phases: the initial “rest” scan, followed by the “stress” scan, which is induced pharmacologically with a drug like Regadenoson that safely mimics the effects of exercise on the heart’s blood vessels.
During the stress phase, the pharmacological agent causes the coronary arteries to widen, increasing blood flow to the heart muscle. The radiotracer is injected again at the peak of this stress, and a second set of images is immediately acquired. Throughout the entire process, the patient is continuously monitored by a healthcare team using an electrocardiogram (EKG) and blood pressure cuff.
After the imaging is complete, patients can generally resume their normal diet and activities immediately. They are encouraged to drink plenty of fluids to help flush the small amount of residual radiotracer from their system. The radiation exposure from the test is low, comparable to other common diagnostic imaging procedures, and the short half-life of the tracer ensures the radioactivity quickly dissipates.