A cardiac Positron Emission Tomography (PET) scan is an advanced medical imaging technique that provides detailed, three-dimensional information about the heart’s function and health. Unlike X-rays or standard Computed Tomography (CT) scans, which primarily visualize physical structure, PET imaging focuses on metabolic activity and blood flow. The procedure involves injecting a small amount of a short-lived radioactive material, known as a radiotracer, into the bloodstream. A specialized scanner detects the energy released as the tracer decays after it travels to the heart muscle. The resulting images reveal how well the heart muscle is using energy and how effectively blood is reaching its various regions.
Why Doctors Use Cardiac PET Scans
Doctors use cardiac PET scans to gain specific insights into the heart’s physiological condition, particularly when diagnosing or managing Coronary Artery Disease (CAD). The test is accurate in assessing myocardial perfusion, which is the flow of blood through the coronary arteries to the heart muscle. By quantifying this blood flow, the scan can identify areas of the heart that are not receiving adequate oxygen and nutrients, often before structural damage occurs.
This diagnostic capability is used to determine the location and severity of blockages within the coronary arteries. The PET scan’s ability to measure Myocardial Blood Flow (MBF) and Myocardial Flow Reserve (MFR) offers a quantitative advantage over other imaging methods. MFR measures how much blood flow can increase during physical or pharmacological stress compared to rest; a low MFR can indicate a problem even if resting blood flow appears normal.
The scan is also used to assess myocardial viability, determining the health of heart muscle damaged, typically after a heart attack. The test differentiates between irreversibly scarred (“dead”) tissue and tissue that is stunned or “hibernating” due to chronic lack of blood flow. Identifying viable, dormant tissue suggests that muscle function could be restored through a revascularization procedure, such as bypass surgery or stent placement.
Preparing For and Undergoing the Scan
Proper preparation ensures the accuracy of the cardiac PET scan, particularly regarding the heart’s metabolic state. Patients are asked to fast, avoiding all food and beverages except water, for at least four to six hours before the procedure. It is also necessary to refrain from consuming caffeine, including coffee, tea, and certain medications, for 24 hours prior, as caffeine can interfere with the stress portion of the test.
For specific scans, such as those assessing for sarcoidosis or myocardial viability, a specialized high-fat, low-carbohydrate diet may be required for 24 to 48 hours beforehand. Patients with diabetes must discuss their medication and insulin schedule with their doctor, as fasting may necessitate adjustments to prevent low blood sugar levels. Strenuous exercise and tobacco products should also be avoided for 24 hours leading up to the scan, as both can affect test results.
The procedure begins with the placement of an intravenous (IV) line, typically in the arm, through which the radiotracer is administered. Electrodes are attached to the chest to monitor the heart’s electrical activity throughout the test. A waiting period, usually between 30 and 60 minutes, allows the tracer to circulate and be absorbed by the heart muscle.
The patient lies on a narrow table that slides into the doughnut-shaped PET scanner. A cardiac PET scan involves both a “rest” image and a “stress” image to compare blood flow under normal and high-demand conditions. If the patient cannot exercise, a pharmacological stress agent, such as a vasodilator medication, is administered through the IV to simulate physical exertion on the heart. The entire process, including the waiting period and imaging, typically takes 90 minutes to two hours.
Interpreting the Scan Results
Interpreting the cardiac PET scan involves a visual and quantitative analysis of tracer uptake patterns in the heart muscle. On the resulting images, the heart appears as a ring or “donut.” Areas with normal blood flow and metabolic activity show uniform, bright tracer uptake, while areas with low or no tracer uptake appear as dim or “cold spots,” signifying poor blood flow or damaged tissue.
The clinical meaning is derived by comparing images captured at rest with those captured during stress. A perfusion defect present during stress but resolving at rest indicates reversible ischemia, meaning the tissue is at risk but viable. If the defect appears as a cold spot in both the rest and stress images, it is categorized as a fixed defect, which represents scar tissue from a prior heart attack.
The quantitative measurement of Myocardial Flow Reserve (MFR) is a valuable part of the interpretation. This measurement provides a numerical ratio of stress blood flow to rest blood flow; a value above \(2\) is considered normal. A reduced MFR, even with visually normal perfusion images, can indicate early-stage coronary artery disease or microvascular dysfunction (damage to the heart’s smallest blood vessels).