Single-Photon Emission Computed Tomography (SPECT) Myocardial Perfusion Imaging (MPI) is a non-invasive diagnostic procedure that evaluates blood flow to the heart muscle, both at rest and during physical or pharmacological stress. It helps diagnose and manage various heart conditions.
Understanding the Purpose
Doctors recommend SPECT MPI scans to assess blood flow to the heart muscle, especially for coronary artery disease (CAD). This imaging technique identifies areas not receiving adequate blood supply, indicating narrowed or blocked coronary arteries. The test diagnoses CAD, evaluates blockages, and assesses severity.
SPECT MPI also investigates chest pain or discomfort, especially when angina is suspected. The scan differentiates healthy heart tissue from areas damaged by a prior heart attack, showing decreased resting perfusion. It also assesses myocardial viability, determining if heart muscle tissue is alive but underperforming from reduced blood flow, potentially benefiting from revascularization procedures like bypass surgery or angioplasty.
The information from a SPECT MPI scan guides treatment decisions and stratifies risk for future cardiovascular events, predicting outcomes like cardiac death or heart attack. The test can also be used for risk assessment before non-cardiac surgery or to follow up after interventions like percutaneous coronary intervention or coronary artery bypass graft.
The Imaging Process
Patients are asked to fast for at least four hours before the rest imaging phase. Diabetic patients should consult their doctor regarding fasting and medication adjustments.
Medication adjustments are needed. Patients should avoid caffeine-containing foods, beverages, and drugs for 12 to 24 hours before the test, as caffeine can interfere with certain stress medications. Some heart medications, such as beta-blockers and calcium channel blockers, may need to be withheld for 24-48 hours before the test.
A small amount of radioactive tracer, such as Technetium-99m Sestamibi or Thallium-201, is administered intravenously. This tracer is taken up by the heart muscle cells in proportion to blood flow and viability.
The procedure involves two phases: a stress test and a rest test, often performed on the same day or over two separate days. For the stress test, patients undergo either exercise stress (like treadmill walking or stationary cycling) or pharmacological stress if unable to exercise adequately. During exercise stress, the tracer is injected at peak exertion.
Pharmacological stress agents, such as adenosine, dipyridamole, or regadenoson, are administered intravenously to simulate exercise by increasing blood flow to the heart. Dobutamine is another option for patients who cannot exercise or have contraindications to vasodilators.
Following the stress injection, a waiting period allows the tracer to distribute within the heart muscle. A separate tracer injection is given for the rest phase. Imaging then begins, where the patient lies still on a table while a SPECT camera rotates around their body, capturing 3D images of the heart.
The SPECT machine, a large circular device, contains a camera that detects gamma rays emitted by the tracer. The images are sent to a computer that reconstructs them into 3D views of the heart. The entire procedure, including waiting periods, takes approximately three to four hours.
Interpreting the Results
Interpreting SPECT MPI images involves comparing stress and rest images to evaluate blood flow to different heart muscle regions. Medical professionals, typically cardiologists or radiologists, analyze the distribution and intensity of the radioactive tracer uptake.
Normal results show uniform tracer uptake across all heart segments, indicating consistent blood flow both at rest and during stress. Conversely, abnormal results present as areas of reduced tracer uptake, known as perfusion defects. These defects are categorized by size (small, medium, or large) and severity (mild, moderate, or severe).
Perfusion defects are further classified based on their behavior between stress and rest images. A “reversible defect” indicates ischemia, meaning reduced blood flow during stress that normalizes at rest, suggesting a temporary lack of blood supply. A “fixed defect” suggests myocardial infarction or scar tissue, where reduced perfusion remains consistent in both stress and rest images, indicating permanent damage.
Comparing the stress and rest images helps differentiate between these conditions. For instance, if a defect appears during stress but resolves at rest, it points to inducible ischemia. If the defect is present in both stress and rest images, it may signify an old heart attack or scar tissue. The interpreting physician considers these findings along with the patient’s clinical history to determine the diagnosis and recommend appropriate next steps, which could range from lifestyle modifications and medication to further diagnostic tests or revascularization procedures.
Safety and Important Considerations
SPECT MPI involves exposure to a small amount of ionizing radiation from the radioactive tracer. This dose is generally considered safe for diagnostic purposes and is comparable to other common diagnostic imaging tests. The body naturally eliminates the radioactive tracer over a few hours to days, primarily through urine.
While generally well-tolerated, some patients may experience mild, temporary side effects from the tracer or stress medications. These can include transient flushing, headache, nausea, or a cold sensation during the tracer injection. More rare complications, such as allergic reactions to the tracer, irregular heartbeat, or low blood pressure, can occur but are uncommon.
Certain conditions may require modification of the test or make it unsuitable. Pregnancy is a contraindication due to potential fetal radiation exposure. Women who are breastfeeding should discuss this with their doctor, as they may need to temporarily stop nursing for about 36 hours after the tracer injection. Patients with severe kidney disease or certain heart conditions should also inform their healthcare provider.
Before undergoing a SPECT MPI scan, patients should discuss their full medical history, any known allergies, all current medications (including over-the-counter drugs and supplements), and any concerns about the procedure with their healthcare provider. Adhering to preparation instructions and communicating thoroughly with the medical team helps ensure the test is performed safely and yields accurate results.