A Nuclear Medicine Myocardial Perfusion Single-Photon Emission Computed Tomography (SPECT) is a non-invasive imaging test that provides detailed pictures of blood flow to the heart muscle. Often called a nuclear stress test, this procedure uses a small, safe amount of a radioactive substance to create images. The test assesses the heart’s ability to receive an adequate blood supply, both at rest and under maximum demand. By capturing images of the heart muscle (myocardium), the test helps clinicians visualize areas experiencing reduced blood flow, which often indicates underlying heart disease.
The Technology Behind the Scan
This imaging technique relies on nuclear medicine, which employs radiotracers to visualize physiological processes within the body. A radiotracer is a pharmaceutical molecule tagged with a small quantity of a radioactive isotope, such as Technetium-99m (Tc-99m) or Thallium-201 (Tl-201). Once injected intravenously, the tracer travels through the bloodstream and is selectively absorbed by heart muscle cells, with the concentration correlating directly to the amount of blood flow in that region.
The Single-Photon Emission Computed Tomography (SPECT) camera detects the gamma rays emitted by the radiotracer within the myocardium. The camera rotates around the patient’s chest, taking numerous two-dimensional images from various angles. A computer then processes these images using reconstruction algorithms to generate a detailed, three-dimensional (3D) cross-sectional view of the heart, mapping the distribution of the radiotracer.
Why and When is it Used?
Myocardial Perfusion SPECT is a functional test primarily used to diagnose or assess the severity of Coronary Artery Disease (CAD). It is often recommended for patients with symptoms suggestive of CAD, such as chest pain or shortness of breath. The test helps determine if these symptoms are caused by ischemia, which is a lack of sufficient blood flow to the heart muscle.
The scan is also valuable for risk stratification, assessing the likelihood of future adverse cardiac events in patients with known CAD or those who have recently experienced a heart attack. Clinicians use the results to evaluate the effectiveness of prior interventions like bypass surgery or stenting. The functional information gathered can also guide treatment decisions, indicating whether revascularization procedures might benefit the patient.
The Rest and Stress Protocol
The diagnostic power of the SPECT scan comes from comparing the heart’s blood flow under two states: rest and stress. This requires two distinct imaging phases, often performed on the same day using a higher dose of radiotracer for the stress phase, or sometimes over two separate days. During the rest phase, a small dose of the radiotracer is injected while the patient is lying still, followed by the first set of SPECT images 30 to 60 minutes later.
The stress phase is designed to increase the heart’s workload and blood flow demand to the maximum safe level. Stress is typically induced through physical exercise, such as walking on a treadmill, while continuously monitoring the patient’s heart rate, blood pressure, and electrocardiogram (ECG). The radiotracer is injected at the peak of exercise, and the second set of images is acquired shortly after the patient stops.
For patients unable to exercise adequately, pharmacological stress agents are used to simulate the effects of exercise. These agents include vasodilators, which chemically widen the coronary arteries, or dobutamine, which increases heart rate and contractility. The radiopharmaceutical is injected near the end of the drug infusion, and the stress images are captured shortly afterward for comparison with the rest images.
Pharmacological Stress Agents
Common agents include:
- Adenosine
- Dipyridamole
- Regadenoson
- Dobutamine
Understanding the Results
Interpretation of the SPECT scan relies on comparing tracer uptake patterns in the rest images against those in the stress images. The heart muscle is segmented into 17 standardized regions, and tracer activity in each segment is scored by its perfusion level. A Normal Scan shows the radiotracer distributed uniformly throughout the heart muscle in both rest and stress images, indicating healthy blood flow.
A key finding is the Reversible Defect, which is reduced tracer uptake during the stress phase that normalizes in the rest phase. This pattern suggests myocardial ischemia, meaning the region lacks sufficient blood only when the heart is under stress, often due to a coronary artery blockage. Conversely, a Fixed Defect shows a persistent reduction in tracer uptake in the same region in both images. This finding usually represents scar tissue or prior heart attack, where the muscle cells cannot absorb the radiotracer.
The SPECT scan also provides functional data through ECG-gated acquisition, allowing assessment of the heart’s mechanical function. This includes calculating the Left Ventricular Ejection Fraction (LVEF), which is the percentage of blood pumped out of the heart’s main chamber with each beat. A severely reduced LVEF or a drop in LVEF after stress can indicate extensive damage or significant ischemia.