Myocardial perfusion imaging, also known as a nuclear stress test, is a non-invasive procedure that shows how well blood flows through the heart muscle. It involves injecting a small, safe amount of a radioactive substance, or radiotracer, into the bloodstream. This tracer is absorbed by healthy heart tissue, allowing a special camera to capture tomographic (cross-sectional) images of blood distribution. The test is performed in two stages: once while the heart is at rest and again when it is under stress from exercise or medication. By comparing these two sets of images, doctors can evaluate blood flow to the myocardium, the muscular tissue of the heart.
Why Myocardial Perfusion Imaging is Performed
A primary reason for this imaging test is to diagnose coronary artery disease (CAD) in individuals with symptoms like chest pain (angina) or shortness of breath. The scan identifies the narrowing of coronary arteries that supply blood to the heart. For patients with known CAD, the test helps determine the condition’s severity to guide treatment. It offers a physiological assessment of blood flow, which can differ from the anatomical view provided by an angiogram.
The test is also used to evaluate the heart muscle after a myocardial infarction, or heart attack, by pinpointing the location and size of damaged tissue. Following procedures to improve blood flow, such as angioplasty or bypass surgery, this imaging assesses the intervention’s effectiveness by confirming if blood flow has been restored.
Another application is assessing cardiac risk before a patient undergoes major non-cardiac surgery. For individuals with multiple risk factors for heart disease, understanding how the heart responds to stress is part of preoperative planning. The results help determine if a patient’s heart is strong enough for surgery or if preliminary cardiac treatment is needed.
The Myocardial Perfusion Imaging Process
Preparation involves several instructions to ensure accurate results. Patients must fast, avoiding all food and drinks except plain water, for a set period before the test. It is also necessary to abstain from caffeine for at least 12 to 24 hours, as it can interfere with stress medications. Patients should wear loose, comfortable clothing and walking shoes.
The test includes a stress component to see how the heart performs when it works hard. This involves exercising on a treadmill or stationary bicycle to reach a target heart rate. If a person cannot exercise, a medication is given intravenously to simulate the effects of exercise on the heart.
A small amount of the radioactive tracer is injected into a vein. The radiation exposure is considered safe and is comparable to other diagnostic imaging procedures. Imaging is then performed in two sessions. For the rest scan, the tracer is injected while the patient is resting, and pictures are taken after it circulates for 30 to 60 minutes. For the stress scan, a second injection is given after the exercise or medication, followed by another set of images. During each 15-minute scan, the patient lies still on a table with their arms above their head while a gamma camera rotates around their chest.
After the test, the intravenous line is removed, and patients can resume normal activities and diet unless instructed otherwise. Drinking plenty of fluids for the next 24 to 48 hours helps flush the tracer from the body. A physician reviews the images and sends a report to the referring doctor.
Interpreting Tomographic Perfusion Images
The images visualize blood flow using colors or shades of gray to represent the radiotracer’s concentration in the heart muscle. Healthy areas with good blood flow show uniform and bright uptake of the tracer. These regions appear consistent in both the rest and stress images, indicating adequate blood supply.
Abnormalities are identified as areas with reduced tracer uptake, called perfusion defects or “cold spots.” These defects signal that a part of the heart muscle is not receiving adequate blood flow. The nature of the defect is determined by comparing the stress and rest images.
A “reversible defect” indicates ischemia. This is a perfusion defect that appears on the stress images but improves or disappears on the rest images. This pattern suggests a blockage in a coronary artery that limits blood flow only when the heart’s demand for oxygen increases during stress.
In contrast, a “fixed defect” points toward a prior heart attack, resulting in scar tissue. This defect is visible as a “cold spot” on both the stress and rest images, showing no change between the two states. Because scar tissue is not living heart muscle, it does not absorb the radiotracer.
Common Tomographic Techniques in Heart Imaging
Single-Photon Emission Computed Tomography (SPECT) is a widely used technique for this imaging. SPECT cameras detect gamma rays from the radiotracer to create three-dimensional images of the heart as they rotate around the patient’s chest. This method offers good image quality.
Positron Emission Tomography (PET) is another advanced technique for evaluating heart perfusion. PET scanners detect pairs of gamma photons produced by the radiotracer. This process allows for images that have higher spatial resolution and can provide more precise measurements of blood flow compared to SPECT.
While both technologies are effective, PET is preferred for its ability to quantify blood flow in absolute terms (mL/min/g) and can sometimes be done with a lower radiation dose. However, SPECT is more widely available and less expensive. This makes it the more common method for cardiac perfusion evaluation in many clinical settings.