Myocardial perfusion scintigraphy (MPS), often called a nuclear stress test, is a non-invasive imaging procedure used by cardiologists to assess blood flow through the heart muscle (myocardium). The test utilizes a small, safe amount of a radioactive substance, called a radiotracer, which is injected into the patient’s bloodstream. This radiotracer is absorbed by healthy heart tissue, and a specialized camera detects the signals it emits to create detailed images of the heart. By performing this scan both when the heart is at rest and when it is under stress, doctors can compare the blood supply under different conditions.
Why Doctors Use Myocardial Perfusion Scintigraphy
MPS is used to diagnose and manage heart conditions related to myocardial blood supply. The scan frequently evaluates patients presenting with chest pain (angina), especially when triggered by exertion or stress. This helps detect Coronary Artery Disease (CAD), which involves narrowing or blockage of the heart’s arteries. The test reveals if a blockage significantly reduces blood flow when the heart’s oxygen demand increases.
The scan also assesses damage following a heart attack (myocardial infarction). It differentiates between permanently scarred, non-functional muscle and tissue that is still alive but receiving insufficient blood flow. This distinction determines if procedures like stenting or bypass surgery could potentially restore function to the under-perfused area. MPS is also used for risk assessment before major non-cardiac surgery to evaluate cardiovascular health.
Step-by-Step: What to Expect During the Procedure
The MPS procedure is divided into a resting phase and a stress phase, often taking three to five hours in total or sometimes split over two days. The process begins with placing an intravenous (IV) line in the arm or hand for injecting the radiotracer. Electrodes are attached to the chest to monitor the heart’s electrical activity (ECG), and a blood pressure cuff is placed on the arm for continuous tracking.
The first set of images is taken during the resting phase, which involves injecting a small dose of the radiotracer through the IV. After a brief waiting period for the tracer to circulate and be absorbed, the patient lies beneath a specialized imaging device, typically a Single-Photon Emission Computed Tomography (SPECT) camera. The camera rotates around the chest to capture three-dimensional images of the tracer distribution, and the patient must remain still during this 15- to 20-minute acquisition.
The stress phase follows the resting scan, elevating the heart rate and blood pressure to simulate exertion. The heart is stressed either through physical exercise, usually walking on a treadmill with increasing speed and incline, or through medication. Pharmacological stress is used for patients unable to exercise adequately, involving injecting a drug that temporarily widens the coronary arteries.
Once the heart reaches peak stress, a second, larger dose of the radiotracer is injected through the IV line. The patient immediately moves to the imaging camera for a second set of pictures, taken while the heart is still under stress. Comparing the rest and stress images allows the cardiologist to detect differences in blood flow caused by the increased demand. After both sets of images are complete, the IV line and monitoring equipment are removed, and the patient is monitored briefly until heart rate and blood pressure return to normal.
Understanding the Images: Results of the MPS Test
The MPS images map blood flow to the heart muscle, with normal flow areas appearing bright. A cardiologist interprets results by comparing tracer uptake between the stress and rest images. Normal results mean the radiotracer was evenly distributed throughout the heart muscle in both scans, suggesting adequate blood supply even under peak demand.
Abnormal findings are categorized as fixed defects or reversible defects. A fixed defect is an area of reduced tracer uptake diminished in both the stress and rest images. This pattern suggests the heart muscle in that region has been damaged, likely by a past heart attack, and has turned into scar tissue that cannot absorb the radiotracer.
A reversible defect is a region that shows reduced tracer uptake during the stress images but a normal uptake pattern in the resting images. This indicates myocardial ischemia: blood flow is insufficient only when the heart is working hard, but returns to normal at rest. This pattern often points to a narrowing in a coronary artery that limits blood supply when oxygen demand increases.
Preparing for Your Scan and Safety Information
Proper preparation ensures the accuracy and safety of the myocardial perfusion scan. Patients are instructed to fast, often four hours, before the test, consuming no food or drink during that time. Patients must also avoid all sources of caffeine for at least 12 to 24 hours before the scan, including coffee, tea, chocolate, and certain over-the-counter medications. Caffeine can interfere with pharmacological stress agents, potentially leading to a false-negative result.
Patients must discuss all medications with their doctor, as some, such as beta-blockers, may need to be temporarily stopped because they affect the heart rate response to stress. The test is safe, though it involves exposure to a small amount of radiation from the radiotracer, comparable to other diagnostic imaging procedures. If a pharmacological stress agent is used, temporary side effects like flushing, headache, or mild chest discomfort may occur, which are closely monitored and resolve quickly. Patients are advised to drink plenty of fluids after the scan to help flush the remaining radiotracer from their system.