Makindo Medical Notes

Myocardial perfusion imaging relies on creating a physiological scenario where well-perfused myocardial regions can be distinguished from those with compromised blood flow. By increasing myocardial oxygen demand—through exercise or pharmacological stress—areas of significant coronary stenosis become ischemic, enhancing the contrast between normal and underperfused myocardium.

About

• All diagnostic tests in this category are based on the principle that increasing myocardial oxygen demand (e.g., via exercise or pharmacological stress) will expose areas of limited blood supply due to coronary artery stenosis, typically greater than 70% lumen narrowing.
• Ischemic regions appear “cold” or underperfused on nuclear imaging techniques or demonstrate wall motion abnormalities during stress echocardiography.

Common Diagnostic Modalities

• Exercise Stress Testing: Uses electrocardiographic ST segment depression as an objective marker of ischemia and impaired perfusion under increased workload.
• Dobutamine Stress Echocardiography: Dobutamine increases heart rate and contractility, elevating oxygen demand. Regional wall motion abnormalities that emerge under stress indicate areas of poor perfusion.
• Technetium-based Radionuclide Studies: A radioactive isotope (e.g., technetium) is injected, and its distribution in the myocardium is imaged. Stress (exercise or dobutamine) reveals areas of reduced uptake corresponding to ischemic myocardium.
• Thallium (Tl), MIBI (Sestamibi), or Myoview Scans: Thallium mimics potassium distribution and is taken up by viable myocardial cells. Following stress, “cold” or reduced uptake areas indicate ischemia. Redistribution of thallium into previously “cold” regions over time suggests reversible ischemia. Technetium-labeled agents provide improved image quality but operate on similar principles.
• Adenosine or Dipyridamole Stress: These vasodilators increase coronary blood flow differentially in normal versus stenotic vessels, mimicking the effects of exercise. This is useful for patients unable to exercise and can be used similarly to dobutamine.
• Coronary Angiography: An invasive, direct visualization of the coronary arteries using a radio-opaque contrast. This gives a 2D representation of the vessel lumen and identifies significant stenoses directly.
• MUGA Scan: A radionuclide ventriculography technique used to assess left ventricular function, providing information on ejection fraction and wall motion without directly assessing perfusion.