Radionuclide Imaging

Nuclear cardiac imaging procedures are noninvasive techniques that provide valuable information for cardiac disease in two main broad categories. The first category involves the assessment of ventricular function by radioactive tracers that remain in the intravascular space during the study. The second category of imaging visualizes myocardial intracellular uptake of radioactive tracers reflecting myocardial perfusion, metabolism and viability.

The multigated equilibrium blood pool imaging study is used to assess the cardiac performance. This study is done using the patient's own red blood cells tagged with Technitium-99m (99mTc). This method provides a relatively stable blood pool for up to 6 to 8 hours with a biologic half-life of approximately 4 hours. This technique is used to determine the left ventricular ejection fraction as well as any wall motion abnormalities.

Thallium-201 is a potassium analog taken up by viable myocardium. Studies using Technetium-99 and Thallium 201 include wall motion evaluation, myocar-dial perfusion and viability assessment. The initial distribution of these agents at rest demonstrates the areas of adeqaute blood flow and viable tissue. Both underperfused and infarcted tissue will appear dark (cold spot) on the resultant images. The ability to re-image several hours later will sometimes demonstrate viable myocardium, where initially blood flow was inadequate, but over time there is enhancement of these regions. Studies have used both dobutamine and nitroglycerine to enhance the filling, further demonstrating the presence of viable myocardium. Presumably these regions are chronically ischemic or hibernating areas of myocardium that may benefit from revascularization.

Exercise radionuclide imaging is the most common application of this technique, especially in patients with suspected coronary artery disease (CAD). These patients have normal resting images (good perfusion to all areas of the myocardium) but decreased uptake (dark or cold spots) after exercise. This demonstrates regions of inadequate blood flow during exertion (perfusion defects) diagnostic of obstructive coronary artery disease. The published sensitivity and specificity of these techniques, as compared to cardiac catheterization to diagnose obstructive CAD, is approximately 85%. A reversible defect (one not present on the resting images but present on exertion) is synonymous with obstruction, while a fixed defect (one present on both exercise and rest) represents infarcted tissue.

Patients who cannot achieve adequate exercise levels are often given agents that will cause differential blood flow during rest and stress. Intravenous persantine is most often used to mimic regional ischemia and allow detection of obstructive coronary artery disease. Persantine is a potent vasodilator that causes selective coronary hyperemia. A differential rate of radionuclide uptake from rest to stress makes the diagnosis. The basis of this test is that a stenotic lesion cannot vasodilate so there is more blood flow to normal regions of the heart, and the myocardium distal to the stenotic lesion appears less perfused (dark). An alternative to this is to use dobutamine or arbutamine stress to increase cardiac requirements (see

Echocardiography, above), thus allowing relative hypoperfusion in areas distal to stenoses.

The limitations of radionuclide testing are the need for intravenous injection, high cost (approximately $2,000 in most laboratories), radiation exposure, as well as a slow imaging protocol. The images take up to 4 hours to obtain, requiring the patient remain immobile. Furthermore, often the rest and stress imaging cannot be done on the same day so the patient must spend two days in testing.

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