Cardiac Blood Pool Imaging
Cardiac blood pool imaging evaluates regional and global ventricular performance after I.V. injection of human serum albumin or red blood cells (RBCs) tagged with the isotope technetium-99m (99mTc) pertechnetate. In first-pass imaging, a scintillation camera records the radioactivity emitted by the isotope in its initial pass through the left ventricle. Higher counts of radioactivity occur during diastole because there is more blood in the ventricle; lower counts occur during systole as the blood is ejected. The portion of isotope ejected during each heartbeat can then be calculated to determine the ejection fraction; the presence and size of intracardiac shunts can also be determined.
Gated cardiac blood pool imaging, performed after first-pass imaging or as a separate test, has several forms; however, most forms use signals from an electrocardiogram (ECG) to trigger the scintillation camera. In two-frame gated imaging, the camera records left ventricular end-systole and enddiastole for 500 to 1,000 cardiac cycles; superimposition of these gated images allows assessment of left ventricular contraction to find areas of hypokinesia or akinesia.
In multiple-gated acquisition (MUGA) scanning, the camera records 14 to 64 points of a single cardiac cycle, yielding sequential images that can be studied like motion picture films to evaluate regional wall motion and determine the ejection fraction and other indices of cardiac function. In the stress MUGA test, the same test is performed at rest and after exercise to detect changes in ejection fraction and cardiac output. In the nitro MUGA test, the scintillation camera records points in the cardiac cycle after the sublingual administration of nitroglycerin to assess its effect on ventricular function.
Blood pool imaging is more accurate and involves less risk to the patient than left ventriculography in assessing cardiac function.
Procedure and posttest care
Normal findingsNormally, the left ventricle contracts symmetrically, and the isotope appears evenly distributed in the scans. Normal ejection fraction is 55% to 65%.
Abnormal findingsPatients with coronary artery disease usually have asymmetrical blood distribution to the myocardium, which produces segmental abnormalities of ventricular wall motion; such abnormalities may also result from preexisting conditions, such as myocarditis. In contrast, patients with cardiomyopathies show globally reduced ejection fractions. In patients with left-toright shunts, the recirculating radioisotope prolongs the downslope of the curve of scintigraphic data; early arrival of activity in the left ventricle or aorta signifies a right-to-left shunt.
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