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.

Purpose

• To evaluate left ventricular function
• To detect aneurysms of the left ventricle and other motion abnormalities of the myocardial wall (areas of akinesia or hypokinesia)
• To detect intracardiac shunting
• To calculate ejection fraction, especially before certain chemotherapies

Patient preparation

• Explain to the patient that this test permits assessment of the heart's left ventricle.
• Describe the test, including who will perform it, where it will take place, and its expected duration.
• Tell him that he needn't restrict food or fluids before the test.
• Explain that he'll receive an I.V. injection of a radioactive tracer and that a detector positioned above his chest will record the circulation of this tracer through the heart.
• Reassure him that the tracer poses no radiation hazard and rarely produces adverse effects.
• Inform him that he may experience transient discomfort from the needle puncture, but that the imaging itself is painless.
• Instruct him to remain silent and motionless during imaging, unless otherwise instructed.
• Make sure the patient or a responsible family member has signed a consent form.

Procedure and posttest care

• The patient is placed in a supine position beneath the detector of a scintillation camera, and 15 to 20 millicuries of albumin or RBCs tagged with 99mTc pertechnetate are injected.
• For the next minute, the scintillation camera records the first pass of the isotope through the heart so that the aortic and mitral valves can be located.
• Then, using an ECG, the camera is gated for selected 60-millisecond intervals, representing end-systole and end­diastole, and 500 to 1,000 cardiac cycles are recorded on X-ray or Polaroid film.
• To observe septal and posterior wall motion, the patient may be assisted to modified left anterior oblique position; or he may be assisted to right anterior oblique position and given 0.4 mg of nitroglycerin sublingually. The scintillation camera then records additional gated images to evaluate abnormal contraction in the left ventricle.
• The patient may be asked to exercise as the scintillation camera records gated images.

Precautions

• Cardiac blood pool imaging is contraindicated during pregnancy.

Normal findings

Abnormal findings

Interfering factors

• None significant