Radionuclide Imaging - Diagnostic Evaluation of Cardiovascular Function

RADIONUCLIDE IMAGING

Radionuclide imaging studies involve the use of radioisotopes to evaluate coronary artery perfusion noninvasively, to detect myo-cardial ischemia and infarction, and to assess left ventricular func-tion. Radioisotopes are atoms in an unstable form. Thallium 201 (Tl²⁰¹) and technetium 99m (Tc^99m) are two of the most common radioisotopes used in cardiac nuclear medicine studies. As they decay, they give off small amounts of energy in the form of gamma rays. When they are injected intravenously into the bloodstream, the energy emitted by the radioisotope can be de-tected by a gamma scintillation camera positioned over the body. Planar imaging, used with thallium, is a technique that provides a one-dimensional view of the heart from three locations. A rela-tively new technique called single photon emission computed tomography (SPECT) provides three-dimensional images. With SPECT, the patient is positioned supine with arms raised above the head, while the camera moves around the patient’s chest in a 180- to 360-degree arc to identify the areas of decreased myocar-dial perfusion more precisely.

Myocardial Perfusion Imaging

The radioisotope Tl²⁰¹ is used to assess myocardial perfusion. It resembles potassium and readily crosses into the cells of healthy myocardium. It is taken up more slowly and in smaller amounts by myocardial cells that are ischemic from decreased blood flow. However, thallium will not cross into the necrotic tissue that re-sults from an MI.

Often, thallium is used with stress testing to assess changes in myocardial perfusion immediately after exercise (or after injec-tion of one of the agents used in stress testing) and at rest. One or two minutes before the end of the stress test, a dose of Tl²⁰¹ is injected into the intravenous line, allowing the radioisotope to be distributed into the myocardium. Images are taken immediately. Areas that do not show thallium uptake are noted as defects and indicate areas of either infarction or stress-induced myocardialischemia. The resting images, taken 3 hours later, help to differ-entiate infarction from ischemia. Infarcted tissue is unable to take up thallium regardless of when the scan is taken; the defect re-mains the same size. This is called a fixed defect, indicating that there is no perfusion in that area of the myocardium. Ischemic myocardium, on the other hand, recovers in a few hours. Once perfusion is restored, thallium crosses into the myocardial cells, and the area of defect on the resting images is either smaller or completely reversed. These reversible defects constitute positive stress test findings. Usually, cardiac catheterization is recom-mended after a positive test result to determine whether angio-plasty or coronary artery bypass graft surgery is needed.

Another radioisotope used for cardiac imaging is Tc^99m. Tech-netium can be combined with various chemical compounds, giv-ing it an affinity for different types of cells. For example, Tc^99m sestamibi (Cardiolite) is distributed to myocardial cells in pro-portion to their amount of perfusion, making this tracer excellent for assessing perfusion to the myocardium. The procedure for cardiac imaging using Tc^99m sestamibi with stress testing is simi-lar to the one using thallium, with two differences. Patients re-ceiving Tc^99m sestamibi can have their resting images recorded before or after the exercise images. Timing of the images is not important because the half-life of Tc^99m is short, and Tc^99m needs to be injected before each scan. Also, SPECT imaging with Tc^99m sestamibi provides high-quality images.

NURSING INTERVENTIONS

The patient undergoing nuclear imaging techniques with stress testing should be prepared for the type of stressor to be used (ex-ercise or drug) and the type of imaging technique (planar or SPECT). The patient may be concerned about receiving a radio-active substance and needs to be reassured that these tracers are safe, the radiation exposure being similar to that of other diag-nostic x-ray studies. No postprocedure radiation precautions are necessary.

When providing teaching for patients undergoing SPECT, the nurse should instruct them that their arms will need to be posi-tioned over their head for about 20 to 30 minutes. If they are phys-ically unable to do this, thallium with planar imaging can be used.

Test of Ventricular Function and Wall Motion

Equilibrium radionuclide angiocardiography (ERNA), also known as multiple-gated acquisition (MUGA) scanning, is a common noninvasive technique that uses a conventional scintillation cam-era interfaced with a computer to record images of the heart dur-ing several hundred heartbeats. The computer processes the data and allows for sequential viewing of the functioning heart. The sequential images are analyzed to evaluate left ventricular func-tion, wall motion, and ejection fraction. MUGA scanning can also be used to assess the differences in left ventricular function during rest and exercise.

The patient is reassured that there is no known radiation dan-ger and is instructed to remain motionless during the scan.

Computed Tomography

Computed tomography (CT), also called computerized axial to-mographic (CAT) scanning or electron-beam computed tomog-raphy (EBCT), uses x-rays to provide cross-sectional images of the chest, including the heart and great vessels. These techniques are used to evaluate cardiac masses and diseases of the aorta and pericardium.

EBCT, also known as the Ultrafast CT, is an especially fast x-ray scanning technique that results in much faster image acqui-sition with a higher degree of resolution than traditional x-ray or CT scanning provides (Woods et al., 1999). It is used to eval-uate bypass graft patency, congenital heart lesions, left and right ventricular muscle mass, chamber volumes, cardiac output, and ejection fraction. For people without previous MI, PTCA, or coronary artery bypass surgery, the EBCT is used to determine the amount of calcium deposits in the coronary arteries and underlying atherosclerosis. From this scan, a calcium score is derived that predicts the incidence of cardiac events, such as MI or the need for a revascularization procedure within the next 1 to 2 years.

The EBCT is not widely used, but it does show great promise for early detection of CAD that is not yet clinically significant and that would not be identified by traditional testing methods, such as the exercise stress test.

NURSING INTERVENTIONS

Patient preparation is the primary role of the nurse for these tests. The nurse should instruct the patient that he will be positioned on a table during the scan while the scanner rotates around him. The procedure is noninvasive and painless. However, to obtain adequate images, the patient must lie perfectly still during the scanning process. An intravenous access line is necessary if con-trast enhancement is to be used.

Positron Emission Tomography

Positron emission tomography (PET) is a noninvasive scanning method that was used in the past primarily to study neurologic dysfunction. More recently, and with increasing frequency, PET has been used to diagnose cardiac dysfunction. PET provides more specific information about myocardial perfusion and via-bility than does TEE or thallium scanning. For cardiac patients, including those without symptoms, PET helps in planning treat-ment (eg, coronary artery bypass surgery, angioplasty). PET also helps evaluate the patency of native and previously grafted vessels and the collateral circulation.

During a PET scan, radioisotopes are administered by injec-tion; one compound is used to determine blood flow in the myo-cardium, and another shows the metabolic function. The PET camera provides detailed three-dimensional images of the dis-tributed compounds. The viability of the myocardium is deter-mined by comparing the extent of glucose metabolism in the myocardium to the degree of blood flow. For example, ischemic but viable tissue would show decreased blood flow and elevated metabolism. For a patient with this finding, revascularization through surgery or angioplasty would be likely to improve heart function. Restrictions of food intake before the test vary among institutions, but, because PET evaluates glucose metabolism, the patient’s blood glucose level should be in the normal range. Al-though PET equipment is costly, it is increasingly valued and available.

NURSING INTERVENTIONS

Nurses involved in PET and other scanning procedures may in-struct the patient to refrain from using tobacco and ingesting caf-feine for 4 hours before the procedure. They should also reassure the patient that radiation exposure is at safe and acceptable lev-els, similar to those of other diagnostic x-ray studies.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a noninvasive, painless technique that is used to examine both the physiologic and anatomic properties of the heart. MRI uses a powerful magnetic field and computer-generated pictures to image the heart and great vessels. It is valuable in diagnosing diseases of the aorta, heart muscle, and pericardium, as well as congenital heart lesions. The application of this technique to the evaluation of coronary artery anatomy, cardiac blood flow, and myocardial viability in con-junction with pharmacologic stress testing is being investigated.

NURSING INTERVENTIONS

Because of the strong magnetic field used during MRI, diagnos-tic centers where these procedures are performed carefully screen patients for contraindications. Standardized questionnaires are commonly used to determine whether the patient has a pace-maker, metal plates, prosthetic joints, or other metallic implants that can become dislodged if exposed to MRI. During an MRI, the patient is positioned supine on a table that is placed into an enclosed imager or tube that contains the magnetic field. People who are claustrophobic may need to receive a mild sedative before undergoing an MRI. As the MRI is performed, there is an inter-mittent clanking or thumping sound from the magnetic coils that can be annoying to the patient, so patients are offered headsets to listen to music. The scanner is equipped with a microphone so that the patient can communicate with the staff. During the scan-ning, the patient is instructed to remain still and not move.