Thallium Imaging

Also called cold spot myocardial imaging or thallium scintigraphy, thallium imaging evaluates myocardial blood flow after I.V. injection of the radioisotope thallium-201. Because thallium, the physiologic analogue of potassium, concentrates in healthy myocardial tissue but not in necrotic or ischemic tissue, areas of the heart with a normal blood supply and intact cells rapidly take it up. Areas with poor blood flow and ischemic cells fail to take up the isotope and appear as cold spots on a Scan.

This test is performed in a resting state or after stress. Possible complications of stress testing include arrhythmias, angina pectoris, and myocardial infarction (MI).

Purpose

Resting imaging

• To assess myocardial scarring and perfusion
• To demonstrate the location and extent of acute or chronic MI, including transmural and postoperative infarction

Stress imaging

• To diagnose coronary artery disease (CAD)
• To evaluate the patency of grafts after coronary artery bypass surgery
• To evaluate the effectiveness of antianginal therapy or balloon angioplasty

Patient preparation

• Explain to the patient that these tests help determine if any areas of the heart muscle aren't receiving an adequate supply of blood.
• If the patient will be undergoing stress imaging, instruct him to restrict alcohol, tobacco, and nonprescribed medications for 24 hours before the test and to have nothing by mouth for 3 hours before the test.
• Describe the test, including who will perform it, where it will take place, and its expected duration (45 to 90 minutes). Explain that additional scans may be required.
• Tell him that he'll receive a radioactive tracer I.V. and that multiple images of his heart will be scanned.
• Explain to the patient that it's very important to lie still when images are taken.
• Warn him that he may experience discomfort from skin abrasion during preparation for electrode placement. Assure him that the test involves minimal radiation exposure.
• Make sure the patient or a responsible family member has signed a consent form.
• Tell the patient undergoing stress imaging to wear walking shoes during the treadmill exercise and to report fatigue, pain, or shortness of breath immediately.

Procedure and posttest care

Resting imaging

• Optimally, within the first few hours of symptoms of MI, the patient receives an injection of thallium I.V. and scanning begins after 3 to 5 minutes.
• If further scanning is required, have the patient rest and restrict food and fluids.

Stress imaging

• The patient, wired with electrodes, walks on a treadmill at a regulated pace that's gradually increased, while the electrocardiogram (ECG), blood pressure, and heart rate are monitored.
• When the patient reaches peak stress, the examiner injects 1.5 to 3 millicuries of thallium into the antecubital vein and then flushes it with 10 to 15 ml of normal saline solution.
• The patient exercises an additional 45 to 60 seconds to permit circulation and uptake of the isotope, and then lies on his back under the scintillation camera.
• If the patient is asymptomatic, the precordial leads are removed. Scanning begins after 3 to 5 minutes with the patient in anterior, 45-degree and 60­degree left anterior oblique, and left lateral positions.
• Additional scans may be taken after the patient rests 3 to 6 hours.

Precautions

• Contraindications include impaired neuromuscular function, pregnancy, locomotor disturbances, acute MI or myocarditis, aortic stenosis, acute infection, unstable metabolic conditions (such as diabetes), digitalis toxicity, and recent pulmonary infarction.
• Stop stress imaging at once if the patient develops chest pain, dyspnea, fatigue, syncope, hypotension, ischemic ECG changes, significant arrhythmias, or critical signs (pale, clammy skin, confusion, or staggering).

Normal findings

Imaging should show normal distribution of the isotope throughout the left ventricle and no defects (cold spots).

Abnormal findings

Persistent defects indicate MI; transient defects (those that disappear after 3 to 6 hours of rest) indicate ischemia from CAD. After coronary artery bypass surgery, improved regional perfusion suggests patency of the graft. Increased perfusion after ingestion of antianginal drugs can show that they "relieve ischemia. Improved perfusion after balloon angioplasty suggests increased coronary flow.

Interfering factors

• Cold spots (possible result of sarcoidosis, myocardial fibrosis, cardiac contusion, attenuation due to soft tissue, apical cleft, coronary spasm and artifacts such as implants and electrodes)
• Absence of cold spots in the presence of CAD (possibly due to insignificant obstruction, inadequate stress, delayed imaging, collateral circulation, or single-vessel disease, particularly of the right or left circumflex coronary arteries).