A myocardial perfusion imaging (MPI) test utilizes radioactive imaging agents, such as ^99mTc sestamibi, thallium 201 (²⁰¹Tl), and ^99mTc tetrofosmin, to diagnose ischemic heart disease and allow differentiation of ischemia and infarction. This imaging test reveals myocardial wall defects and heart pump performance during increased oxygen demands. NMI may also be done before and after streptokinase treatment for coronary artery thrombosis, after surgery for great vessel translocation, and after transplantation to detect organ rejection and myocardial viability. Pediatric indications include evaluation for ventricular septal defects and congenital heart disease and postsurgical evaluation of congenital heart disease. Studies have shown the efficacy of performing SPECT imaging with ^99mTc sestamibi when triaging patients with diabetes who arrive in the emergency department with symptoms suggestive of acute cardiac ischemia.

²⁰¹Tl is a physiologic analogue of potassium. The myocardial cells extract potassium, as do other muscle cells. ^99mTc sestamibi is taken up by the myocardium through passive diffusion, followed by active uptake within the mitochondria. Unlike thallium, technetium does not undergo significant redistribution. Therefore, there are some procedural differences. Myocardial activity also depends on blood flow. Consequently, when the patient is injected during peak exercise, the normal myocardium has much greater activity than the abnormal myocardium. Cold spots indicate a decrease or absence of flow.

A completely normal myocardial perfusion study may eliminate the need for cardiac catheterization in the evaluation of chest pain and nonspecific abnormalities of an ECG. SPECT imaging can accurately localize regions of ischemia.

Administration of dipyridamole or regadenoson is indicated in adults and children who are unable to exercise to achieve the desired cardiac stress level and maximum cardiac vasodilation. These medications have an effect similar to that of exercise on the heart. Physical stress testing may be initiated in children beginning at 4–5 years. Candidates for drug-induced stress testing are those with lung disease, peripheral vascular disease with claudication, amputation, spinal cord injury, multiple sclerosis, or morbid obesity. Dipyridamole stress testing is also valuable as a significant predictor of cardiovascular death, reinfarction, and risk for postoperative ischemic events and to reevaluate unstable angina.

Ejection fraction and wall motion can be assessed by computer analysis.

1. MPI procedure:

   1. There are two phases to this procedure: the rest imaging and the stress imaging. ²⁰¹Tl, ^99mTc sestamibi, or ^99mTc tetrofosmin may be administered.

      1. Rest imaging:

         1. Perform an IV injection of the radioisotope. Allow a 30- to 60-minute delay for the radioisotope to localize in the heart.

         2. Perform SPECT imaging.

      2. Stress imaging:

         1. The patient undergoes an exercise or a pharmacologic cardiac stress test. At the peak level of stress, inject the patient with the radioisotope.

         2. SPECT imaging may begin 30 minutes after injection.

   2. Pharmacologic stress tests may be performed with any of three routine stressing agents:

      1. Infuse dipyridamole over 4–6 minutes. Inject the radiopharmaceutical agent. Two minutes later, administer aminophylline, an antidote to the dipyridamole, at the nuclear medicine physician or cardiologist’s discretion. Patient monitoring may last 20 minutes. Contraindication: caffeine.

      2. Infuse regadenoson over 20 seconds. Inject the radiopharmaceutical agent 3 minutes after the infusion.

      3. Infuse dobutamine until the predicted heart rate is achieved. The infusion protocol lasts 3 minutes at each dose increment.

2. ²⁰¹Tl:

   1. During the cardiac stress test, the patient is monitored by a nuclear medicine physician, cardiologist, a registered nurse, an electrophysiologist, or an ECG technician.

   2. Have the patient begin walking on the treadmill.

   3. When the patient has reached 85%–95% of maximum heart rate, inject ²⁰¹TI. Take the patient for immediate imaging.

   4. SPECT imaging begins within 5 minutes of injection.

   5. Acquire a second image approximately 3–4 hours later, with the patient at rest, to determine redistribution of ²⁰¹TI.

   6. See Chapter 1 guidelines for safe, effective, informed intratest care.

3. ^99mTc sestamibi and ^99mTc tetrofosmin:

   1. Follow MPI procedures.

   2. Observe standard precautions.