S. Ted Treves and Jan Stauss

Myocardial Perfusion

Myocardial perfusion images are obtained by using single photon emission tomography (SPECT) after administration of an intravenous tracer during a time of peak stress and while the patient is at rest. A time of peak stress can be achieved by having the patient exercise in the form of walking on a treadmill or riding a stationary bicycle. For children who are too young to cooperate with exercise testing (usually, children younger than 4 or 5 years), pharmacologic stress testing with use of vasodilators such as dipyridamole and adenosine or inotropic drugs such as dobutamine can be performed safely. Different radiotracers are available for myocardial SPECT in children, including technetium-99m hexakis-2-methoxyisobutylisonitrile (^99mTc-MIBI), technetium-99m (^99mTc) tetrofosmin, and thallium-201 (²⁰¹Tl). ^99mTc-MIBI and ^99mTc-tetrofosmin are both rapidly taken up by the myocardium, reflecting regional perfusion at the time of injection, and show only negligible redistribution compared with ²⁰¹Tl. The use of technetium-labeled compounds in pediatric nuclear cardiology is favorable compared with ²⁰¹Tl because of the lower radiation dose, the potentially higher tracer activities for better counting statistics, the advantageous photon energy, and the longer retention in the myocardium, which facilitates acquisition of gated SPECT to assess ventricular wall motion. The ^99mTc-labeled compounds (MIBI and tetrofosmin) have largely replaced ²⁰¹Tl for the evaluation of myocardial perfusion in children.

Myocardial perfusion SPECT is useful for identification of fixed or stress-induced myocardial perfusion abnormalities in patients with a history of Kawasaki disease, transposition of the great arteries after an arterial switch operation, cardiac transplants, cardiomyopathy, chest pain, chest trauma, an anomalous left coronary artery from the right sinus of Valsalva, an anomalous right coronary artery from the left sinus of Valsalva, and a left coronary artery from the pulmonary artery. Other less frequent indications include hyperlipidemia, supravalvular aortic stenosis, syncope, coarctation of the aorta, and pulmonary atresia with an intact ventricular septum. In children with Kawasaki disease and coronary aneurysms (which has surpassed acute rheumatic fever as the leading cause of acquired heart disease in children in the United States), cardiac stress testing for reversible ischemia is indicated to assess the existence and functional consequences of coronary artery abnormalities (Figs. 68-1 and 68-2). It has been shown that myocardial perfusion SPECT is a safe and sensitive diagnostic method for identifying coronary stenosis in these children.

Because ^99mTc-MIBI does not show significant redistribution, two injections of the radiopharmaceutical agent are necessary to obtain resting and peak exercise myocardial perfusion images. For a single study (rest or exercise), a dose of 0.25 mCi (9.25 MBq)/kg can be used with a minimum total dose of 2 mCi (74 MBq) and a maximum dose of 10 mCi (370 MBq). If rest and exercise studies are done on separate days (i.e., a 2-day protocol), the same dose of ^99mTc-MIBI can be used for both studies. For rest and exercise studies performed on the same day (i.e., a 1-day protocol), 0.15 mCi (5.55 MBq)/kg, with a minimum dose of 2.0 mCi (74 MBq) and a maximum dose of 10 mCi (370 MBq), should be used for the study while the patient is at rest. At 2 to 4 hours after the rest study is completed, the exercise study is performed, using a dose of 0.35 mCi (12.95 MBq)/kg, with a minimum dose of 4 mCi (148 MBq) and a maximum dose of 20 mCi (740 MBq), administered at the time of peak exercise. Imaging is performed 0.5 to 1.0 hour after administration of the tracer. The acquisition protocols should be adapted to individual SPECT systems. SPECT is usually acquired using 120 total projections with a 128 × 128 matrix for a total acquisition of 30 minutes. Appropriate magnification should be used, which depends on the patient’s heart size.

Myocardial perfusion also can be assessed using positron emission tomography (PET) with a variety of tracers such as rubidium-82, nitrogen-13 (¹³N) ammonia, and oxygen-15. Cardiac PET remains underused in the pediatric patient population. Observed agreement between perfusion abnormalities on ¹³N-ammonia PET and coronary angiography suggests a potential for ¹³N-ammonia PET to serve as a valid noninvasive screening tool and an important adjunct to invasive angiography in selected populations. However, PET imaging with short-lived radiotracers such as ¹³N-ammonia requires access to an on-site cyclotron and therefore is not yet widely available.

Myocardial Viability

Fixed defects on 4-hour ²⁰¹Tl or ^99mTc-MIBI images could represent either scarred or viable chronic ischemic (hibernating) myocardium. To differentiate between these two possibilities, delayed images after 12 to 24 hours or reinjection techniques with ²⁰¹Tl traditionally have been used to allow a maximum of redistribution in viable myocardial cells. Alternatively, cardiac PET imaging with fluorine-18-2-fluorodeoxyglucose (¹⁸F-FDG) can be used for viability testing.

¹⁸F-FDG is a glucose analog that is phosphorylated and trapped in the myocardial cell without further metabolism. ¹⁸F-FDG PET images reflect regional myocardial glucose metabolism, which is preserved in viable myocardium but not in scarred tissue. Compared with ¹⁸F-FDG PET, imaging with ²⁰¹Tl may underestimate the extent of viable myocardium. In adult nuclear cardiology, ¹⁸F-FDG PET has become the gold standard for viability evaluation. Only limited data on the role of ¹⁸F-FDG PET in pediatric cardiology has been available until now. In children, the usefulness of ¹⁸F-FDG PET has been evaluated for myocardial viability after arterial switch operation and suspected infarction. In these children, ¹⁸F-FDG PET may provide pertinent information to guide further therapy by identifying patients with viable myocardium who will more likely benefit from revascularization.

Myocardial Function

Nuclear medicine techniques available for the assessment of ventricular function include electrocardiography-gated myocardial perfusion SPECT, gated metabolic PET, gated blood pool scintigraphy, and first-pass radionuclide angiography. The main purpose of these methods is to assess ventricular function such as right and left ventricular ejection fractions and to detect wall motion abnormalities.

Gated scintigraphy is based on synchronization of data recording with the patient’s electrocardiogram, which allows repetitive sampling of the cardiac cycle until an appropriate count density is acquired. The data acquisition during each R-R interval is subdivided into a number of frames. At least 16 frames per cardiac cycle are required to calculate an accurate ejection fraction. In gated blood pool scintigraphy, autologous red blood cells are labeled in vivo or in vitro with ^99mTc, and imaging should be performed using SPECT. Because of its excellent interobserver and intraobserver reliability, SPECT is used clinically to track serial changes in quantitative measurements of left ventricular ejection fraction. In pediatrics, it has been used to monitor the left ventricular ejection fraction in patients undergoing chemotherapy regimens involving drugs with high cardiotoxicity such as Adriamycin to reduce chemotherapy-related morbidity and mortality through early detection of a decline in cardiac function.

Shunts

Regional Pulmonary Blood Flow

A more frequent indication for pulmonary scintigraphy with ^99mTc-MAA in pediatric nuclear cardiology is to assess regional pulmonary blood flow in children with congenital or acquired anomalies of the heart and great vessels. This rapid and safe technique often is used to quantify the percent distribution of total pulmonary blood flow in the left and right lungs before and after interventional procedures to relieve obstruction to pulmonary blood flow (Fig. 68-3). This technique is used, for example, before and after catheter or surgical arterioplasty in patients with tetralogy of Fallot and peripheral pulmonary artery stenosis, in patients with pulmonary vein stenosis, and to assess the effect of intravascular stent placement or coil occlusion of vascular communications.

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