Nuclear Cardiology

Nuclear Cardiology

S. Ted Treves and Jan Stauss

Nuclear cardiovascular examinations complement anatomic imaging modalities by providing noninvasive methods to assess myocardial perfusion, myocardial viability, myocardial function (including ejection fraction and wall motion), cardiac shunts, and regional pulmonary blood flow in children with congenital and acquired anomalies of the heart and great vessels. Among the different nuclear cardiology techniques, myocardial and pulmonary perfusion imaging are the most commonly used in children.

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.

Figure 68-1 Single photon emission tomography images of a patient with Kawasaki disease.
An exercise stress test with technetium-99m MIBI was ordered for a 20-year-old man with a history of Kawasaki disease who had giant coronary artery aneurysms. Single photon emission tomography images were acquired after injection of the tracer material at rest, and the procedure was repeated during a period of stress on the same day. A small fixed perfusion defect on rest, along with stress images involving the distal anterior wall of the left ventricle and the apex (arrows), most likely represent an area of prior infarction. No evidence was found of additional stress-induced ischemia. Ant, Anterior; Inf, inferior; Lat, lateral; MIBI, hexakis-2-methoxyisobutylisonitrile; Sept, septal.

Figure 68-2 Single photon emission tomography images of a patient with Kawasaki disease.
A 4-year-old boy with a history of Kawasaki disease underwent exercise stress testing with technetium-99m MIBI. Short-axis images, as well as vertical and horizontal long-axis stress images (S rows of images), show a perfusion defect (arrows) in the inferolateral wall that is reversible on rest images (R rows of images), consistent with stress-induced ischemia. MIBI, hexakis-2-methoxyisobutylisonitrile.

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 ^99m

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Tags: Caffeys Pediatric Diagnostic Imaging

Dec 20, 2015 | Posted by admin in PEDIATRIC IMAGING | Comments Off

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