Nuclear Imaging Techniques

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Gallium-67 cardiac scintigraphic imaging has been used to evaluate conditions that result in myocardial inflammation. It is currently seldom performed at most centers. However, at centers with extensive experience in the technique, gallium-67 imaging has been reported to be useful as a screening tool and in predicting response to treatment.37,73 O'Connell et al.,73

who studied this methodology most extensively, reported a sensitivity of 36% and a specificity of 98% for histologic detection of myocarditis.

Indium-labeled monoclonal antibody fragments of antimyosin antibodies (directed against heavy chain myosin) bind to cardiac myocytes that have lost the integrity of their sarcolemma membranes and have exposed intracellular myosin to the extracellular fluid space.74 Unlike gallium-67, which detects extent of myocardial inflammation, antimyosin uptake reflects the extent of myocyte necrosis. Because both elements are present in myocarditis, the 2 imaging modalities should provide similar or complementary information. Unfortunately, no studies have been performed that directly examine the utility of these radionuclide techniques when combined or when compared with one another. Nonetheless, published sensitivities and specificities suggest that antimyosin has a higher negative predictive value than gallium-67 scintigraphy.

Dec et al.75 studied the utility of antimyosin imaging in 82 patients with clinically suspected myocarditis. Symptoms at presentation included congestive heart failure and cardiomyopathy (92%), chest pain mimicking myocardial infarction (6%), and life-threatening ventricular tachyarrhythmias (2%). All patients underwent planar and single photon emission computed tomographic cardiac imaging 48 hours after injection of indium-111-labeled antimyosin antibody fragments (Fig. 11-4). Right ventricular biopsy was performed within 48 hours after imaging. On the basis of right ventricular histologic

Anterior

Fig. 11-4. A positive antimyosin image demonstrates diffuse tracer uptake in the cardiac region on both the anterior planar image (Upper left) and in all coronal tomographic reconstructions (Bottom left). Biopsy showed multifocal lymphocytic myocarditis. Antimyosin imaging was repeated after 6 months of immuno-suppressive therapy. Biopsy showed healed myocarditis. No antimyosin uptake is visible on either the planar (Top right) or tomographic reconstructions (Bottom right). (From Dec and Narula.76 By permission of Edizioni Minerva Medica.)

Fig. 11-4. A positive antimyosin image demonstrates diffuse tracer uptake in the cardiac region on both the anterior planar image (Upper left) and in all coronal tomographic reconstructions (Bottom left). Biopsy showed multifocal lymphocytic myocarditis. Antimyosin imaging was repeated after 6 months of immuno-suppressive therapy. Biopsy showed healed myocarditis. No antimyosin uptake is visible on either the planar (Top right) or tomographic reconstructions (Bottom right). (From Dec and Narula.76 By permission of Edizioni Minerva Medica.)

features, antimyosin was highly sensitive but moderately specific for detecting myocardial necrosis (Table 11-10).75 The sensitivity was 83%; specificity, 53%; and predictive value of a negative scan, 92%.

Perhaps more important than the correlation between cardiac antimyosin uptake and histologic findings, improvement in left ventricular function within 6 months of treatment occurred in 54% of patients with a positive antimyosin scan but in only 18% of those with a negative scan.75 Because spontaneous improvement in ventricular function is a well-recognized feature of acute lymphocytic myocarditis, it is suspected that several patients who were scan-positive but biopsy-negative may have, in fact, had myocarditis.27'76 A small cohort of patients who had a negative initial antimyosin scan returned 6 to 12 months later for a second study. All showed no evidence for antimyosin uptake.75 Repeat antimyosin imaging among 17 patients whose initial scan was positive showed persistent uptake in 9 individuals and resolution of uptake in the remaining 8 patients. No correlation could be found between ongoing myocarditis on repeat biopsy and clinical improvement.75

Narula et al.61 also evaluated the role of antimyosin imaging among patients who presented with chest pain mimicking acute myocardial infarction despite normal coronary anatomy. Antimyosin uptake was global in 7 of the 8 patients with confirmed myocarditis on biopsy and equivocal in the 8th patient. Antimyosin uptake was segmental in patients with acute myocardial infarction and almost always confined to the territory of the infarct-related vessel. Thus, antimyosin uptake may be useful in differentiating unstable coronary syndromes from myocarditis.

The low specificity of cardiac antimyosin uptake results from its exquisite affinity with necrotic myocytes. Antimyosin uptake has been reported in systemic diseases that affect the heart such as Lyme disease.77 Positive uptake has also been reported in heart transplant rejection,78 anthracycline-induced cardiomyopathy,79 and alcohol-related cardiomyopathy.80 Its high sensitivity and modest specificity suggest that antimyosin scintigraphy may be useful as an initial screening tool to determine which patients should undergo biopsy. Unfortunately, this imaging agent is not currently available commercially and is restricted to research.

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