cardiovascular disorders
Radionuclide Imaging of the Heart
Infarct Avid Imaging
Infarct avid imaging uses radiolabeled markers that accumulate in areas of damaged myocardium, such as Tc-99m pyrophosphate and antimyosin (indium-111 [In-111]–labeled antibodies to cardiac myosin). Images usually become positive 12 to 24 hours after acute myocardial infarction and remain positive for about 1 week; they may remain positive if myocardial necrosis continues post-myocardial infarction or if aneurysms develop. This technique is rarely used now because other diagnostic tests for myocardial infarction (eg, biomarkers) are more readily available and less expensive and because it provides no prognostic information other than infarct size. Tc-99m pyrophosphate is now being used similarly to evaluate cardiac involvement in Amyloidosis (a type of amyloidosis in which sheets of misfolded transthyretin protein can accumulate in tissues, including the heart). Transthyretin amyloid deposits in the myocardium are particularly avid for Tc-99m pyrophosphate. Absent serum and urine manifestations of a light chain amyloidosis (which also can infiltrate the myocardium and result in a positive scan) or a recent infarct, a high ratio of uptake in the myocardium is specific for transthyretin cardiac amyloidosis and can eliminate the need for myocardial biopsy. 