Pyrophosphate myocardial imaging

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Technetium-99m pyrophosphate myocardial scintigraphy is a sensitive indicator of acute myocardial infarction (AMI). Over 90% of acute myocardial infarctions will result in an abnormal scintigram. The sensitivity is highest for transmural myocardial infarction, reaching levels of 95% or better. The positivity rate for subendocardial infarction ranges from 40% to 88% depending on the criteria used for interpreting the study. The threshold for positivity may be established using the intensity level present within the myocardium, on the presence or absence of localization within a specific wall of the myocardium, or a combination of both. The more stringent the criteria and the higher the threshold for positivity, the greater will be the specificity for acute infarction. The high specificity will, however, be at the expense of a lower sensitivity. Several other pathologic conditions can yield a positive myocardial scintiphotogram. Most commonly these are other forms of coronary artery disease in which frank myocardial infarction is not occurring. The cause of positive scintiphotograms in the absence of acute myocardial infarction is not known. It may be that with ischemia there are small focal areas of necrosis or that pyrophosphate concentration occurs purely on the basis of the ischemia. In the absence of infarction, the pattern of positivity is usually diffuse rather than localized in a specific wall. There are exceptions to this, most notably with ventricular aneurysms; however, the presence of a localized abnormality generally increases the specificity of a positive scintiphotogram for AMI. A positive scintigram in the absence of acute infarction has prognostic value in coronary artery disease for some conditions. A persistently positive study following a remote infarction is associated with an increased morbidity and mortality. Likewise, a positive scintiphotogram before coronary artery bypass surgery portends a higher surgical risk. Other indications for the examination include the diagnosis of perioperative infarction and right ventricular infarction. Less common entities such as metastasis to the myocardium, myocardial trauma, radiation therapy, or any other entity leading to significant myocardial injury or cellular death may result in an abnormal scintiphotogram.