Cardiac Amyloidosis Imaging, Part 1: Amyloidosis Etiology and Image Acquisition

Cardiac amyloidosis is a systemic form of amyloidosis in which protein-based infiltrates are deposited in myocardial extracellular space. The accumulation of amyloid fibrils causes the myocardium to thicken and stiffen, leading to diastolic dysfunction and, eventually, heart failure. Until recently, cardiac amyloidosis was considered rare. However, the recent adoption of noninvasive diagnostic testing, including 99mTc-pyrophosphate imaging, has revealed a previously undiagnosed sizable disease prevalence. Light-chain amyloidosis (AL) and transthyretin amyloidosis (ATTR), the 2 primary types, account for 95% of cardiac amyloidosis diagnoses. AL results from plasma cell dyscrasia and has a very poor prognosis. The usual treatment for cardiac AL is chemotherapy and immunotherapy. Cardiac ATTR is more chronic, usually resulting from age-related instability and misfolding of the transthyretin protein. ATTR is treated by managing heart failure and using new pharmacotherapeutic drugs. 99mTc-pyrophosphate imaging can efficiently and effectively distinguish between ATTR and cardiac AL. Although the exact mechanism of myocardial 99mTc-pyrophosphate uptake is unknown, it is believed to bind to amyloid plaque microcalcifications. 99mTc-pyrophosphate imaging has a 97% sensitivity and nearly 100% sensitivity for identifying cardiac ATTR when the AL form of the disease is ruled out through serum free light-chain and serum and urine protein electrophoresis with immunofixation testing. Although there are no published 99mTc-pyrophosphate cardiac amyloidosis imaging guidelines, the American Society of Nuclear Cardiology, Society of Nuclear Medicine and Molecular Imaging, and others have published consensus recommendations to standardize test performance and interpretation. This article, part 1 of a 3-part series in this issue of the Journal of Nuclear Medicine Technology, describes amyloidosis etiology and cardiac amyloidosis characteristics, including the types, prevalence, signs and symptoms, and disease course. It further explains the scan acquisition protocol. Part 2 of the series focuses on image/data quantification and technical considerations. Finally, part 3 describes scan interpretation, along with the diagnosis and treatment of cardiac amyloidosis.