Imaging of cardiac amyloidosis: Will this become a unique application for dual-isotope imaging?

Excerpt

Cardiac amyloidosis is an infiltrative cardiomyopathy characterized by the deposition of amyloid fibrils within the extracellular space of the heart, which is caused by the misfolding of precursor proteins, most commonly transthyretin (ATTR)—a transport protein synthesized in the liver—or monoclonal light chains (AL), produced by an abnormal plasma cell clone.1 Identifying the type of amyloid deposition has very important treatment and prognostic implications.2,3 Echocardiography and cardiac magnetic resonance imaging are powerful tools for diagnosing cardiac amyloidosis.4,5 However, neither technique can reliably differentiate between ATTR and AL forms of cardiac amyloidosis. Despite initial promise in the late 1970s, bone scintigraphy for the detection of cardiac amyloidosis did not gain a hold in clinical practice. One reason was a perceived lack of sensitivity, as the difference in radiotracer avidity between ATTR and AL amyloidosis was not appreciated. However, the more recent demonstration that bone-avid radiotracers are specifically taken up in hearts of patients with ATTR amyloidosis has renewed clinical interest in this diagnostic approach as it is relatively inexpensive, broadly available, and it has a high sensitivity and specificity in the correct setting.6,7 In fact, a recent multicenter study demonstrated that bone scintigraphy has a specificity and positive predictive value for cardiac ATTR amyloidosis of 100% when there is intense myocardial radiotracer uptake (equal or greater than bone) and absence of a monoclonal protein in serum or urine.8 Indeed, bone scintigraphy has now largely replaced the need for endomyocardial biopsy in a majority of patients with suspected cardiac amyloidosis. Furthermore, the recent FDA approval of novel therapies designed to treat systemic ATTR amyloidosis and the early promising results in patients with cardiac involvement assigns a critical role to bone scintigraphy in differentiating AL from ATTR amyloidosis and guiding patient management.9,10 …