Low burden transthyretin cardiac amyloidosis on cardiac magnetic resonance: comprehensive phenotyping and distinction from hypertrophic phenocopies.

Aims: Transthyretin cardiac amyloidosis (ATTR CA) is a progressive disease arising from the deposition of amyloid fibrils in the myocardium. Cardiac magnetic resonance (CMR) tissue characterization imaging, including myocardial extracellular volume (ECV) fraction, is used to detect amyloid infiltration, but the identification of early-stage disease is challenging. We sought to describe the phenotype of low burden ATTR CA on CMR and identify imaging features that allow differentiation from potential disease mimickers. Methods and results: Eighty-three patients with ATTR CA and prior contrast-enhanced CMR were stratified by quartiles of ECV into low (ECV ≤43%) or higher (ECV >43%) burden groups. Global and regional function and myocardial tissue characterization were used to phenotype disease. Receiver operating characteristic analysis was performed to assess the diagnostic performance of CMR for distinguishing low burden ATTR CA from hypertensive heart disease (HHD) and mild hypertrophic cardiomyopathy (HCM). Among 22 patients with low ECV burden, CMR measures of amyloid infiltration predominantly affected the basal left ventricular (LV) segments with progressive involvement of the mid and apical regions at higher ECV. Global myocardial late gadolinium enhancement (LGE) and ECV showed high accuracy for differentiating low burden ATTR CA from HHD and mild HCM, area under the curve (AUC) of 0.99 and 0.97, respectively, compared to strain-based measures, AUC 0.47-0.82. Conclusion: Tissue characterization imaging (myocardial ECV and LV LGE) can be used to distinguish low burden ATTR from potential disease mimickers and appears to outperform traditional strain-based measures.