Transthyretin amyloid cardiomyopathy disease burden quantified using 99m Tc-pyrophosphate SPECT/CT: volumetric parameters versus SUVmax ratio at 1 and 3 hours

Background Various parameters derived from technetium-99m pyrophosphate ( 99m Tc-PYP) single-photon emission computed tomography (SPECT) correlate with the severity of transthyretin amyloid cardiomyopathy (ATTR-CM). However, the optimal metrics and image acquisition timing required to quantify the disease burden remain uncertain. Methods and results We retrospectively evaluated 99m Tc-PYP SPECT/CT images of 23 patients diagnosed with ATTR-CM using endomyocardial biopsies and/or gene tests. All patients were assessed by SPECT/CT 1 hour after 99m Tc-PYP injection, and 13 of them were also assessed at 3 hours. We quantified 99m Tc-PYP uptake using the volumetric parameters, cardiac PYP volume (CPV) and cardiac PYP activity (CPA). We also calculated the SUVmax ratios of myocardial SUVmax/blood pool SUVmax, myocardial SUVmax/bone SUVmax, and the SUVmax retention index. We assessed the correlations between uptake parameters and the four functional parameters associated with prognosis, namely left ventricular ejection fraction, global longitudinal strain, myocardial extracellular volume, and troponin T. CPV and CPA correlated more closely than the SUVmax ratios with the four prognostic factors. Significant correlations between volumetric parameters and prognostic factors were equivalent between 1 and 3 hours. Conclusions The disease burden of ATTR-CM was quantified more accurately by volumetric evaluation of 99m Tc-PYP SPECT/CT than SUVmax ratios and the performance was equivalent between 1 and 3 hours.