Single-photon emission computed tomography/computed tomography quantification of Tc-99m pyrophosphate uptake to assess tafamidis treatment response in transthyretin cardiac amyloidosis.

Background: Cardiac imaging with bone-avid tracers for the diagnosis of transthyretin amyloid (ATTR) cardiac amyloidosis uses only limited quantification, but single-photon emission computed tomography/computed tomography (SPECT/CT) acquisition can provide volumetric assessment with quantification of tracer uptake. Tafamidis is routinely used in the treatment of cardiac amyloidosis, but there are scant data on changes in imaging results during therapy. The purpose of this study was to perform a longitudinal assessment of Tc-99m-pyrophosphate (PYP) imaging to determine if tafamidis therapy results in any change in quantitative measures of tracer uptake.

Methods: The study incorporated a prospective, single-center study of ATTR patients being treated with tafamidis using Tc-99m-PYP SPECT/CT to quantify cardiac tracer uptake in the whole heart and left ventricle. Standardized uptake values (SUVs) were adjusted for blood pool activity. Comparison of baseline activity was made to values obtained approximately every 6 months during treatment.

Results: Twenty-two patients (77.0±7.5 years old, 86.4% male) were on tafamidis for 15.3±4.0 months, with an average time between baseline and final follow-up study of 16.8±4.7 months. Thirteen patients (59.1%) had multiple follow-up amyloid studies. Statistically significant reductions in total SUVs, SUV volume, and percentage of injected dose were seen. Adjusted for the maximal aortic SUV, the total SUV in the left ventricle decreased by 36.9%, the SUV volume by 38.7%, and the percentage of injected dose decreased by 34.9% (all P values≤0.0001). Over the study duration, there was a decrease of 7.7%/month in the measured metrics.

Conclusion: The quantitative SUV measurements from Tc-99m-PYP SPECT/CT revealed an overall decrease in scintographic amyloid burden during the course of tafamidis therapy, but additional work is needed to determine the optimal metrics and improve the reproducibility of the quantification.