Assessment of Cardiac Amyloidosis with 99Mtc-Pyrophosphate (PYP) Quantitative SPECT.

Background Tc-99m-PYP scintigraphy provides differential diagnosis of ATTR cardiomyopathy (ATTR-CM) from light chain cardiac amyloidosis and other myocardial disorders without biopsy. This study was aimed to assess the diagnostic feasibility and the operator reproducibility of Tc-99m-PYP quantitative SPECT. Method Thirty-seven consecutive patients who underwent a Tc-99m-PYP thorax planar scan followed by SPECT and CT scans to diagnose suspected ATTR-CM were enrolled. For the quantitative SPECT, phantom studies were initially performed to determine the image conversion factor (ICF) and partial volume correction (PVC) factor to recover Tc-99m-PYP activity concentration in the myocardium for calculating the standardized uptake value (SUV) (unit: g/ml). SUVmax was compared among groups of ATTR-CM, AL cardiac amyloidosis, and other pathogens (others) and among categories of Perugini visual scores (grades 0-3). The intra- and inter-operator reproducibility of quantitative SPECT was verified, and the corresponded repeatability coefficient (RPC) was calculated. Results The ICF was 79,327 Bq/ml to convert count rate in pixel to Tc-99m activity concentration. PVC factor as a function of the measured activity concentration ratio in the myocardium and blood-pool was [y = 1.424 x (1 - exp(- 0.759 x x)) + 0.104]. SUVmax of ATTR-CM (7.50 +/- 2.68) was significantly higher than those of AL (1.96 +/- 0.35) and others (2.00 +/- 0.74) (all p < 0.05). SUVmax of grade 3 (8.95 +/- 1.89) and grade 2 (4.71 +/- 0.23) were also significantly higher than those of grade 1 (1.92 +/- 0.31) and grade 0 (1.59 +/- 0.39) (all p < 0.05). Correlation coefficient (R-2) of SUVmax reached 0.966 to 0.978 with only small systematic difference (intra = - 0.14; inter = - 0.23) between two repeated measurements. Intra- and inter-operator RPCs were 0.688 and 0.877. Conclusions Tc-99m-PYP quantitative SPECT integrated with adjustable PVC factors is feasible to quantitatively and objectively assess the burden of cardiac amyloidosis for diagnosis of ATTR-CM.