Assessment of Cardiac Amyloidosis with^99mTc-Pyrophosphate (PYP) Quantitative SPECT

Background: ^99mTc-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 ^99mTc-PYP quantitative SPECT. Method: Thirty-seven consecutive patients underwent a ^99mTc-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 ^99mTc-PYP activity concentration in myocardium for calculating the standardized uptake value (SUV) (unit: g/ml). SUV_max was compared among groups of ATTR-CM, AL cardiac amyloidosis and other pathogens (Others), and among categories of Perugini visual scores (Grade: 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 ^99mTc activity concentration. PVC factor as a function of the measured activity concentration ratio in myocardium and blood-pool was [y=1.424*(1-exp(-0.759*x))+0.104]. SUV_max 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). SUV_max 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 (R²) of SUV_max 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: ^99mTc-PYP quantitative SPECT is a reliable method to quantitatively and objectively assess the burden of cardiac amyloidosis for diagnosis of ATTR-CM. Keywords: ATTR cardiomyopathy; ^99mTc-PYP quantitative SPECT; standardized uptake value; diagnostic feasibility; the operator reproducibility.