Tc-99m pyrophosphate scintigraphy for cardiac amyloidosis: concordance between planar and SPECT/CT imaging

The imaging protocol and the optimal cut-off points for quantitative assessment of technetium-99m pyrophosphate (Tc-99m PYP) cardiac amyloidosis scintigraphy remain controversial. The aim of this study was to evaluate the concordance between planar and SPECT images, and to investigate the contribution of SPECT/CT to diagnostic precision. All patients referred to our department for Tc-99m PYP cardiac imaging between April 2019 and April 2022 were included in the study. Heart-to-contralateral lung (H/CL) ratios were calculated from anterior planar images at both 1- and 3 h, and visual grading was done in SPECT/CT images at both time points. A total of 141 patients were included in the study (median age 59 years, 54% female). There was a strong positive correlation between H/CL ratios calculated at 1- and 3 h (Pearson's ρ = 0.842, p < 0.001). The highest level of concordance between planar and SPECT/CT images was achieved at a H/CL cut-off point of 1.5 for 1-h images, and 1.4 for 3-h images. SPECT/CT imaging contributed to diagnostic precision in both 1- and 3-h images by reducing the rate of equivocal results from 83% (n = 117) to 25% (n = 35), and from 77% (n = 108) to 27% (n = 38), respectively. Our findings have three implications: (1) planar imaging at both 1- and 3 h could be redundant, (2) a lower H/CL cut-off point for 3-h planar images could improve concordance between planar and SPECT imaging, and (3) SPECT/CT in both 1- and 3 h could improve the diagnostic precision by offering markedly reduced equivocal results.