Absolute Quantitation of Cardiac 99mTc-pyrophophate Using Cadmium Zinc Telluride-based SPECT/CT.

Rationale: The primary aims of this study were to determine the correlation between absolute quantitative 99mTc-pyrophosphate (99mTc-PYP) metrics and traditional measures of cardiac amyloid burden and to measure intra-observer repeatability of the quantitative metrics. Methods: We studied 72 patients who underwent 99mTc-PYP SPECT/CT using a novel general purpose CZT-based SPECT/CT system (Veriton, Spectrum Dynamics Inc). The clinical standard for these studies is visual grading (0-3: myocardial uptake none, rib uptake, respectively). Visual grade ≥ 2 was considered positive. For 72 patients, standardized uptake value (SUV) maximum and mean, cardiac amyloid activity (CAA; SUVmean*left ventricular (LV) volume), and percent injected dose (%ID) were calculated, and visual grading, was performed. Correlation of the 4 quantitative metrics and visual grades with LV mass indexed to body surface area (LVMI on echocardiography, 67 patients) was measured. For a subset of 11 patients, correlation of visual and quantitative metrics with extracellular volume (ECV on cardiac magnetic resonance imaging) was measured. Normal linear regression was used to compare the standardized association of each of the 4 quantitative metrics with LVMI, as a surrogate for amyloid burden. Receiver operating curve (ROC) analysis was used to determine the diagnostic accuracy of quantitative metrics, using visual grading as the reference standard. Intra-observer repeatability of generation of quantitative metrics was also determined. Results: All 4 quantitative metrics were highly accurate with area under the ROC curve > 0.96 for diagnosis of ATTR cardiac amyloidosis. SUVmax, SUVmean, CAA, %ID, and visual grade were moderately positively correlated with LVMI (r=0.485 for ID) and strongly positively correlated, albeit in a small cohort, with ECV (r=0.873, SUVmax). Intraobserver repeatability was excellent with 0.324 on LVMI; the largest standardized effect was 0.485 for %ID. Principal Conclusion: In this first study of 99mTc-PYP cardiac imaging using a novel CZT SPECT/CT scanner, SUV maximum and mean, CAA, and %ID measured by absolute quantitation of 99mTc-PYP were moderately correlated with LVMI and strongly correlated, albeit in a small cohort, with ECV. Intra-observer repeatability of generating the quantitative metrics was excellent.