Lateral Planar Imaging of 99mTc-pyrophosphate Scintigraphy in Patients with Suspected Transthyretin Cardiac Amyloidosis.

Background: Lateral planar ^99mTc-pyrophosphate (PYP) imaging is recommended as a standardized acquisition method because it helps separate extracardiac uptake from the myocardium. We evaluated its discriminatory performance in detecting myocardial PYP uptake, using single-photon emission computed tomography (SPECT) as a reference standard. Methods: We retrospectively evaluated 170 patients who underwent PYP imaging for suspected transthyretin cardiac amyloidosis. Anterior and lateral planar imaging and SPECT were performed 3 h after PYP administration. The myocardial PYP uptake on planar and SPECT images was visually assessed and quantified using the heart-to-contralateral lung uptake (H/CL) ratio. The heart-to-mediastinum uptake (H/M) ratio was calculated as the mean count of the region of interest in the heart divided by that in the superior mediastinum on lateral planar images. Results: Patients with PYP SPECT-positive results had significantly higher H/M ratios at 3 h than those with PYP SPECT-negative results (1.23 [interquartile range: IQR, 1.15-1.43] vs. 1.08 [IQR, 1.02-1.16]; P<0.001). A reclassification analysis that added the H/M ratio to visual scores for detecting positive PYP SPECT yielded a significant improvement with a net reclassification improvement (NRI) of 0.56 (95%CI, 0.25-0.87; P<0.001) and integrated discrimination improvement (IDI) of 0.038 (95%CI, 0.005-0.072; P=0.026). The H/M ratio significantly improved the predictive ability of SPECT findings based on the visual scores and H/CL ratio with an NRI of 0.49 (95%CI, 0.18-0.81; P=0.003) and IDI of 0.036 (95%CI, 0.004-0.069; P=0.029). Conclusions: Adding the H/M ratio derived from lateral planar PYP imaging to visual scores or the H/CL ratio on anterior planar images improved the accuracy of detecting significant myocardial uptake on SPECT.