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