Test-retest Reproducibility of Myocardial 99mTc-HMDP Uptake Quantification by SPECT-CT in Suspected Transthyretin Cardiac Amyloidosis.

Background: Diagnosing transthyretin-related cardiac amyloidosis (ATTR-CA) relies on excluding plasma cell dyscrasia and performing bone-avid tracer cardiac scintigraphy. However, visual assessment of cardiac uptake has limitations in evaluating amyloid burden. Whole-body SPECT-CT may overcome these limitations by enabling quantitative assessment, but data on test-retest reproducibility is lacking.

Objective: This study aimed to evaluate the test-retest, intra- and interobserver reproducibility of cardiac amyloid quantification in suspected ATTR-CA using whole-body cadmium zinc telluride SPECT-CT.

Methods: Twenty patients with suspected ATTR-CA underwent repeated planar bone-avid tracer cardiac scintigraphy and thoracic SPECT-CT one week apart (October 2020-December 2022; Clinical Trial: NCT04535349). The Perugini score was assessed using planar acquisition. Cardiac 99mTc-HMDP uptake was quantified on SPECT images with CT-based cardiac contouring and attenuation correction. The evaluated metrics included maximum standardized uptake values (SUVmax), SUVmean, percentage of injected dose (%ID), and cardiac amyloid activity (CAA). Concordance was evaluated by Lin's concordance correlation coefficient (CCC). Test-retest reproducibility was assessed using the mean absolute difference, coefficient of variation, and intraclass correlation coefficient (ICC).

Results: Intra- and interobserver reproducibility was excellent for all metrics (CCC ≥ 0.99). Test-retest reproducibility demonstrated substantial agreement for SUVmean (CCC: 0.79) and almost perfect agreement for SUVmax (CCC: 0.81), %ID (CCC: 0.88), and CAA (CCC: 0.93). All parameters correlated with the Perugini score. Test-retest reliability was high across metrics, with ICC values of 0.80-0.97.

Conclusions: Bone-avid tracer cardiac SPECT-CT shows excellent reproducibility for quantifying cardiac amyloid burden in suspected ATTR-CA, with %ID and CAA being especially reliable for future longitudinal studies. Background: NCT04535349.