Applicability of Effective Atomic Number (Z eff) Image Analysis of Coronary Plaques Measured With Photon- Counting Computed Tomography

Objective: Coronary computed tomography (CT) allows the assessment of cardiovascular risk by imaging calcified plaques in coronary arteries. Because photon-counting CT (PC-CT) can analyze the effective atomic number (Zeff) of the subject, it is expected to be applied to the analysis of plaque components. The purpose of this study was to investigate the applicability of plaque analysis based on Zeff images with continuous gradation.
Methods: Zeff images were generated from virtual monoenergetic images (VMIs) obtained by PC-CT. Zeff values were derived from the difference between linear attenuation coefficients (μ) at low and high energies using an in-house program. Coronary CT images of 64 plaques in 10 patients were analyzed. The Zeff score, calculated as the sum of Zeff values within the plaque region, was calculated and compared with the conventional Agatston score and mean coronary artery calcium (CAC) score.
Results: The systematic uncertainty of Zeff images was estimated to be ±0.08. The Zeff score of actual patient data showed strong positive correlations with the conventional Agatston and mean CAC scores. The Zeff score uses all voxel data in the plaque area, whereas conventional scores consider only data from voxels with a CT value >130. We found that the conventional scores excluded 39% of the plaque area, and the Zeff score permitted the analysis of low- and high-density plaques.
Conclusions: Zeff imaging was shown to be applicable to plaque analysis that reflects the entire plaque volume. This study demonstrated its technical feasibility as a compositional analysis method using the Zeff image.