Impact of contrast medium volume on myocardial extracellular volume measurement accuracy in first-generation photon-counting detector CT.

Photon-counting detector CT (PCD-CT) holds promise for cardiac CT imaging, including the measurement of extracellular volume (ECV), due to its advanced imaging capabilities. The study aimed to compare the protocols with and without additional contrast medium after coronary CT angiography (CCTA) by evaluating (1) the stability of subtraction method-based ECV (ECV_SUB) across various keV images and (2) the correlation and agreement of ECV_SUB with iodine map-based ECV (ECV_IOD). Forty patients with known or suspected coronary artery disease were divided into two groups. In Protocol A (n = 20), only the standard contrast dose for CCTA was administered, while in Protocol B (n = 20), additional contrast medium was given after CCTA. The difference between the largest and smallest ECV_SUB among multiple keV images was defined as the variability of ECV_SUB. Correlations and agreement between the methods were assessed using Pearson's correlation coefficient (r), intraclass correlation coefficient (ICC), and Bland-Altman analyses. The mean variability in ECV_SUB was significantly higher in Protocol A (3.8 ± 2.1) compared to Protocol B (2.1 ± 0.9) (p = 0.008). In Protocol A, the correlation between ECV_SUB and ECV_IOD was poor (r = 0.43, p = 0.059) with a low ICC of 0.40. In this group, Bland-Altman analysis showed a mean difference of 3.7 and limits of agreement from -9.4 to 16.8. In Protocol B, a stronger correlation was observed (r = 0.74, p < 0.001) with an ICC of 0.68. In this group, the mean difference was 2.8 with narrower limits of agreement (-4.8 to 10.4). Additional contrast medium is essential for stable myocardial ECV measurements using PCD-CT.