CT Number Accuracy of Virtual Monoenergetic Images In Photon-Counting CT: Implications for Iodine, Iron, and Fat Quantification
Abstract
Purpose
To systematically evaluate CT number accuracy on virtual monoenergetic images (VMIs) generated by a clinical photon-counting CT (PCCT) system using projection-domain multi-material modeling with a water/iodine basis.
Methods
A medium-sized (25×32.5 cm2) liver phantom was custom-designed and industry-fabricated (CIRS). Shaped in realistic liver, the anthropomorphic phantom consisted of three segments, representing early arterial phase (no iodine), late arterial phase (iodine concentration 0.68 mg/cm3), and venous phase (1.89 mg/cm3) in multi-phase liver CT scans. Each segment contained multiple liver low density, fat, iodine, and iron oxide in clinically relevant shapes (sphere, ellipsoid, lobulated), volumes and concentrations. The phantom was scanned on Siemens NAEOTOM Alpha using abdomen/pelvis protocol (CTDIvol, 8.51 mGy; QR40f; QIR 3; slice thickness 3 mm), at 120 and 140 kV for comparison. CT numbers were measured across VMIs from 40–100 keV using a Python-based analysis. The mean value of three repeated scans was calculated, and compared with CIRS specifications for 50, 60, 80 and 100 keV.
Results
At 120 kV, CT numbers for liver tissue (lower and normal densities, iodinated 0.68-1.89 mg/cm3) were consistent with CIRS specifications (RMS error 4.0 HU; maximum difference 10.1 HU). CT numbers for fat (5–40% volume fraction) were consistent with specifications (4.9 HU; 8.8 HU). CT numbers for iodine (concentration 0.05–1.47 mg/cm3) were consistent with specifications (8.0 HU; 21.3 HU). But CT numbers for Fe₃O₄ (9.79–40 mg/cm3) were lower than specifications by 28.4–195.3 HU (RMS error 92.6 HU). Similar trends were observed at 140 kV.
Conclusion
PCCT virtual monoenergetic CT numbers are quantitatively accurate for materials that are either explicitly modeled or spectrally close to the basis materials, but show substantial systematic bias for high-Z material such as iron, as observed in this study.