Performance of Photon-Counting Detector CT for Electron Density and Effective Atomic Number Maps
Abstract
Purpose
Photon-counting-detector (PCD) CT acquires multi-energy data in a single scan, enabling quantitation of electron density (ρe) and effective atomic number (Zeff). This study assessed the performance of PCD-CT for ρe and Zeff mapping across different phantom sizes and tube potentials.
Methods
Three Gammex phantoms representing different patient sizes (20, 40 and 50 cm lateral diameter) containing nine tissue-equivalent inserts were scanned using PCD-CT (NAEOTOM Alpha) at 120 and 140 kV. Dual-energy energy-integrating-detector (EID) CT (SOMATOM Force) scans were acquired at 80/Sn150, 90/Sn150, and 100/Sn150 kV. Images were reconstructed using a quantitative kernel (Qr40) and 2-mm thickness. Relative ρe and Zeff maps were generated through material decomposition and circular regions-of-interest were placed in each insert. Mean absolute error (MAE) and percentage error (%) were calculated.
Results
For PCD-CT, relative ρe accuracy was higher at 140 kV compared to 120 kV, with MAE 0.0037-0.0052 at 140 kV (0.33-0.45%), and 0.014-0.017 at 120 kV (1.23-1.51%). For EID-CT, relative ρe differences were more variable across tube potentials, with MAE ranging from 0.0046 to 0.0115, and percentage error ranging from 0.39-1.01%. Across all phantom sizes, relative ρe map noise was consistently lower for PCD-CT than for EID-CT. For PCD-CT, noise ranged from 0.008-0.013, while EID-CT noise ranged from 0.012-0.021. For effective Z, MAE was comparable across PCD-CT and EID-CT for all phantom sizes and tube potentials. Across all scans, MAE for Zeff remained around 0.2-0.4 (approximately 2.67-5.68%), with no observed dependance on phantom size or tube potential.
Conclusion
PCD-CT demonstrated higher relative ρe accuracy, lower variations, and decreased noise compared to EID-CT, particularly at 140 kV. Zeff values were comparable between all scans, with minimal dependence on scan parameters and phantom size. These results suggest that PCD-CT is a promising technology for quantitative imaging in both clinical and research applications.