Accuracy of the raw-data-based effective atomic numbers and monochromatic CT numbers for contrast medium with a dual-energy CT technique

Objective: To evaluate the accuracy of raw-data-based effective atomic number (Z(eff)) values and monochromatic CT numbers for contrast material of varying iodine concentrations, obtained using dual-energy CT. Methods: We used a tissue characterization phantom and varying concentrations of iodinated contrast medium. A comparison between the theoretical values of Z(eff) and that provided by the manufacturer was performed. The measured and theoretical monochromatic CT numbers at 40-130 keV were compared. Results: The average difference between the Z(eff) values of lung (inhale) inserts in the tissue characterization phantom was 81.3% and the average Z(eff) difference was within 8.4%. The average difference between the Z(eff) values of the varying concentrations of iodinated contrast medium was within 11.2%. For the varying concentrations of iodinated contrast medium, the differences between the measured and theoretical monochromatic CT values increased with decreasing monochromatic energy. The Z(eff) and monochromatic CT numbers in the tissue characterization phantom were reasonably accurate. Conclusion: The accuracy of the raw-data-based Z(eff) values was higher than that of image-based Zeff values in the tissue-equivalent phantom. The accuracy of Z(eff) values in the contrast medium was in good agreement within the maximum SD found in the iodine concentration range of clinical dynamic CT imaging. Moreover, the optimum monochromatic energy for human tissue and iodinated contrast medium was found to be 70 keV.