Accuracy of electron density, effective atomic number, and iodine concentration determination with a dual-layer dual-energy computed tomography system

Abstract

Purpose: This study aimed to quantitate the accuracy of the determination of electron density (ED), effective atomic number (Z_eff ), and iodine concentration, directed for more accurate radiation therapy planning, with a new dual-layer dual-energy computed tomography (DL-DECT) system. The dependence of the accuracy of these values on the scan and reconstruction parameters, as well as on the phantom size, was also examined.

Methods: Measurements were performed on a commercial DECT system with a DL detector (IQon Spectral CT, Philips Healthcare), using phantoms with various tissue-equivalent inserts as well as iodine and calcium inserts of different concentrations. The expected values of ED and Z_eff for the insert materials were derived from the chemical compositions provided by the vendors. The nominal scan condition for the accuracy measurements was 120 kVp, 20 mGy CTDIvol, 0.812 pitch, 16 × 0.625 mm collimation, and 0.33-second gantry rotation.

Results: The median deviation of ED ranged from -0.1% to 1.1% for all Gammex tissue inserts. The median deviation of Z_eff ranged from -2.3% to 1.7% for soft tissue and bone inserts and was ≤7% for lung inserts. The absolute deviations for ED and Z_eff in lung inserts were within 1% of the ED of water and 1 a.u., respectively. For two different phantom sizes, the ED values agreed to within 0.7% and the Z_eff values agreed to within 2%, except for the lung inserts. When the scan parameters were changed from 120 kVp/20 mGy to 140 kVp/30 mGy, the ED differed within [-0.51%, 0.65%] and the Z_eff differed within [-1.1%, 0.23%] for all materials except lungs, in which Z_eff increased by 2.4%. The accuracy of ED and Z_eff measurement at 120 kVp was no worse than that at 140 kVp. For iodine quantitation, the median absolute deviations from the nominal values were up to 0.3 mg/mL for iodine concentrations of 2-20 mg/mL, with an overall median deviation of -0.1 mg/mL. Iodine and calcium were well separated on the ED-Z_eff scatter plot, even at the lowest concentrations (2 mg/mL for iodine and 50 mg/mL for calcium).

Conclusions: The accuracy of ED measurement, Z_eff determination, and iodine quantitation derived from DL-DECT was demonstrated with phantom measurements. The accuracies were not sensitive to scan and reconstruction parameters, namely tube potential, dose, rotation time, and spectral reconstruction level, especially in the case of electron density.

Keywords: dual-layer CT; effective atomic number; electron density; iodine quantitation; material separation.