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

Abstract

This study aimed to quantitate the accuracy of the determination of electron density (ED), effective atomic number (Zeff ), 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. 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 Zeff for the insert materials were derived from the chemical compositions provided by the vendors. The nominal scan condition for the accuracy measurements was 120kVp, 20mGy CTDIvol, 0.812 pitch, 16×0.625mm collimation, and 0.33-second gantry rotation. The median deviation of ED ranged from -0.1% to 1.1% for all Gammex tissue inserts. The median deviation of Zeff 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 Zeff 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 Zeff values agreed to within 2%, except for the lung inserts. When the scan parameters were changed from 120kVp/20mGy to 140kVp/30mGy, the ED differed within [-0.51%, 0.65%] and the Zeff differed within [-1.1%, 0.23%] for all materials except lungs, in which Zeff increased by 2.4%. The accuracy of ED and Zeff measurement at 120kVp was no worse than that at 140kVp. For iodine quantitation, the median absolute deviations from the nominal values were up to 0.3mg/mL for iodine concentrations of 2-20mg/mL, with an overall median deviation of -0.1mg/mL. Iodine and calcium were well separated on the ED-Zeff scatter plot, even at the lowest concentrations (2mg/mL for iodine and 50mg/mL for calcium). The accuracy of ED measurement, Zeff 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.