Estimation of electron density, effective atomic number and stopping power ratio using dual-layer computed tomography for radiotherapy treatment planning

Abstract

Purpose: Assess the accuracy for quantitative measurements of electron density relative to water (ρ_e/ρ_e,w), effective atomic number (Z_eff) and stopping power ratio relative to water (SPR_w) using a dual-layer computed tomography (DLCT) system.

Methods and materials: A tissue characterization phantom was scanned using DLCT with varying scanning parameters (i.e., tube voltage, rotation time, CTDI_vol, and scanning mode) and different reference materials. Then, electron density ρ_e/ρ_e,w and atomic number Z_eff images were reconstructed, and their values were determined for each reference materials. Based on these two values, SPR_w was calculated. Finally, the percent error (PE) against the theoretical values was calculated for reference materials.

Results: Significant linear relationships (p < 0.001) were observed between the measured and theoretical ρ_e/ρ_e,w (r = 1.000), Z_eff (r = 0.989) and SPR_w (r = 1.000) values. The PE for each reference material varied from -2.0 to 1.2% (mean, <0.1%) for electron density ρ_e/ρ_e,w, from -6.4 to 8.0% (mean, -2.0%) for atomic number Z_eff, and from -2.0 to 1.9% (mean, 0.3%) for stopping power ratio SPR_w. The mean PE of ρ_e/ρ_e,w (<0.1%), Z_eff (<-2.5%) and SPR_w (<0.4%) was verified across the variation of scanning parameters (p > 0.85).

Conclusions: DLCT provides a reasonable accuracy in the measurements of ρ_e/ρ_e,w, Z_eff and SPR_w, and could enhance radiotherapy treatment planning and the subsequent outcomes.

Keywords: Dual-energy CT; Dual-layer CT; Effective atomic number; Electron density.