Influence of Radiation Dose, Photon Energy, and Reconstruction Kernel on rho/z Analysis in Spectral Computer Tomography: A Phantom Study

Abstract

Background/aim: The effective atomic number (Z_eff) and electron density relative to water (ρ_e or Rho) of elements can be derived in dual-energy computed tomography (DECT). The aim of this phantom study was to investigate the effect of different photon energies, radiation doses, and reconstruction kernels on Z_eff and Rho measured in DECT.

Materials and methods: An anthropomorphic head phantom including five probes of known composition was scanned under three tube-voltage combinations in DECT: Sn140/100 kV, 140/80 kV and Sn140/80 kV with incremented radiation doses. Raw data were reconstructed with four reconstruction kernels (I30, I40, I50, and I70). Rho and Z_eff were measured for each probe for all possible combinations of scan and reconstruction parameters.

Results: DECT-based Rho and Z_eff closely approached the reference values with a mean and maximum error of 1.7% and 6.8%, respectively. Rho was lower for 140/80 kV compared with Sn140/100 kV and Sn140/80 kV with differences being 0.009. Z_eff differed among all tube voltages with the most prominent difference being 0.28 between 140/80 kV and Sn140/100 kV. Z_eff was lower in I70 compared with those of I30 and I40 with a difference of 0.07. Varying radiation dose yielded a variation of 0.0002 in Rho and 0.03 in Z, both considered negligible in practice.

Conclusion: DECT comprises a feasible method for the extraction of material-specific information. Slight variations should be taken into account when different radiation doses, photon energies, and kernels are applied; however, they are considered small and in practice not crucial for an effective tissue differentiation.

Keywords: Tomography; X-ray computed; image reconstruction; radiation dosage.

Figure 1. Anthropometric head phantom with 16 cm diameter filled with tap water and cylindrical probes (24).

Figure 2. Syngo.via CT Dual energy «Rho/Z» application map: HU_Rho and Z_eff were measured for each probe in the middle of each cylinder after axis correction. Rho (ρ_e) was further calculated based on HU_Rho as proposed by Saito et al. (16).

Figure 3. Graphs presenting the electron density relative to water Rho (ρ_e) measured in DE Rho/Z imaging [and calculated as proposed by Saito et al. (16)] for each probe at different photon energies (Sn140/100 kV, 140/80 kV, Sn140/80 kV) and radiation doses (20/40/…/200 mAs) for the different reconstruction kernels (I30, I40, I50, I70). SD arising from the different tube currents was in total 0.0002 for all phantom probes. The tube voltage had significant effect on Rho (p<0.001): being significantly lower for 140/80 kV compared with both Sn140/100 kV and Sn140/80 kV with differences of 0.009 and 0.008, respectively. The reconstruction kernels did not affect Rho (p=0.99).

Figure 4. Graphs presenting the effective atomic number (Z_eff) measured in DE Rho/Z imaging for each probe at different photon energies (Sn140/100 kV, 140/80 kV, Sn140/80 kV) and radiation doses (20/40/…/200 mAs) within the different reconstruction kernels (I30, I40, I50, I70). The sinus LAA347 probe results are not presented because Z_eff values could not be extracted. SD arising from the different tube currents was in total 0.03. Both the tube voltage and reconstruction kernel had a significant effect on Z_eff (p<0.001). Zeff was significantly different among all three voltages with the most prominent differences between 140/80 kV and Sn140/100 kV. Z_eff was lower in I70 compared with those of I30 and I40.