# Enhancing Soft Tissue Differentiation with Different Dual-Energy CT Systems: A Phantom Study
## 2 Materials and Methods
### CT Scanners and Acquisition Protocols  
The phantom images were acquired with four different CT scanners: Siemens Somatom Confidence, Siemens Somatom Force, Siemens Somatom Definition Edge, and GE Revolution GSI. Each of these CT scanners represents different DECT technologies.  
* The Siemens Somatom Confidence scanner is a single source-detector system that performs the dual-energy acquisition by two sequential scans of the entire volume at 80 and 140 kVp. The advantage of this approach is that no hardware modification is required. This scanner is equipped with an ultrafast ceramic (UFC) detector [30];
* The Siemens Somatom Force DECT is equipped with a dual-source-dual-detector configuration. This configuration enables optimization of the separation of the low- and high-energy spectra (80 or 100 kV and 140 kVp or 150 kVp generally), with also the addition of tin filters. This scanner is equipped with the Stellar Detector, which reduces the electronic noise during image acquisition [31; 32];
* The Siemens Somatom Definition Edge uses the split-filter technology, achieving spectral separation by using two filters of tin and gold. Also, this scanner is equipped with the Stellar detector;
* The GE Revolution GSI scanner is based on the concept of rapid kVp-switching. This technology is realized due to a high performance generator capable of very rapid transitions in tube potential (between 80 and 140 kVp) and a fast-sampling scintillator detector with very low afterglow (Gemstone [6]).  
The acquisition parameters are summarized in Table 1. Images were first acquired in SECT mode at 120 kVp. The tube current (without automatic tube current modulation) was adjusted to have a similar Volume CT Dose Index (CTDIvol) across CT scanners. A good agreement (\(10\pm 4\%\)) between displayed and measured values of CTDIvol was obtained on all CT scanners using calibrated pencil chambers [33]. All scanners operated in helical mode (pitch 0.95). Then, the images were acquired in DECT mode while maintaining the settings and the dose level as similar as possible to SECT. The use of the same settings was not always possible. For example, the twin-beam DECT mode required a lower pitch (0.25) due to the halved beam used to acquire low- and high-energy images. To test repeatability and obtain sufficient statistical power for the subsequent analyses, CT scans of the phantom were repeated six times for each protocol and for both the SECT and DECT images modes on the same day for each CT scanner, taking care to reposition the phantom between one scan and the next. Both SECT and DECT images were reconstructed through filtered back-projection (FBP) and iterative algorithms. The iterative algorithms available on the various scanners were SAFIRE on Siemens Confidence, ADMIRE on Siemens Force and Edge, and ASIR on GE Revolution GSI [34; 35; 36]. A medium strength of three was used with SAFIRE and ADMIRE, while a percentage of 30% was used with ASIR (standard values used in clinical practice). Medium smooth kernels typical of the various scanner models were used to reconstruct the various SECT and DECT images. The FOV of the abdominal clinical protocol was used (50 cm). A matrix of 512 \(\times\) 512 was used. Moreover, FBP and iterative DECT images were reconstructed at different VMI energies available on the various scanners (40, 50, 70, 100, 120, and 140 keV). DECT images were reconstructed with Monoenergetic plus (Siemens, image domain) and GSI (GE, projection domain) algorithms [37; 38; 39].  
\begin{table}
\begin{tabular}{c c c c c} \hline
**SECT** & **GE** & **Siemens** & **Siemens** & **Siemens** \\
**PROTOCOL** & **Revolution GSI** & **Siemens** & **Edge** & **Force** \\ \hline kVp & 120 & 120 & 120 & 120 \\ mAs & 380 & 380 & 380 & 380 \\ Rotation time (s) & 1 & 1 & 1 & 1 \\ Collimation (mm) & \(20\times 1.25\) & \(16\times 1.2\) & \(32\times 1.2\) & \(96\times 0.6\) \\ FOV (cm) & \(50\) & 50 & 50 & 50 \\ \hline \end{tabular}
\end{table}
Table 1: SECT and DECT scan protocols used for phantom acquisitions.