# Enhancing Soft Tissue Differentiation with Different Dual-Energy CT Systems: A Phantom Study
## 1 Introduction  
Computed tomography (CT) plays an important role in quantitative imaging research. However, materials with different elementary compositions can be represented by very similar CT numbers, hindering the differentiation and classification of biological tissues. These difficulties are due to the similar linear attenuation coefficient, which depends on various factors such as the atomic composition of the material and photon energy. In the diagnostic energy range (up to 150 keV), the attenuation of X-rays is dominated by two physical processes, the photoelectric effect and the Compton effect. The relative probability of photoelectric absorption is approximately proportional to \(Z^{3}/\mathrm{E}^{3}\), where \(Z\) is the atomic number and E is the energy of the incident photon. The relative probability of a Compton interaction increases, compared to photoelectric absorption, as the photon energy increases and is approximately proportional to the \(Z\) of the material. The attenuation coefficient is approximately a linear combination of these two probabilities. Two different materials that show a similar attenuation coefficient (which is not unique for any given material) on images acquired with one energy spectrum, may show substantial differences in their attenuation on the images acquired with two different spectra and, hence, may be easily differentiated [1; 2]. Dual-energy computed tomography (DECT) is a valuable technique that utilizes the dependence of the attenuation coefficient on photon energy to provide additional quantitative and qualitative information about material composition. It uses two different X-ray spectra to acquire two datasets of the same anatomic region, allowing analysis of energy-dependent changes in the attenuation of different materials. This has led to a transition from CT attenuation-based imaging to material-specific or spectral imaging that provides specific information about the composition of materials [3]. Clinical applications of DECT have become possible thanks to the overcoming of some technical issues, such as X-ray spectra generation time, difficulties in X-ray tube cooling, and management of the resulting data amount [4; 5]. Today, five DECT diagnostic techniques are commercially available, based on single or double X-ray sources, dual detectors, or dual beam filters, depending on the manufacturer: sequential scans, fast kVp-switching, dual-layer detector, split filter, and dual source. One of the earliest and simplest DECT systems was based on the sequential acquisition of high- and low-energy data by performing two consecutive scans with different tube potentials. This DECT mode can be obtained with any CT scanner since it does not require special hardware. A DECT scanner equipped with fast kVp-switching acquires data with high- and low-energy spectra by modulating the X-ray generator voltage from low to high kVp between alternating projections. When a dual-layer detector is present, the spectra energy separation is obtained by the detector itself and does not rely on different energies generated by the source. The detector configuration consists of two layers, one on top of another. The top layer preferentially absorbs lower-energy photons, while the bottom one absorbs the remaining photons with higher energy. This configuration takes advantage of the polychromatic nature of the X-ray beam and the fact that detector sensitivities are optimized for different energies. A split-filter DECT consists of a single source-detector combination, and spectral separation is achieved by two filters of different materials that change the beam quality along the bed movement direction. The corresponding halves of the detector are then used for the detection of the low- and high-energy spectra. A dual-source DECT consists of two perpendicular source-detector combinations, allowing for scanning the same volume simultaneously with high- and low-energy spectra. Each X-ray tube has its own high-voltage generator, allowing independent control of both kVp and tube current.