For the VME images, a 70-keV monoenergetic beam was simulated to obtain a “conventional CT” impression (comparable to 120-kV images). The calculation algorithm for the iodine map was based on the three-material decomposition technique using the difference in the attenuation coefficient specific to a substance upon scanning at two tube voltages. Image reconstructionĭECT postprocessing was performed by raw data-based decomposition using a dedicated workstation with the CT scanner. To investigate the effects of x-ray beam hardening, we repeated this DECT scan protocol identically with and without the x-ray attenuation rubber layer. The scan field of view and display field of view were set by assuming a head CT examination. The images were reconstructed by means of adaptive iterative dose reduction (AIDR 3D STD) with the following reconstruction parameters: slice thickness, 2.0 mm reconstruction kernel, FC43 display field of view, 24.0 cm. High tube current was used to suppress the fluctuation of the CT value, and the scan was performed nine times for every 36° to change the start position of the x-ray tube. The DECT scan parameters were as follows: collimation, 320 × 0.5 mm gantry rotation time, 1.0 s tube potential/tube current, 80 kVp/800 mA and 135 kVp/520 mA z-coverage, 160 mm scan field of view, 24.0 cm. The center of the phantom was aligned to coincide with the center of the gantry. Furthermore, a 2-mm-thick rubber material with a radiodensity of ~700 HU was wound around the cylindrical phantom to function as a high x-ray attenuation layer. By assuming brain CT imaging, 10 targets were set Target_1, Target_3, and Target_7: iodine concentration of 10.00, 2.00, and 5.00 Target_2, Target_5, Target_6, Target_9, and Target_10: soft tissues as blood ( ρ e w = 1.07), brain ( ρ e w = 1.02), blood ( ρ e w = 1.10), water ( ρ e w = 1.00), and water ( ρ e w = 1.00) Target_4 and Target_8: mixture of iodine and blood (2.00 mg/mL + blood and 4.00 mg/mL + blood). A total of ten 28.5-mm diameter holes were created, with the resulting background CT value being equivalent to that of water the holes were structured such that they could be filled with rods of various substances. 1, the phantom consisted of a cylinder with a diameter of 200 mm. The DECT images were obtained as two datasets, corresponding to the two x-ray tube energies used for the first and second rotations. The CT equipment used in this study was calibrated with specific phantoms daily for the quality control. We used a 320-row CT scanner (Aquilion One Vision Edition, Canon Medical Systems, Japan) to scan a phantom (Model 1472, Gammex RMI, Middleton, WI, USA) to obtain DECT images. This study was a phantom experiment and did not require ethics committee approval. Consequently, in this study, we investigated the effect of beam hardening on the DECT values of the iodine map, VNC images, and VME images. Whereas beam-hardening correction has been implemented in the calculation of DECT imaging, to the best of our knowledge, no published study has investigated the influence of x-ray beam hardening on the CT values of various materials. Thus, it may also affect the accuracy of iodine maps, VME images, and VNC images. This effect may reduce the accuracy of CT values in both low- and high-energy CT images. Because lower-energy photons are absorbed more rapidly than higher-energy photons, the spectrum of the x-ray beam becomes more intense by the time the x-ray reaches the detector. However, in clinical examinations, x-rays can cause a beam-hardening effect due to differences in the amount of x-ray transmitted through the human tissue therefore, the effective incident x-ray energy on the x-ray detector differs from the output x-ray energy. The calculations to obtain these novel CT images are based on the differences in the attenuation coefficient values of materials at the low and high effective x-ray energies utilized for DECT scanning. Further, the VNC method reduces the need for plain computed tomography (CT) scans and also reduces patient radiation exposure, as VNC images are virtually reconstructed from the DECT images. Moreover, VME imaging, which refers to the generation of virtual images scanned at lower or higher actual effective tube voltages relative to the conventional scan voltage, can be applied to improve the enhancement of contrast media or reduce metal artifacts. An iodine map can be used to identify the contrast media of iodine in any anatomical region. Dual-energy computed tomography (DECT) is an imaging technique that utilizes continuous x-ray energies at two different kVp to generate iodine maps, virtual monoenergetic (VME) images, and virtual non-contrast (VNC) images.
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