**2. Techniques**

Recent generations of MDCTs are able to acquire dual-energy data by applying two X-ray tubes and two corresponding detectors at different kVp and mA settings simultaneously in a dual-source CT (Siemens Healthcare), by ultra-fast kVp switching in a single source CT (GE Healthcare) or by compartmentalization of detected X-ray photons into energy bins by the detectors of a single-source CT operating at constant kVp and mA settings (Philips Healthcare). The dual-source CT scanner is composed of two x-ray tubes and two corresponding detectors. The two acquisition systems are mounted on the rotating gantry with an angular offset of 90°/95° with regard to their kilovoltage and milliamperage settings. For dual-energy CT acquisition, the tube voltages are set at high energy (140 kVp) for tube A and low energy (80 kVp) for tube B. The rapid kilovoltage switching technique from GE Healthcare uses a single x-ray source. A generator electronically switches rapidly the tube energies from low energy (80 kVp) to high energy (140 kVp) and back again to acquire dual-energy images. Each exposure takes about 0.5 msec.
