Dual-energy imaging exploits the energy dependence of the attenuation difference between materials containing elements with a high atomic number, such as calcium or iodine, and those with a low atomic number, such as water or soft tissues (Fig. 1).
Dual-energy CT imaging provides images in which one material is eliminated. Elimination of soft tissue creates so-called iodine maps; elimination of calcium removes bones and calcified plaques; and elimination of iodine creates a virtual unenhanced image. Dual-energy CT can characterise renal calculi and distinguish between gout and other crystal arthropathies.
A prerequisite for dual-energy CT is the nearsimultaneous acquisition of high- and low-energy CT data sets. At present, each CT vendor has chosen a different technique to do so. High- and low-energy CT data sets can be acquired by rapidly alternating the potential of a single tube, by switching the tube potential during successive rotations or by using two separate X-ray tubes (dual source) that use different kilovoltage settings. Finally, there is also a technique that splits the X-ray detector so that high- and low-energy photons can be separated in the detector system.
The principle of the technique is not new, having first been identified in the 1970s, but its clinical applications were beyond the capabilities of the technology at that time. Typically, images are acquired at 135–140 kVp and 100 or 80 kVp. By blending these images together, normal-looking CT images of good signal-to-noise ratio can be created. True dual-energy images eliminate one preselected material but suffer from a much lower signal to-noise ratio than blended images. The technique to do so is called material decomposition and separates the attenuation properties of the examined object into virtual contributions from two base materials, e.g. water and iodine or water and calcium.12 Material decomposition simultaneously eliminates beam hardening effects, which can be used to synthesise virtual monoenergetic images in which the kiloelectron volt value can be chosen arbitrarily. This can be used, for example, to strongly increase the contrast between vessels and surrounding tissues in CT angiography. Because of the very unfavourable signal-to-noise ratio, all vendors apply various degrees of proprietary post-processing to reduce image noise.
FIGURE 1 ■ (A) The Hounsfield units of different structures vary with the energy of the X-ray photons and, more importantly, so does the rate of change. Dual-energy imaging exploits this to identify individual chemical components within a structure. Imaging of a thoracic aortic dissection with (B) conventional unenhanced and (C) arterial phase imaging. A 'virtual unenhanced' image (D), created by subtracting the iodine from the arterial phase study, enhances the conspicuity of the dissection flap and removes the need for the unenhanced phase.
责任编辑：admin
上一篇：没有了
下一篇：医学文章阅读——Planning and Patient Preparation
微信公众号搜索“译员”关注我们，每天为您推送翻译理论和技巧，外语学习及翻译招聘信息。