Principles and Applications of Multi-energy CT Report of AAPM Task Group 291.

In x-ray computed tomography (CT), materials with different elemental compositions can have identical CT number values, depending on the mass density of each material and the energy of the detected x-ray beam. Differentiating and classifying different tissue types and contrast agents can thus be extremely challenging. In multi-energy CT, one or more additional attenuation measurements are obtained at a second, third or more energy. This allows the differentiation of at least two materials. Commercial dual-energy CT systems (only two energy measurements) are now available either using sequential acquisitions of low- and high-tube potential scans, fast tube-potential switching, beam filtration combined with spiral scanning, dual-source, or dual-layer detector approaches. The use of energy-resolving, photon-counting detectors is now being evaluated on research systems. Irrespective of the technological approach to data acquisition, all commercial CT multi-energy systems today provide dual-energy data. Material decomposition algorithms are then used to identify specific materials according to their effective atomic number and/or to quantitate mass density. These algorithms are applied to either projection or image data. Since 2006, a number of clinical applications have been developed for commercial release, including those that automatically 1) remove the signal from bony anatomy and/or calcified plaque; 2) create iodine concentration maps from contrast-enhanced CT data and/or quantify absolute iodine concentration; 3) create virtual non-enhanced images from contrast-enhanced scans; 4) identify perfused blood volume in lung parenchyma or the myocardium; and 5) characterize materials according to their elemental compositions, which can allow in vivo differentiation between uric-acid and non-uric-acid urinary stones or uric acid (gout) or non-uric-acid (calcium pyrophosphate) deposits in articulating joints and surrounding tissues. In this report, the underlying physical principles of multi-energy CT are reviewed and each of the current technical approaches described. In addition, current and evolving clinical applications are introduced. Finally, the impact of multi-energy CT technology on patient radiation dose is summarized.