Dual Energy applications based on Dual Source computed tomography (CT) technology have made their way into daily clinical routine and find large press coverage, such as in Diagnostic Imaging’s cover story, “CT moves ahead with multiple energy sources.” This is because of Dual Energy’s exceptional ability to take over where standard CT reaches its limits.

Hounsfield units, displayed as gray levels on standard CT images, represent the X-ray attenuation of the material mix in each image voxel. It is not possible, however, to differentiate materials of identical X-ray density using this attenuation information alone. A mixture of blood and contrast in a vessel, for example, cannot be distinguished from adjacent bone or from calcifications in the vessel wall if the blood-diluted contrast agent has the same attenuation as these materials.

Dual Energy CT scanners are able to overcome this limitation. Dual Energy applications for CT were proposed as early as 1976, but did, owing to technical limitations, not enter everyday clinical business until recently. Dual Energy CT is now available for clinical use on scanners that integrate two complete detector-tube assemblies into one rotating gantry, such as Siemens SOMATOM^® Definition Dual Source CT. The Dual Source setup allows simultaneous acquisition of data at two tube currents without the limitations imposed by the energy spectrum of a single-tube X-ray or by the necessity of rapid tube current adaption. 

 

Matter penetrated by X-rays will absorb and scatter a certain fraction of the passing photons. This absorbed fraction will be lower for high energy X-rays and increase with decreasing photon energy. The curve that describes this change of attenuation over the X-ray spectrum is characteristic for each element. Imaging of a given material mixture using Dual Source CT is performed with two X-ray beams of different effective photon energies. The combined absorption spectrum of the mixture is sampled at two points of the energy spectrum. These two data points can be used to determine the concentration of three materials in each voxel. Three series of images are typically reconstructed from a Dual Energy scan, one from each tube-detector pair, and a weighed combination of the two sets.