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 blooddiluted contrast agent has the same attenuation as these materials.
Dual-energy CT techniques have the potential to identify different chemical elements by sampling the attenuation of material mixtures at two different x-ray energy levels. This technique applies even when the two materials of interest have similar absorption properties.
The use of dual-energy x-ray techniques in healthcare has traditionally been limited to conventional projection techniques. Dual-energy x-ray absorptiometry bone densitometry is used widely to quantify bone mineralization for osteoporosis screening and therapy monitoring. Dual-energy chest radiography improves the detection of pulmonary masses by specifically highlighting bone and calcified structures or soft tissue.
Dual-energy applications for CT were proposed as early as 1976, but they did not enter widespread clinical practice owing to technical limitations.1,2 The initial approach was to acquire two separate scans, which was shown to be feasible for the quantification of fat3 and iron4 content in the liver. Data at the two energy levels could not be acquired simultaneously, though, and interscan motion had a severe impact on results. The inevitable change in contrast concentration between the scans also hampered dual-energy detection of iodine.
Some CT scanners overcame this limitation by rapidly switching the tube voltage during rotation. These systems were used for bone densitometry.5 They could not, however, adapt the tube current fast enough to achieve equivalent doses at both energy levels. The result was considerably higher noise levels in the lower energy data, resulting in artifacts and loss of resolution.
Another approach to dual-energy CT is to use just one x-ray source and a multilayered detector, each layer registering a specific band of energy for the attenuated x-rays. For optimal performance, the spectrum of energy distribution has to be extremely wide, or even have two distinctive peaks. In practice, this would be hard to achieve with one x-ray source. The latest generation of dual-energy CT systems that work according to this principle are being built with energy-sensitive photoncounting detectors that can be used with a single kVp x-ray source.6
