CT is one of the most important of the noninvasive imaging modalities, providing 3D representations of the x-ray attenuation coefficient with submillimeter spatial resolution. The main limitations of CT relate to soft tissue contrast resolution and quantitative material differentiation. Administration of iodinated contrast material can improve soft tissue contrast, as well as enable vessels and parenchymatous organs to be visualized. Quantitative material differentiation requires more novel techniques.
X-ray attenuation for materials with a high atomic number (Z), such as iodine(Drug information on iodine) or calcium, depends strongly on beam energy during CT scanning. Only minor attenuation changes relative to water are observed in soft tissues with similar attenuation coefficients when the beam energy is altered. This is mainly due to the photoelectric effect, which depends strongly on energy and atomic number. The Compton effect is almost independent of photon energy in the energy range used for diagnostic CT. Using different x-ray spectra thereby allows high-Z materials to be differentiated, leading to new applications for CT.
Dual-energy methods were suggested as early as 1976 for enhancing the soft tissue contrast in CT.1,2 The idea was to determine the coefficients of photoelectric absorption and Compton scatter attenuation independently using two CT measurements with different tube voltages. An alternative, material decomposition, technique was also proposed. That technique used alternate sets of base functions, for example, the attenuation functions of water and bone. The latter was finally implemented into a commercially available CT scanner (Siemens Somatom DR, 1983 to 1987).3
Technical limitations of CT scanners available at this time kept dual-energy scanning from becoming a routine clinical tool. The main problems were unstable CT numbers and long scan times. In addition, x-ray tubes could not provide currents at sufficiently low voltages to achieve an output of photons adequate for reliable dual-energy imaging. This required similar levels of image noise in both image data sets, something that was not achievable at that time.4
These limitations were overcome in 2006 with the introduction of dualsource CT (DSCT) by Siemens Medical Solutions. DSCT technology solves problems of data registration and patient movement that were previously associated with dual-energy imaging. With DSCT, both low- and high-voltage data sets are obtained simultaneously. Synchronous data acquisition is particularly important in contrast-enhanced scans, where temporal changes in contrast enhancement happen extremely quickly. Any temporal offset between the two acquisitions would make dualenergy postprocessing impossible.
Early approaches to dual-energy imaging focused on bone density measurement, 5 detection of intrathoracic calcifications,6 differentiation of urinary calculi,7 and assessment of hepatic iron load.8 Brain tumor differentiation was also evaluated, but with only limited success.9 None of these techniques reached clinical relevance.
DSCT has paved the way for a broad variety of dual-energy clinical applications. Some simple but powerful tools can enhance visualization of baseline images. The dual-energy capability can also be used for two different types of clinical application: material differentiation and semiquantitative depiction of iodine distribution.
