Physics of Dual Energy CT (T. Johnson)

Provided energy

Different photon energies are required for Dual Energy CT. In reality, the photons produced by the X-ray tubes of a Dual Energy CT scanner have various energies, not only two distinct energy levels, as the term may suggest. The X-ray sources available today that can produce enough quanta for diagnostic imaging are tubes with rotating anodes. They have polychromatic spectra consisting of a continuous spectrum of the Bremsstrahlung, interrupted by characteristic lines of the tungsten material of the anode. If both tubes are run with the largest possible difference, i.e. at 140 and 80 kVp, the resulting mean photon energies are 76 and 56 keV, so the difference is smaller than one may expect (1). Second generation Dual Source CT systems have an additional 0.4 mm tin filter that eliminates the 140 kVp spectrum of low-energy quanta. This decreases overlap of the spectra and increases hardness of the spectrum to a mean photon energy of 92 keV (2).

Attenuated energy

Furthermore, the investigated material must have some spectral properties, i.e. differences in X-ray attenuation at different photon energies. Only then does differentiation from other material or quantification become possible. The X-ray attenuation at relevant photon energies is caused by the Compton Effect, coherent scatter, and photo effect. The first two show only small differences in different atoms. However, the photo effect shows a strong relationship with the atomic number, i.e. with the weight of the atomic nucleus of the material. Most of the atoms in the human body, i.e. hydrogen, carbon, nitrogen or oxygen, have a rather weak photo effect. Some ions such as calcium or magnesium have a somewhat stronger effect, while the photo effect of iodine is comparatively very strong. This difference can be observed in CT images obtained at lower tube potential, in which vascular enhancement from iodine contrast material is much stronger compared to the density at high voltages. This is the reason why angiographic exams are partially obtained at 80 kVp. This difference in spectral behavior can be used to detect and semi-quantify iodine in CT images.

Related articles: Dual Energy CT – an Introduction, Technical Implementation, Clinical Applications

References:
1.    Johnson TR, Krauss B, Sedlmair M, et al. Material differentiation by dual energy CT: initial experience. Eur Radiol 2007; 17:1510-1517.
2.    Petersilka M, Bruder H, Krauss B, Stierstorfer K, Flohr TG. Technical principles of dual source CT. Eur J Radiol 2008; 68:362-368.