There are several ways to acquire CT data with spectral information. Today's medical CT systems are typically equipped with solid state detectors. A scintillation crystal absorbs the x rays and converts them into visible light, which is then detected by an attached Si photodiode. These detectors integrate the x-ray flux over a certain period of time (the reading time), with a weighting factor proportional to the energy of the absorbed x-ray quanta. As a consequence, the detectors do not provide energy resolution, and the use of different x-ray spectra is needed to acquire spectral CT data. In CT, different x-ray spectra can be realized by using different kV settings of the x-ray tube. In a standard CT system, the kV setting of the x-ray tube can be changed either between different CT scans (slow kV switching), or more rapidly, ideally, between the different projections of a CT scan (fast kV switching). An alternative is the use of DSCT systems with two x-ray tubes and two corresponding detectors offset by about 90 deg. These systems have the potential to acquire dual-energy data by operating both x-ray tubes at different kV settings.
Energy-resolving detectors enable the acquisition of spectral CT data with a single polychromatic x-ray spectrum. Pertinent examples are dual-layer detectors consisting of two conventional scintillation detectors on top of each other, and direct converting photon-counting detectors. So far, only prototype CT systems relying on both detector technologies have been realized. However, in particular, photon-counting detectors are a promising technology for future CT systems. In this chapter we will discuss the different technologies for acquiring spectrally resolved CT data.