5. CT X-Ray-Spectrum Characterization

The vast majority of x-ray tubes used in diagnostic radiology make use of tungsten anodes, and CT is no exception. Tungsten (alloyed with 5 % to 10 % rhenium) has excellent heat conductivity, a high melting point, and its relatively high atomic number (Z 1⁄4 74) makes for efficient bremsstrahlung x-ray production. Although bremsstrahlung represents the majority of the photons produced by CT x-ray tubes, the characteristic-radiation production from tungsten produces two peaks, at 59 keV and 68 keV (each is a doublet) when tube potential is above tungsten’s K edge of 70 keV. The x-ray spectra used in CT imaging are some of the hardest used in medical radiological x-ray imaging, generally because of the typically higher tube potentials used and the greater amount of added filtration for the central ray. Toward the periphery of the fan beam, the beam-shaping filter provides even more x-ray-beam filtration, and this hardens the x-ray spectrum further. A harder beam is necessary in CT to reduce beam-hardening artifacts, which arise from differing magnitudes of spectrum hardening for different projections around a patient. Adding metallic (and sometimes plastic) filtration to the x-ray beam pre-hardens the x-ray beam and thus reduces beam-hardening artifacts, as discussed in Section 2.4.5. The higher filtration levels also lead to relatively lower absorbed-dose levels in the patient. Although spectroscopy methods have been used to accurately measure x-ray spectra, the experimental setup for x-ray spectroscopy is complicated, the equipment is expensive, and the procedure is time-consuming and requires significant expertise to produce accurate results. Consequently, x-ray spectra have been characterized using the concept of the half-value layer (HVL) for nearly a century. The HVL of an x-ray beam is measured using an air-kerma meter, or other dosimeters calibrated to produce air-equivalent readings. The measurement of the HVL generally requires that the measurement instrument remain fixed in location, as a number (including zero) of different thicknesses of an absorber are placed between the dosimeter and the stationary x-ray source. In the context of CT, the traditional method for HVL measurement requires that the service mode of the scanner be used in order to stop gantry rotation. Aluminum is the predominant material for characterizing the HVL in diagnostic-radiology applications, including CT. For an aluminum thickness t, the measurement is approximated by the polyenergetic form of the Lambert–Beers law: