[Effects of beam weakening and beam hardening on the measured CT values]

The present article reports on the effects of weakening and hardening of radiation on CT values in the environment of highly absorptive materials, using phantoms for carrying out these measurements. First of all, it was found with the help of geometrically simple phantoms (discs, circular rings), that the measured CT value of each volume element depends on the density distribution in the environment and that the CT values of homogeneous object layer are not uniform, but clearly dependent on the location at which measurement is effected. The standard deviation of the CT values of a homogeneous part of an object layer was also found to depend on the environment. Finally, using anatomically relevant phantoms, it was found that density value changes from max. +20 HU to -65 HU occur in the region of the cranium, whereas in the environment of the lumbar vertebra the density varies between max. +20 HU and -10 HU; in the pleural region, the maximum density variations are +/- 10 HU.