X-ray spectra estimation using attenuation measurements from 25 kVp to 18 MV

Attenuation measurements for primary x-ray spectra from 25 kVp to 18 MV wer e made using aluminum filters for all energies except for orthovoltage wher e copper filters were used. An iterative perturbation method, which utilize d these measurements, was employed to derive the apparent x-ray spectrum. A n initial spectrum or pre-spectrum was used to start the process. Each ener gy value of the pre-spectrum was perturbed positively and negatively, and a n attenuation curve was calculated using the perturbed values. The value of x-rays in the given energy bin was chosen to minimize the difference betwe en the measured and calculated transmission curves. The goal was to derive the minimum difference between the measured transmission curve and the calc ulated transmission curve using the derived x-ray spectrum. The method was found to yield useful information concerning the lower photon energy and th e actual operating potential versus the nominal potential. Mammographic, di agnostic, orthovoltage, and megavoltage x-ray spectra up to 18 MV nominal w ere derived using this method. The method was Validated using attenuation c urves from published literature. The method was also validated using attenu ation curves calculated from published spectra. The attenuation curves were then used to derive the x-ray spectra. (C) 1999 American Association of Ph ysicists in Medicine. [S0094-2405(99)00107-8].