An accurate method for computer-generating tungsten anode x-ray spectra from 30 to 140 kV

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Abstract

A tungsten anode spectral model using interpolating polynomials (TASMIP) was used to compute x-ray spectra at 1 keV intervals over the range from 30 kV to 140 kV. The TASMIP is not semi-empirical and uses no physical assumptions regarding x-ray production, but rather interpolates measured constant potential x-ray spectra published by Fewell et al. [Handbook of Computed Tomography X-ray Spectra (U.S. Government Printing Office, Washington, D.C., 1981)]. X-ray output measurements (mR/mAs measured at 1 m) were made on a calibrated constant potential generator in our laboratory from 50 kV to 124 kV, and with 0-5 mm added aluminum filtration. The Fewell spectra were slightly modified (numerically hardened) and normalized based on the attenuation and output characteristics of a constant potential generator and metal-insert x-ray tube in our laboratory. Then, using the modified Fewell spectra of different kVs, the photon fluence Φ at each 1 keV energy bin (E) over energies from 10 keV to 140 keV was characterized using polynomial functions of the form Φ(E)=a0[E]+a1[E] kV+a2[E] kV2+··· +(n)[E] kV(n) · A total of 131 polynomial functions were used to calculate accurate x-ray spectra, each function requiring between two and four terms. The resulting TASMIP algorithm produced x-ray spectra that match both the quality and quantity characteristics of the x-ray system in our laboratory. For photon fluences above 10% of the peak fluence in the spectrum, the average percent difference (and standard deviation) between the modified Fewell spectra and the TASMIP photon fluence was - 1.43% (3.8%) for the 50 kV spectrum, -0.89% (1.37%) for the 70 kV spectrum, and for the 80, 90, 100, 110, 120, 130 and 140 kV spectra, the mean differences between spectra were all less than 0.20% and the standard deviations were less than ~ 1.1%. The model was also extended to include the effects of generator-induced kV ripple. Finally, the x-ray photon fluence in the units of photons/mm2 per mR was calculated as a function of HVL, kV, and ripple factor, for various (water-equivalent) patient thicknesses (0, 10, 20, and 30 cm). These values may be useful for computing the detective quantum efficiency, DQE(f), of x- ray detector systems. The TASMIP algorithm and ancillary data are made available on line at http://www.aip.org/epaps/epaps.html.

Original languageEnglish (US)
Pages (from-to)1661-1670
Number of pages10
JournalMedical Physics
Volume24
Issue number11
DOIs
StatePublished - Nov 1997

Keywords

  • Computer model
  • Computer simulation
  • Detective quantum efficiency
  • Spectra
  • X ray

ASJC Scopus subject areas

  • Biophysics

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