Estimation of a photon energy spectrum for a computed tomography scanner.

Estimated photon energy spectra are derived from transmission measurements using aluminium, copper, and sodium iodide absorbers. Two spectral models are proposed. One is based on a previously published model that analyzes the electron's penetration into the anode, and the production and attenuation of bremsstrahlung photons. The second model does not include details regarding the underlying physics, but treats the spectrum as a sum of delta functions. A nonlinear regularization method is used to overcome ill conditioning in the second model. Both models fit the transmission data to an accuracy of 0.30%, which is consistent with the experimental error. A quantitative comparison of the models is made by calculating the average and variance (over the derived energy spectra) of several relevant mass attenuation coefficients. The maximum variation in the average and variance was 1.5% and 3.2%, respectively, indicating that the spectra exhibit similar attenuation and beam hardening properties. The spectra were tested with a simulation that predicts scanner CT numbers for phantom measurements consisting of dilutions of sodium iodide in a water equivalent background. The agreement between simulation and experiment ranged from 1.5% at 220 HU to 4.4% at 1700 HU.

[1]  L. Silberstein.  XXXIV. Spectral composition of an X-ray radiation determined from its filtration curve , 1933 .

[2]  B W Soole,et al.  The effect of target absorption on the attenuation characteristics of bremsstrahlung generated at constant medium potentials , 1972 .

[3]  B W Soole,et al.  A method of X-ray attenuation analysis for approximating the intensity distribution at its point of origin of bremsstrahlung excited in a thick target by incident electrons of constant medium energy. , 1976, Physics in medicine and biology.

[4]  G. Herman,et al.  Convolution reconstruction techniques for divergent beams. , 1976, Computers in biology and medicine.

[5]  P. Joseph,et al.  A Method for Correcting Bone Induced Artifacts in Computed Tomography Scanners , 1978, Journal of computer assisted tomography.

[6]  R Birch,et al.  Computation of bremsstrahlung X-ray spectra and comparison with spectra measured with a Ge(Li) detector. , 1979, Physics in medicine and biology.

[7]  L. C. Baird.  X-ray spectra vs attenuation data: a theoretical analysis. , 1981, Medical physics.

[8]  J. H. Hubbell,et al.  Photon mass attenuation and energy-absorption coefficients , 1982 .

[9]  P. Huang,et al.  Reconstruction of diagnostic x-ray spectra by numerical analysis of transmission data. , 1986, Medical physics.

[10]  J M Boone,et al.  The three parameter equivalent spectra as an index of beam quality. , 1988, Medical physics.

[11]  G. Barnes,et al.  Semiempirical model for generating tungsten target x-ray spectra. , 1991, Medical physics.

[12]  L J Schreiner,et al.  A comparison of semiempirical models for generating tungsten target x-ray spectra. , 1992, Medical physics.

[13]  G. Barnes,et al.  Normalized average glandular dose in molybdenum target-rhodium filter and rhodium target-rhodium filter mammography. , 1994, Radiology.

[14]  P M Joseph,et al.  A comparison of beam-hardening artifacts in x-ray computerized tomography with gadolinium and iodine contrast agents. , 1995, Medical physics.