Fig. 5.

X-ray spectra estimated for the numerical iodine concentrate phantom data. Model spectra generated using both (a) SpekCalc and (b) Geant4 were employed in the estimation procedure. The initial guesses for the sequential optimization problem correspond to the hardest and softest model spectra for the SpekCalc and Geant4 model spectra estimation, respectively. As the Geant4 model spectra are relatively noisy, spectrum estimated using these spectra is also noisy.