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The characteristics of an x-ray spectrum can greatly influence imaging andrelated tasks. In practice, due to the pile-up effect of the detector, it'sdifficult to directly measure the spectrum of a CT scanner using an energyresolved detector. An alternative solution is to estimate the spectrum usingtransmission measurements with a step phantom or other CT phantom. In thiswork, we present a new spectrum estimation method based on indirecttransmission measurement and model spectra mixture approach. The estimatedx-ray spectrum was expressed as weighted summation of a set of model spectra,which can significantly reduce the degrees of freedom (DOF) of the spectrumestimation problem. Next, an estimated projection can be calculated with theassumed spectrum. By iteratively updating the unknown weights, we minimized thedifference between the estimated projection data and the raw projection data.The final spectrum was calculated with these calibrated weights and the modelspectra. Both simulation and experimental data were used to evaluate theproposed method. In the simulation study, the estimated spectra were comparedto the raw spectra which were used to generate the raw projection data. For theexperimental study, the ground truth measurement of the raw x-ray spectrum wasnot available. Therefore, the estimated spectrum was compared against spectragenerated using the SpekCalc software with tube configurations provided by thescanner manufacturer. The results show the proposed method has potential toaccurately estimate x-ray spectra using the raw projection data. The differencebetween the mean energy of the raw spectra and the mean energy of the estimatedspectra were smaller than 0.5 keV for both simulation and experimental data.Further tests show the method was robust with respect to the model spectragenerator.

An indirect transmission measurement-based spectrum estimation method for computed tomography