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5 June 2003 Differences between mono- and poly-energetic spectra in modeling DQE(f)
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Most theoretical calculations of DQE are done assuming a mono-energetic beam, whereas, clinically, the systems are used with poly-energetic spectra. Using a cascaded stochastic model of a mammographic imaging system, we compared results using mono-energetic beams to mammographic poly-energetic beams. The effects of the depth of x-ray interaction in the phosphor were explicitly included in the model. We calculated DQE, MTF and NPS as a function of spatial frequency and also calculated the Swank noise factor (AS) and quantum detection efficiency (AQ). For mono-energetic beams between 15 and 40 keV, the AS changed from 0.80 to 0.84, and the AQ decreased from 0.90 to 0.16. For the spatial-frequency dependent measures, at 10 cycles/mm, the MTF increased from 0.09 to 0.16, the ratio of MTF squared to the relative NPS increased from 0.13 to 0.29, and DQE decreased from 0.19 to 0.06. These values were compared to poly-energetic x-ray calculations assuming molybdenum and tungsten anodes in the range of 24 to 40 kVp. Good agreement to within 5% in DQE at all spatial frequencies was found. However, at high kVp, the mono-energetic approximation overestimated the AS and underestimated the AQ by as much as 10%.
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Robert M. Nishikawa "Differences between mono- and poly-energetic spectra in modeling DQE(f)", Proc. SPIE 5030, Medical Imaging 2003: Physics of Medical Imaging, (5 June 2003);

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