Accuracy validation of incident photon fluence on DQE for various measurement conditions and X-ray units

Detective quantum efficiency (DQE) is widely used as a comprehensive metric for X-ray image evaluation in digital X-ray units. The incident photon fluence per air kerma (SNRⁱⁿ₂) is necessary for calculating the DQE. The International Electrotechnical Commission (IEC) reports the SNR ⁱⁿ₂ under conditions of standard radiation quality, but this SNRⁱⁿ₂ might not be accurate as calculated from the X-ray spectra emitted by an actual X-ray tube. In this study, we evaluated the error range of the SNRⁱⁿ₂ presented by the IEC62220-1 report. We measured the X-ray spectra emitted by an X-ray tube under conditions of standard radiation quality of RQA5. The spectral photon fluence at each energy bin was multiplied by the photon energy and the mass energy absorption coefficient of air; then the air kerma spectrum was derived. The air kerma spectrum was integrated over the whole photon energy range to yield the total air kerma. The total photon number was then divided by the total air kerma. This value is the SNRⁱⁿ₂. These calculations were performed for various measurement parameters and X-ray units. The percent difference between the calculated value and the standard value of RQA5 was up to 2.9 %. The error range was not negligibly small. Therefore, it is better to use the new SNRⁱⁿ₂ of 30694 (1/(mm² μGy)) than the current SNRⁱⁿ₂ of 30174 (1/(mm² μGy)).