To clarify the factors influencing NPS_digital(u), we conducted verification using Monte Carlo simulation and experimental measurement.

 The simulation revealed the measured NPS_digital(u) was in good agreement with NPS_{quantum mottle}(u), but the NPS accounting for the unsharpness due to the sampling aperture showed different results. These results were verified by experimental measurement, which also supported the simulation result and clarify that NPS_digital(u) is not affected by the sampling aperture.

 Although a detailed simulation was carried in this study, all types of detectors were not verified by the experimental measurement. Therefore, it should be noted that the sampling aperture and aliasing effects may occur in the NPS, depending on the signal transfer process of the detector. In other words, it is important to clarify the characteristics of the detector used in the system. In addition, the effect on clinical images has not yet been studied, and is a subject for further study.

 We believe that the results of this study will improve the measurement accuracy of the NPS for evaluating the noise characteristics of digital radiography and provide further understanding for accurate DQE derivation. It is also expected to lead to a more detailed understanding of noise factors in X-ray detectors.