Evaluation of the DQE should be made under well-defined X-ray beam condition in order to assure intercomparison among different facilities. For this purpose, IEC61267 requires the use of high purity (at least 99.9% or 3N) Al attenuation filter, while IEC62220-1-1 requires lower purity of 99.0% (or 2N) filter since high purity metals are prone to kinds of non-uniformities including unexpected NPS increase in lower spatial frequencies <0.32 mm<sup>-1</sup>. The purpose of this study was to explore a possibility to adopt high purity Al filter without sacrificing NPS degradation in the low frequency region. To this end, we evaluated several types of high purity Al filters with different processing methods: casting, forging and rolling. Since the beam quality of RQA5 requires the use of 21 mm thick Al filter, we compared the following 4 types of 5N purity Al filters with 2N5 purity Al of 21mm thick: <strong>A.</strong> casting (5N) with 21mm x 1 sheet, <strong>B.</strong> forging (5N) with 21mm x 1 sheet, <strong>C.</strong> rolling (5N) with 7mm x 3 sheets, <strong>D.</strong> rolling (5N) with 1mm x 21 sheets. The comparison was made in terms of the Normalized Noise Power Spectrum (NNPS) exposure product in order to eliminate the effect of exposure variability. As a result, thin rolling sheets (D) showed no meaningful difference with 2N5 Al, though casting and forging sheets showed an observable NPS increase throughout the whole frequency range, suggesting that high purity thin rolled sheets could be used as beam attenuating material without suffering from non-uniformity problem.
Evaluation of the effective focal spot size of X-ray tube has been made utilizing the slit or the pin-hole camera, but is not widely used in a daily practice due to the need of specialized tools. The author proposes a simplified method in which only a metal edge and a digital detector are used, together with a process of removing detector blur inherently associated with the adoption of such a detector. The evaluation was made through the OTF (Optical Transfer Function) measurements by using the edge response analysis. Through the whole study, the use of OTF instead of MTF (Modulation Transfer Function) was essential in order to stay within the linear systems theory framework, at cost of handling complex functions. Evaluation steps were as follows; 1. The inherent OTF of the detector (OTF<sub>det</sub>) was measured by acquiring an image of the edge being closely contacted to the detector. 2. The second OTF (OTF<sub>multi</sub>) was measured with the edge placed apart from the detector so as to implement 2 times geometrical magnification of the edge. OTF<sub>multi</sub> is the product of OTF<sub>det</sub> and the focal spot OTF (OTF<sub>focus</sub>). 3. OTF<sub>focus</sub> was obtained by calculating OTF<sub>multi</sub> / OTF<sub>det</sub>, thus removing the detector blur completely. 4. The LSF of the focal spot was obtained through the inverse Fourier transform of OTF<sub>focus</sub>. The resultant LSF<sub>focus</sub> was assured to be a real function due to the fact that original LSF<sub>det</sub> and LSF<sub>multi</sub> were both real functions. Preliminary results well matched those obtained by the pinhole camera.