Test Technologies Contributing to Electrification - 6. X-ray CT Nondestructive Measurement Technology Supporting Vehicle Body Weight Reduction TechnologyFig. 5 X-ray CT image (cross section) of the side sill of an iron vehicle bodyFig. 6 Metal artifacts at the cylindrical section of iron bolts3.3 Noise reduction effect using metal filters3.4 Discussion and hypothesis formulation on noise 63Table 1 Noise reduction effect by metal filter materials Fig. 7 Mass attenuation coefficientreductionverification of the noise reduction effect. This process relies on the expertise and intuition of the operator, and therefore, it requires a long time to determine the imaging conditions each time the measurement target is changed. Metal artifact noise is more likely to be generated in X-ray CT imaging of multimaterial parts made of steel or resin-based materials such as CFRP. The challenge is establishing an engineering technique that can efficiently determine the specifications of the metal filter for obtaining accurate information on the internal structure.(2)An example of the noise-reduction effect achieved by applying a metal filter is illustrated in Fig. 5, which depicts that the metal artifact noise from iron is reduced.Fig. 5 illustrates that metal artifact noise is generated in a complex manner in response to the part profile. The noise reduction effect of the metal filter for a simple iron bolt, as indicated in Fig. 6, is evaluated to reduce the noise effect caused by the part profile.One-mm-thick copper and aluminum plates were prepared as metal filters. A general filtered back projection (FBP) algorithm was used for reconstructing the CT images, and the brightness was corrected to visually emphasize the metal artifact noise, as presented in Fig. 6.The metal artifact noise is generated as white line-shaped false images arising from tangent lines between the two bolts in the CT images of the bolts (Fig. 6). The noise reduction effect increases in the order of copper filter > aluminum filter > no filter.(4)The relative artifact index (AIr)(3), which is used in the medical field, was selected as the index for the quantitative evaluation of the noise reduction effect. The closer AIr is to 1, the better the noise reduction. The results of the calculation of AIr(*) in the area near the bolts presented in Fig. 6 are listed in Table 1. The AIr is 2.3, 2.0, and 1.3 for no filter, aluminum filter, and copper filter, respectively. These results indicate that the noise reduction effect depends on the metal filter material; the noise reduction effect of the copper filter is greater than that of the aluminum filter.Fig. 5 illustrates that a noise reduction effect was observed when using a metal filter, and this suggests that hardening was weakened. The energy distribution on the low-energy side of the incident X-rays can be changed using a metal filter, which is assumed to weaken the hardening.The principle that absorption increases significantly at the energy where transitions occur between electron orbitals, called absorption edges (unique to a material), is applied to the X-ray absorption fine structure (XAFS) for analyzing the absorption spectrum obtained by irradiating X-rays with a material. The K-shell electron orbitals, where transitions occur, are called K-absorption edges. These were studied to evaluate the effect of metal filters.The attenuation coefficient of X-rays, which is a physical quantity affecting absorption, is theoretically estimated to determine the change in X-ray absorption from high to low energies; the results are presented in Fig. 7.
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