医療用X線CTの活用による鉱物試料の密度と有効原子番号を推定する手法の開発

抄録

Since the development in 1972 of X-ray computed tomography (CT) as a medical imaging technique, many studies have applied it in the geoscience field. X-ray CT is a non-destructive technique that enables materials to be observed and analyzed. CT number, which is defined as the ratio of the linear attenuation coefficient μ of the sample to the linear attenuation coefficient μ_w of water, depends on material bulk density, the effective atomic number and the tube voltage of the X-ray CT scanner. A CT image produced by a medical X-ray CT scanner, which applies a polychromatic X-ray beam, includes the artifact of beam hardening (BH). This artifact means that CT numbers for the edge of an image of a sample are higher than for its center, and therefore this effect of BH must be considered to perform accurate quantitative analyses of samples using CT numbers.

 For this study, we investigated a method to estimate the density and effective atomic number using CT values of CT images taken by medical CT for six kinds of mineral samples with known density and average effective atomic number. To estimate the density and effective atomic number, it is effective to find the mode of CT value at the center of the 2D CT image and use the dual energy method except for the sample edge where the BH effect is signifiant. Moreover, since there is a positive correlation between the density and the effective atomic number, the density and the effective atomic number can be directly estimated from the mode of the CT values of the CT images taken with one tube voltage.