CT画像におけるノイズパワースペクトル算出方法の比較評価

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  • Calculation Methods for Noise Power Spectrum Measurement in Computed Tomography

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We have investigated noise power spectrum(NPS)measurement methods suitable for current helical scanning computed tomography(CT)including the multi detector-row CT. Three calculation methods applicable to current CT, including two method for general digital X-ray imaging system, were examined. For the NPS measurement of CT, a method using 2D-FFT combined with a radial frequency averaging and a subtraction technique for background detrending process was reported in previous published papers. We employed this method(radial frequency method)excepting the subtraction technique, because the subtraction technique could not perform successful detrending on the image by the helical reconstruction. In addition, a one-dimensional method using numerical slit scanning(slit method), and the IEC(International Electrotechnical Commission)method using 2D-FFT and a specified frequency bins(2D-method), that both are frequently used for computed radiography(CR)and flat-panel detector(FPD), were employed. We also investigated the appropriate order of the polynomial fitting used in the detrending. The results indicated that the slit method with the numerical slit of 30-pixel height was well comparable with the 2D-method in accuracy and reproducibility, and the radial frequency method had excellent reproducibility because the method needs the averaging of much data in all directions. But shortcoming of the radial frequency method was clarified by noise measurements in peripheral-region with un-isotropic noise property. Also, the linear detrending was enough for CT images with all methods. Therefore, respective methods combining the linear detrending should be used selectively according to the purpose of NPS analysis and the level of demanded accuracy.

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