本学会の大会が150回を迎えました. 50年後に予想される300回大会の折り返し点, 半世紀先のその日にも「医用画像情報学会」は輝きを保っているすばらしい学会と思います. 東京オリンピックの年, 学会を立ち上げて第1回を開催された先達をはじめ, 今日まで44年の長期に亘り大会開催に学会運営にと尽力された役員の方々, 何より研究活動を通して学会を支え, 大会を盛り上げてこられた会員各位にお祝いとお礼を申し上げます. ここでは今後のご参考になろうかと, 古い時代に運営を手伝った立場から歴史を振り返りますが, 記憶違いや偏見もあろうと思います. お許しください.

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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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