计算机X线摄影适宜照射线量的探讨

Investigation of appropriate X ray exposure on a computed radiography system

目的 以噪声的频率特性评价计算机X线摄影系统 (computedradiography ,CR)的适宜曝光量。方法 在相同摄影条件下获得中速CaWO4屏 Fuji片组合照片和CR照片 ,以此为基础 ,改变mAs,获得不同照射剂量下的CR影像 ,经相同的后处理条件输出它们的CR照片。用显微密度计依次对这些照片进行扫描 ,每张获得 10万个密度值 ,作为离散随机信号来处理 ,用快速傅立叶变换计算威纳频谱 (Wienerspectrum ,WS) ,测试CR照片噪声的WS与中速CaWO4屏 Fuji片组合的照片噪声WS值相等的曝光量值。结果 (1)在照射剂量均为 5 6 1μGy、照片密度均为 1 0的情况下 ,在空间频率(ω) =0 1LP/mm处 ,CR系统的WS值为 2 3 1× 10 -6mm2 ,屏 片系统的为 2 0 1× 10 -6mm2 ;在ω =0 5LP/mm处 ,CR系统的WS值为 9 73× 10 -6mm2 ,屏 片系统的为 8 4 1× 10 -6mm2 。 (2 )当ω =0 1LP/mm时 ,照射剂量为 5 6 1μGy的CR照片的WS值分别是照射剂量为 1 4 0、2 80、8 76、13 6 7、2 1 11μGyCR片的 0 5 3、0 75、1 14、1 4 4、1 80倍 ;当空间频率为 0 5LP/mm时 ,照射剂量为 5 6 1μGy的CR片的WS值分别是照射剂量为 1 4 0、2 80、8 76、13 6 7、2 1 11μGyCR片的 0 4 0、0 5 8、1 2 6、1 4 7、2 16倍。 (3)在ω=0 5LP/mm时 ,与照?

Objective To evaluate the appropriate patient X ray exposure when comparing computed radiography (CR) system with film screen combination Methods On the basis of the same radiographic parameters and image density, an image plate was exposed compared with a kind of screen film system Overall Wiener spectrum (WS) sampling films were acquired by changing mAs WS values were calculated by scanning on micro densimeter for every film processed as discrete signal by using fast Fourier transform (FFT) The X ray exposure of CR image, which had the same WS values with film screen combination image, was obtained Results (1) With the same exposure dose, the WS value of CR film was 23 1×10 -6 mm 2 when the spatial frequency was 0 1 LP/mm, while the WS of CaWO 4 screen Fuji film was 20 1×10 -6 mm 2; When the spatial frequency was 0 5 LP/mm, CR film was 9 73×10 -6 mm 2, and screen Fuji film was 8 41×10 -6 mm 2 (2) When the spatial frequency was 0 1 LP/mm, the WS value of CR film exposed at 5 61μGy was 0 53 times of that at 1 40μGy′s, 0 75 times of 2 80μGy′s, 1 14 times of 8 76μGy ′s, 1 44 times of 13 67μGy′s, and 1 80 times of 21 11μGy′s; when the spatial frequency was 0 5 LP/mm, the WS value of CR film exposed at 5 61μGy was 0 40 times of that at 1 40μGy′s, 0 58 times of 2 80μGy′s, 1 26 times of 8 76μGy ′s, 1 47 times of 13 67μGy′s, and 2 16 times of 21 11μGy′s (3) When they had the same WS value, exposure of CR system should be around 7 70 μGy while screen film combination was 5 61 μGy when the special frequency was 0 5 LP/mm Conclusion For identical exposures, the overall Wiener spectrum of the CR was richer than that of the film screen combination With the exposure increased the WS values of CR decreased The exposure of CR must be higher than that of film screen combination when they had the same WS values