摘  要：针对CT值均匀性传统定义只考虑图像的中心和外围部分而不能全面反映整个视野（FOV）内CT值变化的缺点，提出一种改进的评价方法，以适应CT新技术质量保证检测的需要。采用直径为20 cm的圆柱形水模，在GE Discovery CT750 HD扫描仪上选择常用头部协议进行轴向扫描。在水模图像的3，6，9，12点钟方向上距离中心0，4.0，8.0 cm处取面积为1 cm2的圆形感兴趣区，分别计算CT值均值和标准差。定义其中最大、最小均值之差为CT值均匀性指数，最大标准差为噪声指数；单能图像序列中CT值均匀性指数最大值定义为该序列的CT值均匀性指数，噪声指数最大值为该序列噪声指数。结果表明：从FOV中心向外CT值并非单调变化，CT值均匀性评价新方法更加全面、客观地评价整个FOV的图像质量，并能反映双能CT的不同探测器阵列对应图像之间以及不同能级图像序列之间的质量差异。
关键词：计算机体层摄影；双能；CT值均匀性；质量保证
文献标志码：A       文章编号：1674-5124（2016）01-0007-05
A modified evaluation method for CT value uniformity
YANG Kecheng
（School of Biomedical Engineering，Third Military Medical University，Chongqing 400038，China）
Abstract: In current quality assurance （QA） testing guidelines and standards， CT value uniformity （CTVU） is defined only based on image centers and peripheries; instead， the variation in CT value can not be fully reflected within the whole field of view. To solve this problem， a modified method was proposed to meet the QA testing needs of new CT scanners. A 20 cm-diameter water equivalent plastic cylindrical phantom filled with pure water was scanned in a step-and-shot way with a routine dual-energy head protocol （GSI-26） on a GE Discovery CT750 HD scanner. Images were analyzed with a customer designed MATLAB program. Nine circular ROIs （1 cm2） were selected at the spots that were 0 cm， 4.0 cm and 8.0 cm away from the centroid at 3， 6， 9， 12 o’clock positions to calculate the CT mean value and standard deviations. The differences between the maximum and minimum mean value were defined as CTVU indexes; the maximal SD was defined as noise figure; the maximal CTVU index and noise index in virtual monochromatic image sequence were defined as CTVU index and noise index respectively. Experimental results show that the modified evaluation method reflects not only the CT value variation within the whole field of view in a comprehensive and objective manner， but also the quality differences between different images scanned by different detector slices and virtual monochromatic image sequences of dual-energy CT.
Keywords: computed tomography（CT）； dual energy； CT value uniformity； quality assurance