数字射线成像中归一化信噪比指标的分析讨论

Analysis and Discussion on Normalized Signal-to-Noise Ratio in Digital Radiography

就数字射线成像技术中信噪比的内涵、信噪比归一化的意义、与补偿原则的相互关联等问题进行了分析与探讨。胶片照相技术与数字射线照相技术中影像对比度的形成均为影像探测器对主因对比度的乘积变换,只是因子不同:在采用平板探测器的数字射线照相技术中因子为信噪比,而在胶片照相技术中则与梯噪比相关。依据信噪比参数可对工业射线胶片系统进行分类处理,同时也用于实现计算机射线照相系统的分类,而归一化信噪比指标的提出,则将数字成像技术与胶片照相技术在图像信噪比测量方面进行了等价处理。通过提高信噪比来补偿固有不清晰度的方法似与归一化信噪比的要求存在某种关联,但应注意区分探测器的基本空间分辨率指标与图像不清晰度指标。而在实际的射线检测中,应关注探测器的特性及缺陷检出需求,确定适用的检测工艺方法。

Some problems were analyzed and discussed, which include the meaning of SNR in DR and the purpose of normalization of signal-to-noise ratio （SNR） and the relationship between SNR and compensation principles （CP）. Contrast in the testing result of film radiography and DR is both the product of a certain factor and the main subject contrast, and the factor in DR using flat panel detector is SNR, while that in film radiography is related with gradient-to-noise ratio. Film system in radiography testing can be classified according to the SNR parameters （gradient-to-noise ratio）, and computed radiography （CR） system can be classified by SNR parameter likewise. With the help of normalized SNR, the image quality in DR and film radiography can be measured and evaluated equivalently. DR technique suffers from inherent detector unsharpness at present, but the unsharpness can be compensated by increased SNR, which is called CP II. This compensation principle seems to have relation with requirement of normalized SNR, but we should know that basic spatial resolution of a digital detector and the measured unsharpness in the testing is different. In the practical testing, we should know the properties of the detector used and the requirement of detection of certain flaws thus ensuring that applicable process is adopted consequently.