Abstract: For any testing problem, the essential attention for selection a detector system is the basic spatial resolution and the normalized signaltonoise ratio of the digital detector system. The selection of the basic spatial resolution shall follow the sampling theorem. The selection process might be as follows. First of all, the wire IQI sensitivity should be given, and then the maximum spatial frequency of the image upon the wire IQI sensitivity should be determined. Finally, the correspondent sampling frequency with the maximum spatial frequency should be calculated. When radiographic standard was specified, the selection may depend on the specified unsharpness of the image. The normalized signaltonoise ratio value was selected on the requirement in the using radiographic standard.