A sequence of continuously acquired radiographic images are tested. The image sequence is cut from the continuous acquisition of radiographic images. The sequence contains 228 frames of images in total (Frame1025 Frame1252). The section of spiral pipe weld that the image sequence covers contains 6 porosity defects. The appearance of the 6 defects is listed in Table 1. The segmentation based on each single frame extracts the potential regions of defects. The miss detection rate Rm and the false alarm rate Rf are defined as in Eqs. (13) and (14). Fig. 4 shows how the miss detection rate Rm and the false alarm rate Rf change with the increasing of the segmentation threshold. Higher threshold results in less false alarms but more true defects are undetected. Lower threshold can ensure the detection of true defects but the number of false alarms increases significantly. The contradiction cannot be solved because of the similarity between the true defects and the false ones in both gray level distribution and