Extraction of structural features in radiographic images is considered in the context of flaw detection with application to industrial and medical diagnostics. The known approache, like the histogram-based binarization yield poor detection results for such images, which contain small and low-contrast objects of interest on noisy background. In the presented model-based method, the detection of objects of interest is considered as a consecutive and hierarchical extraction of structural features (primitive patterns) which compose these objects in the form of aggregation of primitive patterns. The concept of relevance function is introduced in order to perform a quick location and identification of primitive patterns by using the binarization of regions of attention. The proposed feature extraction method has been tested on radiographic images in application to defect detection of weld joints and extraction of blood vessels in angiography.