Compression degradation metrics for analysis of consistency in microcalcification detection

The authors motivate, define and illustrate the behavior of four scalar metrics which measure the degree to which microcalcification (/spl mu/uCa++) detection is affected by lossy compression distortion. The JPEG method is used at various compression rates determined by a single parameter. The authors' metrics are specifically targeted to measure the degree of invariance of the processing to the presence of a compression-decompression step and not absolute processing success. The detection of the /spl mu/Ca++s is done with a two-step process of selective median filtering and wavelet domain hard thresholding with saturation. This latter step produces a binary image revealing small structures which are the desired objects in the image. The authors have also chosen to perform the enhancement before the compression step as suggested by their previous work to allow higher compression rates. In the system used to do the comparison, the two branches of processing are identical except for the compression/decompression step and their results are binary images. The inputs to the metric computation system are these two images which produce four scalar metrics which are various percentages of differences in pixel and region activity. Using seven image sections from mammograms, the authors illustrate the results at various compression ratios in the range 28:1 to 42:1 for each image and also averaged over the set of seven. The three pixel based metrics decrease monotonically as the JPEG compression quality parameter increases to reduce the compression ratio. The region based metric does not change monotonically due to the more complex nature of region analysis.