Breast region extraction of mammogram is a crucial step for further analysis and processing

and can improve diagnosis accuracy of galactophore diseases. An efficient algorithm of automatically extracting breast region is proposed. The discontinuity of the variety of areas of the isosurfaces is explored and used for computing the accurate segmentation threshold. Then the connected region containing the breast region is extracted by using a scan line based method

which is more efficient than seed-filling based methods. In order to obtain more pure breast region

the unexposed borders of the image connected to the breast region are also removed through a few refinement procedures. Experimental results show that the method has good performances both on the accuracy of breast region segmentation and executing speed.