Clinical determination of mammographic breast density (MD) is subjective and can also suffer from intra- and inter-reader variability. MR-based quantitative assessment of breast density has a few unique advantages over mammography. However, accurate whole breast segmentation is crucial. Manual whole breast segmentation is burdensome and current automated methods also suffer from weaknesses. In this work, a new whole breast segmentation strategy based on image registration is proposed. The test-retest reliability of breast density derived using different segmentation methods was quantitatively assessed. The results demonstrated this new segmentation method yields more reproducible values compared to existing methods.