Masking occurs when surrounding breast tissue obscures a cancer. The cancer is thus indiscernible mammographically, limiting the sensitivity of the screening test. While masking is not a substantial problem in patients with non-dense breasts, mammographic sensitivity is diminished by up to 10-20% in dense breasts. This is a major contributor to the drive for an additional screening modality to be used in conjunction with mammography. MRI has been demonstrated to be a useful screening tool in patients at very high risk for breast cancer. Ultrasound and tomosynthesis are also being studied as potential supplemental screening modalities, with current results suggesting some increase in cancer detection using both modalities, but increased false positives for ultrasound compared to decreased false positives for tomosynthesis.