In 2000, more than 1 million women were diagnosed with breast cancer worldwide (Ferlay et al. 2001). Early detection decreases mortality (Tabar et al. 2003), so many countries routinely screen for breast cancer. X-ray mammography is the screening method of choice (Fletcher and Elmore 2003) but its effectiveness may depend on issues such as the age of the woman, family history of cancer, body mass index, the use of hormone replacement therapy, the use of computer aided detection, and the availability of additional clinical or imaging information (Blarney et al. 2000, Warren 2001, Houssami et al. 2004, Banks et al. 2004). It requires ionizing radiation and its benefits for younger women are unclear (Lucassen et al. 2004). Other imaging modalities such as MRI and ultrasound may be useful in c