<qa>  How many women are scheduled to undergo routine mammography screening?1. After appropriate training and certification of technologists and Mammography Quality Standards Act MQSA qualifications of the participating radiologists, the investigators will recruit sequentially approximately 600 women known to have a defined region of highly dense tissue or with known heterogeneously dense andor primarily dense breast tissue from a prior mammogram who are scheduled to undergo routine mammography screening that includes a DBT examination. 