Dose specified in protocol was not adequate to achieve optimal resolution.

The purpose of this research is to evaluate whether repeating a screening Molecular Breast Imaging (MBI) study two years after an initial screening MBI study further improves breast cancer detection in women with dense breast tissue.

Inclusion Criteria:

- Past prior screening mammogram within the previous 11-24 months interpreted as heterogeneously dense or extremely dense and negative or benign [Breast Imaging Reporting and Data System (BI-RADS) Category 1 or 2]

Exclusion Criteria:

Subjects will be excluded if they:

• Are unable to understand and sign the consent form
• Are pregnant or lactating
• Are physically unable to sit upright and still for 40 minutes
• Have self-reported signs or symptoms that may suggest breast cancer (palpable mass, bloody nipple discharge, axillary mass, etc.)
• Have had needle biopsy within 3 months, or breast surgery or radiation within 1 year prior to the study
• Are currently receiving chemotherapy or tamoxifen, raloxifene, or an aromatase inhibitor for adjuvant therapy or chemoprevention
• Have undergone bilateral mastectomy
• Have had a prior MBI within 20 months of scheduled study MBI.