Increased mammographic density is recognized as an important risk factor for developing breast cancer, however, the underlying mechanism explaining this relationship is unclear. The investigators hypothesize that Molecular Breast Imaging (MBI) can more accurately distinguish dense tissue on mammography which is at high risk from dense tissue at low risk by indicating cellular activity in dense tissue as radiotracer uptake (functional density) in the breast. In this pilot study, the investigators want to compare the histological characteristics of breast tissue in patients with who have similar density on mammography but different levels of functional density on MBI.

Inclusion Criteria:

1. Age 40 or older

2. Be postmenopausal as defined as having at least 12 consecutive months of amenorrhea

3. Screening mammogram performed at Mayo Clinic Rochester within one year prior to the current MBI study which demonstrates

   1. Negative or benign assessment (BIRADs category 1-2)
   2. No proliferative benign lesions (e.g. fibroadenomas) identified
   3. Heterogeneously dense or extremely dense parenchyma (BIRADs density category 3 or 4)

4. MBI performed less than one month prior to biopsy demonstrating either significant FD or photopenic FD.

Exclusion criteria:

1. Using any exogenous hormones (e.g., hormonal contraceptives, sex steroid hormones) or any estrogen receptor modulating drugs (e.g., tamoxifen, raloxifene) or any aromatase inhibitors within six months prior to study biopsy.
2. Personal history of any cancer, except non-melanomatous skin cancer
3. Current breast symptoms
4. Breast implants
5. Known allergy to local anesthetic.
6. History of bleeding complications from prior interventions
7. Current use of anticoagulants (e.g., Coumadin or other blood thinners)
8. Major medical condition

Women with mammographically dense breasts who demonstrate either photopenic or marked background parenchymal uptake on MBI.