The investigators hypothesize that knowledge of the functional behavior of areas of mammographic density will enable more specific identification of dense tissue at-risk for breast cancer, ultimately providing predictive information on an individual's risk of developing breast cancer.

The specific aims are:

• In women with increased mammographic density (MD), determine the histological characteristics of functionally dense tissue compared to that of non-functionally dense tissue via core-needle biopsy of the breast.
• In women with increased mammographic density (MD), determine the correlations between the total percent area of functional density (FD) on MBI, percent density of MD on mammogram, and histological characteristics of dense tissue

Inclusion Criteria:

• Ages 40 - 55 years
• Considered premenopausal or peri-menopausal
• Screening mammogram performed at Mayo Clinic Rochester within one year prior to the current MBI study which demonstrates
• Negative/benign assessment (BIRADS category 1-2) or Additional imaging recommended (BIRADS category 0), where the subsequent diagnostic workup results in a final negative/benign assessment (BIRADS category 1-2).
• No proliferative benign lesions (e.g. fibroadenomas) identified
• Heterogeneously dense or extremely dense parenchyma (BIRADs density category 3 or 4)
• MBI performed less than 3 days prior to biopsy demonstrating either marked FD or photopenic FD in an area of mammographically dense tissue amenable to biopsy.
• No use of exogenous hormonal drugs (e.g. hormonal contraceptives or sex steroid hormones) within six months prior to study biopsy
• Negative pregnancy test or surgically sterilized (for patients in whom a study MBI will be performed)

Exclusion criteria:

• Using any estrogen receptor modulating drugs (e.g., tamoxifen, raloxifene) or any aromatase inhibitors within six months prior to study biopsy.
• Prior diagnosis of bilateral breast cancer.
• Diagnosis of breast cancer in either breast within 3 years prior to study MBI.
• Diagnosis of any cancer, except breast cancer (see #3) or non-melanomatous skin cancer, for which patient has not been disease free for at least 5 years.
• Current breast symptoms
• Breast implants
• Known allergy to local anesthetic.
• History of bleeding complications from prior interventions
• Current use of anticoagulants (e.g., Coumadin or other blood thinners)
• Major medical condition

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