It refers to the effects of a contrast dye injected prior to the MRI to improve the image. Here's an article that came up with a quick google: (certainly more info than I need but you can go as far as you want with it.

Background Parenchymal Enhancement at Breast MR Imaging: Normal Patterns, Diagnostic Challenges, and Potential for False-Positive and False-Negative Interpretation

Catherine S. Giess, Eren D. Yeh, Sughra Raza, Robyn L. Birdwell

Address correspondence to C.S.G. (e-mail: cgiess@partners.org).Disclosures of Conflicts of Interest.—C.S.G.: Financial activities not related to the present article: program reviewer for American College of Radiology Mammography Accreditation; lecturer for National Diagnostic Imaging Symposium, Kaiser-Permanente, and New York Breast Imaging Society. R.L.B.: Financial activities not related to the present article: royalties from Amirsys. S.R.: Financial activities not related to the present article: royalties from Amirsys.

From the Department of Radiology, Brigham and Women's Hospital, 75 Francis St, Boston, MA 02115.

RadioGraphics

Vol. 34: , Issue. 1, : Pages. 234-247
(Issue publication date: January 2014)

https://doi.org/10.1148/rg.341135034

This article reviews the anatomic and physiologic influences on BPE, the typical and atypical patterns of BPE, and the potential of BPE to confound accurate interpretation of MR images.

At magnetic resonance (MR) imaging, both normal and abnormal breast tissue enhances after contrast material administration. The morphology and temporal degree of enhancement of pathologic breast tissue relative to normal breast tissue form the basis of MR imaging's diagnostic accuracy in the detection and diagnosis of breast disease. Normal parenchymal enhancement at breast MR imaging is termed background parenchymal enhancement (BPE). BPE may vary in degree and distribution in different patients as well as in the same patient over time. Typically BPE is minimal or mild in overall degree, with a bilateral, symmetric, diffuse distribution and slow early and persistent delayed kinetic features. However, BPE may sometimes be moderate or marked in degree, with an asymmetric or nondiffuse distribution and rapid early and plateau or washout delayed kinetic features. These patterns cause diagnostic difficulty because these features can be seen with malignancy. This article reviews typical and atypical patterns of BPE seen at breast MR imaging. The anatomic and physiologic influences on BPE in women undergoing diagnostic and screening breast MR imaging are reviewed. The potential for false-positive and false-negative interpretations due to BPE are discussed. Radiologists can improve their interpretive accuracy by increasing their understanding of various BPE patterns, influences on BPE, and the potential effects of BPE on MR imaging interpretation.

© RSNA, 2014

SA-CME LEARNING OBJECTIVES FOR TEST 6click to expand contents

Introductionclick to expand contents

MR Imaging Techniqueclick to expand contents

Factors that Influence BPEclick to expand contents

Common Patterns of BPEclick to expand contents

Atypical Patterns of BPEclick to expand contents

False-Positive Interpretationclick to expand contents

False-Negative Interpretationclick to expand contents

Conclusionclick to expand contents

Recipient of a Cum Laude award for an education exhibit at the 2012 RSNA Annual Meeting.

For this journal-based SA-CME activity, C.S.G., S.R., and R.L.B. have disclosed financial relationships (see "Disclosures of Conflicts of Interest"); the other author, editor, and reviewers have no relevant relationships to disclose.

Abbreviations:BI-RADS

Breast Imaging Reporting and Data System

BPE

background parenchymal enhancement

NME

nonmass enhancement

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You might message djmammo for that. I think there are more factors than breast density that play into the answer.