# Breast MRI: Indications and Interpretation

## Introduction

Breast MRI is a highly sensitive imaging modality used as an adjunct to mammography and ultrasound. It leverages **dynamic contrast-enhanced (DCE)** sequences to evaluate breast lesions based on morphology and enhancement kinetics. Understanding appropriate indications and systematic interpretation is essential for the diagnostic radiology resident.

## Physics and Technical Considerations

### Pulse Sequences

**T1-weighted pre- and post-contrast** sequences form the backbone of breast MRI. **T2-weighted** sequences help differentiate cysts from solid lesions, while **diffusion-weighted imaging (DWI)** provides complementary information, with low ADC values suggesting malignancy. **Short tau inversion recovery (STIR)** or fat-suppressed T2 sequences aid in fluid characterization. Temporal resolution should be approximately **60-120 seconds per acquisition** to capture kinetic data.

### Contrast Kinetics

| Kinetic Curve Type | Pattern | Significance |
|-------------------|---------|-------------|
| Type I (Persistent) | Progressive enhancement | Favors benign |
| Type II (Plateau) | Rapid rise then levels off | Indeterminate |
| Type III (Washout) | Rapid rise then decline | Suspicious for malignancy |

A **Type I curve (persistent)** shows progressive enhancement and favors a benign etiology. A **Type II curve (plateau)** is indeterminate and requires morphologic correlation. A **Type III curve (washout)** shows rapid early enhancement followed by a decline and is suspicious for malignancy. Maximum intensity projection (MIP) images provide an overview of enhancing lesions.

## Indications for Breast MRI

### Screening Indications

Breast MRI screening is indicated for women with a **lifetime risk greater than 20%**, including BRCA1/2 carriers and those with Li-Fraumeni or Cowden syndrome. It is also recommended for women with a history of chest wall radiation between ages 10 and 30 (such as for Hodgkin lymphoma treatment) and for first-degree relatives of known BRCA carriers, even if untested.

### Diagnostic Indications

Diagnostic breast MRI is used for **extent of disease evaluation** in newly diagnosed breast cancer, evaluation of the **contralateral breast** at diagnosis, assessment of **treatment response** to neoadjuvant chemotherapy, and workup of **occult primary breast cancer** when axillary nodal metastasis is present but mammography and ultrasound are negative. Additional indications include evaluation of **silicone implant integrity** using silicone-specific sequences and problem solving when mammography and ultrasound are inconclusive.

## BI-RADS MRI Lexicon

### Mass Descriptors

Masses are described by **shape** (oval, round, irregular), **margin** (circumscribed, irregular, spiculated), and **internal enhancement** pattern (homogeneous, heterogeneous, rim enhancement, dark internal septations).

### Non-Mass Enhancement (NME)

Non-mass enhancement is described by its **distribution** (focal, linear, segmental, regional, multiple regions, diffuse) and its **internal enhancement pattern** (homogeneous, heterogeneous, clumped, clustered ring). **Segmental clumped NME** is highly suspicious for **ductal carcinoma in situ (DCIS)**.

### Background Parenchymal Enhancement (BPE)

BPE is classified as minimal, mild, moderate, or marked. Ideally, breast MRI should be performed on **days 7-14 of the menstrual cycle** to minimize BPE, as marked BPE can obscure lesions and reduce sensitivity.

## Interpretation and Reporting

### Systematic Approach

The interpretation should begin with comparison to prior mammograms and ultrasounds. **Morphology is evaluated first**, followed by kinetics. The radiologist assesses for **skin thickening, chest wall invasion, and axillary lymphadenopathy** and reports findings using standardized **BI-RADS assessment categories** (0-6). Enhancing lesions should be correlated with second-look targeted ultrasound for potential biopsy.

### MRI-Guided Biopsy

MRI-guided biopsy is indicated when a lesion is visible only on MRI and cannot be identified on second-look ultrasound. The procedure is performed with the patient prone using a **grid or freehand technique**, and post-biopsy clip placement is essential for future localization.

## Common Pitfalls

**False positives** can occur with fibroadenomas, fat necrosis, and hormonal enhancement, all of which can mimic malignancy. **False negatives** may occur with low-grade DCIS and mucinous carcinoma, which may show minimal enhancement. Inadequate fat suppression can obscure enhancing lesions, and motion artifact degrades both image quality and kinetic analysis.

## Clinical Pearls

Breast MRI has a sensitivity of approximately **95-100%** for invasive cancer but lower specificity (around 70%). MRI findings should always be correlated with mammography and ultrasound before recommending biopsy. Timing breast MRI to the **second week of the menstrual cycle** reduces background enhancement and false positives. Implant evaluation requires dedicated **silicone-sensitive sequences** that use water suppression rather than fat suppression. The **linguine sign** and **subcapsular line sign** on MRI indicate intracapsular silicone implant rupture.

## References

1. Mann RM, Cho N, Moy L. Breast MRI: State of the Art. *Radiology*. 2019;292(3):520-536.
2. Morris EA, et al. ACR BI-RADS Magnetic Resonance Imaging. In: ACR BI-RADS Atlas. 5th ed. American College of Radiology; 2013.
3. Saslow D, et al. American Cancer Society Guidelines for Breast Screening with MRI as an Adjunct to Mammography. *CA Cancer J Clin*. 2007;57(2):75-89.
4. Kuhl CK. The Changing World of Breast Cancer Management: MRI. *J Clin Oncol*. 2021;39(21):2356-2364.