Residency · Residency · Diagnostic Radiology

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

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.

For the intern

At 3am, if you're called about a breast MRI, confirm the patient is stable and has no contrast reaction. Review the images for obvious findings like rim-enhancing masses with washout kinetics or segmental clumped enhancement—these need documentation but rarely require immediate action. Most breast MRI interpretation, second-look ultrasound correlation, and biopsy planning can wait until morning rounds when you can review with your attending and contact the referring clinician during business hours.

Common pitfalls

• Ignoring menstrual cycle timing: Performing MRI during peak hormonal phases (days 1-6 or after day 21) causes marked background parenchymal enhancement (BPE), obscuring small lesions and generating false positives that waste everyone's time.
• Over-relying on kinetics alone: A Type III washout curve doesn't automatically mean cancer—fibroadenomas and fat necrosis can washout too. Always assess morphology first; an oval circumscribed mass with washout is still likely benign.
• Skipping second-look ultrasound: Before recommending MRI-guided biopsy, always perform targeted ultrasound with the MRI images in hand. Many enhancing lesions have subtle ultrasound correlates that allow easier biopsy.
• Missing extramammary findings: Don't tunnel-vision on the breast tissue. Adenopathy, bone lesions, and lung nodules appear on breast MRI and may change staging dramatically.

<image> Create a simple flowchart showing breast MRI interpretation workflow. Top panel: "Enhancing Lesion Identified on MRI" box. Second panel splits into two branches: left branch "MASS" with three boxes below showing Shape→Margin→Enhancement pattern, leading to BI-RADS category. Right branch "NON-MASS ENHANCEMENT" with two boxes showing Distribution→Internal pattern, leading to BI-RADS category. Bottom panel: Both branches converge to single box "Second-look targeted ultrasound" which splits to "Visible: US-guided biopsy" and "Not visible: MRI-guided biopsy". </image>

Make it stick

Imagine you’re reviewing a breast MRI for a newly diagnosed cancer patient. You see a suspicious mass in the affected breast, but also notice a tiny, enhancing focus in the contralateral breast that wasn't on the mammogram. It's an irregular shape with rim enhancement and rapid washout kinetics. This subtle finding, easily missed without careful review, could be an occult synchronous cancer, changing the entire surgical plan. This is where breast MRI truly shines: finding what other modalities cannot, guiding treatment, and potentially saving lives.

If you remember one thing

Breast MRI is highly sensitive for breast cancer, leveraging dynamic contrast enhancement and morphology to characterize lesions. Always correlate findings with other imaging and clinical context, prioritizing morphology over kinetics, and never skip the second-look ultrasound.

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