FGT — Heterogeneous Fibroglandular Tissue
Heterogeneous fibroglandular tissue is FGT category C in the BI-RADS v2025 breast MRI lexicon. The breast contains a substantial, unevenly distributed mix of fibroglandular and fatty tissue, with fibroglandular tissue comprising approximately 51–75% of breast volume. It is the second-densest category, exceeded only by extreme FGT (category D).
Definition
Fibroglandular tissue is heterogeneously distributed throughout the breast, constituting a significant — but not nearly exclusive — portion of total breast volume. Interspersed fatty regions remain visible, distinguishing this category from extreme FGT where fat is minimal. The distribution is characteristically patchy and asymmetric, with areas of dense parenchyma alternating with fatty lobules.
Board Pearl
FGT assessment on MRI is performed on the non-fat-suppressed T1-weighted sequence (or the first pre-contrast image), NOT on post-contrast images. Evaluating FGT on contrast-enhanced sequences conflates tissue density with enhancement.
Imaging Appearance
T1-Weighted (Pre-contrast)
- Fibroglandular regions: intermediate-to-low signal intensity, often appearing as irregular confluent patches
- Fatty regions: high T1 signal, scattered between parenchymal islands
- The heterogeneous interdigitation of fat and parenchyma is the hallmark — no uniform gradient from anterior to posterior
T2-Weighted
- Fibroglandular tissue shows intermediate-to-high signal on T2W
- Cysts and fluid-filled ducts within dense parenchyma are conspicuous on T2
Post-contrast (Dynamic)
- Background parenchymal enhancement (BPE) is often moderate to marked given the substantial parenchymal volume
- Enhancing lesions may be obscured within areas of heterogeneous BPE — this is the primary clinical concern
DWI
- Dense fibroglandular tissue can show mildly restricted diffusion at baseline, complicating ADC interpretation in heterogeneous breasts
Board Pearl
Heterogeneous FGT with moderate-to-marked BPE is the most challenging combination for lesion detection on breast MRI. Consider recommending follow-up during the optimal menstrual cycle window (days 7–14) or after BPE suppression strategies if BPE is problematic.
FGT Categories Comparison
| Category | Label | Approximate Volume | Key Feature |
|---|---|---|---|
| A | Almost entirely fat | <25% FGT | Rare parenchyma |
| B | Scattered | 25–50% FGT | Islands within fat |
| C | Heterogeneous | 51–75% FGT | Patchy dense + fat mix |
| D | Extreme | >75% FGT | Nearly all parenchyma |
Clinical Significance
- Masking effect: heterogeneous FGT increases the likelihood that an enhancing lesion will be obscured by surrounding enhancing parenchyma, particularly when BPE is moderate or marked
- Sensitivity reduction: MRI sensitivity for small cancers decreases in heterogeneous and extreme FGT, though this effect is less pronounced than on mammography
- Risk association: increased FGT on MRI, like mammographic density, is an independent risk factor for breast cancer — women with heterogeneous or extreme FGT have approximately 1.5–2× increased risk compared to fatty breasts
- Screening implications: heterogeneous FGT supports the role of supplemental MRI screening in women with dense breasts, where mammographic sensitivity is lowest
Edition Conflict
BI-RADS 5th edition used qualitative visual estimation for FGT categories with no percentage guidance. BI-RADS v2025 encourages more consistent categorization with approximate volumetric thresholds, though visual estimation remains acceptable. Automated volumetric tools may assist but are not required.
Distinction from Adjacent Categories
- vs. Scattered FGT (B): scattered FGT has clearly dominant fatty tissue with isolated fibroglandular islands; heterogeneous FGT shows a more balanced or parenchyma-dominant mixture
- vs. Extreme FGT (D): extreme FGT shows near-complete parenchymal replacement with minimal visible fat; heterogeneous FGT retains clearly identifiable fatty regions interspersed throughout
Pitfalls
- Overestimating density on post-contrast images — BPE makes parenchyma appear more extensive; always assess FGT on pre-contrast T1W
- Confusing BPE with FGT — a breast can be heterogeneously dense (FGT C) with minimal BPE, or scattered (FGT B) with marked BPE; these are independent assessments
- Hormonal variation — FGT appearance can change with menstrual cycle phase, hormonal therapy, or menopausal status; note timing context
- Asymmetric density — one breast may qualify as heterogeneous while the contralateral is scattered; report each breast’s FGT if significantly discrepant
- Implant-related compression — subglandular implants compress native parenchyma posteriorly, which can simulate higher FGT category; assess the native tissue, not the compressed configuration
Board Pearl
FGT and BPE are reported independently in the BI-RADS MRI assessment. A common error is conflating the two: FGT describes tissue composition (structural), while BPE describes tissue enhancement behavior (functional). Both affect lesion conspicuity but through different mechanisms.
v2025 Updates
- The four-category system (a–d) is retained from the 5th edition, relabeled A–D in v2025
- v2025 emphasizes that FGT assessment should be made on pre-contrast images and reported as a global bilateral assessment unless significant asymmetry warrants separate reporting
- Quantitative volumetric assessment tools are acknowledged but not mandated
- The link between breast density on MRI and cancer risk is more explicitly stated in v2025