FGT — Almost Entirely Fatty
Almost entirely fatty is the first of four fibroglandular tissue (FGT) categories in BI-RADS v2025, designated Category A. The breast is predominantly composed of fat with only scattered, minimal fibroglandular elements visible on MRI.
Definition
The relative volume of fibroglandular tissue is minimal — approximately ≤25% of the breast volume. The breast parenchyma is composed almost entirely of adipose tissue with only trace islands of glandular tissue. This category corresponds to ACR density category A on mammography (almost entirely fatty), though MRI and mammographic density assessments do not always correlate precisely due to differences in tissue contrast mechanisms.
Imaging Appearance
T1-Weighted (Pre-contrast)
- Breast appears uniformly hyperintense (bright) on T1W images due to the short T1 of fat
- Minimal scattered low-signal fibroglandular tissue may be seen, typically in the retroareolar region or upper outer quadrant
- On T1W fat-suppressed sequences, the breast appears predominantly dark with only faint residual glandular signal
T2-Weighted
- Fat remains hyperintense on non-fat-suppressed T2W images
- On T2W fat-suppressed (e.g., STIR), breast is predominantly hypointense with minimal glandular tissue appearing intermediate to mildly hyperintense
- Cysts, if present, are conspicuous against the suppressed fatty background
Post-contrast (T1W Fat-Suppressed)
- Background parenchymal enhancement (BPE) is typically minimal by definition — there is very little enhanceable parenchyma
- Enhancing lesions are highly conspicuous against the dark, non-enhancing fatty background
- This represents the optimal detection environment for contrast-enhancing lesions on breast MRI
DWI
- Fat demonstrates T2 shine-through on DWI, which can mimic restricted diffusion if ADC maps are not reviewed
- True restricted diffusion from a lesion is easier to identify due to the paucity of surrounding glandular tissue
Board Pearl
Almost entirely fatty breasts provide the highest lesion conspicuity on contrast-enhanced MRI. A new enhancing focus in a fatty breast should never be dismissed — it is almost certainly real and warrants characterization.
Clinical Significance
- Cancer risk: Lower mammographic density is associated with lower breast cancer risk. Women with almost entirely fatty breasts have the lowest density-associated risk among the four categories.
- Lesion detection: The fatty background provides excellent contrast for enhancing lesions, making MRI highly sensitive in this tissue type. False-negative rates are lowest in fatty breasts.
- BPE impact: Minimal FGT means minimal BPE regardless of hormonal status or menstrual cycle timing, reducing the need for precise cycle-based scheduling of MRI.
- Screening implications: While fatty breasts have high mammographic sensitivity, MRI may still be indicated based on lifetime risk (≥20%), genetic predisposition, or prior chest radiation — density alone does not determine MRI screening eligibility.
Relationship to Mammographic Density
| Modality | Category A Designation | Basis |
|---|---|---|
| Mammography | Almost entirely fatty | X-ray attenuation (2D projection) |
| MRI | Almost entirely fatty | Water content / enhancement (3D volumetric) |
| Ultrasound | Homogeneous fat | Echogenicity |
Board Pearl
MRI-based FGT assessment and mammographic density do not always agree. MRI evaluates the volume of enhanceable tissue in 3D, while mammography projects overlapping tissue in 2D. A breast that appears heterogeneously dense on mammography may appear almost entirely fatty on MRI if the fibroglandular tissue is thin but widely distributed.
FGT Categories — Comparison
| Category | Label | Approximate FGT Volume | BPE Expectation |
|---|---|---|---|
| A | Almost entirely fatty | ≤25% | Minimal |
| B | Scattered fibroglandular tissue | 25–50% | Minimal to mild |
| C | Heterogeneous fibroglandular tissue | 50–75% | Mild to moderate |
| D | Extreme fibroglandular tissue | ≥75% | Moderate to marked |
Pitfalls
- Do not conflate low density with low risk in high-risk patients. BRCA carriers with fatty breasts still have significantly elevated lifetime risk and require MRI screening.
- Post-treatment changes: A breast that was previously dense may become almost entirely fatty after chemotherapy, radiation, or hormonal therapy (especially aromatase inhibitors). This change should be documented but does not alter screening protocol based on prior risk category.
- Implant-related: Breasts with implants may appear fatty around the implant. FGT assessment should focus on the native tissue, not the implant itself.
- DWI artifact: Fat-related artifacts on DWI (chemical shift, incomplete suppression) can simulate or obscure findings — always correlate with ADC map and post-contrast sequences.
- Age-related involution: Almost entirely fatty breasts are more common in postmenopausal women due to glandular involution. In a premenopausal woman, this category is less common and the assessment should still be made objectively.
Board Pearl
FGT category is a visual assessment, not a precise volumetric measurement. BI-RADS v2025 intentionally avoids strict percentage thresholds — the radiologist assigns the category based on overall impression. Consistency matters more than precision.
v2025 Updates
- BI-RADS v2025 retains the same four FGT categories (A through D) used in the 5th edition
- The v2025 text emphasizes that FGT assessment is qualitative and subjective — inter-reader variability is expected and acceptable
- v2025 reinforces that FGT category should be reported on every breast MRI examination as it provides context for interpretation confidence
Reporting
FGT category should appear in the Breast Composition section of the MRI report:
The breast tissue is almost entirely fatty (ACR FGT Category A).
This informs the referring clinician that lesion conspicuity on MRI is excellent and that minimal BPE is expected.