Residency · Residency · Diagnostic Radiology
Mammography: Technique, Positioning, and BI-RADS Lexicon
Overview
Mammographic Techniques
Full-field digital mammography (FFDM) is the standard 2D mammographic technique, using a digital detector in place of traditional film-screen systems. Digital breast tomosynthesis (DBT), also known as 3D mammography, acquires images during an arc sweep to produce thin-slice images through the breast, reducing tissue overlap and improving lesion detection. The combination of DBT with a synthetic 2D image has become the standard of care in many practices, reducing recall rates and improving cancer detection compared to FFDM alone. Contrast-enhanced mammography (CEM) is an emerging modality that uses dual-energy acquisition after intravenous iodinated contrast to show enhancing lesions in a manner similar to breast MRI.
Standard Views
The craniocaudal (CC) view directs the x-ray beam from superior to inferior and evaluates medial and lateral breast tissue. The mediolateral oblique (MLO) view angles the beam at 30-60 degrees and includes the pectoralis muscle to the level of the nipple as well as the inframammary fold. Adequate positioning requires the pectoralis muscle to extend to the posterior nipple line on the MLO view, the nipple in profile, and no skin folds.
Additional Views
Spot compression reduces tissue overlap over a focal area to clarify whether a finding is real. Magnification improves spatial resolution for calcification characterization. The 90-degree lateral view (ML or LM) helps localize lesions seen on one view and evaluates milk of calcium. Rolled views (CC rolled lateral or medial) separate overlapping tissue to confirm or localize a finding. The exaggerated CC (XCCL) view images far lateral tissue not included on the standard CC. The cleavage view images deep medial tissue, and a tangential view confirms the dermal location of a calcification.
BI-RADS Lexicon (ACR BI-RADS 5th Edition)
Masses
Masses are described by shape, margin, and density. Shape may be oval (typically benign), round, or irregular (suspicious). Margins may be circumscribed (sharp and well-defined, favoring a benign process), obscured, microlobulated (suspicious), indistinct (suspicious), or spiculated (highly suspicious for malignancy). Density is categorized as fat-containing (benign -- seen in fat necrosis, hamartoma, lipoma, or galactocele), low density, equal density, or high density (which raises suspicion).
Calcifications
Calcifications are divided into typically benign and suspicious morphologies. Typically benign calcifications include skin (dermal) calcifications with lucent centers confirmed on tangential view, vascular calcifications in a railroad-track pattern, coarse or popcorn calcifications from involuting fibroadenomas, large rod-like secretory calcifications from plasma cell mastitis, round calcifications less than 1 mm, rim or eggshell calcifications from fat necrosis or oil cysts, milk of calcium that layers on the 90-degree lateral view producing a tea-cup appearance, dystrophic calcifications that are irregular and greater than 0.5 mm at post-surgical or post-radiation sites, and suture calcifications along surgical sites.
Suspicious calcification morphologies include amorphous calcifications (indistinct and often hazy, with low suspicion), coarse heterogeneous calcifications (irregular, 0.5-1 mm, intermediate suspicion), fine pleomorphic calcifications (varying shapes and sizes less than 0.5 mm, moderate suspicion), and fine linear or fine linear branching (casting) calcifications, which carry the highest suspicion for DCIS.
The distribution of calcifications also matters. Diffuse (scattered) distribution is typically benign. Regional distribution covers more than 2 cm in a non-ductal pattern and carries lower suspicion. Grouped or clustered distribution involves 5 or more calcifications in less than a 2 cm area. Linear distribution arranged along a line suggests ductal involvement. Segmental distribution, with a triangular shape and apex pointing toward the nipple, suggests ductal involvement and carries high suspicion for extensive DCIS.
Architectural Distortion
Architectural distortion appears as lines radiating from a point without a visible mass. It is the most common mammographic presentation of invasive lobular carcinoma and is also caused by radial scar or complex sclerosing lesion, post-surgical scar, and fat necrosis. DBT significantly improves detection of architectural distortion compared to 2D mammography.
Asymmetries
An asymmetry is an area of fibroglandular density seen on one view only. Global asymmetry is a larger area of increased density in one breast and is usually a normal variant. Focal asymmetry is seen on two views but lacks features of a true mass, and may warrant additional evaluation. A developing asymmetry is a new or enlarging focal asymmetry on comparison and is suspicious, warranting tissue sampling.
BI-RADS Assessment Categories
Categories
| BI-RADS Category | Assessment | Malignancy Risk | Management |
|---|---|---|---|
| 0 | Incomplete | — | Additional imaging needed |
| 1 | Negative | Essentially 0% | Routine screening |
| 2 | Benign | Essentially 0% | Routine screening |
| 3 | Probably benign | <2% | 6-month follow-up |
| 4A | Low suspicion | 2-10% | Biopsy recommended |
| 4B | Moderate suspicion | 10-50% | Biopsy recommended |
| 4C | High suspicion | 50-95% | Biopsy recommended |
| 5 | Highly suggestive of malignancy | >=95% | Biopsy required |
| 6 | Known malignancy | — | Staging or treatment monitoring |
BI-RADS 0 indicates an incomplete study requiring additional imaging evaluation. BI-RADS 1 is negative, with routine screening recommended. BI-RADS 2 is benign (such as a calcified fibroadenoma, cyst, or fat necrosis), with routine screening recommended. BI-RADS 3 is probably benign with less than 2% malignancy risk, warranting 6-month follow-up; it should never be assigned on an initial screening mammogram but only after a complete diagnostic workup. BI-RADS 4 is suspicious and biopsy is recommended, subdivided into 4A (2-10% suspicion), 4B (10-50%), and 4C (50-95%). BI-RADS 5 is highly suggestive of malignancy (95% or greater) and biopsy is required. BI-RADS 6 designates a known biopsy-proven malignancy and is used for staging or monitoring neoadjuvant therapy.
Breast Density Categories
Breast density is classified into four categories. Category a is almost entirely fatty (less than 25% fibroglandular tissue). Category b shows scattered fibroglandular densities (25-50%). Category c is heterogeneously dense (51-75%), which may obscure small masses. Category d is extremely dense (greater than 75%), which significantly lowers mammographic sensitivity. Dense breasts (categories c and d) are associated with both increased cancer risk and reduced mammographic sensitivity. Many states mandate density notification, and supplemental screening may be recommended.
<image>A mammographic image panel demonstrating the BI-RADS mass descriptors. Four CC view mammograms showing: (1) An oval, circumscribed mass (fibroadenoma -- BI-RADS 3/benign). (2) A round, obscured mass (needs ultrasound -- BI-RADS 0). (3) An irregular, indistinct mass (suspicious -- BI-RADS 4). (4) An irregular, spiculated mass (highly suspicious -- BI-RADS 5). Each mass is magnified in an inset with the shape and margin descriptors labeled.</image>
<image>A magnification mammography panel showing the spectrum of calcification morphologies from benign to suspicious. Arranged left to right: (1) Round calcifications (typically benign). (2) Coarse/popcorn calcifications in an involuting fibroadenoma (benign). (3) Amorphous calcifications in a grouped distribution (suspicious -- BI-RADS 4A). (4) Fine pleomorphic calcifications in a grouped distribution (suspicious -- BI-RADS 4B). (5) Fine linear branching calcifications in a segmental distribution (highly suspicious for DCIS -- BI-RADS 4C/5). Each morphology is labeled with its BI-RADS descriptor and level of suspicion.</image>
<image>A four-panel mammogram comparison showing the ACR breast density categories. (a) Almost entirely fatty breast with clear visualization of all structures. (b) Scattered fibroglandular densities with some areas of white tissue. (c) Heterogeneously dense breast where fibroglandular tissue could obscure lesions. (d) Extremely dense breast with very limited mammographic sensitivity. Each panel is labeled with the density category letter and the approximate percentage of fibroglandular tissue.</image>
Clinical Pearls
BI-RADS 3 (probably benign) should never be assigned on an initial screening mammogram; it requires a complete diagnostic workup first and indicates a finding with less than 2% malignancy risk warranting short-interval follow-up. Spiculated margins are the most suspicious mass feature on mammography and carry the highest positive predictive value for malignancy. Fine linear branching calcifications in a segmental or linear distribution are the most suspicious calcification pattern and strongly suggest DCIS. Architectural distortion is the most commonly missed finding on 2D mammography, and DBT (tomosynthesis) significantly improves its detection. A developing asymmetry (new or enlarging on comparison) should be treated as suspicious regardless of its morphology and typically warrants biopsy. Dense breast tissue (categories c and d) both reduces mammographic sensitivity and independently increases breast cancer risk, so patients should be informed and supplemental screening may be appropriate.
References
- ACR BI-RADS Atlas, 5th Edition. American College of Radiology, 2013
- Sickles EA, et al. "ACR BI-RADS Mammography." ACR BI-RADS Atlas, 2013
- Conant EF, et al. "Association of Digital Breast Tomosynthesis vs Digital Mammography with Cancer Detection and Recall Rates." JAMA Internal Medicine, 2019
