Residency · Residency · Diagnostic Radiology
Breast Ultrasound and BI-RADS Assessment
Overview
Indications
Breast ultrasound is used to evaluate palpable findings (especially in women under 30 or those who are pregnant or lactating), to characterize mammographic masses as solid or cystic, to guide interventional procedures such as biopsy and cyst aspiration, for supplemental screening in women with dense breasts, to evaluate breast implants, and to assess axillary lymph nodes.
Technique
A high-frequency linear transducer (12-18 MHz) is used with systematic scanning in radial and anti-radial planes. Orthogonal images of any finding are obtained with measurements in three dimensions, and axillary evaluation is included when indicated.
BI-RADS Ultrasound Lexicon
Shape
Masses may be oval (wider than tall, favoring a benign process and parallel to the skin), round (equidimensional and indeterminate), or irregular (neither oval nor round, which raises suspicion).
Orientation
Parallel orientation (wider than tall) favors a benign process because the mass grows along tissue planes. Non-parallel orientation (taller than wide) is suspicious because it suggests infiltrative growth crossing tissue planes.
Margin
A circumscribed margin is well-defined with a sharp boundary and is a benign feature. Not circumscribed margins include indistinct (poor definition, not clearly delineated from surrounding tissue), angular (acute angles at the lesion margin), microlobulated (small 1-2 mm undulations along the margin), and spiculated (sharp lines radiating from the mass, which is highly suspicious).
Echogenicity
Anechoic masses have no internal echoes and represent simple cysts. Hyperechoic masses are brighter than fat and usually benign, suggesting fat necrosis or fibrosis. Isoechoic masses match the echogenicity of surrounding fat. Hypoechoic masses are darker than fat and include most solid masses. Complex cystic and solid masses have mixed components and are suspicious. Heterogeneous masses show mixed echogenicity within a solid mass.
Posterior Acoustic Features
Enhancement (increased echogenicity deep to the lesion) is seen with cysts and some solid masses. Shadowing (decreased echogenicity deep to the lesion) may indicate fibrosis or malignancy from desmoplastic reaction. No posterior features means the beam is not significantly altered. Combined patterns show a mix of enhancement and shadowing.
Special Cases
| Cyst Type | US Features | BI-RADS | Management |
|---|---|---|---|
| Simple cyst | Anechoic, thin imperceptible wall, posterior enhancement, oval/round | 2 | No intervention |
| Complicated cyst | Low-level internal echoes, no solid component, no vascularity | 2-3 | Follow-up or aspiration |
| Complex cystic and solid | Thick walls, thick septations, or solid mural component | 4 | Biopsy recommended |
A simple cyst is anechoic with a thin imperceptible wall, posterior acoustic enhancement, and oval or round shape. It is classified as BI-RADS 2 (benign) and requires no intervention. A complicated cyst has low-level internal echoes without a solid component or vascularity and may need follow-up or aspiration. A complex cystic and solid mass has thick walls, thick septations, or a solid mural component, is classified as BI-RADS 4, and biopsy is recommended.
Benign Lesion Characteristics
Simple Cyst
The simple cyst is the most common breast mass on ultrasound, presenting as anechoic, circumscribed, oval, with a thin imperceptible wall and posterior enhancement. It is classified as BI-RADS 2 and requires no follow-up.
Fibroadenoma
Fibroadenomas are well-circumscribed, oval, parallel in orientation, and homogeneously hypoechoic. They may have gentle lobulations (3 or fewer) and are classified as BI-RADS 3 if they show classic features, permitting 6-month follow-up. Over time, fibroadenomas may calcify, producing the coarse popcorn calcifications seen on mammography.
Fat Necrosis
Fat necrosis has a variable appearance, ranging from an oil cyst (anechoic with an echogenic rim) to a complex mass or hyperechoic mass. Clinical history of trauma or surgery is key. On mammography, an oil cyst with rim calcification is pathognomonic.
Lymph Node (Intramammary)
Normal intramammary lymph nodes have a reniform shape, an echogenic fatty hilum, and hilar blood flow. They are typically found in the upper outer quadrant or axilla. Abnormal features include cortical thickening greater than 3 mm, an absent hilum, rounded morphology, and non-hilar blood flow.
Suspicious Lesion Characteristics
Features Suggesting Malignancy
Features that suggest malignancy include irregular shape, non-parallel (taller-than-wide) orientation, spiculated or angular margins, posterior acoustic shadowing, intranodal vascularity on color or power Doppler, and associated findings such as skin thickening, edema, architectural distortion, and ductal changes.
Invasive Ductal Carcinoma (Most Common)
Invasive ductal carcinoma typically appears as an irregular, hypoechoic mass with spiculated margins, non-parallel orientation, and posterior shadowing, often with associated microcalcifications visible as echogenic foci.
Invasive Lobular Carcinoma
Invasive lobular carcinoma may be subtle on ultrasound, appearing as a hypoechoic area with posterior shadowing but often without well-defined mass margins. It can present as architectural distortion only.
Ultrasound-Guided Biopsy
Indications
Ultrasound-guided biopsy is indicated for BI-RADS 4 or 5 lesions, BI-RADS 3 lesions when the patient cannot comply with follow-up or prefers biopsy, palpable concerns with an imaging correlate, and suspicious lymph nodes (via FNA or core biopsy).
Technique
Core needle biopsy using a 14-gauge spring-loaded or vacuum-assisted device is preferred over FNA for breast masses. Clip or marker placement after biopsy is critical for marking the site, and a post-biopsy mammogram is obtained to confirm clip position relative to the target.
Concordance Assessment
After biopsy, the radiologist must assess pathology-imaging concordance. A discordant result, such as a benign pathology result for a BI-RADS 5 lesion, should prompt recommendation for repeat biopsy or excision. Certain benign results are considered "high-risk" and may warrant excision, including atypical ductal hyperplasia, lobular carcinoma in situ, radial scar, and papilloma with atypia.
<image>A breast ultrasound panel showing the spectrum from benign to malignant lesions. (1) Simple cyst: anechoic, circumscribed, oval, with posterior acoustic enhancement (BI-RADS 2). (2) Fibroadenoma: circumscribed, oval, parallel, homogeneously hypoechoic (BI-RADS 3). (3) Suspicious mass: irregular, taller-than-wide, indistinct margins, hypoechoic, posterior shadowing (BI-RADS 4C/5). (4) Invasive carcinoma: spiculated, irregular mass with posterior shadowing and architectural distortion. Each panel is labeled with the BI-RADS descriptors and assessment category.</image>
<image>An ultrasound image demonstrating ultrasound-guided core needle biopsy technique. The image shows a suspicious hypoechoic mass with the biopsy needle approaching from the side (parallel to the transducer). The needle tip is visible within the mass. A pre-fire and post-fire comparison shows the needle throw through the lesion. An inset image shows the post-biopsy clip (echogenic marker) deployed at the biopsy site. Labels indicate the transducer, needle, mass, and clip.</image>
<image>A comparison panel of normal and abnormal axillary lymph nodes on ultrasound. Left: normal lymph node showing reniform shape with a preserved echogenic fatty hilum and hilar blood flow on color Doppler. Right: abnormal lymph node showing rounded morphology, cortical thickening greater than 3 mm, loss of the echogenic hilum, and peripheral blood flow on color Doppler. An intermediate panel shows focal cortical thickening (asymmetric cortex >3 mm) suggesting early metastatic involvement. Arrows and labels highlight the key features.</image>
Clinical Pearls
A taller-than-wide (non-parallel) orientation is one of the most specific ultrasound features for malignancy, indicating a mass growing across tissue planes rather than along them. Simple cysts meeting all strict criteria (anechoic, thin imperceptible wall, circumscribed, posterior enhancement) are BI-RADS 2 and require no follow-up or aspiration regardless of size. After ultrasound-guided biopsy, always place a clip marker and obtain a post-biopsy mammogram to confirm clip position, as this is essential if the patient later requires surgical excision. Concordance assessment is a critical radiologist responsibility: a benign biopsy result for a highly suspicious lesion (BI-RADS 5) is discordant and requires re-biopsy or surgical excision. Color Doppler showing internal vascularity within a solid mass increases suspicion for malignancy but is not specific, as avascular solid masses can still be malignant. When evaluating axillary lymph nodes, focal cortical thickening greater than 3 mm (even with a preserved hilum) is the earliest sign of metastatic involvement and should prompt ultrasound-guided FNA.
References
- ACR BI-RADS Atlas, 5th Edition -- Ultrasound. American College of Radiology, 2013
- Stavros AT, et al. "Solid Breast Nodules: Use of Sonography to Distinguish between Benign and Malignant Lesions." Radiology, 1995
- ACR Practice Parameter for the Performance of a Breast Ultrasound Examination, 2016
