Sonography has emerged as an essential tool in breast evaluation. Although originally used as an adjunct to mammography to distinguish solid from cystic masses, advances in instrumentation and extensive clinical experience have allowed expansion of its roles to include characterization of solid masses, guidance for interventional procedures, and screening in the appropriate clinical setting.

Sonography is very operator dependent by nature,² and this is especially true in the breast where standard anatomic references are limited. To achieve acceptable levels of sensitivity and specificity, it is important to optimize technical parameters, develop standard exam protocols, and carefully integrate sonographic findings with other imaging modalities and the clinical circumstances. Operator experience is especially important in limiting false positive and negative results.

Diagnostic

Diagnostic evaluation remains the main indication for breast sonography and can be performed as either the primary imaging test or as an adjunct to other imaging procedures. In young women and women who are pregnant, sonography is generally the first test performed for a new mass or other focal breast abnormality.³ Application of ultrasound in this setting limits the radiation dose in younger women and also increases sensitivity in patients who often have dense breast tissue on mammography. If the sonographic evaluation satisfactorily explains the clinical abnormality, no further testing may be required. If sonography does not explain a persistent focal finding, additional evaluation with mammography or other modality would be warranted.

In other women, diagnostic evaluation of a focal clinical abnormality would generally begin with mammography, reserving sonography for supplemental assessment, either to characterize a mammographic abnormality or to increase the sensitivity of the evaluation should no mammographic abnormality be found.

Implant assessment can be performed with mammography, sonography, or magnetic resonance imaging (MRI). Although less accurate than MRI, sonography is often a simple first test when rupture of a silicone implant is suspected. When present and discernible, the pathognomonic “snowstorm appearance” can give an immediate diagnosis of free silicone.⁴

In asymptomatic women, sonography may be used to characterize abnormalities on screening mammography, and increasingly, sonography is the next modality to be used for evaluation of MRI-detected abnormalities that are not seen on mammography (so-called “second look” sonogram).

Breast sonography is not indicated for generalized breast complaints such as diffuse background nodularity or nonfocal mastodynia. However, when complaints are focal, targeted ultrasound to these areas can be performed.

Screening

Sonography as a screening method for breast cancer detection remains controversial.⁵ There is certainly evidence to support its use, especially in high-risk patients with dense breast tissue,^6,7 but the issues of operator dependency and length of exam time have hindered its expansion into routine clinical practice. Information from the American College of Radiology Imaging Network⁸ confirms an increase in sensitivity for women with dense breast tissue and increased risk, but at the expense of a large number of false-positive studies and unnecessary biopsies. If performed, screening breast sonography should be limited to facilities with well-trained breast sonologists and experienced breast imagers. Because it does not reliably depict calcifications or a variety of other mammographic signs of malignancy, it should not be used as a substitute for screening mammography. As breast MRI emerges as a reliable screening tool for high-risk women, the role of screening sonography remains in question.

Tomosynthesis can be used for screening as well, and it is gaining increasing acceptance. Tomosynthesis is essentially a three-dimensional acquisition of images that allows the user to “scroll” through the images and isolate overlapping breast tissue.⁹ As a modality, it decreases the user dependency when compared to screening breast ultrasound.

Mammography remains the standard of care for screening of breast disease in women over the age of 30. The standard bilateral CC (craniocaudal) and MLO (mediolateral oblique) views are evaluated for interval changes or suspicious baseline findings. If further evaluation is warranted, the patient then enters the diagnostic realm where additional mammographic views are obtained, including, but not limited to, spot compression views and magnification views. Depending on mammographic findings, supplemental evaluation with sonography may then be needed for further evaluation.

Intervention

Most breast biopsies are now performed with imaging guidance. For those lesions that are only seen on mammography, biopsy is usually performed with stereotactic guidance. For those things well seen with sonography, however, ultrasound guidance offers the advantage of more comfortable patient positioning and simple targeting.¹⁰ Sonography can be used to direct fine-needle aspiration biopsies, large-gauge core needle biopsies, and vacuum-assisted core biopsies.¹¹ The large-gauge (8- to 12-gauge) vacuum-assisted core biopsies are well suited for percutaneous removal of small fibroadenomas, which may obviate the need for follow-up. Sonographic guidance is especially useful for the aspiration of cysts, seromas, abscesses, and other fluid collections in the breast, and it is increasingly used for deposition of marking clips in patients undergoing neoadjuvant therapy for breast cancer.¹² In addition, ultrasound is sometimes used by surgeons in the operating room to localize their target for benign excisional biopsies or malignant segmental mastectomies.

Much of the expansion of sonography in breast imaging is related to technical advances in instrumentation. Breast sonography requires high-resolution near-field capability that is not available on most standard transducers. Transducers for the breast typically use linear arrays and operate at bandwidth frequencies ranging from 5 to 10 MHz and up to 8 to 17 MHz. Color and power Doppler are useful for assessment of vascularity, but detailed Doppler characterization is less well established than in other areas of ultrasound.^13-15

Spatial compounding and tissue harmonic imaging have allowed us to take advantage of the greater detail by helping manage the artifacts generated by higher frequencies and bandwidth. Compound imaging is performed by processing information from multiple sound beams that are generated from across the transducer face. Harmonic imaging takes advantage of echoes of harmonic frequencies that can be detected and separated from the fundamental frequency and associated artifacts. When using supplemental processing algorithms, care must be taken to not eliminate “true” internal echoes.

At a minimum, one should use a good linear array transducer operating at least at 7 to 10 MHz¹⁶ and capable of focusing well in the near field.

Considerable attention is given to this section as many errors are directly related to improper technique.^16,17

Technique is adapted to the purpose of the examination. Most breast sonographic evaluations will be performed in the diagnostic setting as focused examinations targeted to a specific clinical or imaging abnormality. If a persistent focal clinical abnormality is being assessed, targeting of the sonographic exam is usually straightforward. For subtle or inconsistently reproducible clinical findings, targeting may be more difficult, but in all circumstances every attempt should be made to determine whether a sonographic finding does or does not correlate with a given clinical finding. Research into the medical record may be needed if the patient does not clearly identify the area of clinical concern, or if the clock-face position and distance from the nipple have not been indicated on the order.

If sonography is performed for an abnormality identified on another imaging test, it is equally important to determine if a sonographic abnormality corresponds to the imaging abnormality for which ultrasound was undertaken. This correlation can be difficult given differences in breast position and orientation on sonography, mammography, and MRI. It is helpful to adopt a reproducible system for three-dimensional localization in the breast to facilitate correlation with other modalities, as well as to allow more accurate follow-up of sonographic findings placed into imaging surveillance. The use of clock-face position is a nearly universal way of providing some degree of localization in the breast, but this does not provide radial or depth information. Radial coordinates can be given as distance from the nipple or in zones (eg, three concentric rings from the nipple). Depth coordinates are best given as three zones or layers from the skin to the chest wall, as the actual distance may vary considerably depending on patient position.

Position of the patient is usually supine, although it may occasionally be useful to scan the patient in an upright position, especially if it is difficult for the patient to reproduce a palpable finding when supine. The ipsilateral arm is raised to stretch out the breast tissue and decrease tissue thickness. Initially, the breast should be positioned in such a way that it falls symmetrically on the chest wall with the nipple in the center of the breast. Multiplanar coordinates should be derived from this position, as radial and depth coordinates may change significantly in other positions. Patient position can then be changed to maximize scanning detail. For example, when evaluating lesions deep in the upper outer quadrant of the breast, it may be helpful to have the patient roll farther away from the examiner so that most of the breast tissue falls medially, thinning out the tissue laterally, reducing depth, and improving detail.

All focal abnormalities should be thoroughly evaluated in real time and recorded in two planes. While transverse and sagittal scan planes are common in most areas of the body, the breast and its ductal system are arranged radially. The use of radial and antiradial planes is useful for orientation, and likewise, radial scanning may help identify subtle ductal pathology such as duct extension, which may be seen in invasive malignancy. Regardless of the scan planes chosen, all lesions should be measured in three dimensions, and all images should have appropriate identifying information.

Gain should be adjusted so that there is even echogenicity throughout the depth of the tissue being imaged and so that mammary fat is mid-gray.¹⁸ Tissue is then characterized as hypoechoic or hyperechoic relative to the mammary fat on ultrasound. Doppler evaluation is suggested for all masses that are not completely anechoic; this is not so much to characterize the nature of the blood flow, but simply to determine if there is blood flow within the mass, which would indicate that it is solid. However, the lack of detectable internal Doppler flow does not exclude a solid matrix. For superficial lesions, one should use a standoff pad or a thick layer of gel to improve near-field detail.

Transducer pressure and angle can dramatically alter the appearance of the underlying tissue. In general, it is best to use light to moderate pressure on the transducer. Increasing pressure may help resolve edge shadows and other artifacts, and decreasing pressure may help identify subtle vascularity that can be obscured by too much pressure. Angling or rocking the transducer back and forth can improve margin assessment, and rotation of the transducer helps confirm that a finding is three-dimensional. Utilizing a two-handed technique where one hand provides “back-pressure” while the other hand holds the transducer can also help in certain situations, especially in evaluation of the retroareolar region.

When performing screening breast sonography, it is necessary to develop a systematic approach to be certain that the entire breast has been scanned in two planes. This may be very time consuming, especially with large breasts. Benign sonographic findings are frequently encountered in screening and should not distract the examiner from the primary goal of searching for suspicious sonographic lesions. Any value of screening sonography can easily be lost by under-calling subtle cancers or by over-calling benign findings.

Just as it is important to generate images to completely characterize a sonographic finding, it is important to accurately describe that finding in the breast imaging report. The American College of Radiology’s Breast Imaging-Reporting and Data System (BI-RADS) lexicon has been developed to allow standardized reporting for mammography, breast sonography, and breast MRI. Use of standard lexicon terminology is strongly recommended to facilitate communication and understanding of results. Although BI-RADS outcome coding is only mandated for mammography, its use in sonography and other breast imaging modalities is encouraged for standardization.