The first known clinical use of breast ultrasound (US) was reported in 1951 by Wild and Neal,  who used A-mode sonography to describe the features of one benign and one malignant breast mass. Several attempts were subsequently made to develop automated or multi-transducer scanners to evaluate the whole breast, both to detect subclinical disease (screening) as well as for lesion characterization. 
The 1970s saw a declining interest in breast ultrasound, and, for more than a decade thereafter, ultrasound was relegated to differentiating solid from cystic masses. However, with increasing resolution and quality of US imaging, interest in its use for evaluation of the breast has resurged. 
An image depicting the breast structures visible on ultrasound can be seen below.
Ultrasound is primarily used in the diagnostic setting in the presence of a palpable abnormality or when a previous mammogram has demonstrated a focal lesion. To its advantage, both dense and fat replaced breasts can be well evaluated.
Major indications include the following:
Focal abnormality on mammography
Follow-up of lesions not biopsied (mostly BIRADS-3 lesions)
Determination of lesion extent in patients with suspicious or malignant nodules
Assessment of regional lymph nodes in patients with suspicious or malignant lesions
Guiding interventional procedures
Attempts have been made to establish a place for ultrasound in screening. The American College of Radiology Imaging Network has published data on whole breast ultrasound screening (ACRIN6666), showing that the cancer detection rate with ultrasonography is comparable to that with mammography. 
Given the laborious protocol of screening whole breasts, widespread screening applications are expected to appear only when automated breast scanners have demonstrated ease of use and a sensitivity and specificity comparable to handheld breast screening. [5, 6]
Ultrasound is inherently operator dependent, breast ultrasound even more so; thus, a major contraindication would be inadequate operator experience.
Ultrasound equipment with a high-resolution linear transducer is necessary. Transducers with 7-MHz to 12-MHz frequency are preferred because of better near-field resolution. 
Too deep a focus causes volume-averaging artifacts and gives erroneous results about tissue consistency. Matrix probes that can be focused in the short axis as well offer better resolution.
Even with high-resolution probes, the focal zone (indicated by moving small arrowheads at the side of the screen) must be kept either at the lesion or just deep to it for best resolution.
Machines with panoramic view (extended field of view) options are often useful because this allows very large areas to be collated into a single image. Some machines offer an extended imaging field (trapezoid imaging) while in others, the 2 split fields can be appended to image more of a larger lesion. However, note that one must avoid using the panoramic images for measurements.
Current-generation ultrasound machines offer additional image processing facilities, including spatial compounding, speckle reduction, harmonic imaging, and speed-of-sound imaging that offer enhanced resolution. These modes should be used and optimized for each patient. 
Additional features include elastography that color-codes tissue hardness, adding information that can be useful in certain conditions, as well as microcalcification imaging that enhances the presence of calcium particles.
An acoustic stand-off pad is helpful in imaging very superficial lesions.
Machine settings should be optimized for breast ultrasound: this includes appropriate focal depth, an optimal gain, and time gain compression (TGC; usually the default, without any curve), a high dynamic range, contrast and high edge enhancement settings. The depth of field should extend to the pleural echoes.
When optimized, the fat should appear medium gray in the premammary as well as the retromammary space.
Generally the following positions are used:
Medial lesions: The patient is supine, with the ipsilateral arm over patient’s head.
Lateral lesions: The patient is opposite posterior oblique, with the ipsilateral arm over the patient’s head. The degree of obliquity depends on the breast size, pendulousness, and the location of the lesion within the breast. In a large breast, a complete decubitus position might be the best.
If a lesion is superficially located and is palpable in the erect position, the patient should be examined, at least for a part of the procedure in this position. Remember that the breast appears thicker in the erect position, so this is not appropriate for posterior lesions.
Superior lesions: The patient is supine, opposite posterior oblique or sitting, with the ipsilateral arm over the patient’s head.
Inferior lesions: The patient is supine, and the breast can be held superiorly.
Varying degree of pressure is used to flatten the breast parenchyma as well as bring the conical surface of the glandular tissue into a more orthogonal plane.
Given the real-time dynamic nature of ultrasound imaging, and the tendency of the breast to change its shape with gravity, the patient position should be optimized to the examination.  This may be done several times to allow the acquisition of high-quality, reproducible images. Different positions are especially useful in large breasts. Inadequate positioning may cause portions of the breast to become inaccessible, the breast to fall over the transducer or the lesions to appear displaced on follow-up examinations. Small breasts with little soft tissue to glide over the chest wall can be examined in the supine position.
The breast is a modified sweat gland located between two layers of the superficial pectoral fascia on the anterior thorax, between the second and sixth ribs (see the image below).  The 2 layers can sometimes be seen behind the skin echo and in front of the pectoral muscle echoes.
Twelve to 20 ducts exist, and their lobules, along with fat and stroma, make up the parenchyma of the breast. The breast ducts converge at the nipple at one end, and, at the other end, they branch to terminate in lobules. The duct size varies from about 1 mm at the lobules to up to 5-8 mm near the nipple. The lobules are on an average 3 mm in the greatest diameter. The ratio of the parenchyma to the fat varies by the individual and even in the same woman at different ages and during pregnancy and lactation.
Before moving on to breast ultrasound anatomy, understand that the reference tissue in the breast is the fat, and structures are labeled as hypo, iso, or hyperechoic with reference to fat.
Ultrasound shows certain discrete structures that can be recognized. From superficial to deep these are as follows:
Skin: This is recognized as a thin hyperechoic to isoechoic zone between 2 thinner hyperechoic lines. Usually this is less than or equal to 2 mm in thickness but is slightly thicker over the areolae.
Subcutaneous fat (premammary fat): This is the reference tissue. Premammary fat is more hypoechoic than fat elsewhere and has a lobular structure with the fat lobules surrounded by a very thin echogenic layer.
Parenchyma: This layer appears as an echogenic plate; this can be homogenous in some women but can be heterogeneous, almost tigroid in appearance in others, especially in young women. This can also vary in echogenicity from echogenic in usual cases to almost isoechoic in lactating breasts. Within the parenchyma ducts, the terminal ductal lobular unit (TDLU) can be seen occasionally.
The ducts can be seen as thin echogenic lines representing collapsed and apposed duct walls, or lines surrounded by a hypoechoic zone that is the loose stromal tissue surrounding the ducts. Frequently, only the isoechoic periductal stromal tissue is seen, giving the almost striped (tigroid) appearance. With progressive accumulation of intraductal fluid, the ducts can appear as tubes that can be up to 5 mm across. Duct ectasia becomes more frequent with age, and almost 50% women have ectatic ducts by the age of 50 years. Duct ectasia is mostly an incidental finding but may be associated with discharge and periductal mastitis.
With very high-resolution images, TDLUs are sometimes seen as small hypoechoic elongated or rounded structures. These are usually located peripherally in the parenchymal plate, more numerous in the anterior part than the posterior part. 
Cooper ligaments attach to the parenchymal surface from the anterior mammary fascia and can be seen as thin echogenic bands, widening as these insert into the anterior parenchymal surface. A few Cooper ligaments are seen posteriorly as well.
For more information about the relevant anatomy, see Breast Anatomy.
The patient is examined in the appropriate position already described. The breast is examined in a defined pattern to ensure that the whole organ is visualized; this usually means scanning the breast in a raster pattern in both the longitudinal as well as horizontal planes. Scanning then proceeds radially and finally in oblique planes, for measuring the long and short axes of lesions and also along ducts.
Special attention must be given to the nipple and the peri-nipple subareolar region. The ducts within the nipple and immediately near it cannot be seen with the usual technique. Usually the nipple is compressed when scanned orthogonally and appears as a poorly shadowing nodule within or even deep to the skin as it is pushed into the breast substance by probe pressure. The nipple and its ducts achieve a more favorable geometry with the ultrasound beam if it is rolled to one side by the probe with varying degrees of peripheral pressure. The breast can be supported by the other hand to really flatten the breast contour against the probe to give additional information.
Ultrasound guidance for biopsies is both safe and effective. Taking a biopsy of any lesion that is visible on ultrasound is technically possible. One must keep in mind that the breast flattens against the chest wall during ultrasound, so an oblique needle path carries a risk of injury to underlying muscles or even the pleura and lungs. Therefore, the needle path should be as parallel to the chest wall as possible to minimize the risk of injury to deeper tissues.
Follow-up imaging, biopsy, or surgery must have a reproducible system of lesion localization. At the least, one must include the side (right or left breast), the clock face with the center at the nipple, the distance from the nipple or the zone (typically, the breast is divided into 5 zones comprising the subareolar zone [SA], 3 circular concentric zones outside the SA, and the axillary zone [AX]). The probe orientation is also mentioned, which could be horizontal, vertical, radial, or antiradial (at right angle to the radial section at that point). Additional oblique sections are often required to assess the longest diameter and the short axis of lesions. Including the depth of the lesion from the skin may be helpful, but this is not always necessary.
All lesions should be recorded in at least the long axis and short axis, appropriately labeled with the annotation information.
Using ultrasound criteria a lesion can be classified into a BIRADS-US system developed by the ACR with the following interpretation:
BIRADS-0: Incomplete assessment needs additional evaluation
BIRADS-3: Probably benign (2% of less chances of malignancy)
BIRADS-4: Suspicious (2-95% chances of malignancy)
BIRADS-5: Malignant (>95% chances of malignancy)
BIRADS-6: Biopsy-proven malignancy
|BIRADS category||Conditions that fall into the category||Management|
|BIRADS-1||Normal tissue, focal thickening causing “lump”||Routine screening|
|BIRADS-2||Simple cysts, ductal ectasia, intramammary nodes, lipomas||Early follow-up to document stability unless clinical indications suggest a more aggressive evaluation|
|BIRADS-3||Complex cysts, small intraductal papillomas, fibroadenomas||Short interval follow-up or biopsy|
|BIRADS-4||One feature of malignancy||Biopsy|
|BIRADS-5||More than one feature of malignancy||Biopsy|
The breast parenchyma can appear thin and membranelike in old age or thick and almost isoechoic during lactation. The breast in a young woman who has never been pregnant demonstrates great variation in echogenicity and pattern that ranges from a homogenously echogenic to a diffusely heterogeneous almost striped appearance due to ducts and periductal hypoechoic tissue. The parenchyma may also show focal thickening, which may occasionally cause a palpable abnormality.
Other age-related changes include ductal ectasia, which can be completely asymptomatic or be associated with nipple discharge. The fluid in ectatic ducts can be clear, have low-level echoes, or dependent debris. A solid component is highly suspicious for intraductal papilloma or cancer.
This includes simple and complex cysts and solid masses. The following imaging characteristics are evaluated:
Size - Measured in the long and short axes, to document stability, regression or growth
Shape - Taller than wide, or wider than tall, extension along ducts
Surface - Smooth, irregular, macro or microlobulated, number of lobulations (up to 3 can be considered nonsuspicious), angular margins, spiculations, capsulation
Internal texture - Isoechoic, hyperechoic, or hypoechoic; heterogeneous or homogenous; complex; calcification; presence of echogenic, cystic center, or calcification
Surrounding tissue - Echogenic halo, radiating branches, whether the tissue pulled in toward the mass, and acoustic shadowing
Fixity - On real time ultrasound, fixity to surrounding tissue and underlying muscles can be assessed.
Doppler - Vascularity does not help in differentiating between benign and malignant, although benign lesions tend to be more hypovascular. Color Doppler can assist in planning an appropriate needle trajectory to avoid large vessels.
A simple cyst, if it fulfils the appropriate ultrasound criteria (thin regular walls, no mural nodules, no septations, "clear" anechoic interior, posterior enhancement) is invariably benign. Asymptomatic simple cysts need not be aspirated, and the patient reverts to routine screening mammography.
These have echogenic foci in the contained fluid but have other features of simple cysts. These are usually benign but a short term follow-up may be indicated to document stability or resolution.
Cysts with mural nodules or thick septations should be evaluated further using aspiration cytology of the fluid or a needle biopsy from the solid component of the thickened cyst wall or septum, if present. The presence of solid tissue or a detectable linear vascularity consistent with a vascular stalk is concerning for an intracystic mass.
These are evaluated according to the criteria given in Table 2 (below). To be labeled as benign, the lesion should demonstrate only benign features. The presence of any suspicious finding warrants a biopsy.
|Shape||Round, wider than tall||Taller than wide|
|Margins||Smooth||Irregular, angular, spicular|
|Lobulations||None or up to 3||Multiple|
|Fixity||None||Fixed to surrounding issue and/or underlying muscles|
|Shadowing or enhancement||Enhancement, edge shadowing||Shadowing behind lesion|
|Substance echogenicity||Anechoic (cystic), Hyperechoic||Hypoechoic, calcification|
Image depicting breast ultrasonography can be seen below.
Sonographic guidance for interventional procedures offers several advantages: it is less expensive, quicker, available in real time, avoids ionizing radiation, and is portable. However, a learning curve exists, and the operator must be appropriately trained and skilled to undertake the intervention.
Ultrasound-guided interventions are used for cyst aspiration, fine-needle, and core biopsies, sentinel node injections, localizing wire placement, guiding the placement of brachytherapy catheters, and directional vacuum-assisted biopsy. During biopsy, care must be taken to orient the needle as parallel to the chest wall as possible, especially when performing a core biopsy with a spring-loaded biopsy gun or automated needle. The needle "throw," which may be over 2 cm in some cases, should be taken into account, so that when fired, the needle does not injure the underlying tissues.
Ultrasound of breast implants
Breast implants have been used both for purely cosmetic reasons and postmastectomy. Saline implants have replaced silicone implants as the most common breast prosthesis. Silicone implant imaging is important because the clinical signs of rupture are often nonspecific. In contrast, saline implant ruptures are more obvious due to the decrease in breast volume from extravasated saline. [13, 14]
An intact implant shows a smooth, thin linear membrane with a sonolucent anechoic interior, reverberation artifacts from the proximal wall, radial folds, and linear echoes. With intracapsular rupture, a "stepladder" appearance may be seen. Additionally, one may observe focal irregular bulges and the margins can become ill-defined. With extracapsular rupture, hyperechoic noise or a snowstorm appearance may be observed. Silicone granulomas may be seen that can appear as focal lesions with a snowstorm appearance, hyperechoic, hypoechoic, or even anechoic lesions. 
As ultrasound imaging technology advances, new methods offer incremental information over what basic scanning can yeild. Some of the newer ways of processing the image and display are already noted,  and these include elastography and microcalcification imaging.
The World Federation of Ultrasound in Medicine and Biology (WFUMB) has published guidelines on the use of ultrasound elastography for breast examination to measure the stiffness of breast tissue. According to the WFUMB, ultrasound elastography has been shown to be highly accurate in characterizing breast lesions as benign or malignant. Cancer tissue is stiffer than normal breast tissue, and it is believed that the tissue stiffening begins in the early stage of cancer. The Tsukuba score is a 5-point scale for grading the stiffness of a mass, but other methods of interpretation (eg, lesion-to-fat ratio) have been shown to be effective. 
Three-dimensional ultrasound adds additional features to the image and is slowly refining benign and malignant characteristics of breast masses. It is now known that many malignant lesions can have smooth contours, be wider than tall, and show acoustic enhancement, while benign lesions can have marked irregularity of contour.  Other areas of progress include contrast-enhanced ultrasound, which might find use in sentinel node scanning and complement or replace radionuclide methods. Automated breast ultrasound systems are already commercially available that scan the whole breast volume and increase the cancer pick-up rate by almost 100% as compared with mammography alone.  This technique is especially useful in women with dense breasts in whom mammography offers limited information and sensitivity.