Elastography

Introduced in 1991, elastography, also known as elasticity imaging, has recently shown its usefulness in improving the diagnosis of diseases in many parts of the body. This technology produces images based on the relative stiffness of tissues and maps their elastic properties. By applying compression to the breast, elastography is able to produce images, which show the differences in firmness in breast tissue. Breast cancers, generally firmer than benign lesions, appear darker when compared with surrounding tissue or as red in commonly seen green/red color scale. Although elastography appears as a promising adjunct to gray-scale imaging, it is still being studied for its value in improving diagnostic performance. With researchers actively redesigning elastography hardware and software, elastography may soon show its benefits in diagnostic sonographic breast imaging.

Automated whole-breast sonography

When used on a localized area of the breast, sonography is an important tool in a patient’s diagnosis. As previously mentioned, whole-breast sonography is not routinely done for screening purposes due to it being completely operator dependent. It is difficult to have exact documentation of the entire breast with a handheld device projecting a small field of view. In response to this issue, there has been commercial interest in developing an automated ultrasound system for the breast.

Automated whole-breast sonography may be performed with the patient supine or upright, depending on the design of the equipment. This water path system allows the breast to be compressed, which provides excellent visualization of the mammary tissue. The automated transducer has the ability to obtain images over the entire breast. The machine then correlates the B-mode data and appropriately places the lesion oriented to the nipple and quadrant of the breast. Finally, the data are then put together in a series of sonographic gray-scale cine images, which form a whole-breast image.

Three-dimensional imaging

Three-dimensional (3D) imaging has been successfully used in other areas of sonography such as obstetrics and gynecology. Currently its diagnostic capabilities are comparable to two-dimensional (2D) imaging producing sensitivities and specificities that are close statistically. Unlike 2D imaging, 3D offers images of the coronal plane. In this plane, masses can be described in two new sonographic patterns: compression and converging, also known as retraction. Clinical findings show that the compression patterned masses have an oval shape and are associated with benign lesions, whereas the converging pattern masses appear with spiculations and are associated with malignant tumors.

Normal anatomy

The breast is a modified sweat gland located in the superficial fascia of the anterior chest wall. The major portion of the breast tissue is situated between the second and third rib superiorly, the sixth and seventh costal cartilage inferiorly, the anterior axillary line laterally, and the sternal border medially. In many women, the breast extends deep toward the lateral upper margin of the chest and into the axilla. This extension is referred to as the axillary tail of the breast, or the tail of Spence ( Figure 21-1 ).

The mammary milk line is the anatomic line along which breast tissue can be found in some women. The axillary tail of Spence is an extension of breast tissue into the axilla that is present in some women.

The surface of the breast is dominated by the nipple and the surrounding areola. A few women may have ectopic breast tissue or accessory (supernumerary) nipples. Ectopic breast tissue and accessory nipples are usually located along the mammary milk line, which extends superiorly from the axilla downward and medially in an oblique line to the symphysis pubis of the pelvis.

Sonographically, the breast is divided into three layers located between the skin and the pectoralis major muscle on the anterior of the chest wall. These layers are the subcutaneous layer, the mammary (glandular) layer, and the retromammary layer ( Figure 21-2 and Box 21-1 ). The subcutaneous and retromammary layers are usually quite thin and consist of fat surrounded by connective tissue septa. Although fat is often highly echogenic in other parts of the body, it is the least echogenic tissue within the breast. The fatty tissue appears hypoechoic, and the ducts, glands, and supporting ligaments appear echogenic ( Figure 21-3 ).

Breast anatomy.

Fifteen major ductal systems are present within the breast. Each gives rise to many separate terminal ductal lobular units (TDLUs) containing the terminal ducts, at least one lobule, and the separate acinar units (milk-producing glands) within each lobule. Each TDLU is surrounded by varying amounts of loose and dense connective tissue. The TDLU represents the site of origin of nearly all pathologic processes of the breast. Cooper’s ligaments surround and suspend each of the TDLUs within the surrounding fatty tissue. The ligaments extend to the subcutaneous layer of the skin and the deep retromammary layer next to the pectoralis fascia overlying the chest wall.

Sonographic layers of breast tissue.

The three layers of breast tissue are bordered by the skin and chest wall muscles (arrows). The subcutaneous fat layer and the retromammary fat layer are usually very thin (arrowheads). The mammary layer (asterisks) varies remarkably in thickness and in echogenicity, depending on location within the breast (most glandular tissue is located in the upper outer quadrant) and the patient’s age, hormonal status (e.g., pubertal, mature, gravid, lactating, postmenopausal), and inherited breast parenchymal pattern.

The mammary/glandular layer includes the functional portion of the breast and the surrounding supportive (stromal) tissue. The functional portion of the breast is made up of 15 to 20 lobes, which contain the milk-producing glands, and the ductal system, which carries the milk to the nipple. The lobes emanate from the nipple in a pattern resembling the spokes of a wheel. The upper outer quadrant of the breast contains the highest concentration of lobes. This concentration of lobes in the upper outer quadrant of the breast is the reason why most tumors are found here, as most tumors originate from within the ducts. The lobes of the breast resemble a grapevine branch; the major duct branches into smaller branches called lobules. Each lobule contains acini (milk-producing glands; singular acinus ), which are clustered on the terminal ends of the ducts like grapes on a vine. Literally hundreds of acini are present within each breast ( Figures 21-4 and 21-5 ). The terminal ends of the duct and the acini form small lobular units referred to as terminal ductal lobular units (TDLUs), each of which is surrounded by both loose and dense connective tissue. The TDLUs are invested within the connective tissue skeleton of the breast (see Figures 21-2 , 21-4 , and 21-5 ). Normal TDLUs measure 1 to 2 mm and usually are not differentiated sonographically. The TDLU is significant in that nearly all pathologic processes that occur within the breast originate here. The space between the lobes is filled with connective and fatty tissue known as stroma. These stromal elements are located both between and within the lobes and consist of dense connective tissue, loose connective tissue, and fat. The connective tissue septa within the breasts form a fibrous “skeleton,” which is responsible for maintaining the shape and structure of the breast. These connective tissue septa are collectively termed Cooper’s ligaments; they connect to the fascia around the ducts and glands and extend out to the skin.

Galactogram (contrast injected retrograde into a single ductal system) showing opacification of individual glands (terminal ductal lobular units [TDLUs]). Normally, TDLUs are 2 mm or less in diameter. The TDLU is the site of origin of most pathologic processes within the breast.

Three-dimensional histology showing normal terminal ductal lobular units (TDLUs) and dilated TDLUs. Normal TDLUs are usually no larger than 2 mm. Fibrocystic condition and other pathologic processes can cause marked enlargement of the TDLU. Note the difference between the normal TDLU within the black box and the dilated TDLU filling the right half of the image. The white box shows a single dilated acinus within the enlarged TDLU, showing cellular changes of apocrine metaplasia (one of the tissue changes of fibrocystic condition recognized by pathologists), and causing the lining cells to enlarge and overproduce fluid.

The pectoralis major muscle lies posterior to the retromammary layer. It originates at the anterior surface of the medial half of the clavicle and anterolateral surface of the sternum and inserts into the intertubercular groove on the anteromedial surface of the humerus ( Figure 21-6 ). The lower border of the pectoralis major muscle forms the anterior margin of the axilla. The pectoralis minor muscle lies superolateral and posterior to the pectoralis major. The pectoralis minor courses from its origin near the costal cartilages of the third, fourth, and fifth ribs to where it inserts into the medial and superior surface of the coracoid process of the scapula. These muscles sonographically appear as a hypoechoic interface between the retromammary layer of the breast and the ribs (see Figure 21-6 ). Although most lesions are found within the glandular tissue of the breast, it is important to evaluate tissue all the way to the chest wall.

The hypoechoic pectoralis muscle (arrows) is seen between the retromammary layer and the ribs.

Sonographic appearance

The boundaries of the breast are the skin line, nipple, and retromammary layer. These generally give strong, bright echo reflections. The areolar area may be recognized by its slightly lower echo reflection compared with the nipple and the skin. The internal nipple may show low to bright reflections with posterior shadowing, and it has a variable appearance ( Figure 21-7 ).

Shadow from the areolar area prevents imaging directly posterior to the nipple. The transducer should be moved away from the nipple area to image the mammary and retromammary layers.

Subcutaneous fat generally appears hypoechoic, whereas Cooper’s ligaments and other connective tissue appear echogenic and are dispersed in a linear pattern ( Figure 21-8 ). Cooper’s ligaments are best identified when the beam strikes them at a perpendicular angle; compression of the breast often enhances the ability to visualize them.

Subcutaneous fat is hypoechoic, whereas Cooper’s ligaments appear echogenic within the subcutaneous layer (arrows).

The mammary/glandular layer lies between the subcutaneous fatty layer anteriorly and the retromammary layer posteriorly ( Figure 21-9 ). The fatty tissue interspersed throughout the mammary/glandular layer dictates the amount of intensity reflected from the breast parenchyma. If little fat is present, a uniform architecture with a strong echogenic pattern (because of collagen and fibrotic tissue) is seen throughout the mammary/glandular layer. When fatty tissue is present, areas of low-level echoes become intertwined with areas of strong echoes from the active breast tissue. Analysis of this pattern becomes critical to the final diagnosis, and one must be able to separate lobules of fat from a marginated lesion.

Mammary-glandular layer lies between the subcutaneous fatty layer anteriorly and the retromammary layer posteriorly (arrows).

The retromammary layer is similar in echogenicity and echotexture to the subcutaneous layer, although the boundary echoes resemble skin reflections ( Figure 21-10 ). The pectoral muscles appear as low-level echo areas posterior to the retromammary layer. The ribs appear sonographically as hyperechoic rounded structures with dense posterior shadowing. They are easily identified by their occurrence at regular intervals along the chest wall.

The retromammary layer is similar in echogenicity and echo texture to the subcutaneous layer.

Several normal structures within the breast can appear abnormal, unless care is taken to prevent this during the sonographic examination. The ducts immediately behind the nipple frequently cause acoustic shadowing and can be mistaken for a suspicious breast mass. Angling of the transducer in the retroareolar tissue will usually improve visualization and eliminate doubt. The distinction of a subtle isoechoic or hypoechoic sonographic mass from normal fibroglandular tissue in the breast can sometimes be troublesome. The adipose or fatty tissue can situate itself in and among the areas of glandular tissue and, in some scanning planes, can mimic isoechoic or hypoechoic masses. It is helpful to turn on the structures to see if they are consistent or lengthen out within the scanning plane. To see whether a structure will lengthen out geometrically, one should rotate the transducer 90 degrees during real-time. In the case of a true sonographic mass, the mass will maintain its shape in both dimensions, confirming its 3D character, whereas glandular tissue elements will elongate and appear less like a mass.

Parenchymal pattern

The size and shape of the breasts vary remarkably from woman to woman. Some women have more glandular tissue, some have less. Some have more fatty tissue than others, and some have more connective tissue, thus resulting in firmer breasts. Some women have very little breast tissue. The size and shape of the breasts also vary over time because of changes that occur during the menstrual cycle, with pregnancy and breast-feeding, and during menopause. Most differences in breast size between women are due to the amount of fatty tissue within the breasts.

The involutional changes that occur in the breast throughout life affect the appearance and pattern of the breast parenchyma. Involution is hallmarked in breast imaging by the remodeling process that causes glandular tissue to be slowly replaced by fatty tissue. This accounts for differences in the size, shape, and architecture of breast tissue.

Generally, in a young woman, fibrous tissue elements predominate, and the resulting appearance on mammography and ultrasound is a dense echogenic pattern of tissue ( Figure 21-11 ). In a pregnant or lactating woman, the glandular portions of the breast proliferate remarkably in both density and volume, creating interfaces that are less echogenic. As a woman ages, the glandular breast tissue undergoes cell death and is remodeled by the infiltration of fatty tissue. The tissue is progressively replaced by fat and, with the onset of menopause, the ducts atrophy, resulting in a mammographic and sonographic pattern with less fibrous tissue elements ( Figure 21-12 ). This fatty breast is most difficult to image by sonography, as all three layers of the breast appear hypoechoic, with less distinction between the layers. Sonographically, cancers can be difficult to differentiate in the fatty breast because most cancers appear hypoechoic and can be difficult to differentiate from normal breast tissue. Although sonography of the fatty breast is difficult, mammography images this type of breast very well.

Dense breast.

A, Example of dense breast tissue on mammogram. Mammographic technique emphasizes tissue contrast. As a result, the skin often is not visible on routine images. The skin is separated in this case by nearly 2 cm from the outer margins of the dense mammary layer. B, Example of ultrasound appearance of dense breast tissue. C, Example of variation in breast tissue pattern at different locations even within the same breast.

Fatty breast.

A, Example of predominantly fatty tissue on mammogram. B, Example of ultrasound appearance of predominantly fatty tissue. Note the loss of sonographic detail in the deeper layers of the breast and chest wall. Fat deflects the ultrasound beam and degrades detail. C, Example of improved visualization of skin and subcutaneous tissues with a stand-off pad.

Vascular supply

The main arterial supply to the breast comes from the internal mammary and the lateral thoracic arteries. More than half of the breast—mainly the central and medial portions—is supplied by the anterior perforating branches of the internal mammary artery. The remaining portion—the upper outer quadrant—is supplied by the lateral thoracic artery; intercostals and subcapsular and thoracodorsal arteries contribute in lesser ways to the blood supply.

Venous anatomy largely parallels the arterial anatomy in the deep breast. However, venous drainage is mainly provided by unpaired superficial veins that can be seen sonographically just under the skin. These surface veins are often enlarged with superior vena cava syndrome or chronic venous thrombosis of the subclavian vein, as well as when arteriovenous shunts are placed in patients with chronic renal insufficiency. Figure 21-13 shows an example of a grossly dilated surface vein in the breast. When there is doubt concerning the vascular nature of a long, tubular, anechoic structure on breast ultrasound, such as the distinction between a dilated duct and a vessel, color flow vascular imaging or Doppler ultrasound techniques can easily resolve this situation.

Dilated veins in the breast.

A, Mammographic image of a breast showing markedly dilated surface veins in an elderly woman. B and C, Ultrasound images of veins just under the skin of the breast in the same patient. In cases in which there is doubt, color flow mapping or Doppler techniques will easily confirm the vascular nature of these dilated tubular structures and will distinguish them from dilated ducts.

Lymphatic system

Lymphatic drainage from all parts of the breast generally flows to the axillary lymph nodes. The flow of lymph is promoted by valveless lymphatic vessels that allow the fluid to mingle and proceed unidirectionally from superficial to deep nodes of the breast. The flow of lymph moves from the intramammary nodes and deep nodes centrifugally toward the axillary and internal lymph node chains. It has been estimated that only about 3% of lymph is eliminated by the internal chain, whereas 97% of lymph is removed by the axillary chain.

Part of the standard surgical therapy of invasive breast cancer involves axillary lymph node dissection. This is vital in the staging and management of breast cancer because nodal status affects the patient’s prognosis and is important in guiding adjunctive therapy. Although most tumors can infiltrate and spread via the axillary lymph nodes, they may begin their infiltration by using alternative lymph channels, such as the internal mammary chain within the chest, across the midline to the contralateral breast, deep into the interpectoral (Rotter’s) nodes, or into the supraclavicular nodes ( Figures 21-14 , 21-15 , and 21-16 ).

Lymphatic drainage of the breast.

A, General position of the major axillary lymphatic groups I, II, and III in relation to the pectoralis major and minor muscles of the chest wall. On the right side of the figure, the major lymphatic flow from the periareolar plexus toward the axilla is shown. Alternative routes of lymphatic flow include (1) retromammary nodes, (2) contralateral flow to the opposite breast, (3) interpectoral (Rotter’s) nodes located between the pectoralis major and minor muscles, (4) supraclavicular nodes, and (5) diaphragmatic nodes. B, Same information in cross section.

Normal and abnormal lymph nodes.

A and B, Sonographic images of a normal lymph node showing a smooth homogeneously hypoechoic cortex and an echogenic fatty internal hilum. C–E, Images of abnormal lymph nodes. Signs of suspicion for metastatic involvement of lymph nodes include an irregular, inhomogeneous cortex and loss of the fatty hilum. C, Lymph node that has nearly lost the fatty hilum and has a poorly defined but relatively homogeneous cortex. This may be a reactive lymph node or one with early metastatic involvement. D, Similar lymph node that has completely lost its fatty hilum but has a smooth homogeneous cortex. E, Large hypoechoic mass that has no sonographic characteristics of a normal lymph node. This mass was sampled by ultrasound-guided core biopsy, confirming a low axillary lymph node nearly completely replaced with metastatic cancer.

Metastatic spread of breast cancer to the opposite breast.

A, Mammogram showing several moderate- to low-density, relatively benign-appearing masses (arrows) in a patient with a history of mastectomy for cancer in the opposite breast. B, Image of one of the masses shows a solid mass with an irregular, poorly defined margin and heterogeneous echogenicity. These lesions were all metastatic lesions from the opposite breast, likely from an alternative route of lymphatic spread across the midline.

The male breast

In males, the nipple and the areola remain relatively small. The male breast normally retains some ductal elements beneath the nipple, but it does not develop the milk-producing lobular and acinar tissue. The ductal elements usually remain small but can hypertrophy during puberty and later in life under the influence of hormonal fluctuations, disease processes, or medications. This condition, in which the ductal elements hypertrophy, is called benign gynecomastia ( Figure 21-17 ). Imaging with mammography and ultrasound is often requested to exclude breast cancer as a cause.

Two male patients with palpable breast masses.

A, Mammographic appearance of benign gynecomastia. This condition is nearly as often unilateral as bilateral. B, Ultrasound example of the frequently masslike appearance of gynecomastia. C, Mammography example of male breast cancer. D, Sonographic image with stand-off pad of palpable suspicious breast mass in elderly male. Note the heterogeneous echogenicity and the elevation of skin over the mass.

Although breast cancer is uncommon in males, it does occur. Approximately 1300 new cases are diagnosed each year within the United Sates. The occurrence approximates 1% of the incidence in women. Box 21-2 lists male patients who have an increased risk for breast cancer.

Breast screening

The primary purpose of breast screening is the detection and diagnosis of breast cancer in its earliest and most curable stage. Accurate identification of benign breast lesions during cancer screening is also important for good care because it can save the patient from unnecessary surgical procedures and resultant tissue scarring.

Three general categories of diagnostic breast imaging are available, two of which involve breast ultrasound. These three categories include breast cancer screening (generally performed by physical breast evaluation with mammography), diagnostic interrogation (consultation, problem solving, workup), and interventional breast procedures (histologic diagnosis and/or localization).

Breast cancer screening is recommended in women without clinical signs of breast cancer ( Box 21-3 ). According to the American Cancer Society, breast cancer screening involves monthly breast self-examination (BSE), regular clinical breast examination (CBE) by a physician or other health care provider, and annual screening mammography. Monthly BSE is best performed at the end of menses and should begin at age 20. CBE should be performed once every 3 years from ages 20 to 39 and at least yearly from age 40 on. Screening mammography should be performed yearly starting at age 40. BSE and CBE are important steps in breast cancer screening because 70% of cancers are found as lumps felt during BSE and CBE. BSE and CBE may also identify other signs or symptoms of possible breast cancer that require further evaluation by diagnostic breast imaging ( Box 21-4 ).

Mammography, sonography, and magnetic resonance imaging (MRI) are the primary imaging tools used for diagnostic breast evaluation. Mammography provides a sensitive method of screening for breast cancer, whereas ultrasound and MRI are used to provide additional characterization and further interrogation of breast lesions that are not well visualized by mammography. The mammographic signs of breast cancer are listed in Box 21-5 . Because ultrasound examination is performed by scanning in cross-sectional planes, it is difficult to adequately screen the entire breast in most patients. Ultrasound may be used for screening purposes in young, dense breasts, which are difficult to penetrate by mammography; to evaluate palpable masses that are not visible on a mammogram; and to image the deep juxtathoracic tissue not normally visible by mammography. Ultrasound is also useful in differentiating structures within uniformly dense breast tissue in which mammography is limited (e.g., in differentiating solid, round masses from fluid-filled cysts and in visualizing tissue adjacent to implants or other structures that limit visualization by mammography). MRI is also a useful tool in breast imaging but is prohibitively expensive for screening purposes. Because a strong magnetic field is used to create images, not all patients are good candidates for MRI (e.g., patients with pacemakers or artificial joints). Patients who suffer from uncontrolled claustrophobia are also not good candidates for MRI.

Breast evaluation

The overall goal of breast evaluation is the proper classification of a breast lesion according to the level of suspicion for breast cancer. Thorough evaluation takes into account the results of both the breast imaging assessment and the clinical assessment. The appropriate next step in patient management is dictated by the level of suspicion for cancer in any breast lesion and takes into account the age and individual risk factors for each particular patient. Risk factors for breast cancer are listed in Box 21-6 .