Breast Ultrasound

Chapter 5 Breast Ultrasound

Ultrasound is a useful adjunct to mammography for the diagnosis and management of benign and malignant breast disease. Technical advances have resulted in consistent, reproducible, high-resolution clinical ultrasound images. Although whole-breast automated scanners are now available, most practices use high-resolution hand-held transducers. Scientific evidence and clinical experience support the use of hand-held real-time breast ultrasound to distinguish cysts from solid masses, determine the sonographic characteristics of solid masses, evaluate palpable lumps in young women, and provide guidance for percutaneous biopsy. Given improvements in image quality and data processing, studies suggest roles for breast ultrasound in breast cancer screening and evaluation of breast calcifications identified on mammography, with specific caveats on the technologic limitations of ultrasound for these indications. This chapter explores these and other indications for breast ultrasound.

Technical Considerations

Real-time hand-held scanners provide easy and rapid direct visualization of breast lesions for diagnosis or ultrasound-directed breast biopsy. Hand-held units should include a linear array, high-frequency transducer operating at a frequency of 7.5 to 10 MHz or greater, which provides good tissue penetration to 4 or 5 cm. All scanners should also include a marking system to document and annotate ultrasound images.

Technical issues must be overcome to obtain good sonographic image quality. Superficial lesions in the near field of the transducer may be distorted, but they can be imaged by using a high-frequency transducer or a soft fluid offset. The heterogeneity of breast tissue results in absorption of the ultrasound beam with increasing distance from the transducer. Extensive diffraction of the ultrasound beam leads to beam defocusing and poor image quality the further the lesion is from the transducer. Thus, accurate diagnosis of cysts and deep lesions depends on appropriate power, gain, and focal zone settings. Improper adjustments of any of these parameters can result in misdiagnosis by producing suboptimal images or artifactual echoes within simple cysts. Routine calibration of the unit and evaluation of the unit’s performance with a breast phantom help prevent these technical errors.

The amount of shadowing at normal breast tissue interfaces depends on the transducer’s diameter and the distance of the tissue from the transducer. Artificial shadowing can be caused by Cooper ligaments and normal breast structures, which usually angle up toward the skin and the transducer.

Flattening or compressing the breast tissue decreases the amount of tissue penetrated by the ultrasound beam and diminishes edge artifacts by straightening Cooper ligaments parallel to the transducer. To flatten the breast tissue in the upper outer quadrant, the patient is scanned supine with her hand behind her head in a posterior oblique position, with her back supported by a wedge. Facilities often use the sponge wedges used for positioning posterior oblique lumbar spine radiographs to help the patient achieve this position. For medial lesions, the patient lies flat on her back, which flattens the medial breast tissue. Sonographers use moderate compression during scanning by lightly but firmly pressing the breast with the transducer. This decreases the thickness of the tissue to be scanned, reduces beam absorption and defocusing, allows better penetration, and decreases shadowing from ligaments and glandular elements.

To ensure that the field of view includes all the breast tissue from the skin surface to the chest wall, the sonographer makes sure that the image includes the pectoralis muscle and chest wall at the bottom of the screen. The time-compensated gain (TCG) curve should be adjusted so that fat is uniformly gray from the subcutaneous tissues to the chest wall. This adjustment enables accurate evaluation of masses as cystic or solid at any depth in the breast. Incorrect settings that make the fat look anechoic may also make a solid mass look like an anechoic cyst.

Once the sonographer identifies an area of interest or a mass, he or she magnifies the finding to fill the monitor or screen appropriately because it is hard to see and analyze a lesion if it is too small on the screen. The sonographer then resets the focal zone, TCG curve, and depth-compensated gain (DCG) curves on the lesion to evaluate the finding’s shape, margins, and internal characteristics.

When scanning a breast with palpable findings, the sonographer asks the patient to point out the mass or symptomatic area to ensure evaluation of the area prompting investigation. This ensures that the patient’s area of concern is addressed and that the patient is more confident that her questioned area was investigated (because she pointed it out). If the patient is unsure of the location of the mass, the sonographer scans the quadrant or area requested by the referring physician on the order or requisition.

To scan palpable masses, the sonographer scans the palpable finding, then places a finger over the mass. The sonographer scans over the finger on the mass, and then removes the finger to scan only the palpable finding. This ensures that the palpable finding is in the field of view. Alternatively, the palpable finding can be trapped between two fingers. The sonographer scans the trapped mass between two fingers so that the mass does not roll out of the field of view from under the transducer.

The American College of Radiology (ACR) has made specific recommendations for ultrasound labeling. The sonographer labels each finding according to its location in right or left breast, quadrant or clock position, scan plane (radial or antiradial, longitudinal or transverse), and number of centimeters from the nipple, along with the sonographer’s initials (Box 5-1). The sonographer takes images of the mass with and without measuring calipers. Any other pertinent clinical information, such as whether the lesion is palpable, may also be helpful to note.

Normal Sonographic Breast Anatomy

The breast is composed of fibrous connective tissue (Cooper ligaments) arranged in a honeycomb-like structure surrounding the breast ducts and fat (Fig. 5-1A and B). The proportion of supporting stroma to glandular tissue varies widely in the normal population and depends on the patient’s age, parity, and hormonal status. In young women, breast tissue is composed of mostly dense fibroglandular tissue. In later years, dense tissue involutes into fat in varying degrees, producing a mixed fatty and dense breast or an all-fatty breast (see Fig. 5-1C to F).

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Figure 5-1 Normal breast ultrasound images. A and B, Normal breast ultrasound scans in fatty, mixed, and dense breasts. Unlabeled (A) and labeled (B) ultrasounds of a normal fatty breast show the thin, white, superficial skin line (arrow); dark subcutaneous fat; dark fatty lobules separated by sharp thin Cooper ligaments; and the muscle and chest wall at the posterior aspect of the image. Note that the fat is uniformly gray throughout the image, and multiple fatty lobules are interspersed between the thin, linear Cooper ligaments. Unlabeled (C) and labeled (D) normal breast ultrasounds of fibroglandular and fatty breast tissue show the thin, white, superficial skin line; dark subcutaneous fat; and white glandular tissue interspersed by dark hypoechoic fatty lobules. Note how the white glandular tissue is thicker than the Cooper ligaments seen in A and how the fatty islands might be mistaken for breast masses. Unlabeled (E) and labeled (F) ultrasounds of a normal dense breast show mostly white glandular tissue with scant amounts of fat and hypoechoic ducts over an area of thickening in a young patient. Note how the ducts appear as small, round, dark structures interspersed in the dense glandular tissue when caught in cross-section and appear like long tubes when caught in longitudinal section. Unlabeled (G) and labeled (H) ultrasounds of a normal lymph node show a lobulated hypoechoic mass with an echogenic center that represents the fatty hilum. Note that the fat outside the lymph node in the adjacent breast tissue is hypoechoic (dark), whereas fat inside the lymph node is echogenic (white). This is the typical appearance of a normal lymph node. I and J, Cooper ligaments and normal fatty tissue in a thin fatty breast. Unlabeled (I) and labeled (J) normal breast ultrasounds show the thin echogenic skin line at the top of the image; dark hypoechoic subcutaneous fat; thin, gently curving Cooper ligaments coursing through the fat; and the thin, parallel, tightly packed lines of muscle just above the chest wall and the rib. K and L, Landscape ultrasound view of normal dense tissue in a thin dense breast. Unlabeled (K) and labeled (L) landscape ultrasounds over a thin dense breast show the echogenic skin line, dark subcutaneous fat, dense white glandular tissue, and the pectoralis muscle and chest wall overlying the periodic round shadows of the ribs and intercostal muscles. The ribs cause acoustic shadowing. However, the ribs should not be mistaken for breast masses because they are located behind the muscle and chest wall just above the lung and are not within the breast.

Breast tissues are either echogenic (white) or hypoechoic (black) on ultrasound. The skin is an echogenic line immediately under the transducer in the near field. It is normally about 2 to 3 mm thick and has a hypoechoic layer of dark subcutaneous fat immediately beneath it (Box 5-2). Unlike echogenic or white-appearing fat around the superior mesenteric artery in the abdomen, fat in the breast appears dark or hypoechoic. The only exception to hypoechoic fat in the breast is the echogenic fat in the middle of a lymph node. The normal lymph node is an oval, well-circumscribed mass with a hypoechoic cortex and fatty echogenic hilum, often seen in the upper outer quadrant and axilla, and often near an artery (see Fig. 5-1G and H).

Breast glandular tissue and connective tissue are echogenic or white. Connective tissue has the highest acoustic impedance, fat has the lowest, and glandular parenchyma is of intermediate echogenicity. The Cooper ligaments are thin, sharply defined linear structures that support the surrounding fat and glandular elements (see Fig. 5-1I and J). Cooper ligaments in a fatty breast look like thin, white, gently curving lines surrounding hypoechoic fat. Normally, Cooper ligaments are thin and sharply demarcated. In breast edema, the fat becomes gray and the normally sharp Cooper ligaments become blurred.

Subareolar ducts are dark, hypoechoic, tubular structures leading to the nipple. The glandular tissue elements are echogenic (white), so normal hypoechoic ducts appear like dark tubes against the normal white background when imaged along their long axis. In cross-section, the ducts are dark, hypoechoic, round or oval circles seen against the white echogenic normal glandular tissue.

The pectoralis muscle is a hypoechoic structure of varying thickness that contains thin lines of supporting stroma coursing along its long axis at the chest wall near the bottom of the image. The pectoralis muscle abuts the intercostal muscles and fascia of the chest wall (see Fig. 5-1K and L). Ribs in between the intercostal muscles are round or oval in cross-section, shadow intensely, and are seen at regular intervals along the chest wall. High-resolution transducers may display calcifications in the anterior portions of the cartilaginous elements of the ribs. Newcomers to breast ultrasound may mistake the ribs for masses, but their periodicity along the chest wall and the fact that one can palpate the ribs along their course will help prevent newcomers from making this mistake.

The nipple is a hypoechoic structure at the skin surface that occasionally produces an intense acoustic shadow as a result of the dense connective tissue within it (Fig. 5-2A and B). Because of the presence of retroareolar ducts and blood vessels, there may be marked vascularity in the retroareolar region on color or power Doppler imaging. Newcomers to breast ultrasound may mistake the nipple for a breast mass because of its hypoechoic appearance, shadowing, and the intense vascularity beneath it. However, knowledge of the shadowing, vascularity, and the ability to correlate the mass with the nipple on physical examination will help prevent newcomers from making this mistake.

In children, the breast bud that develops into the adult breast is right underneath the nipple. The breast bud may produce an asymmetric lump under the nipple that may be mistaken for a mass rather than a normal developing structure (see Fig. 5-2C and D). This normal structure should be left alone because surgical removal of the breast bud results in no breast formation on the ipsilateral side.

Ultrasound Evaluation of Mammographically Detected Findings

The ACR Breast Imaging Reporting and Data System (BI-RADS®) committee developed an ultrasound lexicon to provide descriptors for findings seen by ultrasound and recommended specific descriptors for breast masses (Table 5-1). Use of the words in the ACR BI-RADS® lexicon helps clarify one’s impression of the finding, improves communication between the radiologist and referring physician, and may trigger specific patient managements. This is because specific ultrasound features described by the lexicon suggest either benign masses or cancer. Although there is some overlap in benign versus malignant ultrasound features, the radiologist can use the lexicon to be reminded of what features should be searched for on the image (Table 5-2).

The BI-RADS® ultrasound lexicon descriptors for breast masses and their effect on the surrounding breast tissue are illustrated in Figures 5-3 to 5-6. Mass shapes are reported as oval, round, or irregular. Mass margins are circumscribed, angular, indistinct, microlobulated, or spiculated. The internal echo pattern is described as anechoic (all black inside), hyperechoic (white), complex (mixed black and white), isoechoic (equal), or hypoechoic (dark). Posterior acoustic features are described as no posterior acoustic features, enhancement (white), shadowing (dark), or a combined pattern. The boundary between the mass and the surrounding tissue is described as having an abrupt interface or as containing an echogenic halo (a white blurry band surrounding the mass). Calcifications are described as no calcifications, macrocalcifications (>0.5 mm), microcalcifications within the mass, or microcalcifications outside the mass. Effects of the mass on surrounding breast tissue are described using the terms no effect, duct changes, changes in Cooper ligaments, edema, architectural distortion, skin thickening, skin retraction, and skin irregularity.

The terms parallel or not parallel relate to tumor growth patterns with respect to normal tissue planes. They are important because they indicate if the mass is growing along or in between tissue planes versus growing through them. A parallel growth pattern indicates a benign finding (wider than tall, as described by Stavros and colleagues) because it indicates a growth pattern along tissue planes. Not parallel or taller than wide indicates that the mass is growing through the normal tissue planes, which is not normal and indicates cancer (see Fig. 5-6E).

Finally, the ultrasound BI-RADS® lexicon suggests standard reporting for masses, as in Box 5-3.

Breast Cysts, Intracystic Tumors, and Cystic-Appearing Masses

The most frequent clinical application of breast ultrasonography is to characterize masses initially detected by mammography as cystic or solid. Cysts are the most common breast mass and occur in an estimated 7% to 10% of all women. Cysts are lined by apocrine cells that actively secrete material, predisposing these types of cysts to recur after aspiration. Sometimes, cysts are lined by a flat epithelial lining that is less active. The accuracy of ultrasound in distinguishing cystic from solid masses can be as high as 98% to 100%, as reported by Hilton and colleagues.

Strict ultrasound criteria for a simple cyst include a mass with well-circumscribed margins, sharp imperceptible anterior and posterior walls, a round or oval contour, absence of internal echoes, and posterior acoustic enhancement (Box 5-4 and Fig. 5-7A). Cysts may be single or multiple, gathered into small clusters, or contain thin septations (see Fig. 5-7B to D). Cysts are not malignant or premalignant, but examination of them is important because they may cause lumps that mimic round cancer on physical examination or mammography. When palpable, a cyst is a smooth, mobile mass on physical examination. Occasionally cysts appear as a visible mass if the patient is supine and the cyst is large. Cysts may be painful and may wax and wane with the patient’s menstrual cycle. If a mass is proven to be a cyst by ultrasound, the patient can be monitored by screening mammography because cysts are not cancer. Symptomatic cysts that are painful or cause a lump that disturbs the patient can be treated by aspiration. Cysts may be simple or “complicated,” meaning that the cyst contains sloughed debris. These complicated cysts contain material within them rather than being anechoic. Some complicated cysts require aspiration to confirm that they are cysts rather than solid masses (see Fig. 5-7E).

Attention to technical detail is especially important because increasing the TCG curve may produce artifactual echoes in benign cysts that suggest a solid mass. An improperly set DCG curve may inaccurately evaluate the internal matrix of the mass and make a cyst look solid and may make a solid mass appear to be a cyst. On real-time imaging, cyst contours may be flattened with compression, whereas solid masses are less compressible. Alternatively, small, clustered, or deeply located cysts may be at the technical limits of ultrasound to distinguish the usually anechoic cyst from a solid mass.

Deeply located cysts may not show enhanced through-sound transmission because of their location close to the chest wall, and lateral cyst walls may be obscured by refractive shadows (see Fig. 5-7F). These problems may be resolved by repositioning the patient or the transducer to scan from a different angle. This permits visualization of distal acoustic enhancement or eliminates the refractive shadows obscuring the sharp cyst walls. Acorn cysts contain a fluid/fluid level, with the dark dependent portion of the cyst representing clear fluid (the acorn) and the lighter top representing layering fluid above it (the acorn cap) (see Fig. 5-7G). Changing the patient’s position may cause the layer to move dependently, clinching the diagnosis of an acorn cyst.

The internal characteristics of cysts must be analyzed to exclude mural masses or irregular thick walls, which indicate complex masses. Complex masses contain cystic and solid components; intracystic tumors and necrotic neoplasms are in the differential diagnosis. Complex masses are different from complicated cysts, which contain debris. Complex masses might be cancer, but complicated cysts are benign (Table 5-3 and Boxes 5-5 and 5-6).

Table 5-3 BI-RADS® Ultrasound Special Cases (Cystic)

Cystic Mass Type Description Differential Diagnosis
Clustered microcysts
Complicated cysts
Complex mass Has cystic and solid components

Modified from American College of Radiology: ACR BI-RADS®—ultrasound, In ACR Breast Imaging and Reporting and Data System, breast imaging atlas, Reston, VA, 2003, American College of Radiology.

Real-time ultrasound imaging can help distinguish speckle artifact from debris in cyst fluid from a solid mass. Real-time ultrasound can show particulate matter slowly moving inside the cyst. On real-time imaging, the debris causes speckle artifact, which swirls in the cyst like fake snow in a snow globe (Fig. 5-8A to C). Placing the patient in the decubitus position can cause a difference in the sedimentation pattern in the complicated cyst, but not always. Color Doppler or power Doppler ultrasound can detect movement of particulate matter within complicated breast cysts or blood vessels in solid masses (see Fig. 5-8D and E). Doppler imaging will show no blood vessels in breast cysts. Unfortunately, the absence of blood flow in a mass is not diagnostic of a cyst because Doppler imaging does not always detect blood flow in solid masses or even in cancers.

In everyday clinical practice, cysts do not always fulfill all the strict sonographic criteria for cysts because of a variety of technical factors, or they may contain echoes from debris within the fluid. Posterior enhanced through-transmission of sound was not seen on all images in 25% of 80 cysts reported by Hilton and colleagues. Internal cyst echoes may be produced by reverberation artifacts, although the near-field reverberations may be reduced by scanning through an offset. However, Berg and colleagues (2003, 2005) and other researchers have shown that complicated cysts with low-level internal echoes, no mural masses, thin walls, and thin septations rarely represent cancer and can either be monitored or aspirated with little or no morbidity.

Differentiation of complicated cysts from benign or malignant cystic masses can be tricky (see Table 5-3). Some cysts contain true internal echoes as a result of thick tenacious fluid or hemorrhage from previous aspirations. Some cysts have thick walls as a result of inflammation from cyst fluid leaking into the surrounding tissues. In cases in which all the sonographic criteria of a simple cyst have not been met, fine-needle aspiration may obviate the need for core needle biopsy or surgical biopsy. Once the needle is within the mass, the presence of cyst fluid rather than solid tissue can be confirmed by moving the needle, as suggested by Stavros and colleagues (Fig. 5-9). Cysts that do not fulfill all criteria for simple cysts, in the right clinical setting, require aspiration (see Fig. 5-9C to E). Cyst fluid should be sent for cytologic analysis if it is bloody, if there is an intracystic mass on ultrasound or pneumocystography, or if the patient has had prior intracystic carcinoma. Clear cyst fluid can be discarded if there are no clinical factors that would require cytologic examination.

On the other hand, complex cystic masses (i.e., fluid-filled masses with thick walls or mural projections) require biopsy to exclude the rare intracystic papilloma, intracystic carcinoma, phyllodes tumor with a marked cystic component, or solid cancers with central necrosis. Other complex masses include hematoma, abscess, galactocele, and seroma; management of these masses is based on their appearance and the clinical situation (Fig. 5-10).

Intracystic carcinomas are a rare subgroup of tumors that arise from the walls of a cyst; they represent 0.5% to 1.3% of all breast cancers (Fig. 5-11A). These tumors have a better prognosis than other malignant breast neoplasms do. On ultrasound, intracystic carcinomas often appear as solid mural excrescences projecting into the cyst fluid. Differentiation of intracystic carcinoma from benign intracystic papilloma is not possible, and surgical biopsy is thus necessary. The finding of a mural nodule within a cyst has the differential of an intracystic carcinoma, papilloma, a cyst with debris, and reverberations in a simple cyst produced by high gain settings (see Fig. 5-11B to D). Color or power Doppler imaging may be helpful if a blood vessel can be identified in the intracystic mass.

Benign Solid Masses: Fibroadenoma and Fatty Pseudolesions

Fibroadenomas arise from breast lobules and are the most common solid benign masses in women younger than age 30 years. Once diagnosed, fibroadenomas may remain stable in 80% of cases, regress in about 15%, and grow in 5% to 10%. Fibroadenomas are benign, although cancer can occur within a fibroadenoma. Women with a specific histologic diagnosis of complex fibroadenomas have cysts or histologic elements other than the fibroadenoma and have a small increased risk of future breast cancer, as described by DuPont and colleagues. Fibroadenomas may be single or multiple and are called giant fibroadenomas if larger than 8 cm.

On ultrasound, Cole-Beuglet and colleagues describe typical fibroadenomas as solid masses with well-circumscribed, round or oval borders and containing weak low-level homogeneous internal echoes with enhanced, decreased, or unchanged sound transmission. Stavros and colleagues and Fornage and colleagues have described fibroadenomas as smooth, wider than tall solid masses. Stavros and colleagues further characterize fibroadenomas as having at most four gentle lobulations and homogeneous internal echo texture (Box 5-7 and Fig. 5-12A to C).

Dec 24, 2015 | Posted by in BREAST IMAGING | Comments Off on Breast Ultrasound
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