Ultrasound anatomy of the breast

The anatomical structures of the mammary gland are easily differentiated using modern ultrasound equipment. The image of the mammary gland normally varies widely and depends on the ratio of fatty, connective and glandular tissue. Unlike X-ray mammography, ultrasound of the mammary glands allows visualization of a tomographic section of the image of a fragment of the mammary gland from the skin to the chest wall.

On the ultrasound image of the mammary gland of a woman of childbearing age, the following components and structures can be differentiated.

1. Leather.
2. Nipple.
3. Subcutaneous zone (subcutaneous fat layer, anterior layer of the split fascia).
4. Cooper's ligaments.
5. Parenchyma of the mammary gland, fibroglandular zone (glandular part with delicate fibrillar fibers, interparenchymal intraorgan lymphatic network, adipose tissue).
6. Milk ducts.
7. Retromammary adipose tissue (not always visualized).
8. Pectoral muscles.
9. Ribs.
10. Intercostal muscles.
11. Pleura.
12. Lymph nodes (not always visualized).
13. Internal thoracic artery and vein.

Skin. On the echogram, the skin is represented by a more or less even hyperechoic line with a thickness that is normally 0.5-7 mm. When the skin thickens, it can be visualized as two hyperechoic lines separated by a thin echogenic layer. Changes in the contours and thickness of the skin may indicate an inflammatory, postoperative or malignant process in the superficial or deeper parts of the mammary gland.

The nipple is visualized as a round, well-demarcated formation with medium to low echogenicity. An acoustic shadow is often observed behind the nipple. This acoustic phenomenon is caused by connective tissue structures of the milk ducts. Ultrasound examinations of the subareolar region in oblique projections allow for clear visualization of the retronipple region. The skin in the areola area is less echogenic than over the rest of the mammary gland, and the subareolar structures are always more echogenic due to the absence of subcutaneous tissue in this area.

Subcutaneous zone. In early reproductive age, subcutaneous fat is almost completely absent. In very young women, adipose tissue may be represented by a thin hypoechoic layer or elongated hypoechoic inclusions under the skin. With age, an increase in the thickness of the hypoechoic subcutaneous layer is noted on echograms. With the onset of involution processes, adipose tissue becomes less homogeneous. Hyperechoic linear inclusions of connective tissue begin to be determined in its hypoechoic echostructure. Adipose tissue takes the form of rounded hypoechoic structures located in several rows. This occurs due to thickening of Cooper's ligaments, which, as it were, with a hyperechoic capsule, envelop individual accumulations of fat, forming a fat lobule. Very often, symmetrical lateral acoustic shadows are determined on the sides of the fat lobule. With excessive fat content in the mammary gland, multiple alternation of lateral acoustic shadows from adjacent fat lobules interferes with clear differentiation of the organ's echostructure. Compression of the mammary gland tissue with a sensor allows to reduce or eliminate these unwanted artifacts. At the border of the fat tissue and parenchyma, there is an anterior leaf of the split fascia in the form of a hyperechoic strip. Hyperechoic septa, Cooper's ligaments, extend from it perpendicular to the skin.

Cooper's ligaments are also visualized as linear hyperechoic threads covering hypoechoic fat lobules. With age, ultrasound differentiation of Cooper's ligaments improves. Sometimes an acoustic shadow is determined behind Cooper's ligaments, which can imitate pathological processes in the mammary gland. Changing the angle of incidence of the ultrasound wave by moving the sensor or changing the position of the mammary gland allows you to get rid of this artifact.

In a normal state, intraorgan lymphatic vessels are not visualized. In case of their expansion due to inflammation or tumor infiltration, the intraorgan lymphatic network of vessels can be visualized as longitudinal and transverse hypoechoic tubular structures going to the skin.

The anterior contour of the parenchyma is wavy due to bulging at the attachment sites of Cooper's ligaments. Usually, the echogenicity of the parenchyma is intermediate between the echogenicity of fat and fascial structures. In young patients, the parenchyma (fibroglandular part) of the mammary gland is represented by an image of a single granular layer with a high to medium degree of echogenicity. In the echostructure of this single massif, it is almost impossible to differentiate the presence of delicate, collagen-free connective tissue fibrillar fibers. Echography allows us to detect changes in the parenchyma in the form of an increase in the "grain" of the fibroglandular complex from the 16th to the 28th day of the menstrual cycle. During this period, the echostructure of the parenchyma is an alternation of more echogenic areas of fibroglandular tissue with tubular hypoechoic structures of the milk ducts. The echostructure of the parenchyma also depends on the amount and ratio of fibroglandular and adipose tissue. This proportion changes with age and hormonal status (pregnancy, lactation, menopause), and the number of previous pregnancies.

The central sections of the mammary gland are occupied by milk ducts. In a hormonally calm mammary gland, milk ducts are always collapsed and are practically not visualized. If they are determined, the diameter of the terminal and interlobar ducts does not exceed 2 mm. The largest diameter of the ducts (up to 3 mm) is noted in the area of the milk sinus (behind the nipple). In the lactating mammary gland, as well as in the 2nd phase of the menstrual cycle, milk ducts are visualized as linear and tortuous hypoechoic tubular structures more than 2 mm in diameter, radially converging from the base of the mammary gland to the nipple. Often, both transverse and longitudinal fragments of different ducts are visualized on one section in the form of alternating rounded and elongated hypoechoic structures. In young women with a rich glandular component, hyperechoic strands located along the main axis of the duct can be visualized along the internal contour of the ducts. The posterior border of the mammary gland is the image of the posterior leaflet of the split fascia in the form of a hyperechoic line parallel to the skin.

The retromammary region consists of the retromammary fat pad, pectoral muscles, ribs, intercostal muscles, and pleura.

Retromammary fat is visualized as small hypoechoic lobules between the hyperechoic lines of the posterior layer of the split fascia and the anterior fascial compartment of the pectoralis major muscle. In the absence of a retromammary fat layer, the image of the posterior layer of the split fascia may merge with the image of the anterior fascia of the pectoralis major muscles.

The pectoralis major and minor muscles are visualized as differently directed hypoechoic layers parallel to the skin, separated by transverse hyperechoic partitions. On both sides of the muscles, the pectoral fascia are visualized as hyperechoic lines. Identification of the muscle layers is a guarantee that the entire mass of the mammary gland has been examined.

In addition, identifying the posterior border of the gland allows us to differentiate tumors of the soft tissues of the chest wall from tumors of the mammary gland itself.

The ultrasound image of the ribs varies depending on whether the portion is cartilaginous or bony. The transverse image of the cartilaginous portion of the ribs shows an oval formation with a small amount of reflections from the internal structure. This image can be mistaken for a benign solid breast mass or a lymph node. The distinction between these structures is aided by the fact that the rib is located under the muscle, and the lymph node is located in front of or against the muscle. With increased calcification, a weak acoustic shadow may appear behind the cartilaginous segment of the ribs. The lateral, always ossified segments of the ribs are visualized as hyperechoic crescents with a pronounced acoustic shadow.

The intercostal muscles are defined in the intercostal spaces as hypoechoic structures of varying thickness with a typical muscle pattern.

The pleura, in the form of a hyperechoic line, is the deepest structure that can be distinguished during breast ultrasound.

In most cases, regional lymph nodes of the mammary gland are normally not differentiated from surrounding tissues. When using high-end ultrasound devices equipped with specialized high-frequency sensors, it is sometimes possible to visualize a normal lymph node, especially in the projection of the axillary part of the mammary gland near the pectoral muscles. Normal lymph nodes have an elongated shape with a hypoechoic rim of the marginal sinus around the echogenic center - the gate of the lymph node. The horizontal diameter of a normal lymph node usually does not exceed 1 cm. Most often, internal lymph nodes of the mammary gland can be visualized in the projection of the upper outer quadrant. With an increase in size and a change in the echomorphological structure, all groups of lymph nodes are well visualized as hypoechoic spherical formations. According to Pamilo (1993), echography allows detection of breast cancer metastases in the axillary lymph nodes in 73% of cases, while palpation and X-ray mammography - only in 32%.

The internal mammary artery and vein are visualized on longitudinal echograms parallel to the pectoral muscles in the 1st and 2nd intercostal spaces as hypoechoic tubular structures. According to Adler (1993), normal blood flow in the mammary glands is determined by color Doppler mapping in 69% of cases. There are studies in which the authors differentiate normal blood flow in the mammary gland from changes that occur in these vessels when a malignant tumor occurs (the ratio of maximum and minimum blood flow velocities). Other publications emphasize the impossibility of such differential diagnostics using Dopplerography. Thus, due to the lack of experience in these studies and the inconsistency of the published results, it is inappropriate to recommend using the Doppler method as an independent diagnostic technique separately from ultrasound examination in B-mode.

Ultrasound image of mammary glands at different ages

The mammary gland of puberty consists of fat, underdeveloped ducts, glandular elements and is visualized as mixed echogenicity of structures behind the nipple.

The postpubertal mammary gland is characterized by a hyperechoic appearance of glandular tissue surrounded by small hypoechoic areas of fatty structures.

The mammary gland of an adult woman has many variants of ultrasound imaging, in particular, the following types can be distinguished.

Juvenile type. The skin is visualized as a thin hyperechoic line 0.5-2.0 mm thick. The main mass of the gland is represented by the image of glandular structures in the form of a single fine-grained layer of increased echogenicity. In the second phase of the menstrual cycle, the hyperechoic image of glandular structures alternates with hypoechoic tubular (in longitudinal section) or rounded (in cross section) structures of the milk ducts.

Early reproductive type. The skin is visualized as a thin hyperechoic line 0.5-2.0 mm thick. Subcutaneous fat is determined either as a small number of elongated hypoechoic structures, or as a single hypoechoic layer 2-3 cm thick. The glandular part is visualized as a single hyperechoic fine-grained layer, or hypoechoic rounded accumulations of adipose tissue are determined against its background. In the second phase of the menstrual cycle, the image of hyperechoic glandular tissue alternates with the image of hypoechoic fragments of the milk ducts. The anterior contour of the glandular parenchyma has a wave-like shape due to protrusions at the attachment sites of Cooper's ligaments. Cooper's ligaments, fascia, and fibrillar interlobar tissue are poorly differentiated.

Premenopausal type. The skin is visualized as a hyperechoic line 2.0-4.0 mm thick. A well-defined subcutaneous fat layer is determined in the form of rounded hypoechoic structures. Clusters of hypoechoic fat surrounded by hyperechoic rims of connective tissue represent fat lobules. Partial replacement of glandular tissue with fat is characterized by the appearance of numerous areas of hypoechoic fat against the background of hyperechoic glandular tissue. In the 2nd phase of the menstrual cycle, multiple images of hypoechoic structures of the milk ducts appear against this background. Fat tissue is often determined in the retromammary space in the form of hypoechoic small rounded inclusions. Cooper's ligaments, fascia, and fibrillar interlobar tissue are well differentiated as multidirectional hyperechoic strands.

Postmenopausal type. The skin is visualized as two hyperechoic lines, between which a thin hypoechoic layer is determined. The thickness of the skin may vary. Almost the entire mammary gland consists of hypoechoic fat lobules in the form of rounded hypoechoic structures with a pronounced hyperechoic rim. Sometimes, single inclusions of hyperechoic glandular tissue are determined between the fat lobules. Connective tissue structures are characterized by thickened hyperechoic Cooper's ligaments, as well as hyperechoic linear inclusions in the adipose tissue and in the image of the outer contour of the milk ducts.

The mammary gland during pregnancy and lactation. The skin is visualized as a thin hyperechoic line 0.5-2.0 mm thick. Almost the entire image of the gland consists of coarse-grained hyperechoic glandular tissue (hypoechoic fat is pushed to the periphery). In the late stages of pregnancy and during lactation, hypoechoic, more than 2.0 mm in diameter, milk ducts are well differentiated against the background of hyperechoic glandular tissue.