Sonographic Anatomy of the Breast and Axilla

4 Sonographic Anatomy of the Breast and Axilla

Gross Anatomy

Breast

Anatomically the breast is a modified sweat gland lying within the deep and superficial layers of superficial pectoral fascia. Viewing a schematic sagittal section of the breast turned 90° to correlate with ultrasonographic positioning, we observe the following anatomic structures (from anterior to posterior), which can also be visualized with ultrasound (Tables 4.1, 4.2): skin, subcutaneous fat, Cooper ligaments, the superficial mammary fascia, the breast parenchyma (with ducts and lobules), interlobar fibrofatty tissue, the deep mammary fascia, retromammary fat, muscle fascia, the pectoralis major and minor muscles, the ribs and intercostal spaces, and finally the pleura and lung (Fig. 4.1).

The breast consists of a varying mixture of tissue components, and its composition depends on age, hormonal influences, structural changes (congenital, degenerative, or pathologic), and individual characteristics (Tables 4.3, 4.4). As a general rule, the breast tissue of young women consists mostly of parenchyma and contains little fat. With aging, the glandular tissue of the breast is replaced by connective tissue and fat. But there is great individual variation, so that the breasts of young multiparous women who have nursed their infants are predominantly fatty, and even in young girls a substantial portion may consist of fat, especially if the breasts are large.

Conversely, the breasts of older women who receive postmenopausal hormone replacement may respond by increasing fibro-glandular density, and many older women may have mammographically dense breasts, the density reflecting the fibrous (rather than glandular) predominance of their breast tissue. This should be considered in breast examinations as it will influence the overall interpretation of clinical, sonographic, and mammographic findings.

The structural composition of the breast varies with age, functional status, and individual differences in tissue distribution and quantitative make-up.

Fig. 4.1 Gross anatomy of the breast. PMa = pectoralis major muscle, PMi = pectoralis minor muscle.

**Table 4.1** Gross anatomy of the female breast (sequence of tissue layers from anterior to posterior)
Skin
Subcutaneous fat
Cooper ligaments
Superficial mammary fascia
Breast parenchyma with
– Lobules
– Lactiferous ducts
– Interlobar connective tissue
– Fat
Deep mammary fascia
Retromammary fat
Muscle fascia
Pectoralis major muscle
Pectoralis minor muscle
Ribs and intercostal muscles
Pleura

**Table 4.2** Echogenicity of the various breast tissues
Anatomic structure	Echogenicity
Skin	Hyperechoic
Nipple	Hypoechoic
Parenchyma	Hyperechoic
Connective	tissue Hyperechoic
Subcutaneous fat	Hypoechoic
Fatty infiltration	Hypoechoic
Retromammary fat	Hypoechoic
Cooper ligaments	Hyperechoic
Lactiferous ducts	Anechoic*

* Intraductal secretions appear as echo-free fluid, but proliferative changes and inspissated secretions may produce low-level internal echoes.

**Table 4.3** Principal components of the female breast
Fat
Parenchyma
– Alveoli
– Lactiferous ducts
– Intralobar connective tissue (hormonally responsive)
Interlobar connective tissue
Axilla

**Table 4.4** Parenchymal patterns in breast ultrasound
Homogeneous hyperechoic pattern
– Young women
– Good readability
Heterogeneous hyperechoic pattern
– Scant fatty infiltration
– Middle-aged women
– Good or moderate readability
Partially involuted or involuted pattern
– Predominantly hypoechoic with connective-tissue septa
– Older women
– Moderate readability
Fibrotic pattern (heterogeneously hypoechoic)
– Young and middle-aged women
– Poor readability