Breast Imaging. Beverly Hashimoto
is the conversion of sound into heat. A shadow results if the material completely absorbs the sound beam. In the body, absorption is directly proportional to transducer frequency. Therefore, absorption is double for 14 MHz compared with 7 MHz. This increased absorption is the main reason that high frequency is associated with more shadowing than lower-frequency sonography.
Upon encountering a confusing shadow, a breast imager should initially judge whether the shadow is due to reflection or absorption. After making this decision, one should eliminate or reduce the shadow. If the shadow is due to reflection, then the shadow is either due to the acoustic impedance of the material or due to the angle of the sound beam. If the shadowing is due to acoustic impedance, the imager cannot eliminate the shadow, but the imager would have a short list of materials that would produce this shadowing (i.e., air, bone, metal). However, if the shadow is due to the angle of the sound beam, then changing the position of the transducer can eliminate the shadow. Many structures within the breast are curved and create this type of shadow. Pseudomasses may be created by several of these shadows that are close together. To distinguish a true mass from an artifactual one, an imager should routinely study the mass from multiple transducer angles (Figs. 2–8 and 2–9).
If the shadow results from absorption, then one should decrease the frequency of the transducer. This technique is useful to better characterize lesions associated with severe shadowing. Both benign and malignant masses appear as areas of focal shadowing. By lowering the transducer frequency, one may reduce or eliminate the shadowing and be able to visualize the lesion causing the shadow. This technique is useful to differentiate scars or highly absorbing fibroglandular tissue from masses. With lower frequency, no mass will be evident with scars or fibroglandular tissue. The main problem associated with this technique is that lower-frequency imaging still has the disadvantages of relatively poor resolution and contrast. One may miss masses because they may blend into the surrounding fibroglandular tissue. Margins and architectural distortion are poorly defined. To avoid missing masses, one should have a high degree of suspicion. One should closely examine the area for subtle inhomogeneity of architecture and echogenicity. (See Section 3 Cases 12 and 30 and Section 4 Case 47.)
Figure 2–8. (A). Schematic of breast sonogram: When a transducer (T) is positioned over a curved object, the sound at the edges of the object is reflected so shadowing (S) is produced at the edges of the mass. (B). Schematic of breast sonogram: When the transducer (T1-T3) is repositioned around the curved object, shadowing is reduced.
Even after lowering the frequency, shadowing sometimes persists. In this situation, one should first attempt to discover whether the shadowing is caused by a mass or results from the patients dense fibroglandular tissue. To differentiate shadowing from these sources, one should compare the region of interest with normal fibroglandular tissue of the same breast or the opposite breast. Most women have symmetric fibroglandular tissue. Therefore, if the region of interest is in the right upper outer quadrant, then one should compare this area with the left upper outer quadrant. Or if the patients mammogram is completely white, then one may compare the region of interest with another area of the same breast. If the normal fibroglandular tissue does not shadow, then one should be suspicious that the shadowing represents an active process. If the patient's normal fibroglandular tissue shadows, then one should try to use a lower frequency in which the fibroglandular tissue appears hyperechoic. One may then use this new lower frequency to evaluate the region of interest.
Finally, if shadowing persists, one should evaluate the appearance of the shadow. Is it focal? Does it persist consistently in multiple angles? Masses that shadow have borders. Therefore, if the shadow has edges that define a focal area, then the shadowing should be considered suspicious for a mass.
Architectural Distortion and Focal Asymmetric Density
Mammographic architectural distortion and asymmetric density are difficult problems to sonographically correlate. When these findings are present without a mass, they are extremely subtle and commonly only visible in one mammographic view. Because obvious architectural distortion or asymmetry may generally be well characterized with mammography alone, sonography is most valuable when the mammographic findings are uncertain. Therefore, if one utilizes sonography for this purpose, one should only use high-resolution equipment and be experienced in cross correlating cases.
The principles for cross correlating architectural distortion and focal asymmetric density are the same as for a mass. One should match the internal sonographic landmarks to the mammogram. If the area of architectural distortion or focal asymmetry is dark sonographically, then one must suspect a mass (Fig. 2–10). Otherwise, the area will appear to be white fibroglandular tissue.
Generally, the sonographic findings are complementary to the mammographic information, so one is more confident about the final recommendation. However, occasionally, the sonographic and mammographic findings are discordant. In this case, if one of the modalities has information indicating malignancy, then one should recommend biopsy of the abnormality (American College of Radiology Breast Imaging Reporting and Data System Categories 4 and 5).
Figure 2–9. (A). This 35-year-old woman presents with a palpable lump. Sonographically, the lump has been found to be an ill-defined hypoechoic mass. The mass is considered suspicious and biopsy is recommended. The patient returns 2 days later for the biopsy. When the patient returns, the original hypoechoic area is identified (A), but by slightly changing the angle of the transducer, the hypoechoic area disappears (B). The hypoechoic “mass” represents unusual shadowing from a Cooper's ligament attachment. This attachment is also the etiology of the small superficial palpable lump. (A). Left radial breast sonogram of hypoechoic “mass.” (B). Left radial breast sonogram of same area as Figure 2–9A: Pressure on the transducer has slightly changed the incident sonographic angle. Cooper's ligament attachment (arrow).
Figure 2–10. (A). Right MLO mammogram. (B). Left MLO mammogram. (C). Right CC mammogram. (D). Left CC mammogram. (E). Left MLO spot compression mammogram. (F). Left CC spot compression mammogram. (A–F). Subtle architectural distortion (square) is present in the left upper outer quadrant. The architectural distortion blends into the normal central white fibroglandular density. (G). Left radial breast sonogram: Sonographic examination of the left upper outer quadrant demonstrates that the architectural distortion corresponds to an irregular hypoechoic mass with shadowing The mass is connected to hyperechoic tissue (arrows) that corresponds to the central mammographic fibroglandular density. The mass is a lobular carcinoma.
Section III
Circumscribed Masses
This is a schematic diagram of the diagnostic approach to mammographic circumscribed masses. For further discussion, see Chapter 1.
Case 1
Case History
A 39-year-old woman has just stopped breast-feeding her 10-month-old infant and now finds a new breast lump.
Physical Examination
• left breast: 3 cm lump in the upper outer quadrant
• right breast: normal exam
Mammogram
Mass (Fig. 1–1)
• margin: circumscribed
• shape: oval
• density: fat-containing
Figure 1–1. In the upper outer quadrant of the left breast, there is a well-defined