The second cross correlation imaging rule is that most focal breast masses have dissimilar appearances on the two modalities. For example, lymph nodes, cysts, and fibroadenomas appear white (dense) on mammography but dark (hypoechoic) on sonography. Neoplasms appear white (dense) on mammography but dark (hypoechoic or heterogeneous echogenicity) on sonography. The main common exceptions to this rule are benign scars, radial scars, and occasionally fat necrosis. These lesions are sonographically hyperechoic (white).
If one is familiar with mammographic and sonographic appearance of breast structures, then one will be able to sonographically localize a focal mammographic abnormality using internal anatomic landmarks. The steps that I use when faced with an anatomically difficult breast problem are the following:
1. Review the mammogram and identify the abnormality. Notice the parenchymal pattern between the nipple and the lesion. Also study the pattern of the breast tissue surrounding the abnormality. Estimate the general location of the abnormality (i.e., quadrant and finger breadths from nipple).
2. Place the transducer with one edge at the nipple and sonographically examine the breast in a radial orientation. Look for the same parenchymal patterns that one noticed on the mammogram. When one recognizes the parenchymal landmarks that surround the abnormality, sonographically focus the examination in this area.
3. Sonographically characterize the lesion (Fig. 2–6). Utilizing this method, one would first approach finding the mass in Figure 2–6 by noting the general mammographic location of the lesion near the left 9:00 position. The irregular mass is surrounded by fatty density but linked to the subareolar density by linear densities (Fig. 2–6A-C). One would initially place the transducer near the nipple and confirm the presence of dense (hyperechoic) subareolar tissue. In the inner breast one would extend the transducer away from the nipple following the strands of hyperechoic tissue that extend farthest away from the nipple. This “trail” of hyperechoic tissue will lead to the hypoechoic shadowing mass (Fig. 2–6D) that corresponds to the mammographic mass.
Figure 2–5. These schematic diagrams illustrate the anatomic cross correlation between mammography and sonography. In this patient, transducer A is positioned over the dense fibroglandular tissue in the upper outer quadrant. Sonographically, this tissue is hyperechoic (C). Transducer B demonstrates the fatty parenchyma in the inferior inner quadrant that is sonographically hypoechoic (D). (A). Schematic image of an MLO mammogram show two transducers, labeled A and B. (B). Schematic image of a CC mammogram showing position of transducers A and B. (C). Schematic image of breast sonogram that corresponds to tissue imaged by transducer A. (D). Schematic image of breast sonogram that corresponds to tissue imaged by transducer B.
This method is particularly useful when the mammographic abnormality is not easily visualized. Section 6 Case 120 demonstrates an asymmetric density that is only visible in one mammographic view. In this case, the abnormal density is at the distal border of the fibroglandular density. Therefore, to find this type of lesion, one should start near the nipple and confirm the presence of diffuse hyperechoic fibroglandular tissue. One should then move the transducer toward the outer breast until one visualizes fatty hypoechoic tissue. Then, one should only scan the border between the hyperechoic and hypoechoic tissue. One will then discover the hypoechoic malignant mass.
When evaluating the location of a lesion from the mammogram, internal parenchymal landmarks one should notice include: (1) location of the edge of the parenchyma—many lesions are at the border of the fibroglandular (white) tissue and the fat (dark); (2) configuration of fibroglandular tissue—lesion may be linked to the largest area of this tissue; (3) adjacent masses—mammogram may demonstrate another mass next to the questionable lesion. By cross correlating these mammographic landmarks on the breast sonogram, one will be more successful in identifying mammographic lesions (Fig. 2–7).
Figure 2–6. Figure 6A. (A). Left MLO mammogram. (B). Left CC mammogram. (C). Left LM spot compression mammogram. (A–C). At the 9:00 position of the left breast, there is an irregular mass (circle). This mass is primarily surrounded by fatty density but is linked with dense spiculations (arrows) to the main subareolar fibroglandular density. (D). Left radial breast sonogram: The mammographic irregular density corresponds to a heavily shadowing focal mass. These mammographic spiculations correspond to the hyperechoic curvilinear tissue (arrows). This mass is infiltrating ductal carcinoma.
Special Sonographic Problems
Shadowing
One difficulty in cross correlating mammography with sonography is sonographic shadowing. Shadowing is confusing. It is a nonspecific finding because it is associated with both benign and malignant entities. Shadowing is particularly a problem for those who use high-frequency equipment as all tissues more readily attenuate high frequencies and, therefore, shadowing is more frequent. To analyze shadowing, one should (1) be familiar with the etiologies of shadowing; (2) if the shadow hides the lesion, reduce or eliminate the shadow; (3) characterize the tissue that causes the shadow.
Figure 2–7. (A). Left MLO digital mammogram. (B). Left CC digital mammogram. (C). Left CC spot magnification mammogram. (A–C). A partially obscured oval density (arrow) is visible in the central area of the CC view. This density is not visible on the MLO view. In the CC view, the density is at the distal edge of the fihroglandular density. (D). Left radial breast sonogram: A hypoechoic mass is present at the 12:00 position of the breast. In this view, the parenchymal anatomy matches the mammographic anatomy of the CC view. On the side closest to the nipple, the tissue is white fihroglandular parenchyma. On the side away from the nipple, the tissue is hypoechoic fat. N, arrow points toward direction of nipple. (E). Left antiradial breast sonogram: The antiradial view demonstrates irregularity of the margins of the mass. This mass is a mixed infiltrating ductal and lobular carcinoma.
The etiologies of shadowing can be divided into two main categories: (1) reflection and (2) absorption. Reflection of sound is affected by two factors: (1) acoustic impedance and (2) angle of incidence. Acoustic impedance is a fundamental properly of matter and is related to the density of the material and the speed of sound in the material. A portion of a sound wave is reflected whenever the wave strikes an interface between two substances with different acoustic impedances. This principle is the basis for diagnostic sonography. The reflected sound is received by the ultrasound machine and transformed into visual information. The amount of reflection is dependent upon the difference in acoustic impedance between the substances. If the difference is great, then a large percentage of the sound wave is reflected. Acoustic impedance differences between most tissues within the breast such as fat and fibroglandular tissue are very small, so generally less than 1 % of the sound wave is reflected. However, air and bone have acoustic impedances that are very different from breast tissue. When the sound wave strikes a rib, about 90% of the sound is reflected and when the wave strikes the lung over 99% of the wave is reflected.
The second factor that affects the amount of reflected sound is the angle of incidence or the angle at which the sound strikes an object. The closer the sound beam is to a right angle (or perpendicular to the surface of the object), the less the reflection. The proportion of reflected sound increases with decreasing angles. When the sound beam strikes the object at an extremely acute angle (i.e., critical angle), all of the sound is reflected. This phenomenon is evident when sound hits the side of a curved mass such as a cyst. In this situation, the reflected sound produces thin shadows at the edges of the cyst.
Besides reflection, shadowing