Dr. Hashimoto is the Section Head, Ultrasound, Virginia Mason Medical Center, Seattle, WA.

Breast imagers commonly encounter focal asymmetric densities as well as densities that are seen on only one mammographic view. Traditionally, these lesions have been worked up with additional mammographic views. However, in some cases, mammographic evaluation does not resolve the diagnostic question completely. In these situations, high-resolution sonography has the potential to facilitate the diagnosis.

Definition of Focal Asymmetric Densities

Mammographic focal asymmetric densities and densities visible on one view are closely related findings in that both do not meet the criteria for a mammographic mass, a real mass exhibiting a consistent structural identity on two orthogonal mammographic views. Some focal asymmetric densities may be visible on two views, but they do not appear to have the same appearance on both views. With spot compression classically, a focal asymmetric density that is not a mass will disappear or blend into the surrounding parenchymal density.

Mammographic Evaluation of Focal Asymmetric Densities

The evaluation of focal asymmetric densities should start with additional mammographic views. The goals of this assessment should be to: 1) determine the location of the density, and 2) determine if the density is a true mass. When the density is only visible on one view, exaggerated, rolled, or oblique views are useful to locate it if it is outside of the field-of-view (FOV) of standard screening mammographic views. If the lesion is expected to be within the FOV but is not visible because it is obscured by the surrounding fibroglandular tissues, spot compression views are also useful to identify it.

If a density disappears on focal spot compression views, then it is usually benign breast tissue. However, breast imagers have realized that focal asymmetric densities that disappear with spot compression may on occasion represent neoplasms that are either extremely pliable or very small. In order to avoid the pitfall of missing a pliable neoplasm, breast imagers have advocated doing large FOV mammograms performed at different angles (eg, rolled views, 90° mediolateral views, oblique views). These large FOV mammograms not only demonstrate the three-dimensional characteristics of the density, but also provide information about the location of the density.

Even after carefully evaluating these densities with additional mammographic views, it may not always be possible to determine whether or not a focal asymmetric density represents a real mass. Furthermore, since these densities are subtle in other views, it may not be possible to identify the exact location of the density. If the mammographic assessment of a focal asymmetric density or of a density visible only on one view is not adequate, then high-resolution sonography should be considered.

Sonographic Evaluation of Focal Asymmetric Densities

Sonographic evaluation of focal asymmetric densities is one of the most challenging situations in breast sonography. The sonographic examination of a focal asymmetric density consists of two steps. The first step is a survey of the general area in which the density may be located. Usually, it is only necessary to examine a quadrant of the breast, but sometimes, half of the breast must be examined. During this stage, it is generally best to use a lower frequency to cover the entire thickness of the breast from the skin to the chest wall. In order to quickly cover a large area, a high frame rate and a larger transducer are useful. Finally, for this survey stage, it is advantageous to use an imaging program that provides a high sensitivity for identifying focal breast masses. In our institution, we use the Advanced Imaging Package available on the Sequoia 15L8W ultrasound system (Acuson, Mountain View, CA) for these examinations.

Several features of this system are well adapted to breast imaging. The transducer's width is ideal for surveying large areas of the breast. The 14 MHz center frequency has sufficient depth penetration. When used with the Tissue Equalization Control (TEQ) button, the 14 MHz can usually penetrate approximately 4 to 5 cm, so most breasts may be surveyed at this frequency. The imaging package also makes it possible to use multiple transmit zones with the 14-MHz transducer without significant reduction in frame rate. Since it is possible to create custom programs within the Sequoia, a low dynamic range program is routinely used for the survey mode. This program produces superior tissue contrast, and allows the sonographer to spot focal lesions within the breast quickly.

Once a focal abnormality is identified, the second step of the breast sonographic examination is the diagnostic evaluation. High resolution is important to evaluate margin irregularities. Gray-scale contrast manipulation is useful to characterize lesions. We have found that the Sequoia Advanced Imaging Package technology is also useful for the diagnostic assessment of lesions. The TEQ allows the sonographer to optimize the appearance of the lesion rapidly. For cysts or fibroadenomas, some sonographers may prefer to use a compound frequency, which increases the conspicuity of focal masses and produces sharp margins. Compound frequency imaging is the result of sending multiple frequencies simultaneously and combining the image information from all of these frequencies. For lesions with irregular margins the highest center frequency technique is preferred to the use of compound frequencies. For calcifications, a high delta setting strongly highlights small hyperechoic foci. Delta is a type of postprocessing technique that preferentially enhances or supresses signals on a region-by-region basis rather than the traditional point-by-point method. Elevated delta settings appear superior to high-contrast postprocessing or low-dynamic range programs in accentuating subtle margin irregularities and microcalcifications.

Using the above techniques, the diagnosis of a mammographic focal asymmetric density can generally be made. If a hypoechoic, irregular, or heterogeneous mass is identified, then an ACR Category 4 (suspicious--biopsy recommended) is given. If benign hyperechoic tissue is demonstrated, then an ACR Category 3 finding is given (probably benign--recommend 6-month follow-up). If the density is a benign lesion, such as a cyst or lymph node, then a Category 2 is reported (benign) (Figure 1). If short-term follow-up is needed, a recommendation is made for a mammographic follow-up. Rarely, the mammographic density cannot be explained sonographically. In these cases, the mammographic findings are used to plan the next course of action.

Conclusion

There are several advantages to utilizing high-resolution sonography to characterize focal asymmetric densities. High-resolution sonography is useful to localize lesions that cannot be identified on two mammographic views. Furthermore, high-resolution sonography increases diagnostic confidence by characterizing these lesions and may prevent a highly compressible neoplasm, such as lobular carcinoma, from being missed. Finally, if a focal asymmetric density is a sonographic mass, sonography can be used to biopsy the mass. *