Breast Ultrasound Principles

B-Mode Gray Scale Image Optimization

Real-time handheld scanners can be used to differentiate between cysts and solid breast masses and to assess sonographic features used to characterize masses as benign, probably benign, or suspicious. Handheld units used for breast imaging should include a linear array, high-frequency transducer operating at a center frequency of 10 MHz or greater ( Fig. 5.1 ), which provides good tissue penetration to 4 cm or 5 cm. ) recommends use of a linear array transducer with a center frequency of at least 10 MHz. All scanners should also include a marking system to document and annotate US images.

Good sonographic image quality relies on the application of technical principles. Superficial lesions in the near field of the transducer can be imaged well by using a thin standoff pad or a thick layer of gel to bring the lesion into the focal zone of the transducer. Accurate diagnosis of deep lesions depends on appropriate power, gain, and focal zone settings. Improper adjustments of technical parameters can result in misdiagnosis, such as mistaking a cancer for a cyst by producing suboptimal images ( Fig. 5.2 ).

The patient should be positioned to minimize the breast thickness to allow adequate tissue penetration. To flatten the breast tissue in the upper outer quadrant, the patient is scanned in a supine oblique position with the patient’s hand behind the head and the back and shoulder supported by a wedge. For the inner breast, a supine position serves to flatten the medial breast tissue.

To ensure that the field of view includes all the breast tissue from the skin surface to the chest wall, the operator includes the pectoralis muscle and chest wall at the base of the image. Blank areas are caused by excessive depth; the field of view selected should be appropriate to the thickness of tissue in the area being scanned ( Fig. 5.3 ).

Fine adjustments can be made to the time gain compensation (TGC) curve so that fat is uniformly gray from the subcutaneous tissues to the chest wall. A gentle downward slope enables accurate evaluation of masses as cystic or solid at any depth in the breast. Incorrect gain and contrast settings that make the fat look black (anechoic) may also erroneously make a solid mass look like an anechoic cyst.

Once the sonographer identifies an area of interest or a mass, he or she may zoom in on the area to fill the screen appropriately, because it is difficult to evaluate the features of a very small lesion. The sonographer then resets the focal zone to the center of the finding or depth range to include the lesion as well as the TGC curve for optimal characterization. When the indication for a breast US is evaluation of a palpable finding, the patient is asked to locate the mass or symptomatic area to ensure inclusion of the area of concern. If the patient is unaware of a physician-detected abnormality, the quadrant or area requested by the referring physician is scanned.

In the , the ACR has made specific recommendations for US labeling. Labeling annotations should include laterality (right or left), clock face position, number of centimeters from the nipple (eg, “9 o’clock 5 cm fn”), scan plane such as radial or antiradial and longitudinal or transverse, and the initials of the person performing the scan ( Box 5.1 ). In accordance with the Practice Parameter and ACR Breast US Accreditation instructions, orthogonal images of the mass with and without measuring calipers should be documented. Measuring calipers overlying the margin of a mass, particularly if it is small, may prevent definitive evaluation. Any other pertinent clinical information, such as whether or not the lesion is palpable, may also be helpful to note. It is important that any annotation not be placed on the image of the breast itself.

Doppler Sonography

The reflected echoes that form the US image have both frequency and amplitude. The frequency of US changes by the motion of red blood cells. This Doppler shift frequency, which is the difference between the frequency of the transmitted US and the reflected US, is used to form the B-mode color Doppler image. The power Doppler image, on the other hand, is formed by the amplitude of the frequency-shifted echoes. Unlike color Doppler imaging, power Doppler is independent of flow direction and relative flow velocity ( Fig. 5.4 ). Power Doppler imaging is slightly more sensitive than color Doppler imaging and less affected by the Doppler angle. Technical aspects to remember include modest compression of the probe in the area of insonation, so as not to occlude flow, and selection of a sensitive frequency setting to enable low flow vascularity to be depicted.

Doppler sonography is used to assess the vascularity of a mass visualized on gray scale US and may be helpful in determining whether a nearly anechoic mass is solid. It is important to note that there is significant overlap between benign and malignant blood flow patterns. Color Doppler imaging does not always detect increased flow in breast cancer, and absence of color flow should not dissuade biopsy of a mass with suspicious shape or margin. In the appropriate clinical setting, vascularity in the rim of a mass with suspicious marginal characteristics can suggest abscess ( Fig. 5.5 ) rather than cancer. In such cases, US-guided aspiration should be the initial approach followed by core biopsy if aspiration is unsuccessful.

Elastography

Elastography is a measurement of the stiffness of a lesion compared with that of subcutaneous fat or fibroglandular tissue outside of the lesion at the same depth, independent of the lesion morphology. Usually cancers tend to be stiff ( Fig. 5.6 ), whereas benign masses are softer ( Fig. 5.7 ). The elastogram often shows a cancer to be larger compared with B-mode gray scale US imaging (see Fig. 5.6 ), whereas there is no significant difference in size of a benign mass on gray scale or elastographic depiction. At the current time, using elastography to increase specificity above that of gray scale morphology is operable only for Breast Imaging Reporting and Data System (BI-RADS) categories 3 and 4A; downgrading soft masses assessed as categories 4B, 4C, and 5 to avoid biopsy is not permitted. Other technologies such as optoacoustics are under study for improving the specificity of US but have not been validated for use.

Other Techniques

The use of contrast agents has been proposed as a means of improving the sensitivity of US vascular imaging. Three-dimensional gray scale US ( Fig. 5.8 ), currently without either vascularity or elastographic assessment, is undergoing rapid development for whole-breast automated US in which the coronal view of the entire breast can enable rapid perception of architectural distortion and masses (see Fig. 5.8 ). Other new algorithms for use with high-resolution linear transducers have been in development, such as one that aids in the detection of microcalcifications.

Tissue Composition

As in mammography and magnetic resonance imaging (MRI), US can depict the many variations in breast tissue composition. According to BI-RADS fifth edition for mammography, loosely correlating with the four densities are three US categories: homogeneous background echotexture — fat; homogeneous background echotexture — fibroglandular; and heterogeneous background echotexture.

The proportion of fat-to-fibroglandular parenchyma varies widely in the normal population variably dependent on the patient’s age, hormonal influences, and individual characteristics. In young women the breast tissue is composed predominantly of fibroglandular tissue; in older women, breast tissue has a greater proportion of fat. However, there are substantial individual variations.

In breasts with homogeneous background echotexture—fat, most of breast tissue is occupied by oval fat lobules surrounded by a thin rim of connective tissue or Cooper’s ligaments, which provide connective tissue support for the breast from the posterior layer of superficial pectoral fascia to the superficial layer lying just beneath the skin ( Fig. 5.13 ). Breasts with homogeneous background texture—fibroglandular have a thick zone of homogeneously echogenic fibroglandular parenchyma beneath a layer of subcutaneous fat of variable thickness ( Fig. 5.14 ). Many lesions, both benign and malignant, arise within the fibroglandular zone or at its junction with the layer of fat.

Heterogeneous background echotexture is characterized by multiple hyperechoic and hypoechoic areas, which can be focal or diffuse ( Fig. 5.15 ). Heterogeneous parenchyma may confound interpretation of small hypoechoic areas of fat with small hypoechoic masses.

Masses

The BI-RADS lexicon suggests standardized reporting for masses using feature analysis. A mass has three dimensions and occupies space. It should be seen in two planes on two-dimensional imaging and in three planes with volumetric acquisitions. Using multiple sonographic descriptors when describing a mass increases specificity and diagnostic confidence. The three most important feature categories, taken together for assessment of likelihood of malignancy, are shape, orientation, and margin . Additional feature categories such as echo pattern, posterior features, and architectural distortion (included in associated features ) may aid in assessing a mass.

Shape

Mass shapes are classified as oval, round, or irregular ( Figs. 5.16 and 5.17 ).The descriptor oval is used for a mass that is elliptical and includes gentle lobulation with up to three undulations. A round mass is one that is spherical, ball shaped, circular, or globular. Round is the least common shape. With US, to call a mass round, it must be circular in perpendicular projections. By definition, a mass that is neither round nor oval is irregular.

Orientation

This feature is unique to US imaging. The orientation of a mass is categorized as parallel or not parallel ( Fig. 5.18 ). Orientation is defined with reference to the skin surface. A parallel, or wider-than-tall, orientation is a property of most benign masses, but it may be seen with some cancers as well. Not parallel, or taller-than-wide, orientation indicates that a mass may be growing through the normal tissue planes and is the most common orientation of malignant masses. Orientation alone should not be used in assessing a mass.