If the lesion is small or the breast composition is dense, the architectural distortion may only be visible on one view. In these cases, sonography may be useful to demonstrate a mass and localize biopsy. Sonographically, normal fibroglandular tissue and fibrosis will be uniformly hyperechoic. Fat necrosis may either be heterogeneous or hyperechoic echogenicity. Both surgical and radial scars strongly attenuate the sonographic beam so shadowing may be the predominant feature. If heavy shadowing is present, a lower-frequency examination may penetrate the scar and demonstrate no mass. However, in some cases, surgical scars cannot be differentiated from neoplasms and biopsy is necessary. If radial scar is suspected, then excision is necessary. Using high-frequency sonography, malignant masses are either hypoechoic or heterogeneous echogenicity. Sonographic architectural distortion is common. This distortion may appear as hyperechoic haze or spiculations radiating from the tumor. Cooper's ligaments and the anterior fascial line may be distorted.
Sonography may miss a malignant lesion for several reasons. The sonographic examination may miss small masses. However, usually if the neoplasm has created mammographic architectural distortion, there is enough desmoplasia or direct tumor extension for sonographic detection. Therefore, commonly the reasons for error are because the location of the neoplasm is missed or the examiner does not recognize the mass. Reasons for missing the location or not recognizing the mass include examiner inexperience and poor sonographic technique. Since the mammographic findings may be subtle, this sonographic application requires that the examiner be highly skilled in breast sonography. One should be able to cross correlate mammographic and sonographic landmarks to find the lesion.
Even if the examiner is experienced, poor technique or equipment may prevent the imager from identifying masses. Masses are difficult to identify with lower-frequency transducers (<10 MHz). Lower frequencies commonly penetrate masses better than higher frequencies so there is little shadowing and the masses appear hyperechoic. The hyperechoic tumors blend into the surrounding hyperechoic parenchyma. Furthermore, lower frequencies have poorer resolution so the indirect signs of malignancy such as spiculations or distortion of Cooper's ligaments are not visible. Although generally higher frequencies are better than lower frequencies, high-frequency examination also has pitfalls. If the frequency is too high, there may be excessive shadowing. A shadowing malignancy may be hidden by the shadows produced by surrounding fibroglandular tissue. One should use a frequency that allows one to visualize the chest wall. Normal fibroglandular parenchyma will be hyperechoic, but a malignancy will be shadowing, hypoechoic, or heterogeneous echogencity.
Even if one initially does not see a suspicious mass, one may use mammographic and sonographic landmarks (see Section 2) and identify the suspicious breast tissue. The imager may then increase the sonographic frequency and perform a targeted evaluation of the spot to find a subtle mass. If a mass is not discovered, then the sonographic examination should demonstrate the etiology of the mammographic architectural distortion. If the sonographic examination does not clarify the cause of architectural distortion, then the lesion should be classified as Category 4, suspicious, and should be biopsied.
Section II
Sonographic Technique and Cross-Correlation with Mammography
Chapter 2
The main applications of ultrasound are identification of a palpable breast lump and clarification of a confusing mammographic finding. In the past, a common reason that sonographic applications were limited was that the ultrasound examiner could not cross correlate the physical examination or mammographic findings with the sonographic information. This inability to cross correlate the ultrasound examination with the physical examination or mammographic asymmetry is frustrating and leads to long, sonographic examinations. In order to develop the ability to cross correlate sonography with mammography, one should have optimal sonographic equipment and technique, be able to palpate breast masses, be familiar with normal mammographic and sonographic breast anatomy, recognize mammographic and sonographic signs of malignancy, and apply anatomic knowledge to sonographic scanning.
Ultrasound Equipment and Technique
Equipment
In the past, breast ultrasound has required the least sophisticated equipment because there were few sonographic breast applications and these applications required only simple equipment. However, if one wishes not only to have a high rate of localizing solid masses as well as an optimal image to characterize the mass, then one needs a sophisticated machine. In this book, when I refer to high frequency, I am generally referring to imaging with frequencies equal or greater than 10 MHz. To have maximal flexibility, one should have a machine that has linear transducers with frequencies ranging from 7 to 15 MHz.
High frequency is important as many breast structures are small. An important aspect of the technical advancement of mammography is the improvement of spatial resolution. Because of high mammographic resolution, mammographers are identifying smaller and subtler abnormalities. To be an effective adjunctive test, high sonographic spatial resolution is needed to clarify these subtle findings. Because normal breast structures such as ducts and terminal duct lobular units are small, high spatial resolution allows the ultrasound examiner to quickly recognize normal breast architecture and identify small malignant masses in the ductal system.
Besides being able to identify smaller malignancies, high spatial resolution improves the sonographic image so one can better characterize masses. This improved image is similar to the visual effect experienced by a nearsighted person who starts wearing glasses. The image is sharper, and subtle or smaller details are clearer (Fig. 2–1; also see Section 4 Case 57). For breast malignancies, the most important information produced by high-frequency ultrasound is the improved ability to see the margin of the mass and identify secondary signs of malignancy such as spiculation or architectural distortion. Occasionally, one can trace the path of the malignancy through the ductal system and identify the extent of the disease better than mammography (see Section 10, Cases 200 and 201).
Imaging Techniques
Besides the availability of high-frequency transducers, sophisticated ultrasound machines have more technical factors that can be manipulated to improve imaging quality. For breast ultrasound, an important factor in image quality is being able to obtain excellent contrast resolution. The reason contrast resolution is important is that one must be able to distinguish a variety of masses from the background parenchyma. When the breast is fatty, focal masses such as complex cysts, lymph nodes, fibroadenomas, and cancers may be difficult to identify. When the breast is dense, fat necrosis and surgical or radial scars may be hard to locate. Many factors can improve contrast resolution. High spatial resolution produced by high-frequency imaging improves contrast resolution because the assignment of gray shades is more precise. Reducing the dynamic range may improve contrast resolution as this method exaggerates differences between the gray shades of structures. A variety of proprietary postprocessing programs improve contrast resolution by enhancement of specified gray shades on a point-by-point basis. One type of proprietary postprocessing program improves contrast resolution by enhancement of specified gray shades on a region-by-region basis. One should consult the manufacturer of one's equipment to learn which imaging techniques are available (Fig. 2–2; also see Section 3 Case 18).
Figure 2–1. (A). Right MLO mammogram. (B). Right CC mammogram. (A,B). In the upper outer right breast there is a lobulated density (circled). (C). Right radial breast sonogram: A 5 MHz transducer does not clearly demonstrate the lobulated mass (arrows). The inadequate size and contrast resolution results in poor definition of the mass. (D). Right radial breast sonogram: An 8 MHz transducer improves the definition of the mass. This result allows an observer to confidently localize this mass. This mass is a fibroadenoma.
Besides adjusting image contrast, one should be aware of software methods to optimize resolution. These methods include increasing the line density of the image, increasing the persistence, and adjusting the focal zones.