High-quality Breast Ultrasonography
Janice S. Sung, MD
KEYWORDS Breast Ultrasonography Imaging Image quality
KEY POINTS Ultrasonography is an important modality that is frequently used in all aspects of breast imaging, including breast cancer screening, the evaluation of palpable abnormalities, further characteriza- tion of lesions seen mammographically, and for determining the method of percutaneous biopsy. Understanding the basic technical aspects of ultrasonography equipment is critical to ensure high breast ultrasonography image quality.
INTRODUCTION ultrasonography, including the development of three-dimensional transducers and automated Ultrasonography is an important imaging modality whole-breast systems, breast ultrasonography for the detection and characterization of lesions in still requires real-time imaging in many situations the breast. Appropriate indications for breast ultra- for accurate interpretation. sonography as recommended by the American In recent years, multiple states have passed College of Radiology Practice Guidelines include legislation mandating that women with dense the following: breasts be notified of their breast density and that Evaluation and characterization of palpable they may benefit from supplemental screening. masses and other breast-related signs and/ This development has led to the proliferation of or symptoms whole-breast screening ultrasonography as a sup- Evaluation of abnormalities detected on plemental screening modality to mammography in mammography or breast magnetic resonance women with dense breasts. Particularly when ultra- (MR) imaging sonography is used in the screening setting, it is Determining the method of guidance for imperative to understand the various technical percutaneous biopsy factors affecting image optimization in order to Supplemental screening to mammography in maximize sensitivity while reducing the number certain populations of unnecessary biopsies and recommendations for short term follow-up studies. This article re- Advantages of breast ultrasonography include views the technical factors that should be con- that it is a rapid, widely available, and inexpensive sidered in order to perform high-quality breast modality that does not involve breast compression ultrasonography. or ionizing radiation. As with all imaging modal- ities, ultrasonography’s value in the detection PATIENT POSITIONING and characterization of breast lesions largely de- pends on the quality of the images. Ultrasonogra- The optimal position of the patient should minimize phy is highly operator dependent, and erroneous the thickness of the portion of the breast being conclusions may be caused by technique or the imaged. During breast ultrasonography, the pa- application or misapplication of image processing tient should be positioned with the ipsilateral arm algorithms. Although there have been advances in over the head. In general, medial lesions should
Department of Radiology, Memorial Sloan-Kettering Cancer Center, New York, NY 10065, USA E-mail address: [email protected]
Radiol Clin N Am 52 (2014) 519–526 http://dx.doi.org/10.1016/j.rcl.2014.02.012
0033-8389/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved. radiologic.theclinics.com 520 Sung
be scanned with the patient in the supine position, scale. Transducers with higher frequencies have whereas lateral lesions should be scanned with improved contrast resolution. the patient in a supine oblique position in order Spatial resolution is composed of both axial to reduce the thickness of the breast. and lateral resolution. Axial resolution is the ability to resolve structures along the axis of the ul- TRANSDUCER SELECTION trasound beam or the Z plane (the depth). Axial resolution depends on pulse length, which is High-quality breast ultrasonography begins with determined by the frequency and the bandwidth selection of an appropriate transducer. The trans- of the transducer. A transducer with a higher fre- ducers used in breast imaging must have a high quency and a broader bandwidth has a shorter frequency (between 10 and 15 MHz) because of wavelength and pulse length relative to a lower the superficial nature of the breast and the need frequency transducer, which improves axial to resolve small structures (Fig. 1). However, resolution. higher frequency sound waves are more strongly Lateral resolution is the ability to resolve struc- attenuated by tissue than lower frequency waves. tures in the X and Y planes that are at the same Therefore there is a trade-off between higher reso- depth. Lateral resolution is related to the trans- lution and reduced penetration. With proper posi- ducer beam width, and lateral resolution is not tioning and the patient in the supine or supine adequate when lesions positioned side by side oblique position, most breasts are only a few cen- are within the same beam width. Because a higher timeters thick and high-frequency transducers frequency transducer has a narrower beam width provide optimum image quality for all of the breast relative to a lower frequency transducer, lateral tissue. When evaluating deep tissue in patients resolution is improved. with particularly large breasts, it may be helpful to have lower frequency transducers available to FOCAL ZONE be used only in this specific situation. Bandwidth is another consideration in trans- As discussed earlier, lateral resolution is related ducer selection. The bandwidth is the spread of to the beam width. In addition to selecting a frequency around the central frequency of the high-frequency transducer to maintain a narrow transducer, and transducers with a broader band- beam width, the beam width can be further width have improved resolution. Transducers used reduced by adjusting the focal zone. in breast imaging may either be linear or matrix Linear array transducers have multiple piezo- array, which affects image resolution. electric crystals or elements arranged side by side. If a single element is used to both transmit IMAGE RESOLUTION and receive the signal, the beam diverges quickly after traveling a few millimeters, resulting in poor The goal of equipment selection is to maximize im- lateral resolution because of beam divergence. age resolution, which is the ability to distinguish Linear array transducers pulse adjacent elements structures that are close together as separate le- simultaneously as a single element group to over- sions. Image resolution is composed of contrast come beam divergence and then in succession to and spatial resolution (Fig. 2). Optimal contrast form the image. Delaying the timing of firing of el- resolution is necessary to differentiate subtle le- ements within a single element group adjusts the sions from the surrounding breast tissue, and focal zone so that the beam is narrowed along these lesions may have subtle variations of gray the long-axis plane. The specific time delay
Fig. 1. Transducer frequency. High-quality breast ultrasonography requires the use of high-frequency trans- ducers. The same mass (arrows) is imaged using 9-MHz (A) and 12-MHz (B) transducers. The higher frequency (12 MHz) transducer results in better detail of the mass and the surrounding breast parenchyma. High-quality Breast Ultrasonography 521
transducer, compensating for increased attenua- tion of the ultrasound beam as it penetrates deeper into the tissues. Increasing the gain am- plifies the intensity of all signals returning to the transducer so that the image is brighter and more visible on the display screen. The initial gain setting should be adjusted so that the subcutaneous fat is a medium level of gray. Le- sions may be mischaracterized if the gain is inap- propriately set. For example, a subcentimeter hypoechoic solid mass or a complicated cyst can mimic a simple anechoic cyst if the gain is Fig. 2. Image resolution. Static ultrasonography im- Fig. 6 age shows a subtle 2-mm invasive cancer (arrow) set too low ( ). In contrast, by inappropriately that would easily have been missed if image quality setting the gain too high, a simple or complicated was not optimized. cyst may appear solid. Unlike the gain, which adjusts all signals return- determines the depth of focus for the transmitted ing to the transducer, the time gain compensation beam. Matrix array transducers, which have multi- (TGC) function allows selective amplification of ple rows of elements, allow focusing in both the weaker signals from areas deeper in the breast. short and long axes. The focal zone represents The TGC should be set so that all echoes from the narrowest part of the beam and is the area similar structures are displayed with the same where lateral resolution is optimized. Therefore brightness from the near to far field. For example, the focal zone should be placed at or slightly below the subcutaneous fat should be the same shade of the area of interest (Fig. 3). Because of transducer medium gray as the retromammary fat. limitations, even the shallowest focal zone setting may not achieve the narrowest beam width SPATIAL COMPOUND IMAGING possible for lesions near the skin. Therefore a standoff pad may be required to improve resolu- With standard imaging, the ultrasound pulses are tion (Fig. 4). propagated perpendicular to the long axis of the transducer. Spatial compounding uses electronic beam steering to obtain multiple images at DEPTH different angles, which are then combined to During an initial survey, the depth on the ultraso- form a single image in real time.1 Spatial com- nography image should be set so that the breast pounding enhances returning echoes from real parenchyma is imaged to the pectoralis muscle, structures, thereby improving image resolution, which should be along the far field of view so that image features such as lesion margins (Fig. 5). Once a finding is identified, the depth may be better characterized. Artifacts, such as may be adjusted. posterior acoustic enhancement characteristic of simple cysts and posterior shadowing seen with GAIN AND TIME GAIN COMPENSATION some solid masses, tend to be averaged out and reduced. However, some small cancers are The overall gain control provides uniform amp- detected primarily because of their posterior lification of all echo signals returning to the acoustic shadowing. When scanning for a subtle
Fig. 3. Focal zone. Images of the same mass with the focal zone set too deep (A, arrow) and at the appropriate depth (B, arrow). When the focal zone is at the level of the mass, the margins and internal features can be better characterized. 522 Sung
Fig. 4. Focal zone of superficial lesion. Because of transducer limitations, even the shallowest focal zone setting may not achieve the narrowest beam width possible for lesions near the skin. Therefore a standoff pad may be required to improve resolution. (A) A superficial hypoechoic mass (arrow). With the use of a standoff pad (B) al- lowing optimal resolution at the level of the skin, a skin tract (arrow) is now seen, confirming the diagnosis of a benign sebaceous cyst.
lesion, it may be beneficial to use standard ultraso- color Doppler.4 Power Doppler imaging may be nography imaging, in which the posterior shadow- used to increased sensitivity compared with color ing may help identify the lesion, and then apply Doppler in detecting small vessels and low flow. spatial compound imaging for analysis once the The presence or absence or type of tumor lesion has been detected. vascularity alone is not sufficient to characterize a lesion as benign or malignant. Features suggest- HARMONIC IMAGING ing malignant lesions include hypervascularity, irregular branching central vessels, and more Harmonic imaging is another signal processing than one vascular pole.5,6 Demonstrating internal technique to improve contrast and lateral resolu- vascularity within a sonographic lesion confirms tion and reduce artifacts.1 Harmonic imaging ap- that it is either solid or at least contains a solid plies a filter to remove image harmonics, which component. Doppler imaging is most useful in dis- are multiples of the transmitted frequency. This tinguishing a high-grade invasive cancer or meta- technique is advantageous because much of the static lymph node, both of which can appear artifact degrading image resolution is contained anechoic, from a simple or complicated cyst within the lower frequency components of the (Fig. 8). Color Doppler may also be useful in differ- beam (Fig. 7). entiating between debris within a duct and an in- traductal mass. COLOR DOPPLER AND POWER DOPPLER When using Doppler imaging, it is important to apply light transducer pressure to prevent occlu- Tumor angiogenesis plays a fundamental role in sion of slow flow within vessels. The position and local tumor growth, invasion, and progression to size of the color box should also be focused to metastases.2,3 As tumors outgrow their native the lesion of interest to maximize sensitivity to flow. blood supply, hypoxia ensues, which induces expression of multiple angiogenic factors such as REAL-TIME IMAGING vascular endothelial growth factor. These factors induce the growth of existing capillaries and Real-time imaging may be essential for accurate the formation of abnormal vessels that are often interpretation in many situations. In patients with tortuous and disordered, which may be seen on palpable complaints, real-time ultrasonography by
Fig. 5. Depth. The depth should be set so that the image focuses on the area of interest without including the lung, which provides no useful information (A). When the depth is set appropriately (B), the features of the mass (arrows) are better seen. High-quality Breast Ultrasonography 523
Fig. 6. Gain. Lesions can be mischaracterized if the gain is set too low or too high. (A) The gain is set too low and the lesion appears anechoic (arrow), suggesting a simple cyst. With the gain set correctly (B), low-level internal echoes (arrow) are now seen in this complicated cyst. (C) The gain is set too high, creating artificial internal echoes (arrow) mimicking a solid mass. When the gain is corrected (D), the lesion is seen to be a simple cyst (arrow). the interpreting radiologist may be prudent in cases biopsy is faster, more comfortable for the patient, in which initial scanning by the technologist does and allows greater access to breast tissue, espe- not identify an abnormality. Another situation in cially for far posterior and medial lesions that which real-time imaging may be important is when may not be amenable to either stereotactic or differentiating between mobile debris in a compli- MR imaging–guided biopsy. In addition, adequate cated cyst versus a complex cystic mass (Fig. 9). sampling is more consistently obtained because the needle can be seen traversing the target in TARGETED ULTRASOUND: LESION real time. CORRELATION When targeted ultrasonography is performed to evaluate a mammographic finding, careful trian- Targeted ultrasonography is often performed in gulation of lesion location should be performed. order to identify a sonographic correlate to an Adjustments should be made when using cranio- abnormality identified on mammography or caudal and mediolateral-oblique (MLO) views to breast MR imaging before percutaneous biopsy. determine the clock axis for targeted ultrasonogra- Ultrasonography-guided biopsy is the method of phy. The location of the lesion may be higher or choice to sample any finding that is sonographi- lower than expected based on the MLO view for cally evident. Compared with stereotactic or MR medial and lateral lesions respectively. In addition, imaging–guided biopsy, ultrasonography-guided careful attention should be paid to both lesion depth
Fig. 7. Harmonic imaging. (A) The 12:00 axis of the right breast shows 2 subtle structures (arrow) that appear almost isoechoic to the fat. (B) Harmonic imaging of the same area allows better delineation of the margins and internal features (arrow), which allows the two lesions to be characterized as complicated cysts. 524 Sung
Fig. 8. Color Doppler. (A) A hypoechoic mass (arrow) in the far field with posterior enhancement, possibly rep- resenting a complicated cyst. With color Doppler (B), vascularity (arrow) is seen within the mass, confirming that it is solid and not a complicated cyst. Biopsy yielded invasive ductal carcinoma.
and the surrounding anatomic landmarks. The pa- enhancement.7–11 However, true ultrasonogra- tient is positioned differently during breast ultraso- phy–MR imaging correlation can only be con- nography, MR imaging, and mammogram, which firmed if follow-up MR imaging is performed, affects lesion location. A superficial mass at the showing the localizing clip placed at the time of fat-gland interface on mammography or MR imag- ultrasonography-guided biopsy within the area of ing should also be at that interface on targeted ultra- enhancement on MR imaging. In one study, the sonography (Fig. 10). Other findings such as an presumed sonographic correlate biopsied yielding adjacent cyst or dilated duct may also be used as a benign, concordant diagnosis did not correspond anatomic landmarks. with the lesion originally detected on MR imaging in When sampling a potential sonographic corre- 12% of cases.10 For this reason, 6-month follow-up late to a mammographic abnormality, a localizing MR imaging is recommended following benign clip should be placed in the biopsied lesion using concordant biopsy of a sonographic correlate to ultrasonography guidance. A postbiopsy mammo- an MR imaging–detected lesion.12 gram should be obtained to confirm correlation between the biopsied lesion and the mammo- AMERICAN COLLEGE OF RADIOLOGY graphic abnormality. ACCREDITATION Targeted ultrasonography is often performed to evaluate for a sonographic correlate to an MR The American College of Radiology offers a Breast imaging finding in order to facilitate biopsy. A Ultrasound Accreditation Program. Accreditation potential correlate is more frequently identified requires the application of many of the concepts for enhancing masses compared with nonmass discussed in this article to obtain high-quality
Fig. 9. Real-time imaging. (A) In the right breast at the 6:30 axis is a cyst with a possible intracystic mass (arrow). (B) When the patient is rolled into the left lateral decubitus position, the questionable mass moves into the dependent portion of the cyst (arrow), consistent with mobile debris. High-quality Breast Ultrasonography 525
Fig. 10. Lesion miscorrelation. (A) A superficial enhancing mass (arrow) in the superior breast. Targeted ultraso- nography identified a mass (arrow) that was thought to represent a correlate (B). Although the sonographic mass was of a similar size and clock axis to the MR finding, it was deep within the breast, whereas the MR mass was at the fat-gland interface. Ultrasonography-guided biopsy yielded fibroadenoma. On 6-month follow-up MR imag- ing, the clip (arrow) from ultrasonography-guided biopsy is seen deep to the MR mass (C). Subsequent biopsy yielded invasive ductal carcinoma. images. The center frequency of the transducer including breast cancer screening, the evaluation must be at least 10 MHz. Unlike breast MR imag- of palpable abnormalities, further character- ing accreditation, in which images from each indi- ization of lesions seen mammographically, and vidual magnet must be submitted, a facility must for determining the method of percutaneous only provide a single example of a cyst and solid biopsy. Understanding the basic technical as- mass in the breast for ultrasonography accredita- pects of ultrasonography equipment is critical tion regardless of the number of ultrasonography to ensure high breast ultrasonography image units at that facility. Two orthogonal mammo- quality. graphic views must be submitted with the lesion circled and visible on both views. Sonographic REFERENCES clinical images include the cyst and solid mass on 2 orthogonal views without calibers, and 1 im- 1. Harvey CJ, Pilcher JM, Eckersley RJ, et al. Ad- age with appropriate measurements. Images vances in ultrasound. Clin Radiol 2002;57(3): should be taken along the longest axis of a lesion 157–77. and then an orthogonal image. The longest axis 2. Schneider BP, Miller KD. Angiogenesis of breast of the lesion is not necessarily in the radial or anti- cancer. J Clin Oncol 2005;23(8):1782–90. radial plane, but may be in an oblique position. 3. Weidner N, Semple JP, Welch WR, et al. Tumor Criteria for a simple cyst include anechoic, circum- angiogenesis and metastasis–correlation in inva- scribed margin, and posterior enhancement. For sive breast carcinoma. N Engl J Med 1991; submitted images of cysts, spatial compound im- 324(1):1–8. aging should be avoided if the resulting image fails 4. Less JR, Skalak TC, Sevick EM, et al. Microvascular to show posterior enhancement. architecture in a mammary carcinoma: branching patterns and vessel dimensions. Cancer Res 1991; SUMMARY 51(1):265–73. 5. Yang W, Dempsey PJ. Diagnostic breast ultrasound: Ultrasonography is an important modality that is current status and future directions. Radiol Clin frequently used in all aspects of breast imaging, North Am 2007;45(5):845–61, vii. 526 Sung
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