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Ultrasound artifacts affecting the diagnosis of breast masses
Ultrasound in Med. & Biol. Vol. 14, Sup. i, pp. 203-210, 1988 Printed in the U.S.A.
0301-5629/88 $3.00 + .00 (c) 1988 Pergamon Press plc
ULTRASOUND ARTIFACTS AFFECTING THE DIAGNOSIS OF BREAST MASSES
Carolyn Kimme-Smith, PhD Peter A. Rothschild, MD Lawrence W. Bassett, MD Richard H. Gold, MD Delma Westbrook, RT Department of Radiological Sciences UCLA School of Medicine Los Angeles, California 90024
ABSTRACT An investigation of the outcome of over 1600 breast ultrasound examinations revealed a number of scanning artifacts. Artifactual echoes in cysts resulted from inappropriate scanning factors, including focal zone placement, gain, TGC, and gray scale, and from partial volume effect. The absence of a s&gn suspicious for carcinoma, posterior shadowing to a solid mass,resulted from faulty focal zone placement, or from the mass resting on chest wail tissue. Pathology was sometimes simulated by postsurgical scars, ducts, and benign calcifications. Corrective procedures to alleviate the various scanning artifacts are recommended. Key words:
Ultrasound instrumentation, techniques
phased arrays, time gain compensation,
scanning
INTRODUCTION During the last four years, over 1600 women were examined with ultrasound (US) as an adjunct to mammography at the UCLA Center for the Health Sciences. US was performed with a variety of instruments. The results were entered into a computer data system which facilitated a comparison of the instruments as well as clinical data and clinical outcome (Bassett et al, 1987, and Kimme-Smith et al, 1988). This data base also identified those cases in which different scanning methods and/or different instruments produced varying findings and hence varying diagnoses. When such findings do not represent a faithful image of the breast but are merely artifactual, their recognition becomes essential for making the true diagnosis, as has already been shown for organs other than the breast (Sarti, 1987, and Laing, 1983). The technical factors which, if improperly applied, are most likely to produce scanning errors are: the position in the breast where the technologist places the focal zone of the transducer, either by varying the depth of the water path (for automatic breast scanners) or by adding a stand-off pad (for hand-held real-time equipment) (Kimme-Smith et al 1988); and the technologist's settings of output power and time gain compensation (TGC) (Black et al, 1979). Most sonographers who use hand-held real-time instruments assume that the first 3 cm of the image are in focus because of the fluid stand-off which is an integral part of the hand-held probe (Winsberg, 1983). Because phased arrays are electronically focused, users expect that by selecting a i or 2 cm focus, the subcutaneous tissue will be well visualized. The users may be unaware that the asymmetry of the phased array element requires manufacturers to place a fixed-focus lens on the elevation or slice thickness direction of the array, resulting in a partial volume effect for masses at "out of focus" depths (Goldstein and Madrazo, 1981). Furthermore, for both phased and mechanical units, high resolution hand-held US requires high-frequency (5MHz to IOMHz) transducers (Kimme-Smith et al, 1988). Because the increased attenuation of high-frequency sound in tissue limits penetration, high-frequency transducers must be sharply focused. Although a sharply focused transducer has a short focal region and spreads and dissipates the sound waves abruptly in the far field (Walter, 1985), it has been found effective for examinations of the compressed breast (Kelly-Fry and Harper, 1983). Automated US breast scanners were designed to avoid these focusing problems by allowing the sonographer to adjust the focal zone placement independently for each breast (Kossoff and Jellins, 1982 and Jackson et al, 1986). The water offset of these instruments avoids near field artifacts and allows the sonographer to reduce the length of th~ water path, permitting the focal zone to be placed deeper in the breast.
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Other artifacts associated with hand-held real-time units are documented elsewhere (Bassett et al, 1986). The present paper compares the appearances of lesions with and without artifacts, and describes methods to create artifacts to aid in the differentiation of benign from malignant masses. MATERIALS AND METHODS Two automated units, an Ausonics (with 3.9 and 4.5 MHz transducers) or a Labsonics (with a single P o l y v i n y ~ d e n e Fluoride transducer naturally resonant at 7.5 MHz but tunable to lower frequencies), were used to examine over 1300 of the 1600 patients examined with breast US at the UCLA Medical Center during the past four years (Ausonics:985, Labsonics:355). The remainder were scanned by one or more of ten real-time hand-held units operating at 5 MHz or higher (Diasonics 200:280, Philips 2500, 2000, 1550, 1200:700). The units and scanning techniques are described elsewhere (Bassett et al, 1987, and Kimme-Smith et al, 1988). Over 500 patients were scanned by two or more units. This practice often resolved difficult diagnoses resulting from artifactual scans with a single unit. Most of the hand-held examinations utilized a Diasonics 200 unit using a i0 MHz lead zirconate probe with a fluid offset. In addition, scans were obtained with an Ausonics Micro Imager 7.5 MHz unit and with Philips 3000, 2500, 1200, and 1550 units, using a variety of mechanical and phased probes operating at 5 MHz or 7.5 MHz. Acuson, T~chnicare, Picker, and ATL scans were done on a smaller population of patients for comparison purposes. Scans were usually made of both breasts, even when only one breast contained an area of clinical or mammographic concern. One technologist with eight years of clinical experience in ultrasound performed every examination throughout the time period covered by this report. The illustrative cases in this paper were retrieved by two methods: I. When a patient with a cystic mass was found to have a solid mass at the same location on subsequent examinations, she was re-examined with at least two different units in an attempt to establish the reason for the initial misdiagnosis. 2. Using the computer data system described in the Introduction, we were also able to identify examinations in which masses were detected in the automated scan but not the hand-held scan, and vice versa. From the US images, we recorded the following mass characteristics on our computer data system: solid versus cystic, smooth borders versus irregular b o r d e r s , ~ d posterior echo enhancement versus shadowing. Image differences in any one of these features led to a re-examination of the images for artifacts. The accompanying illustrations (Figures 1-11 are from some of the cases retrieved by these two methods. RESULTS AND DISCUSSIONS MASS BORDERS Of the three imaging features for differentiating benign from malignant solid masses, (i.e., interior echo characteristics, mass borders, and posterior echo enhancement or shadowing), mass borders were less prone to artifactual misdiagnosis than the other features. When a smooth bordered, round mass is imaged in h e far field, it sometimes appears to have irregular side walls due to its elongated texture and otherwise poor imaging characteristics in that location; but even then the mass does not manifest the lobulated or stellate characteristics of carcinoma. None of our cases showed this diagnostic error. POSTERIOR ENHANCEMENT The varying presence or absence of posterior enhancement or shadowing was the most common artifact among scans of the same breast obtained with different instruments. Although posterior enhancement was dependent on the type of gray scale, the tissue lying posterior to the mass, and the position of the mass with respect to the TGC and focal zone, it was the gray scale that was most likely to affect posterior enhancement. Gray scale is associated with that electronic circuitry in the US unit which maps the received voltage into a discrete shade of gray. The relationships of the voltage to the shade of gray displayed in the image is called the "gray scale." Many US units allow the user t o s e l e c t from several mapping functions, or to change the dynamic range so that the contrast in the i m ~ e ~s changed. When low amplitude reflections are represented with the same number of gray levels as high amplitude reflections, the gray scale map is called linear, even when a logarithmic receiver has preferentially amplified the low amplitude signals resulting from weak reflectors or scatter. A logarithmic gray scale map assigns more gray level values to low amplitude reflections hence increasing contrast. Thus, a low voltage level resulting from scattering will be represented by brighter gray levels (for a white-on-black image) if a logarithmic rather than linear gray level map is employed. Manufactureres vary the number of gray levels displayed and the proportion of gray levels reserved for displaying a low range of voltage signals. Users can identify logarithmic gray scale maps by the increased contrast in image scatter when the logarithmic rather than the
5th International Congress on the Ultrasonic Examination of the Breast
linear gray logarithmic "quadratic". the contrast
205
scale map is selected. Different manufacturers of ultrasound units may label gray scale maps differently. We have seen the labels "root", "exponential", "B map" applied to alternative gray scales which had the effect of increasing in low amplitude reflections and scatterers.
Because a high frequency pulse is attenuated more than a lower frequency pulse, we would expect posterior echo enhancement, such as that associated with a cyst, to be more visible on an image made with a high frequency transducer. However, since 99% of the incident US wave amplitude is attenuated within 3 cm of the skin for a 5 MHz focused pulse, even posterior enhancement from a 1 cm cyst whose anterior edge is 2 cm deep in the breast will have low amplitude. The gray scale at which this low amplitude voltage signal is mapped will affect the appearance of posterior enhancement of the cyst. Thus a linear gray scale map may not have sufficient differences in gray levels to differentiate between the signals returning from behind the cyst and those returning from adjacent tissue at this same level. Unfortunately, a more logarithmic gray scale map, while it may help image posterior echoes, may also create artificial interior echoes in anechoic masses (Figure i). However, the character of these echoes help to identify them as artifactual. This subject is discussed in the section on the diagnosis of cystic versus solid masses.
Figure i.
Superficial logarithmic distributed selected by
1.6 cm diameter cyst imaged on Diasonics 200 (IOMHz) with a gray scale map to insure posterior enhancement. Note uniformly interior echoes within the cyst. These are due to the gray scale map the operator and will disappear when a more linear map is used.
The specific nature of the tissue distal to the mass may also affect enhancement. When a cyst lies deep in the breast, the muscle and chest wall deep to the cyst attenuate the beam and thus enhancement is not apparent (Figure 2A). If the patient is repositioned so that her breast is scanned from the side, breast tissue introduced behind the cyst will show posterior echo enhancement (Figure 2B).
A Figure 2.
The .Scm diameter cyst in this image, which lies against the chest wall, does not show posterior enhancement when scanned by the Diasonics 200 unit (IOMHz). B) When the position of the rib cage is changed by obliquing the patient, breast tissue can be positioned behind the cyst and will show posterior enhancement. The fine echoes in the cyst are due to the logarithmic gray scale map used for these images in an attempt to increase posterior enhancement.
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DISTAL SHADOWING Shadowing distal to a mass indicates a higher attenuation in the mass than in the surrounding tissue, and is suspicious for malignancy. Unfortunately, many normal structures, such as ducts and Cooper's ligaments, can also cause posterior shadowing, particularly in the far field (that depth where the beam width is two times that at the focus) of sharply focused transducers (Figure 3). Nipple shadowing can also be misdiagnosed as pathologic if images are not properly labelled with position information. Since masses behind and beside the nipple may be obscured by the nipple shadow (Figure 4), an experienced sonographer will always try to reposition or redirect the transducer obliquely behind such shadowing. Shadows less than 2 mm in width which begin at the skin usually represent air bubbles, (Figure 5), while larger shadows beginning near the skin are usually scars (Figure 6) or other skin irregularities, and are only misinterpreted when the technologist fails to notate their true origin. Large benign calcifications may cause prominent shadows, and should be obvious in correlative mammo~rams (Figure 7).
Figure 3.
The arrow at the left of this longitudinal, outer quadrant breast image marks the end of the focal zone in the Labsonics automated unit. The focal zone of this unit can be positioned by the operator. Shadows posterior to the focal zone are due to the large beam aperture in the far field imaging ducts and other reflectors which are smaller than the beam profile. These shadows should not be mistaken for malignant masses.
Figure 4.
The shadow from a carcinoma, (arrow) beside the nipple (N) merges with the nipple shadow to make a difficult diagnostic problem on this longitudinal Labsonics scan.
Figure 5.
An air bubble on the skin causes a 2 mm shadow within the breast on an Ausonics water path scan (permission to reproduce from Bassett LW, Gold RH, Kimme-Sm~th C. "Hand-held and Automated Breast Ultrasound", Little Brown and Company).
5th International Congress on the Ultrasonic Examination of the Breast
Figure 6.
207
A biopsy scar produced this shadow in an Ausonics water path scanner (permission to reproduce from Bassett LW, Gold RH, Kimme-Smith C. "Hand-held and Automated Breast Ultrasound," Little Brown and Company).
A Figure 7.
A. B.
Mammogram of a 4 m m benign calcification imaged on a CGR 500T Senographe. Diasonics 200, 10 MHz, with a fluid offset, images the calcification.
ANECHOIC MASSES The differential diagnosis of a cyst versus solid mass depends entirely on the echoes or lack of them within the mass. Unfortunately, echoes can be produced in any cystic mass by imaging out of the focal region, increasing the gain or TGC, reducing the dynamic range, and/or incorrectly using a logarithmic gray scale. Of the instruments we used, the Labsonics was the least susceptible to these problems, since it does not have operator controlled gray scales or changeable TGCs, but our Labsonics unit also did not show posterior echo enhancement as frequently as our real-time hand-held units. In general, increased gain, TGC, or power, will fill in only the proximal portion of a cyst, while changing the gray scale or dynamic range to increase contrast may cause the center or all of the interior of the cyst to be filled in with fine textured noise (i.e., homogeneous texture). A mass should only be
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suspected of being solid when its interior echoes are heterogeneous when compared to the very fine "noise" from artifactual cyst fill-in. While some carcinomas have very coarse interior echoes compared to fibroadenomas, this is not a necessary finding for malignancy. When 3 or 4 bright echoes are seen in what would otherwise be diagnosed as a cyst, these may represent debris within the cyst fluid. Debris can sometimes be diagnosed by changing the position of the patient while continuing to scan the mass with a real-time hand-held transducer. If the debris moves about relative to the cyst wall, the mass is a cyst. When a cyst fiils in because it is imaged beyond the focal region of the transducer, repositioning the patient may enable the amount of tissue between the mass and transducer to be reduced. However, more subtle problems arise for masses imaged in the near field. Because of Huygen's principle, the intensity pattern of the US wavelets forming the pulse are irregular and have local maximums in the near field. These interference effects cause apparent echoes to occur in small anechoic masses. Because the US beam is wider in the near field than in the focal zone, the edge of the beam may strike a random scatterer outside of the cyst, which will be misinterpreted by the US unit as being within a cyst. Mechanical real-time units have circular transducers, so the beam shape is symmetrical. Phased array transducers elements are rectangular, with the long dimension parallel to the slice thickness direction. The slice thickness direction is the plane perpendicular to the image plane. As in computerized tomography, the depth of the slice encompassed by the beam contributes information to the image. High frequency phased linear or sector arrays (as opposed to annular arrays) are always permanently focused in the slice thickness direction by attaching an external lens to each element in the array. Lateral direction focusing is controlled by electronic phasing, while slice thickness focus is always at a single depth, which is determined by the transducer's designers. Phased arrays almost always image superficial masses improperly unless an offset is used to bring the mass to the depth of the slice thickness focus (Figure 8). Offset pads or water filled bags should always be used with phased or linear arrays when examining the skin and subcutaneous tissue (Figure 9). Near field reverberations may also occur wfth the use of mechanical sector units if hard plastic is used to separate the offset fluid from the patient. The reverberations interfere with visualizing the first cm Of tissue (Figure i0).
A Figure 8.
B
A 1.2 cm superficial cyst imaged on the Acuson (A) without, and (B) with a 2 cm offset pad. Despite correct dynamic focusing, the 5MHz probe has a fixed slice thickness focus (see text) at 3.5 cm, which contributes in (A) to the fill-in of the cyst by the partial volume effect.
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Figure 9.
Figure i0.
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A 5.0 MHz Picker phased array distorts the image of the near field parenchyma because it operates without an offset. This 1.1 cm diameter solid mass is imaged with two different gray scales. The gray scale on the right causes the mass to resemble a cyst with a septum.
The hard plastic in the ATL Mark 4 Access transducer operating at 7.5 MHz produces reverberations which can only be corrected by scanning through an offset pad (or water-filled bag).
FAT LOBULES AND DUCTS Several misdiagnoses occurred in our series because of failure to obtain transverse slices of fat lobules and ducts. When imaged longitudinally, these structures may be mistaken for masses (Figure Ii).
A Figure ii.
B
A. Ducts ranging in diameter from .3 to .5 mm appear to be masses in the transverse plane, but are correctly diagnosed in B by a longitudinal scan. Both images were produced by an Ausonics Microimager operating at 7.5 MHz with a built-in fluid offset.
To avoid this error, all masses should be imaged in two planes. A fat lobule while round in one cross sectional plane is elongated in a second plane perpendicular to the first. Thus, an image of a fat lobule should not be misdiagnosed as a round mass with heterogeneous interior echoes because in a plane perpendicular to the initial scan plane the resultant excessively elongated structure is easily distinguished from a true mass. The Labsonics
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automated breast unit is superior to a hand-held unit in distinguishing fat lobules from masses because it can pivot on the exact location of a suspected mass, while a hand-held unit is not as precise in identifying the plane perpendicular to the area of concern. CONCLUSIONS The user should obtain from the manufacturer precise data regarding the frequency, F#, depth of focus, and maximum penetration of each transducer. If phased arrays are used, the manufacturer should also provide slice thickness focusing information; i.e., where the image slice is narrowest. This information may help the user to avoid problems related to lack of posterior echo enhancement and/or increased shadowing in the far field. Suitable offset materials, either commercially manufactured pads or water filled bags (or gloves), should always be available wherever hand-held imaging is performed. The interiors of all masses should be filled in, either by increasing the gain, by using a logarithmic gray scale map, or by selecting a low dvpamic range. The nature of the resultant artifactual interior echoes will aid in determining the presence or absence of true anechoic interiors. Changing the position of the patient and/or the transducer may yield an image with more diagnostic information. Because the breast is a soft organ its position can be shifted on the chest wall, enabling masses to be moved into the focal zone of the transducer. It may also be scanned from a variety of angles to increase posterior echo enhancement. The US examination of the breast is an adjunct to mammography. As such it is a problemsolving modality. The diagnosis of e~ery mass depends on the intelligence and skill of both the technologist and the image interpreter. REFERENCES Bassett LW, Gold RH, Kimme-Smith C (1986). Instrumentation and technique. In Hand-Held and Automated Breast Ultrasound (By Bassett LW, Gold RH, Kimme-Smith C) Little Brown and Company, New Jersey. 33-60. Bassett LW, Kimme-Smith C, Sutherland LK, Gold RH, Sarti D, King W (1987). Automated versus hand-held breast ultrasound: effect on patient management. Radiology, 165:103-108. Black EB, Ferrucci JT, Jr., Wittenberg J, Kirkpatrick RH, Hann LE (1979). Acoustic contrast enhancement: value of several system gain variations in gray scale ultrasonography. AJR 133, 689-693. Goldstein A, Madrazo BL (1981). 365-375.
Slice-thickness
artifacts in gray-scale ultrasound JCU 9,
Jackson VP, Kelly-Fry E, Rothschild PA, Holden RW, Clark SA (1986). Automated breast sonography using a 7.5 MHz PVDF transducer: preliminary clinical evaluation. Works in Progress, Radiology 159, 679-684. Kelly-Fry E and Harper (1983). Factors Critical to Highly Accurate Diagnoses of Malignant Breast Pathologies by Ultrasound Imaging. In Ultrasound 82, Ed. R.A. Leraki and P. Morley, Pergamon Press, New York. Kimme-Smith C, Bassett LW, Gold RH (1985). High frequency breast ultrasound: Hand-held versus automated units; examinations for palpable mass versus screening. J Ultrasound in Med 7, 77-81. Kimme-Smith C, Hansen M, Bassett LW, Sarti D, King W (1985). effects of focal zone placement. Radiographics 5, 955-969. Kossoff G and Jellins J (1982).
Ultrasound mammography:
The Physics of Breast Echography.
Semin in Ultr~sound 3,5-12
Laing FC (1983). Commonly encountered artifacts in clinical ultrasound. In Diagnostic Ultrasound: Text and Cases (By Sarti, DA) Yearbook Medical Publihsers, Inc., Chicago, 1-69. Walter JP (1985). N A m e r 23, 3-11.
Physics of high resolution ultrasound -- practical aspects.
Wi~mberg F (1983). Semin in Ultrasound,
Physical and design considerations Ct and MR 4, 44-49
Rad Clin of
in real-time ultrasonography.
0301-5629/88 $3.00 + .00 (c) 1988 Pergamon Press plc
ULTRASOUND ARTIFACTS AFFECTING THE DIAGNOSIS OF BREAST MASSES
Carolyn Kimme-Smith, PhD Peter A. Rothschild, MD Lawrence W. Bassett, MD Richard H. Gold, MD Delma Westbrook, RT Department of Radiological Sciences UCLA School of Medicine Los Angeles, California 90024
ABSTRACT An investigation of the outcome of over 1600 breast ultrasound examinations revealed a number of scanning artifacts. Artifactual echoes in cysts resulted from inappropriate scanning factors, including focal zone placement, gain, TGC, and gray scale, and from partial volume effect. The absence of a s&gn suspicious for carcinoma, posterior shadowing to a solid mass,resulted from faulty focal zone placement, or from the mass resting on chest wail tissue. Pathology was sometimes simulated by postsurgical scars, ducts, and benign calcifications. Corrective procedures to alleviate the various scanning artifacts are recommended. Key words:
Ultrasound instrumentation, techniques
phased arrays, time gain compensation,
scanning
INTRODUCTION During the last four years, over 1600 women were examined with ultrasound (US) as an adjunct to mammography at the UCLA Center for the Health Sciences. US was performed with a variety of instruments. The results were entered into a computer data system which facilitated a comparison of the instruments as well as clinical data and clinical outcome (Bassett et al, 1987, and Kimme-Smith et al, 1988). This data base also identified those cases in which different scanning methods and/or different instruments produced varying findings and hence varying diagnoses. When such findings do not represent a faithful image of the breast but are merely artifactual, their recognition becomes essential for making the true diagnosis, as has already been shown for organs other than the breast (Sarti, 1987, and Laing, 1983). The technical factors which, if improperly applied, are most likely to produce scanning errors are: the position in the breast where the technologist places the focal zone of the transducer, either by varying the depth of the water path (for automatic breast scanners) or by adding a stand-off pad (for hand-held real-time equipment) (Kimme-Smith et al 1988); and the technologist's settings of output power and time gain compensation (TGC) (Black et al, 1979). Most sonographers who use hand-held real-time instruments assume that the first 3 cm of the image are in focus because of the fluid stand-off which is an integral part of the hand-held probe (Winsberg, 1983). Because phased arrays are electronically focused, users expect that by selecting a i or 2 cm focus, the subcutaneous tissue will be well visualized. The users may be unaware that the asymmetry of the phased array element requires manufacturers to place a fixed-focus lens on the elevation or slice thickness direction of the array, resulting in a partial volume effect for masses at "out of focus" depths (Goldstein and Madrazo, 1981). Furthermore, for both phased and mechanical units, high resolution hand-held US requires high-frequency (5MHz to IOMHz) transducers (Kimme-Smith et al, 1988). Because the increased attenuation of high-frequency sound in tissue limits penetration, high-frequency transducers must be sharply focused. Although a sharply focused transducer has a short focal region and spreads and dissipates the sound waves abruptly in the far field (Walter, 1985), it has been found effective for examinations of the compressed breast (Kelly-Fry and Harper, 1983). Automated US breast scanners were designed to avoid these focusing problems by allowing the sonographer to adjust the focal zone placement independently for each breast (Kossoff and Jellins, 1982 and Jackson et al, 1986). The water offset of these instruments avoids near field artifacts and allows the sonographer to reduce the length of th~ water path, permitting the focal zone to be placed deeper in the breast.
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Other artifacts associated with hand-held real-time units are documented elsewhere (Bassett et al, 1986). The present paper compares the appearances of lesions with and without artifacts, and describes methods to create artifacts to aid in the differentiation of benign from malignant masses. MATERIALS AND METHODS Two automated units, an Ausonics (with 3.9 and 4.5 MHz transducers) or a Labsonics (with a single P o l y v i n y ~ d e n e Fluoride transducer naturally resonant at 7.5 MHz but tunable to lower frequencies), were used to examine over 1300 of the 1600 patients examined with breast US at the UCLA Medical Center during the past four years (Ausonics:985, Labsonics:355). The remainder were scanned by one or more of ten real-time hand-held units operating at 5 MHz or higher (Diasonics 200:280, Philips 2500, 2000, 1550, 1200:700). The units and scanning techniques are described elsewhere (Bassett et al, 1987, and Kimme-Smith et al, 1988). Over 500 patients were scanned by two or more units. This practice often resolved difficult diagnoses resulting from artifactual scans with a single unit. Most of the hand-held examinations utilized a Diasonics 200 unit using a i0 MHz lead zirconate probe with a fluid offset. In addition, scans were obtained with an Ausonics Micro Imager 7.5 MHz unit and with Philips 3000, 2500, 1200, and 1550 units, using a variety of mechanical and phased probes operating at 5 MHz or 7.5 MHz. Acuson, T~chnicare, Picker, and ATL scans were done on a smaller population of patients for comparison purposes. Scans were usually made of both breasts, even when only one breast contained an area of clinical or mammographic concern. One technologist with eight years of clinical experience in ultrasound performed every examination throughout the time period covered by this report. The illustrative cases in this paper were retrieved by two methods: I. When a patient with a cystic mass was found to have a solid mass at the same location on subsequent examinations, she was re-examined with at least two different units in an attempt to establish the reason for the initial misdiagnosis. 2. Using the computer data system described in the Introduction, we were also able to identify examinations in which masses were detected in the automated scan but not the hand-held scan, and vice versa. From the US images, we recorded the following mass characteristics on our computer data system: solid versus cystic, smooth borders versus irregular b o r d e r s , ~ d posterior echo enhancement versus shadowing. Image differences in any one of these features led to a re-examination of the images for artifacts. The accompanying illustrations (Figures 1-11 are from some of the cases retrieved by these two methods. RESULTS AND DISCUSSIONS MASS BORDERS Of the three imaging features for differentiating benign from malignant solid masses, (i.e., interior echo characteristics, mass borders, and posterior echo enhancement or shadowing), mass borders were less prone to artifactual misdiagnosis than the other features. When a smooth bordered, round mass is imaged in h e far field, it sometimes appears to have irregular side walls due to its elongated texture and otherwise poor imaging characteristics in that location; but even then the mass does not manifest the lobulated or stellate characteristics of carcinoma. None of our cases showed this diagnostic error. POSTERIOR ENHANCEMENT The varying presence or absence of posterior enhancement or shadowing was the most common artifact among scans of the same breast obtained with different instruments. Although posterior enhancement was dependent on the type of gray scale, the tissue lying posterior to the mass, and the position of the mass with respect to the TGC and focal zone, it was the gray scale that was most likely to affect posterior enhancement. Gray scale is associated with that electronic circuitry in the US unit which maps the received voltage into a discrete shade of gray. The relationships of the voltage to the shade of gray displayed in the image is called the "gray scale." Many US units allow the user t o s e l e c t from several mapping functions, or to change the dynamic range so that the contrast in the i m ~ e ~s changed. When low amplitude reflections are represented with the same number of gray levels as high amplitude reflections, the gray scale map is called linear, even when a logarithmic receiver has preferentially amplified the low amplitude signals resulting from weak reflectors or scatter. A logarithmic gray scale map assigns more gray level values to low amplitude reflections hence increasing contrast. Thus, a low voltage level resulting from scattering will be represented by brighter gray levels (for a white-on-black image) if a logarithmic rather than linear gray level map is employed. Manufactureres vary the number of gray levels displayed and the proportion of gray levels reserved for displaying a low range of voltage signals. Users can identify logarithmic gray scale maps by the increased contrast in image scatter when the logarithmic rather than the
5th International Congress on the Ultrasonic Examination of the Breast
linear gray logarithmic "quadratic". the contrast
205
scale map is selected. Different manufacturers of ultrasound units may label gray scale maps differently. We have seen the labels "root", "exponential", "B map" applied to alternative gray scales which had the effect of increasing in low amplitude reflections and scatterers.
Because a high frequency pulse is attenuated more than a lower frequency pulse, we would expect posterior echo enhancement, such as that associated with a cyst, to be more visible on an image made with a high frequency transducer. However, since 99% of the incident US wave amplitude is attenuated within 3 cm of the skin for a 5 MHz focused pulse, even posterior enhancement from a 1 cm cyst whose anterior edge is 2 cm deep in the breast will have low amplitude. The gray scale at which this low amplitude voltage signal is mapped will affect the appearance of posterior enhancement of the cyst. Thus a linear gray scale map may not have sufficient differences in gray levels to differentiate between the signals returning from behind the cyst and those returning from adjacent tissue at this same level. Unfortunately, a more logarithmic gray scale map, while it may help image posterior echoes, may also create artificial interior echoes in anechoic masses (Figure i). However, the character of these echoes help to identify them as artifactual. This subject is discussed in the section on the diagnosis of cystic versus solid masses.
Figure i.
Superficial logarithmic distributed selected by
1.6 cm diameter cyst imaged on Diasonics 200 (IOMHz) with a gray scale map to insure posterior enhancement. Note uniformly interior echoes within the cyst. These are due to the gray scale map the operator and will disappear when a more linear map is used.
The specific nature of the tissue distal to the mass may also affect enhancement. When a cyst lies deep in the breast, the muscle and chest wall deep to the cyst attenuate the beam and thus enhancement is not apparent (Figure 2A). If the patient is repositioned so that her breast is scanned from the side, breast tissue introduced behind the cyst will show posterior echo enhancement (Figure 2B).
A Figure 2.
The .Scm diameter cyst in this image, which lies against the chest wall, does not show posterior enhancement when scanned by the Diasonics 200 unit (IOMHz). B) When the position of the rib cage is changed by obliquing the patient, breast tissue can be positioned behind the cyst and will show posterior enhancement. The fine echoes in the cyst are due to the logarithmic gray scale map used for these images in an attempt to increase posterior enhancement.
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DISTAL SHADOWING Shadowing distal to a mass indicates a higher attenuation in the mass than in the surrounding tissue, and is suspicious for malignancy. Unfortunately, many normal structures, such as ducts and Cooper's ligaments, can also cause posterior shadowing, particularly in the far field (that depth where the beam width is two times that at the focus) of sharply focused transducers (Figure 3). Nipple shadowing can also be misdiagnosed as pathologic if images are not properly labelled with position information. Since masses behind and beside the nipple may be obscured by the nipple shadow (Figure 4), an experienced sonographer will always try to reposition or redirect the transducer obliquely behind such shadowing. Shadows less than 2 mm in width which begin at the skin usually represent air bubbles, (Figure 5), while larger shadows beginning near the skin are usually scars (Figure 6) or other skin irregularities, and are only misinterpreted when the technologist fails to notate their true origin. Large benign calcifications may cause prominent shadows, and should be obvious in correlative mammo~rams (Figure 7).
Figure 3.
The arrow at the left of this longitudinal, outer quadrant breast image marks the end of the focal zone in the Labsonics automated unit. The focal zone of this unit can be positioned by the operator. Shadows posterior to the focal zone are due to the large beam aperture in the far field imaging ducts and other reflectors which are smaller than the beam profile. These shadows should not be mistaken for malignant masses.
Figure 4.
The shadow from a carcinoma, (arrow) beside the nipple (N) merges with the nipple shadow to make a difficult diagnostic problem on this longitudinal Labsonics scan.
Figure 5.
An air bubble on the skin causes a 2 mm shadow within the breast on an Ausonics water path scan (permission to reproduce from Bassett LW, Gold RH, Kimme-Sm~th C. "Hand-held and Automated Breast Ultrasound", Little Brown and Company).
5th International Congress on the Ultrasonic Examination of the Breast
Figure 6.
207
A biopsy scar produced this shadow in an Ausonics water path scanner (permission to reproduce from Bassett LW, Gold RH, Kimme-Smith C. "Hand-held and Automated Breast Ultrasound," Little Brown and Company).
A Figure 7.
A. B.
Mammogram of a 4 m m benign calcification imaged on a CGR 500T Senographe. Diasonics 200, 10 MHz, with a fluid offset, images the calcification.
ANECHOIC MASSES The differential diagnosis of a cyst versus solid mass depends entirely on the echoes or lack of them within the mass. Unfortunately, echoes can be produced in any cystic mass by imaging out of the focal region, increasing the gain or TGC, reducing the dynamic range, and/or incorrectly using a logarithmic gray scale. Of the instruments we used, the Labsonics was the least susceptible to these problems, since it does not have operator controlled gray scales or changeable TGCs, but our Labsonics unit also did not show posterior echo enhancement as frequently as our real-time hand-held units. In general, increased gain, TGC, or power, will fill in only the proximal portion of a cyst, while changing the gray scale or dynamic range to increase contrast may cause the center or all of the interior of the cyst to be filled in with fine textured noise (i.e., homogeneous texture). A mass should only be
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suspected of being solid when its interior echoes are heterogeneous when compared to the very fine "noise" from artifactual cyst fill-in. While some carcinomas have very coarse interior echoes compared to fibroadenomas, this is not a necessary finding for malignancy. When 3 or 4 bright echoes are seen in what would otherwise be diagnosed as a cyst, these may represent debris within the cyst fluid. Debris can sometimes be diagnosed by changing the position of the patient while continuing to scan the mass with a real-time hand-held transducer. If the debris moves about relative to the cyst wall, the mass is a cyst. When a cyst fiils in because it is imaged beyond the focal region of the transducer, repositioning the patient may enable the amount of tissue between the mass and transducer to be reduced. However, more subtle problems arise for masses imaged in the near field. Because of Huygen's principle, the intensity pattern of the US wavelets forming the pulse are irregular and have local maximums in the near field. These interference effects cause apparent echoes to occur in small anechoic masses. Because the US beam is wider in the near field than in the focal zone, the edge of the beam may strike a random scatterer outside of the cyst, which will be misinterpreted by the US unit as being within a cyst. Mechanical real-time units have circular transducers, so the beam shape is symmetrical. Phased array transducers elements are rectangular, with the long dimension parallel to the slice thickness direction. The slice thickness direction is the plane perpendicular to the image plane. As in computerized tomography, the depth of the slice encompassed by the beam contributes information to the image. High frequency phased linear or sector arrays (as opposed to annular arrays) are always permanently focused in the slice thickness direction by attaching an external lens to each element in the array. Lateral direction focusing is controlled by electronic phasing, while slice thickness focus is always at a single depth, which is determined by the transducer's designers. Phased arrays almost always image superficial masses improperly unless an offset is used to bring the mass to the depth of the slice thickness focus (Figure 8). Offset pads or water filled bags should always be used with phased or linear arrays when examining the skin and subcutaneous tissue (Figure 9). Near field reverberations may also occur wfth the use of mechanical sector units if hard plastic is used to separate the offset fluid from the patient. The reverberations interfere with visualizing the first cm Of tissue (Figure i0).
A Figure 8.
B
A 1.2 cm superficial cyst imaged on the Acuson (A) without, and (B) with a 2 cm offset pad. Despite correct dynamic focusing, the 5MHz probe has a fixed slice thickness focus (see text) at 3.5 cm, which contributes in (A) to the fill-in of the cyst by the partial volume effect.
5th International Congress on the Ultrasonic Examination of the Breast
Figure 9.
Figure i0.
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A 5.0 MHz Picker phased array distorts the image of the near field parenchyma because it operates without an offset. This 1.1 cm diameter solid mass is imaged with two different gray scales. The gray scale on the right causes the mass to resemble a cyst with a septum.
The hard plastic in the ATL Mark 4 Access transducer operating at 7.5 MHz produces reverberations which can only be corrected by scanning through an offset pad (or water-filled bag).
FAT LOBULES AND DUCTS Several misdiagnoses occurred in our series because of failure to obtain transverse slices of fat lobules and ducts. When imaged longitudinally, these structures may be mistaken for masses (Figure Ii).
A Figure ii.
B
A. Ducts ranging in diameter from .3 to .5 mm appear to be masses in the transverse plane, but are correctly diagnosed in B by a longitudinal scan. Both images were produced by an Ausonics Microimager operating at 7.5 MHz with a built-in fluid offset.
To avoid this error, all masses should be imaged in two planes. A fat lobule while round in one cross sectional plane is elongated in a second plane perpendicular to the first. Thus, an image of a fat lobule should not be misdiagnosed as a round mass with heterogeneous interior echoes because in a plane perpendicular to the initial scan plane the resultant excessively elongated structure is easily distinguished from a true mass. The Labsonics
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automated breast unit is superior to a hand-held unit in distinguishing fat lobules from masses because it can pivot on the exact location of a suspected mass, while a hand-held unit is not as precise in identifying the plane perpendicular to the area of concern. CONCLUSIONS The user should obtain from the manufacturer precise data regarding the frequency, F#, depth of focus, and maximum penetration of each transducer. If phased arrays are used, the manufacturer should also provide slice thickness focusing information; i.e., where the image slice is narrowest. This information may help the user to avoid problems related to lack of posterior echo enhancement and/or increased shadowing in the far field. Suitable offset materials, either commercially manufactured pads or water filled bags (or gloves), should always be available wherever hand-held imaging is performed. The interiors of all masses should be filled in, either by increasing the gain, by using a logarithmic gray scale map, or by selecting a low dvpamic range. The nature of the resultant artifactual interior echoes will aid in determining the presence or absence of true anechoic interiors. Changing the position of the patient and/or the transducer may yield an image with more diagnostic information. Because the breast is a soft organ its position can be shifted on the chest wall, enabling masses to be moved into the focal zone of the transducer. It may also be scanned from a variety of angles to increase posterior echo enhancement. The US examination of the breast is an adjunct to mammography. As such it is a problemsolving modality. The diagnosis of e~ery mass depends on the intelligence and skill of both the technologist and the image interpreter. REFERENCES Bassett LW, Gold RH, Kimme-Smith C (1986). Instrumentation and technique. In Hand-Held and Automated Breast Ultrasound (By Bassett LW, Gold RH, Kimme-Smith C) Little Brown and Company, New Jersey. 33-60. Bassett LW, Kimme-Smith C, Sutherland LK, Gold RH, Sarti D, King W (1987). Automated versus hand-held breast ultrasound: effect on patient management. Radiology, 165:103-108. Black EB, Ferrucci JT, Jr., Wittenberg J, Kirkpatrick RH, Hann LE (1979). Acoustic contrast enhancement: value of several system gain variations in gray scale ultrasonography. AJR 133, 689-693. Goldstein A, Madrazo BL (1981). 365-375.
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artifacts in gray-scale ultrasound JCU 9,
Jackson VP, Kelly-Fry E, Rothschild PA, Holden RW, Clark SA (1986). Automated breast sonography using a 7.5 MHz PVDF transducer: preliminary clinical evaluation. Works in Progress, Radiology 159, 679-684. Kelly-Fry E and Harper (1983). Factors Critical to Highly Accurate Diagnoses of Malignant Breast Pathologies by Ultrasound Imaging. In Ultrasound 82, Ed. R.A. Leraki and P. Morley, Pergamon Press, New York. Kimme-Smith C, Bassett LW, Gold RH (1985). High frequency breast ultrasound: Hand-held versus automated units; examinations for palpable mass versus screening. J Ultrasound in Med 7, 77-81. Kimme-Smith C, Hansen M, Bassett LW, Sarti D, King W (1985). effects of focal zone placement. Radiographics 5, 955-969. Kossoff G and Jellins J (1982).
Ultrasound mammography:
The Physics of Breast Echography.
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Laing FC (1983). Commonly encountered artifacts in clinical ultrasound. In Diagnostic Ultrasound: Text and Cases (By Sarti, DA) Yearbook Medical Publihsers, Inc., Chicago, 1-69. Walter JP (1985). N A m e r 23, 3-11.
Physics of high resolution ultrasound -- practical aspects.
Wi~mberg F (1983). Semin in Ultrasound,
Physical and design considerations Ct and MR 4, 44-49
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in real-time ultrasonography.