June 2001, VOLUME 176
NUMBER 6

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June 2001, Volume 176, Number 6

Genitourinary Imaging

Color and Power Doppler Twinkling Artifacts from Urinary Stones
Clinical Observations and Phantom Studies

+ Affiliations:
1 Department of Radiology, Seoul National University College of Medicine, The Institute of Radiation Medicine, 28, Yongon-dong, Chongno-ku, Seoul, 110-744, Korea.

2 Department of Radiology, Sungkyunkwan University School of Medicine, Samsung Cheil Hospital, 1-19, Mookjung-dong, Chung-Ku, Seoul, 100-380, Korea.

Citation: American Journal of Roentgenology. 2001;176: 1441-1445. 10.2214/ajr.176.6.1761441

ABSTRACT
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OBJECTIVE. The purpose of this study was to determine whether color and power Doppler twinkling artifacts could be considered an additional diagnostic sonographic feature of urinary stones.

SUBJECTS AND METHODS. A prospective study was performed in 32 patients with 20 renal stones and 16 ureteral stones to assess how often urinary stones show twinkling artifacts on Doppler sonography. Gray-scale images and color, power, and spectral Doppler images were obtained in all patients. All sonographic examinations were performed with a 3.5- or 5-MHz curvilinear phased array probe. The images were then analyzed for the presence, appearance, and intensity of the artifacts. Phantom experiments were performed with various kinds of urinary stones with high-megahertz linear phased array probes. The effects on the artifacts of the composition of the stones, of the Doppler velocity scale, and of the focal zone were investigated.

RESULTS. Thirty (83%) of 36 urinary stones showed color and power Doppler twinkling artifacts, which appeared as a rapidly changing color complex seen persistently behind stones like a comet's tail. Twenty-two of 30 stones with the twinkling artifacts showed strong intensity artifacts. Spectra with saturated amplitude were obtained from all 30 stones showing color Doppler artifacts. In phantom experiments, the artifacts originated from all stones. The velocity range did not affect the artifacts, whereas focal zone did.

CONCLUSION. Color Doppler twinkling artifacts from urinary stones occur frequently and may be considered an additional sonographic feature of urinary stones. The observation of these artifacts may be helpful in determining the presence of urinary stones.

Introduction
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Detection of urinary stones on sonography may be problematic when they are obscured by sonic beam—attenuating tissue, such as renal sinus fat, mesenteric fat, and bowel, or when their posterior acoustic shadowing is weak. Despite the technical advances of sonography, radiologists have difficulty confirming or excluding the presence of urinary stones if they are obscured. If an additional sonographic feature associated with the urinary stone is found, determining its presence would be easier. Frequently, we encounter in clinical practice color Doppler twinkling artifacts that originate from urinary stones. This study was conducted to determine whether color and power Doppler twinkling artifacts could be considered an additional sonographic feature of urinary stones.

Subjects and Methods
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Thirty-six stones (20 renal stones, 16 ureteral stones) in 32 patients were included in our prospective study. The urinary stones were confirmed with unenhanced abdominal radiography and excretory urography. Unenhanced abdominal radiography was performed in all patients, and excretory urography was performed in 11 of 32 patients. Four of 36 stones were radiolucent.

Gray-scale sonography, color and power Doppler sonography, and spectral Doppler sonography were prospectively performed in all patients from March 1999 to January 2000. All sonographic examinations were performed by one of two experienced radiologists with a 3.5- or 5-MHz curvilinear phased array probe (Ultramark 9; Advanced Technology Laboratories, Bothell, WA; or Sonoace 8800; Medison, Seoul, South Korea). For visualization of posterior acoustic shadowing, focal zones were always placed at the depth of or slightly deeper than the stone, with careful control of the B-mode gain setting. In color Doppler sonography, a red-and-blue color map was used, and the color window size was adjusted to cover the concerned lesion and adjacent tissue. The color Doppler gain was set to the point just below the threshold for color noise. The presence of a color signal was assessed relative to adjacent soft tissue. The color signal was used as a guide to obtain the Doppler spectrum.

All images were evaluated by two observers with decisions made by consensus. The sonographic appearances of urinary stones were analyzed for size, echo difference between stone and adjacent tissue, and posterior acoustic shadowing. Stone size was determined on sonography because of the variable magnification associated with abdominal radiography. Echo difference between stone and adjacent tissue was recorded as marked, slight, or indistinct. Posterior acoustic shadowing was recorded as discrete and indiscrete. On color and power Doppler images, the presence, appearance, and intensity of the artifacts were assessed. The intensity of the color signal was recorded as zero, absent; +, present; or ++, strong. An artifact with a length of more than 1 cm was classified as having strong intensity. At spectral Doppler sonography, the pattern of the spectrum was analyzed.

In the phantom study, natural stones that were removed by surgery or percutaneous nephrostolithotomy were used. Four oxalate, seven struvite, and three uric acid stones with a range of 0.4-2 cm in diameter were located between two gel pads (Aquaflex; Parker Laboratories, Fairfield, NJ). These stones were scanned with a 7-MHz linear phased array probe (Ultramark 9; Advanced Technology Laboratories) or a 7.5-MHz linear phased array probe (Sonoace 8800; Medison). The probes were fixed with a device to prevent motion artifact. The appearance of the artifacts and the effects on the artifact of the composition of the stones, of velocity scale, and of focal zone were investigated.

Results
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Of the renal stones and ureteral stones studied, the average diameter was 0.9 and 1.0 cm, respectively. Twenty-four (67%, 16 renal stones, eight ureter stones) of 36 urinary stones showed marked echo difference, eight urinary stones (22%, four renal, four ureteral stones) showed slight echo difference, and four ureter stones (11%) showed indistinct echo difference. Sixteen (44%) of 36 urinary stones had discrete posterior shadowing, whereas 20 urinary stones (56%) had indiscrete posterior shadowing.

On color Doppler sonography, the twinkling artifacts were generated from 30 (83%) of 36 stones and from 26 (81%) of 32 patients (Tables 1 and 2 and Figs. 1A,1B,1C,1D,2A,2B,2C,2D,3A,3B,3C). Four renal stones and two ureter stones did not have these Doppler artifacts. Those artifacts appeared as a rapidly changing color complex seen persistently behind stones, like a comet's tail. Eighty percent of renal stones and 88% of ureter stones had the artifacts; 100% of stones less than 0.5 cm, 75% of stones 0.6-1.0 cm, and 100% of stones greater than 1.0 cm had the artifacts. All four ureteral stones with indistinct echo difference showed the twinkling artifacts. Seventeen (85%) of 20 urinary stones with indiscrete posterior acoustic shadowing showed twinkling artifacts. Twenty-two (73%) of 30 stones with twinkling artifacts had signals with strong intensity. Color Doppler twinkling artifacts occurring from regions other than stones were not seen during our study.

TABLE 1 Number of Urinary Stones and Frequency of Color Doppler Twinkling Artifacts Classified by Location and Size of Urinary Stones

TABLE 2 Number of Urinary Stones and Frequency of Color Doppler Twinkling Artifacts Classified by Radiodensity, Echo Difference, and Posterior Acoustic Shadowing

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Fig. 1A. 45-year-old woman with 1.5-cm renal stone. Sonogram shows well-marginated hyperechoic lesion (arrow) with indiscrete acoustic shadowing.

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Fig. 1B. 45-year-old woman with 1.5-cm renal stone. Color Doppler sonogram shows rapidly changing color band (arrow) seen persistently behind stone with comet's tail appearance.

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Fig. 1C. 45-year-old woman with 1.5-cm renal stone. Power Doppler sonogram shows artifactual power signal (arrow) behind and around stone.

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Fig. 1D. 45-year-old woman with 1.5-cm renal stone. Spectral Doppler sonogram with sample volume located in stone shows artifactual spectral signal with saturated amplitude.

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Fig. 2A. 51-year-old woman with 1-cm ureteral stone. Sonogram shows echogenic lesion (solid arrow) near lower pole of right kidney. It is poorly distinguished from adjacent echogenic tissue (open arrow) and does not show discrete posterior acoustic shadowing. Therefore, it is not easy to determine that this lesion is ureteral stone.

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Fig. 2B. 51-year-old woman with 1-cm ureteral stone. Color Doppler sonogram shows twinkling artifact (arrow) from echogenic lesion.

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Fig. 2C. 51-year-old woman with 1-cm ureteral stone. Artifactual signal (arrow) is also seen on power Doppler sonogram.

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Fig. 2D. 51-year-old woman with 1-cm ureteral stone. On spectral sonogram, spectrum is composed of close vertical bands without definable waveform.

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Fig. 3A. 35-year-old man with 0.9-cm stone near ureterovesical junction (UVJ). Axial sonogram of bladder shows that right UVJ (arrow) is swollen, and right UVJ area is slightly more echogenic, compared with opposite site. However, existence of ureteral stone is not definite because of surrounding echogenic tissue and indiscrete posterior acoustic shadowing.

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Fig. 3B. 35-year-old man with 0.9-cm stone near ureterovesical junction (UVJ). Color Doppler sonogram shows prominent twinkling artifact (arrow) from right UVJ area.

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Fig. 3C. 35-year-old man with 0.9-cm stone near ureterovesical junction (UVJ). Spectral Doppler sonogram with sample volume located in right UVJ area shows artifactual spectral signal with saturated amplitude.

On power Doppler sonography, artifactual signal also appeared around and behind the stones that showed the artifacts on color Doppler sonography (Figs. 1A,1B,1C,1D,2A,2B,2C,2D,3A,3B,3C). Spectra with noise and saturated amplitude were obtained from all 30 patients with the color Doppler twinkling artifacts (Figs. 1A,1B,1C,1D,2A,2B,2C,2D,3A,3B,3C). The spectrum was composed of close vertical bands without a definable waveform. All spectra originated at the echogenic stones, not at the color artifacts.

In the phantom study, we observed color Doppler twinkling artifacts with various intensity from all stones (Fig. 4A,4B,4C). In each stone, the location of the origin of the artifact and its shape were not changed during repeated examinations. Change of artifact by change of velocity range was not detected from any stone on color Doppler imaging. However, focal zone influenced the artifact. When the focal zone was moved below the stone, prominent change of artifact was not shown from any stone. However, when the focal zone was located at or above a stone, the artifact disappeared or was weakened in all stones that were included in the phantom study.

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Fig. 4A. Calcium oxalate stone (0.7 cm) used as phantom and located between gel pads. Color Doppler sonogram obtained with 7.5-MHz linear array transducer shows that artifact is not produced by stone when focal zone (arrowhead) is located above stone.

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Fig. 4B. Calcium oxalate stone (0.7 cm) used as phantom and located between gel pads. When focal zone (arrowhead) is below stone, color Doppler sonogram shows rapidly changing and continuing color bands.

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Fig. 4C. Calcium oxalate stone (0.7 cm) used as phantom and located between gel pads. Color Doppler sonogram, obtained when focal zone (arrowhead) is moved below stone, fails to show prominent change of artifact.

Discussion
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Urinary stones can be detected easily when they show both distinct echogenicity and discrete posterior acoustic shadowing on sonography. However, in clinical practice, we encounter many equivocal cases in which it is difficult to determine whether a urinary stone is present because of its indistinct echogenicity and indiscrete posterior acoustic shadowing. Indistinct echogenicity of stones results from surrounding sonic beam—attenuating tissue, such as prominent renal sinus fat, mesenteric fat, and bowel. For example, when a renal stone is poorly distinguished from echogenic renal sinus fat and has an indiscrete posterior acoustic shadowing, it is difficult to determine its presence on sonography.

In a previous study [1], three radiologists interpreted 31 sonograms with a sensitivity of 81% and a specificity of 86% for detecting renal stones. If an additional sonographic feature associated with urinary stones is found, it would be helpful in minimizing the false-positive and false-negative findings and in avoiding an unnecessary study, such as CT.

Our study was designed to determine whether the color Doppler twinkling artifacts from urinary stones are frequent enough to be considered an additional finding on sonography. We chose patients who had urinary stones found on unenhanced abdominal radiography or contrast-enhanced studies of the renal collecting system. In our study, most stones (83%) showed color and power Doppler twinkling artifacts. In addition, 22 (61%) of 36 stones showed artifactual signals with strong intensity. This finding indicates that color Doppler twinkling artifacts from urinary stones are frequent, and most of the artifacts can be detected easily.

Rahmouni et al. [2] found color Doppler twinkling artifacts originating from parenchymal calcifications, including bladder calculi. They explained that when an incidental sonographic beam impinges a rough interface composed of sparse reflectors, the artifact is generated by the phase shift resulting in a faint variation of the incidental sonographic beam at the interface and by the increase of pulse duration resulting in multiple reflections in the medium. Because urinary stones become larger particles by aggregation or agglomeration of primary crystal forms, they are predominantly composed of a highly reflecting crystalline aggregate of varying chemical composition with a mucoprotein organic matrix [3]. On the basis of the explanation of Rahmouni et al., the twinkling artifacts from urinary stones are likely to be generated by a random strong reflection and multiple inner reflections of the incidental sonographic beam at a rough interface formed by a crystalline aggregate of stones.

In our phantom study, artifacts with each different intensity occurred regardless of the composition of stones. In addition, the artifacts originated from a fixed site of each stone during repeated scanning. This finding indicates that the twinkling artifact is related to some structural factor in the stone.

Rahmouni et al. [2] suggested that the artifact could be influenced by sonic beam attenuation of tissues interposed between the probe and a calcification. In our study, four renal stones and two ureteral stones did not show any artifactual signals. Because the ureter is deep-seated below abundant fatty tissue without a proper acoustic window, ureteral stones may be influenced more than renal stones by sonic attenuation of interposed tissues. However, we could not find any correlation between the location of stones and the genesis of the artifacts. Therefore, color Doppler twinkling artifacts seem to be more affected by the architecture of the stones themselves than by sonic beam attenuation from interposed tissue. In terms of architecture of stones, the characteristics of the interface and the geometric arrangement of the individual reflectors in a stone could influence the genesis of the artifacts.

The location of focal zone can influence the occurrence and intensity of the artifact. When the focal zone was placed below urinary stones, artifactual color signal was prominent and strengthened in our phantom study.

One of the limitations of this study is that we could not determine whether the detection of these twinkling artifacts would actually improve the detection of stones, although these color Doppler twinkling artifacts from urinary stones are a frequent phenomenon. However, we note the potential usefulness of these artifacts in clinical practice in confirming the presence of urinary stones, especially with indistinct echo difference and indiscrete posterior acoustic shadowing. This study is also limited because echogenic foci with color artifacts seen in the area of the renal sinus do not always suggest stones. Renal artery calcification should be considered in the differential diagnosis, especially in patients with long-standing diabetes, hypertension, or other systemic diseases associated with atherosclerotic vascular disease [4]. Real-time scanning can help differentiate arterial calcifications from renal calculi because arterial calcifications are seen to pulsate. The artifacts may also develop from calcifications of renal tumor, renal cyst, and renal parenchyma. These calcifications usually can be differentiated from renal stones on the basis of their location on real-time scanning and of the patient's history.

In spite of these limitations, the color Doppler twinkling artifacts were frequent and characteristic enough to be considered additional findings of urinary stones on sonography because more than 80% of urinary stones had the twinkling artifacts and because false-positive findings were absent during the scanning of 36 stones in our study. Therefore, our study suggests the color Doppler twinkling artifacts can help rule out or confirm the presence of a stone in equivocal cases, for example, when a stone is suspected but not definite or when an echogenic interface in the renal sinus simulates a stone.

In summary, color Doppler twinkling artifacts can be considered an additional sonographic feature of urinary stones, and they may be useful in determining the presence of urinary stones especially urinary stones with indistinct echo difference and indiscrete posterior acoustic shadowing.

Supported by Medison Research Fund, Medison, Company, Ltd. 997-4, Daechi-dong, Kangnam-ku, Seoul, Korea, 135-280.

Address correspondence to S. H. Kim.

References
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1. Kimme-Smith C, Perrella RR, Kaveggia LP, Cochran S, Grant EG. Detection of renal stones with real-time sonography: effect of transducers and scanning parameters. AJR 1991; 157:975-980 [Abstract] [Google Scholar]
2. Rahmouni A, Bargoin R, Herment A, Bargoin N, Vasile N. Color Doppler twinkling artifact in hyperechoic regions. Radiology 1996; 199:269-271 [Google Scholar]
3. Segal AJ, Banner MP. Radiological characteristics of urolithiasis. In: Pollack HM, ed. Clinical urography. Philadelphia: Saunders, 1990: 1758-1767 [Google Scholar]
4. Kane RA, Manco LG. Renal arterial calcification simulating nephrolithiasis on sonography. AJR 1983; 140:101-104 [Abstract] [Google Scholar]

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