Introduction

Twinkling is a color Doppler artifact, also known as "color comet tail artifact". The Doppler color twinkling artifact during abdominal ultrasound can aid in lesion location, mainly described in urinary calculi but not limited to other tissue calcifications, biliary stones and encrusted indwelling urinary stents.¹ The postulated mechanism for the generation of the twinkling artifact is that it occurs with a strongly reflective medium with a rough interface. The presence of a rough interface implies that the acoustic wave splits and forms multiple reflections that are interpreted by Doppler units and displayed as multiple colors.¹ The more surface irregularity, the more the twinkling artifact will be noticed.² An important advantage of the twinkling artifact is that in tissues surrounded by echogenic tissue (e.g., in the renal parenchyma) where shadowing does not occur - a common artifact to help diagnose calculi-nephroliths can still be detected.² The main constituent in order for the twinkling artifact to occur is calcium.

For optimization of the twinkling artifact there are several machine settings that have to be made:

1.  The focal zone should be placed below the rough interface.

2.  Increased color white priority enhances twinkling artifact.

3.  Reducing the grey scale gain enhances twinkling artifact in most cases.

4.  Reducing pulse repetition frequency enhances twinkling artifact.

Uses for Twinkling Artifact in Abdominal Ultrasound

As mentioned previously, the twinkling artifact can be detected in various calcified tissues and therefore helps distinguish disease entities. The main use of this artifact is for urinary and kidney stones but can also participate in the identification of biliary tract lithiasis, pancreatic ductal or parenchymal calcifications and granulomatous lesions in liver and spleen.²

Park SJ, et al.,² have documented the importance of this artifact in improving diagnostic sensitivity for small stones in urinary bladder. Also, the twinkling artifact can help diagnose nephrolithiasis in >95% of patients making it a valuable tool for calculus identification where B mode falls short.² It is important to acknowledge that about 80% of renal stones exhibit twinkling.

Although the surface of the lith is one of the most important factors that determine the twinkling artifact, there has been a correlation with the chemical composition of the stones. It is now known that this artifact is more frequent in stones made of calcium oxalate dihydrate, whereas calcium oxalate monohydrate does not exhibit this artifact. This, particularly in human medicine, where lithotripsy is a frequent procedure, becomes more useful to determine the efficacy of fragmentation.³

In the imaging of cholelithiasis, the major advantage of the twinkling artifact is the biochemical composition, since these stones are readily observed by B-mode. The choleliths with the greater concentration of cholesterol produce a more prominent twinkling. Where this artifact becomes more useful is in evaluating liths in the common bile duct, where gas can obscure their visualization.¹

In chronic pancreatitis the twinkling artifact is clinically useful to evaluate small calcifications, which, could be missed in normal B mode sonography since they would not cause posterior shadowing. Finally, if the trajectory of the needle after a fine needle aspirate is desired, this can be done with the twinkling artifact, probably secondary to the entrapped air and rough surface left behind.¹

Despite the benefits of the twinkling artifact, it is important to know that it can confuse the clinician with vascular flow within a tissue, when only power Doppler images are obtained.³ Also it is important to keep in mind that not all calcifications will cause twinkling, and therefore a disease cannot be discarded based solely on the absence of twinkling. Furthermore, the calcification of renal arteries can cause twinkling; however, it can be distinguished from a nephroliths by the pulsating nature of the arteries observed. Lastly, after renal biopsies, the formation of arteriovenous fistulas is common and because of their high velocity turbulent flow can cause a similar visual image to that of twinkling, however, the clinical history of a renal biopsy helps to differentiate both entities.¹

Conclusion

The twinkling artifact can help distinguish between different pathologies in the abdominal ultrasound, however, it must be kept in mind that certain machine settings should be met. Furthermore, the absence of this artifact does not discard certain pathologies, for example, only 80% of calcium uroliths present twinkling. The knowledge of this artifact is important since it could be confused with vascular flow within tissue and confuse the clinician.

References

1.  Kim HC, Yang DM, Jin W, Ryu JK, Shin HC. Color Doppler twinkling artifact and various conditions during abdominal and pelvic sonography. J Ultrasound Med. 2010:29;621–632.

2.  Hirsch MS, Palavecino TB, León RB. Color Doppler twinkling artifact: A misunderstood and useful sign. Revista Chilena de Radiología. 2001;17(2):82.

3.  Tsao T-F, Kang RJ, Gueng M-K, Tyan Y-S, Lin Y-C, Lee S-K. Color Doppler twinkling artifact and clinical use. J Med Ultrasound. 2009;17(3):157–166