A 32 year old man presents with right flank pain. He had an episode of renal colic 2 years ago and presented to the Emergency Department back then. He had a CT KUB done at the time which showed a small stone in his right VUJ and a larger 7mm stone in his right renal calyx. He is in agony and cannot lie still. You give him opiates and nonsteroidal antiinflammatories. When his pain settles a little you do a focussed bedside ultrasound. Below is his right kidney and left kidneys taken longitudinally

The right kidney is 10.3cm in length and his left kidney is 10.4 cm. Note the large 14 mm stone in the pelvis of his right kidney.

Note that there is no hydronephrosis. This is more evident on the transverse view of the kidney

An examination of the bladder shows an echogenic foci which appears to be in the right VUJ, which, with colour Doppler shows a twinkling artifact. The echogenic focus is not producing an acoustic shadow and therefore it is difficult to be sure that this is a stone. The twinkle artifact, which is the colour Doppler signal behind the echogenic foci confirms that this is indeed a stone

Discussion

Clinical Utility and limitations

Sensitivity: Ranges from 54% to 86% depending on stone size and composition. (Remember, if it doesn’t twinkle, you still cannot rule out a stone!

Specificity: Extremely high (up to 94%–100%). If you see a robust twinkle at the VUJ without color bleeding into surrounding tissue, you can be highly confident it is a stone.

The “False Positive” Pitfall: Always check your B-mode. Strongly reflective, non-stone structures like parenchymal calcification, renal sinus fat, bowel gas, or indwelling stents can also trap microbubbles and produce a false-positive “twinkle.”

What causes the twinkling artifact?

The Microbubble Theory

The twinkling artifact is not caused by probe movement, patient respiration, or simple surface roughness.

Instead, it is caused by microscopic gas bubbles (microbubbles) trapped within the tiny, rough crevices on the stone’s surface.

When the ultrasound wave hits these trapped microbubbles, it causes them to oscillate. This rapid, microscopic vibration scatters the Doppler ensemble pulses, which the ultrasound machine’s autocorrelation algorithm misinterprets as rapidly changing velocities (turbulent blood flow).

This is why rough-surfaced stones (like calcium phosphate or calcium oxalate dihydrate) twinkle intensely—they have plenty of crevices to trap gas.

Smooth stones (like uric acid) often do not twinkle.

Teaching point: The twinkle artifact improves sensitivity and specificity for the detection of small renal calculi. This is especially true if the calculus is not associated with acoustic shadowing. It is worth remembering however, that not all renal stones will be associated with a twinkle artifact. Stones which have a rough surface will produce a twinkle artifact while smoother stones may not.

References

  1. J Endourol 2013 Feb; 27(2): 149–153.doi: 10.1089/end.2012.0430PMCID: PMC3573723B-mode Ultrasound Versus Color Doppler Twinkling Artifact in Detecting Kidney Stones Mathew D. Sorensen, MD, MS,corresponding author1 Jonathan D. Harper, MD,1 Ryan S. Hsi, MD,1 Anup R. Shah, MD,1 Manjiri K. Dighe, MD,2 Stephen J. Carter, MD,2 Mariam Moshiri, MD,2 Marla Paun, BS, RDMS, RVT,3 Wei Lu, MS,3 and Michael R. Bailey, PhD3
  2. Radiopaedia Dr Saeed Soltane Hosn The twinkle artifact
  3. Radiology. 1996;199:269–271. Rahmouni A. Bargoin R. Herment A, et al. Color Doppler twinkling artifact in hyperechoic regions.
  4. Abdom Imaging. 2013;38:863–869. doi: 10.1007/s00261-012-9946-7.Ripollés T, Martínez-Pérez MJ, Vizuete J, et al. Sonographic diagnosis of symptomatic ureteral calculi: usefulness of the twinkling artifact.
  5. AJR Am J Roentgenol. 2003;180:215–222 Kamaya A. Tuthill T. Rubin JM. Twinkling artifact on color Doppler sonography: Dependence on machine parameters and underlying cause.
  6. Int Braz J Urol. 2009;35:532–541. Mitterberger M. Aigner F. Pallwein L, et al. Sonographic detection of renal and ureteral stones. Value of the twinkling sign.
  7. J Clin Ultrasound. 1999;27:433–439. Aytac SK. Ozcan H. Effect of color Doppler system on the twinkling sign associated with urinary tract calculi.