Twinkle artifact

Twinkling artifact sign appears as a quickly fluctuating mixture of Doppler signals that imitate turbulent flow with an associated characteristic flat Doppler spectrum suggestive of noise, behind a strongly reflecting granular interface such as urinary tract stones (Figure 1) or parenchymal calcifications (1).

Physics:

The twinkling artifact is generated by a narrow band of intrinsic machine noise called phase (or clock) jitter. The appearance of this sign is highly dependent on machine settings. System noise measured on a flat surface generates a band-limited Doppler shift on spectral displays with a mean frequency shift of 0 Hz and a mean (- SD) absolute fluctuation of 86 - 10 Hz over a pulse repetition frequency range of 1250-10,000 Hz. Rough surfaces increase the spectral bandwidth. With a strongly reflecting, rough surface such as a renal stone, the high amplitude, broadband signal appears as random motion on color Doppler sonography (2).

DISCUSSION
In 1996, “twinkling artifact” was described by Rahmouni et al (3) as an artifact generated by a strongly reflecting medium composed of individual reflectors (2). Its appearance is dependent on color-write priority and gray-scale gain. As color write priority decreases, the amount of twinkling artifact decreases behind the stone (2). The twinkling artifact can be seen behind any granular reflecting surface. It has primarily been described as occurring with urinary tract calculi and parenchymal calcifications but it can also be seen with biliary calculi and any material with an irregularly reflective, granular surface, such as iron filings, emery paper, ground chalk, wire mesh, an aneurysmal coil during transcranial Doppler sonography, gall bladder adenomyomatosis, and recently encrusted stents (4).
In a study of 32 patients by Lee et al (5) 86% of urinary calculi demonstrated this artifact. The authors suggested that the presence of twinkle artifact may aid in stone detection, especially if the stone lacks a strong echo or discrete shadowing.
The chemical composition of stones appears to be a factor in whether the twinkling artifact will appear. Chelfouh et al (6) reported that calculi composed of calcium oxalate dihydrate and calcium phosphate always produce a twinkling artifact. Absence of the twinkling artifact was noted only in stones composed of calcium oxalate monohydrate and urate. A twinkling artifact emanating from an erroneously placed intravesicular Proline suture after bladder neck suspension surgery also has been reported (7).
Twinkling artifact has been shown in a case of gall bladder adenomyomatosis (GADM) on color Doppler sonography. Sonographically, GADM is characterized by diffuse or focal thickening of the gallbladder wall associated with small intramural cystic spaces known as Rokitansky-Aschoff sinuses. These sinuses may contain tiny echogenic foci that commonly cause a reverberation artifact (8). On color Doppler sonography, some of these echogenic foci show a persistent posterior signal composed of a rapidly changing mixture of red and blue, interpreted as the twinkling artifact. Some of the intramural echogenic foci in GADM are composed of calcifications with a rough interface. This conclusion is supported by previously described computed tomographic findings of GADM which include small intramural calcifications within the gallbladder wall (9). Conversely, perhaps it is the cholesterol deposits themselves which cause the artifact. The recognition of the presence of the twinkling artifact in GADM is important because it may simulate high-velocity blood flow within a gallbladder mass, a finding that may indicate the presence of gallbladder carcinoma, one of the major differential diagnoses of GADM (10).Twinkling artifact may also be seen in gall bladder cholesterolosis on color Doppler imaging.(Figure 2).Cholesterolosis is characterized by abnormal deposition of cholesterol and triglyceride in macrophages in the lamina propria of the gall bladder.
The artifact has also been described in transcranial Doppler Sonography following a coil embolization of an anterior choroidal artery aneurysm (11). It is important to recognize this artifact as it could be mistaken for residual flow or aneurysmal cavity recanalization.
Identification of encrustation of ureteral stents, one of the main complications of these materials, is possible with color-flow Doppler sonography in the presence of a twinkling artifact. (4)
Twinkling artifact has also been observed in Color Doppler sonography of the orbit behind strongly reflecting surfaces; recognition of this artifact is important in order to avoid misinterpreting it as abnormal blood flow caused by a pathologic lesion. Detection of a color mosaic should always prompt further spectral Doppler analysis particularly if therapeutic decisions hinge on criteria based on the presence of blood flow within the tissue (12).
The clinical importance of understanding the twinkling artifact is that it helps the identification of renal calculi and foreign bodies with ease because of the presence of color behind them, especially small sized calculi, which may be missed in the absence of an acoustic shadowing on gray scale imaging.(7,13)

Therefore, to avoid misdiagnosis, the presence of a color signal close to calcifications should be interpreted with caution, and a flow spectrum should always be recorded to eliminate the twinkling artifact. (3)