J JACOB, MRCS, MRCP, DTMH;       P S SIDHU, MRCP, FRCR
Department of Radiology, King’s College Hospital, Denmark Hill, London SE5 9RS, UK

Introduction
The use of contrast enhanced ultrasound in elucidating the aetiology of disease in the gallbladder and biliary system in fast gaining wide acceptance. One further benefit over conventional ultrasound is in determining the presence and cause of some of the well known ultrasonic artifacts that can catch out the unwary. One such example is the so called twinkle artifact which occurs at strongly reflecting granular interfaces and can be mistaken for hypervascularity and thus pathology. We illustrate a case of a patient that had an ill-defined mass seen at the fundus of the gallbladder on conventional ultrasound but showed increased signal when colour Doppler was applied. The use of microbubble contrast, clearly delineated the lack of vascularity in this region - which was not seen to enhance - and confirmed the high signal seen earlier had in fact been twinkle artifact. A diagnosis of gallstones and gallbladder debris could then be confidently made, and the patient immediately reassured.


Case History
A 65-year old lady with end stage renal disease complained of intermittent right upper quadrant discomfort during an inpatient stay in hospital. She was referred for a conventional ultrasound scan where the only abnormality visible was an irregular, loculated hypoechoic 14 x 14 mm mass situated around the fundus of her gallbladder. This mass showed increased signal with colour Doppler, suggesting a vascular, potentially aggressive lesion.

The patient then underwent a contrast enhanced ultrasound which clearly demonstrated that the region did not have increased vascularity in the early or late phase. Indeed it became clear that the increased colour signal was actually twinkle artifact, and that the hypoechoic region was comprised of a cluster of gallstones localized to the fundus.

No other abnormalities were seen on examination of her liver and biliary tree and it was possible to reassure the patient about the benign nature of the ultrasound findings immediately after the examination.


Discussion
Ultrasound examination of the gallbladder is the primary modality used in the assessment of gallbladder and biliary disease. The use of contrast enhanced ultrasound is now well recognized as a valuable adjunct to conventional ultrasound in studying pathology in the liver. However its role in helping to delineate the aetiology of disease in the gallbladder and biliary tree is still being developed.

One constraint of ultrasonography is the presence of various types of signal artifact that can be generated by tissues and interfaces when using colour or power Doppler. If these artifacts are not recognized, then their presence can be misinterpreted as pathology leading to unnecessary investigation and anxiety for the patient and clinical team.

One such example is the so called twinkle artifact which occurs at strongly reflecting granular interfaces such as with gallstones, urinary tract stones or parenchymal calcification. It appears as a rapidly fluctuating mixture of Doppler signals (red and blue pixels) that imitate turbulent flow. Spectral analysis should demonstrate noise and a lack of flow, but if this remains unclear, then contrast enhanced ultrasound can be used as an adjunct.

The microbubble contrast as in this case will remain within the vascular compartment and initially show enhancement within the hepatic artery and gallbladder wall in the arterial phase. The portal phase will then increase echogenicity in the remainder of the liver, with the gallbladder wall becoming relatively hypoechoic compared to the liver parenchyma. No abnormal enhancement will be seen in the region that previously demonstrated twinkle artifact. In this case, no enhancement was seen in the region of the gallbladder fundus, other than the outline of the gallbladder wall during the arterial phase and no enhancement was present in the portal phase. The increased signal seen on the colour Doppler was thus localized to a non vascular region within the gallbladder lumen and confirmed as twinkle artifact secondary to the granular interface of gallstones.

The use of contrast enhanced ultrasound thus can act not only as an extremely useful tool in confirming the aetiology of disease in the liver, gallbladder and biliary tree, but can also help with establishing the cause of ultrasonic artifact. In so doing, not only can it filter out those benign findings that are the consequence of artifact as opposed to true pathology, but can also prove confirmatory in those cases where artifact such as twinkle artifact highlights real pathology such as gallstones.

References

Meacock LM, Sellars ME, Sidhu PS. Contrast-Enhanced Ultrasound Beyond the Liver: Evaluation of Gallbladder and Biliary Duct Disease Processes. British Journal of Radiology 2010;83:615-627.

Campbell SC, Cullinan JA, Rubens DJ. Slow flow or no flow? Color and power Doppler US pitfalls in the abdomen and pelvis. Radiographics 2004;24:497-506.



Figure 1. (a) There is an area of mixed echogenicity at the fundus of the gallbladder (arrow). (b) This demonstrates increased signal with colour Doppler (arrow) which could be interpreted as increased vascularity thus indicating concerning liver or gallbladder pathology.


Fig 2. (a) On the contrast enhanced ultrasound images, in the early portal phase the area of mixed echogencity in the gallbladder fundus (arrow) does not enhance with contrast implying it is not vascular. (b+c) This is confirmed on the delayed phase contrast studies. The increased signal seen previously on conventional ultrasonography can thus be identified as twinkle artifact and the mixed echogenicity confirmed as gallstones within the gallbladder fundus.