The "snow globe" sign: a microvascular flow imaging artifact in cystic lesions, and the role of dispersed internal debris.

Cystic lesions with heterogeneous content are often encountered during ultrasound examinations. The cause can be debris such as milk of calcium (MOC), or sediment caused by hemorrhage. Microvascular flow imaging (MVFI) techniques are often used for assessing the vascularity of cystic lesions. Here we report a MVFI artifact, which we termed the snow globe sign. We have observed a distinct MVFI phenomenon of pixelated intracystic artifacts within a subset of renal and liver cysts. Examinations were performed with an MVFI imaging technique (MV-flow™), as well as conventional color/power Doppler ultrasound. An imaging phantom was made to simulate MOC cysts using a suspension of calcium carbonate. The snow globe artifact is characterized by temporary, pixelated signal in the lesion. The artifact can co-occur with debris, demonstrating color comet tail artifact. We have also observed it occasionally in the sludge-filled gallbladder. The imaging phantom demonstrated a similar artifact. The MVFI snow globe artifact can be seen within cystic lesions with or without concomitant color Doppler twinkling artifacts. We hypothesize that reflective microparticles, such as MOC are the main culprit behind this observation. Furthermore, the unique post-processing of MVFI methods (spatiotemporal coherence-based filtering) can lead to their increased sensitivity to such Doppler artifacts.