Searchlight phenomenon: a novel artifact of the gallbladder

Abstract

Background: "Searchlight phenomenon" is an ultrasound artifact that appears as thin, bright, and posteriorly extending lines in the gallbladder lumen. It mimicks the comet-tail artifact, but it does not decrease in intensity in the depth of the view field, unlike the comet-tail artifact.

Purpose: To determine the mechanism of this unpublished ultrasound artifact on the basis of clinical ultrasound images as well as using an in vitro system.

Materials and methods: (1) CLINICAL STUDY: to determine the detection rate of the searchlight phenomenon in 453 subjects with a normal abdominal US examination result according to the medical chart. We also analyzed the appearance of the searchlight phenomenon in which US showed the searchlight phenomenon in the gallbladder. (2) Basic study: we placed a thin (thickness: 12 μm) polyethylene membrane at the depth of 2.5 cm from the transducer inside a water bath. Then, we inserted air bubbles (50-400 μm) below the membrane and observed when the ring-down artifact and searchlight phenomenon appeared.

Results: (1) CLINICAL STUDY: the searchlight phenomenon was observed in 30 out of 453 (6.6 %) subjects. When the searchlight phenomenon was seen, duodenal gas was present dorsally in the same line of sight, and the searchlight phenomenon appeared only when duodenal gas was accompanied by the ring-down artifact on US. (2) Basic study: a stepwise increase in the number of air bubbles gave rise to (a) multiple echo spots (single layer of air bubbles), (b) ring-down artifact (two layers), and (c) ring-down artifact below the membrane and searchlight phenomenon in front of the membrane simultaneously. The ring-down artifact and searchlight phenomenon always appeared in the same line of sight.

Conclusion: The searchlight phenomenon is thought to be a range-ambiguity artifact of the ring-down artifact. Duodenal gas is thought to give rise to this artifact. When examining the gallbladder with ultrasound, knowledge of many artifacts including the searchlight phenomenon can prevent us from making a hazardous misdiagnosis.

Keywords: Artifact; Gallbladder; Range-ambiguity artifact; Ring-down artifact; Ultrasound.