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Gas at Abdominal US: Appearance, Relevance, and Analysis of Artifacts

¹ Departments of Medical Imaging (S.R.W., D.M.)
² Medical Biophysics (P.N.B., L.M.W., D.H.S.)
³ University of Toronto; the Department of Medical Imaging, The Toronto Hospital (S.R.W., D.M.), 200 Elizabeth St, Toronto M5G 2C4, Ontario, Canada
⁴ Department of Imaging Research, Sunnybrook Health Science Centre (P.N.B., L.M.W., D.H.S.), Toronto.

PURPOSE: To describe the spectrum of ultrasonographic (US) appearances of intraluminal gas, including two clinically relevant gas artifacts.

MATERIALS AND METHODS: Observations were made in patients and reproduced in an animal model, an ex vivo gut preparation, and a tissue-mimicking phantom. Appearances were classified according to a physical model of the interaction between sound and collections of gas.

RESULTS: Free bubbles of gas appeared as bright echogenic foci extending artifactually owing to lateral and axial blooming. This causes bubbles that abut the gut wall to enhance the layer one echo, which corresponds to the interface between the mucosa and the luminal contents. Such bubbles can also falsely appear to be within the gut wall itself owing to elevation averaging and thereby cause the artifact pseudo–pneumatosis intestinalis. Isolated groups of small bubbles created a characteristic periodicity and tapering of the distal echo pattern. In the extreme case, in which many such echoes are superimposed, "dirty shadowing" occurs. A contiguous pocket of gas may cause the gut wall to appear artifactually thickened (ie, pseudo–thickened gut). This was shown to be a form of mirror image artifact.

CONCLUSION: Classification of the effects of gas on US images according to their physical characteristics may aid in their interpretation. Appreciating two previously undescribed artifacts, pseudo–pneumatosis intestinalis and pseudo–thickened gut, will improve the usefulness of abdominal US.

Index terms: Gastrointestinal tract, US, 74.12989, 75.12989 • Intestines, cysts, 74.782, 75.782 • Ultrasound (US), artifact, 74.1298, 75.1298 • Ultrasound (US), experimental, 74.12989, 75.12989

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