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Free Air: Signs to Watch Out for In Abdominal Imaging Part II

March 8, 2010
Written by: , Filed in: Abdominal Imaging
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Pneumoperitoneum must be diagnosed promptly when it occurs, which is often abruptly and without warning.

Author: Stephen R. Baker, MD. Special Presentation for Practical Reviews in Radiology

Signs of Free Air on Radiography

The best place to look on plain films for findings of pneumoperitoneum is in the right upper quadrant, because the homogeneous shadow of the liver affords an excellent background upon which even very small collections of free air can be displayed.

However, to take advantage of the hepatic background, the entirety of the peritoneal cavity from the top to bottom must be encompassed.

That is, the radiograph or radiographs must extend from the obturator foramina inferiorly to the apex of the higher hemidiaphragm superiorly.

For most adults, a standard KUB (Kidney-ureter-bladder) cannot include the full expanse of the peritoneal cavity, and a second film of the upper abdomen must be obtained.

The most common right upper quadrant sign of free air is the anterior superior bubble which, contrary to traditional notions, can be discerned even when its diameter is much less than 1 cm and its volume less than ice.

Summary:
The most common right upper quadrant sign of free air is the anterior superior bubble which, contrary to traditional notions, can be discerned even when its diameter is much less than 1 cm and its volume less than ice.

Other signs of free air superimposed on the liver or even within it will be outlined in Part III of this discussion.

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