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Pneumoperitoneum refers to the presence of free gas within the peritoneal cavity. The plain films signs of pneumoperitoneum are both diverse and sometimes difficult to identify. This page provides an overview of the plain film diagnosis of pneumoperitoneum.

Why is Pneumoperitoneum Important?

Pneumoperitoneum is most often caused by perforated abdominal viscus and can present an acute medical emergency.

“The Radiological signs of pneumoperitoneum are among the most important signs in radiology, indeed in Medicine. Sometimes the amount of free gas is small and you may have to work to demonstrate it (i.e. modify the film technique). Miss it and the patient may die”
Abdominal X-rays made easy. 2nd edition, James D. Begg
Churchill Livingstone, Elsevier, 2006

Plain Film Signs of Pneumoperitoneum

Radiographers are more likely to demonstrate pathologies that they are familiar with. As James Begg stated, sometimes you have to 'work to demonstrate pneumoperitoneum'. What he meant by this is that the supine signs of pneumoperitoneum may be subtle- the radiographer must be able to identify the signs on the supine image and know what supplementary views will prove the existence of free intraperitoneal gas.

“We diagnose what we look for
and look for what we know”

The plain film signs of pneumoperitoneum are well established in the literature as follows
1 Anterior Subhepatic Space Air
2 Doges Cap Sign (free Air in Morrison's Pouch)
3 Air Anterior to Ventral Surface of Liver
4 Rigler’s sign on supine AXR (also known as double-wall or bas-relief sign)
5 Falciform Ligament Sign
6 The ‘football’ sign
7 The cupola. Air accumulation beneath the central tendon of the diaphragm
8 Continuous diaphragm sign
9 The triangle- air trapped between three loops of bowel
10 Air under diaphragm on erect cxr
11 Air outlined against liver/flank on decub AXR
diaphragmatic muscle slips, ligamentum teres air, Double Gastric Fundus sign, The Inverted-V sign, Scrotal air
13 Abscess Gas
14 Pneumoretroperitoneum

RUQ/liver signs on supine AXR

There are 3 separate signs of free air around the liver as follows.

Pneumoperitoneum - wikiRadiography

Pneumoperitoneum: Importance of Right Upper Quadrant Features
Am J Roentgeno! 127:753-756. 1976

1. Anterior Subhepatic Space Free Air (RUQ sign 1)
Pneumoperitoneum - wikiRadiography
Pneumoperitoneum: Importance of Right Upper Quadrant Features
Am J Roentgeno! 127:753-756. 1976
Anterior subhepatic space free air tends to be vaguely linear in shape (arrowed). A visible medial border of the liver is often seen outlined by fat. A careful examination of this image (left) shows the arrowed density to be air density rather than fat density.

Pneumoperitoneum - wikiRadiographyThe differentiation between fat and air density becomes easier with experience. This image of normal fat surrounding the liver shows a consistent density continuous with the properitoneal fat stripe.

2. Doges Cap Sign (RUQ sign 2)
Pneumoperitoneum - wikiRadiography
Pneumoperitoneum: Importance of Right Upper Quadrant Features
Am J Roentgeno! 127:753-756. 1976
Doges Cap sign refers to free air in Morrison's pouch. Morrison's pouch is normally a potential space between the right kidney and the liver. This is a particularly difficult sign of pneumoperitoneum for several reasons. Firstly, it may be the only sign of pneumoperitoneum and may be very subtle. Secondly, it can be easily misinterpreted as gas in the duodenum.

Gas in Morrison's pouch may have the following features

  • Triangular in shape
  • concave medial border
  • positioned inferior to the right 11th rib
  • positioned superior to the right kidney

Pneumoperitoneum - wikiRadiography

Pneumoperitoneum - wikiRadiography
Morrison’s pouch free gas demonstrated on supine Radiographs typically show the following Characteristics
1.Typically triangular shaped
2.The lower lateral corner is commonly sharp
3.The lateral border is typically concave and outlines the medial border of the liver
4.It is positioned inferior to the 11thrib
5.It is positioned superior to the right kidney

3. Air Anterior to Ventral Surface of Liver(RUQ sign 3)
Pneumoperitoneum - wikiRadiography Air sitting against the ventral surface of the liver can be any shape and, as in this case, is frequently "geographical" in shape. The liver is a homogenous organ and should be homogenous in density on plain film. If the liver is seen to demonstrate an uneven density, pneumoperitoneum should be considered.

Note also Rigler's sign

4. Decubitus Abdomen Sign
Pneumoperitoneum - wikiRadiography This patient is in the left lateral decubitus position. It is conventional in radiography to mark the side the side that is up.

There is evidence of free air between the abdominal wall and the liver (white arrow). There is also evidence of free fluid in the peritoneum (black arrow).

5. Rigler’s Sign on supine AXR
Pneumoperitoneum - wikiRadiography Rigler's sign is named after Leo G. Rigler. The sign refers to the appearance of the bowel wall on plain film when it is outlined by intraluminal and extraluminal air (arrowed). The extraluminal air is free peritoneal gas.

6. Falciform Ligament Sign
Pneumoperitoneum - wikiRadiography The falciform ligament connects the anterior abdominal wall to the liver. The ligament continues to extend inferiorly beyond the liver where it becomes the round ligament (white arrow). Given that the falciform ligament is situated against the anterior abdominal wall, it is not surprising that it becomes outlined with air in a supine patient with free abdominal gas.
Pneumoperitoneum - wikiRadiographyThis is an axial CT scan image of a patient with pneumoperitoneum. The free gas is seen outlining the anterior abdominal wall and several loops of bowel. The arrowed structure is the falciform ligament surrounded by free intraperitoneal gas.

The falciform ligament sign is almost never seen in isolation. If there is enough free air to outline the falciform ligament, there is usually enough air to also provide at least a Rigler's sign. In this case(left), there is a Rigler's sign as well as RUQ signs. Note also bilateral nephrostomy tubes insitu.

7. The ‘football’ sign

Pneumoperitoneum - wikiRadiography

The football sign likens the massively air-filled peritoneum to an American football. To extend the simile a little further, the falciform ligament has been likened to the seam in the football, and the rarely seen medial and lateral umbilical ligaments are likened to the football laces.

This neonatal patient has massive pneumoperitoneum and could reasonably be said to display football sign. There is also falciform ligament sign, Rigler's sign and air in the scrotum.

Pneumoperitoneum - wikiRadiography

John W. Rampton, MD
The Football Sign
(Radiology 2004;231:81-82.)
© <a class="external" href="" rel="nofollow" target="_blank">RSNA</a>, 2004

8. Continuous Diaphragm Sign
Pneumoperitoneum - wikiRadiography
image source unknown
Another manifestation of massive pneumoperitoneum is the continuous diaphragm sign. Where there is sufficient air beneath the diaphragm, the continuous nature of the diaphragm is demonstrated. Note that the left and right hemidiaphragms contrasted by the free gas appear as a continuous structure.

9. Double Bubble Sign
Pneumoperitoneum - wikiRadiography The double bubble sign is an appearance of subdiaphragmatic gas under the left hemidiaphragm in which there are two collections of overlapping gas- one of these collections is subdiaphragmatic free gas and the other is normal gas within the fundus of the stomach. Note that the diaphragm (black arrow) is a thinner walled structure than the stomach wall (white arrow). This distinction is sometimes useful in distinguishing between the two structures.

Note also free subdiaphragmatic gas under the right hemidiaphragm

10. The Cupola Sign.
Pneumoperitoneum - wikiRadiography The Cupola Sign refers to an air accumulation beneath the central tendon of the diaphragm (white arrows)

Pneumoperitoneum - wikiRadiographyThe term cupola comes from a dome such as this famous dome of the Duomo in Florence.

11. Lesser Sac Gas
Pneumoperitoneum - wikiRadiography This image of free gas has a cupola sign (white arrows) and a lesser sac gas sign (black arrows). The lesser sac is positioned posterior to the stomach and is usually a potential space. There is free connection between the lesser sac and the greater sac through the foramen of Winslow.

12. The Triangle Sign
Pneumoperitoneum - wikiRadiography The triangle sign refers to small triangles of free gas that can typically be positioned between the large bowel and the flank(black arrow)

12. The Others
There are a number of other signs of pneumoperitoneum that are less commonly seen. These signs are sufficiently rare to not warrant close examination. Equally, for reasons of completeness, they have been included on this page.

Leaping Dolphins Sign
Air under hemidiaphragm and diaphragmatic muscle slips visible
Urachus Sign
Air contrasted urachus. Appears as vertical line between bladder and umbilicus. Outline of medial umbilical ligament
<a class="external" href="" rel="nofollow" target="_blank">google</a>
The Inverted “V” Sign
" in infants the “inverted V” is undoubtedly caused by the large umbilical arteries, in adults I believe it is the inferior epigastric vessels that produce the “inverted V” sign.”
The “Inverted V” Sign of Pneumoperitoneum
John F. Bray, M.B., Ch.B., F.R.C.S. (Ed.),
Radiology 1984; 151: 45-46
Air in the Fissure for the Ligamentum Teres
Air in the Fissure for the Ligamentum Teres. May appear in isolation. Appears as a lucent vertical stripe over liver Radiology; 1991. 178: 489 - 492
Coronary Ligament Outlined by Air
The coronary ligament is sited anterior to the liver.
Pneumo-gall bladder Air in the gall bladder fossa outlining the gall bladder
<a class="external" href="" rel="nofollow" target="_blank">google</a>

13. Abscess Gas
Pneumoperitoneum - wikiRadiography This patient has an abscess (proven on CT). The arrowed bubbles of gas are suspicious in that they are not clearly contained within normal hollow abdominal viscus. If they were, for example, contained within the colon, they would tend to be aligned in a more linear fashion and may outline normal haustral features.

14. Pneumoretroperitoneum
Pneumoperitoneum - wikiRadiography This patient has free air in the retroperitoneal space. The air is seen surrounding the lateral border of the right kidney (white arrow). There is other evidence of free gas including Rigler's sign.

If you are not confident that the appearance is pneumoretroperitoneum, you can try an erect and decubitus view to see if the gas moves. If the gas is seen to move, it's not in the retroperitoneum.

Pneumoperitoneum - wikiRadiographyAn axial CT scan image is shown with air around the right kidney (black arrow).

Case 1
PNEUMOPERITONEUM This in-patient was referred for chest radiography following open mitral valve surgery. There appears to be free air under the right hemidiaphragm. This was interpreted as 'colonic interposition' (Chilaiditis sign) but I suspect that it is a 'leaping dolphins sign'. The appearance of septa within the air collection under the right hemidiaphragm is probably outlining the costal muscle slips of the hemidiaphragm rather than representing outline of bowel wall.

There also appears to be a negative 'silhouette sign' of the liver (black arrow).

There is an azygous lobe (white arrow).

There is a pleural effusion on the left.

leaping dolphins sign
[fromMorton A. MeyersDynamic radiology of the abdomen: normal and pathologic anatomy]
PNEUMOPERITONEUM The lateral chest image similarly demonstrates a negative 'silhouette sign' involving the liver (black arrow).

There appears to be a triangle of pleural fluid at the base of an inferior accessory fissure.

There are three metallic rings associated with the mitral valve surgery.

There is loss of visualisation of the left hemidiaphragm associated with a left pleural effusion.

Case 2
Pneumoperitoneum - wikiRadiography This 78 year old man presented to the Emergency Department with a history of abdominal pain. He was examined and referred for an acute abdominal and chest X-ray examination.

There appears to be gas under the right hemidiaphragm. Note that there is bowel gas superimposed over the liver suggesting colonic interposition- a known cause of false pneumoperitoneum. If this appearance represented pneumoperitoneum, you would expect to see air-contrasted liver.
Pneumoperitoneum - wikiRadiography The lateral chest projection image demonstrates the appearance again of air under the right hemidiaphragm. The is an associated air-fluid level. This appearance is more likely to indicate colonic interposition than pneumoperitoneum. Where there is a clinical question of pneumoperitoneum, a decubitius abdominal plain film would be helpful.


Pneumoperitoneum - wikiRadiography
Pneumoperitoneum - wikiRadiography
Colonic Interposition
Pneumoperitoneum - wikiRadiography
It is useful to be able to distinguish between the appearance of air under the right hemi-diaphragm, colonic interposition and pneumothorax.

The black arrow identifies the aircontrasted liver. The aircontrasted costal muscle slip is marked with a white arrow
This paient has a pneumothorax. The right hemidiaphragm contrasted with air in the pleural space resembles the liver contrasted with free air in the peritoneum(left) This appearance of colonic interposition does bear some similarity to the appearance of pneumoperitoneum (above left).

The white arrowed structure is probably a haustral marking and the black arrowed structure is diaphragmatic

Case 3

Pneumoperitoneum - wikiRadiography This is a 60 year old lady with known Crohn's disease and abdominal pain. She was referred for an abdominal plain film. The large bowel wall is visible along both flanks. The liver is also outlined by fat.
false pneumoperitoneum There is fat interposed between the inferior border of the liver and the hepatic flexure of the colon (white arrow)

There is properitoneal fat contrasting the caecum (black arrow)

These appearances are not associated with pneumoperitoneum. The scattered air densities over the liver probably represent air within bowel.

Case 4

Pneumoperitoneum - wikiRadiography
This is a supine bedside chest X-ray image post intubation. At a cursary glance there may appear to be air under the right hemidiaphragm
Pneumoperitoneum - wikiRadiography
This is in fact air above the diaphragm rather than under it. This patient has a right sided pneumothorax with a deep sulcus sign. The pneumothorax is also visible at the right lung apex (arrowed)


Pneumoperitoneum is an important plain film finding. Untreated, pneumoperitoneum has poor morbidity and mortality outcomes. Radiographers who are familiar with the plain film appearances of pneumoperitoneum, particularly the supine appearances, are more likely to undertake supplementary views to concusively demonstrate the pathology. Moreover, they will also have the opportunity to report the finding immediately to the referring doctor.