The interpretation of AXR


The AXR, although frequently requested, diagnoses a relatively small number of pathologies and in very unwell patients; CT is more sensitive and specific for diagnosis. It is, however a simple and quick test to perform as CT may not always be immediately available.

Remember that an AXR carries approximately 70 times the radiation dose of a CXR. A CXR will give approximately the same dose as 3 days of background radiation in Cornwall.

Because the AXR is a high dose examination, make sure that you ask females of childbearing age about their pregnancy status. Irradiating an unborn foetus is a big deal because radiation has potential teratogenic effects. Consider whether ultrasound might be a viable alternative in young patients.


Abdominal pain

Suspected obstruction

Suspected perforated viscus (with an erect CXR)

?toxic megacolon

How it is performed

The “film” cassette is placed behind a supine patient and x-rays pass through the abdomen. Technical factors such as rotation and penetration are not as big an issue as they are in CXR interpretation. Just ensure that the film is adequate and covers the whole abdomen and hernial orifices.

System for AXR interpretation

There are many ways of reading a film and there are fewer areas to check than on a CXR. It is acceptable to just describe any abnormality you see. The following is a possible checklist for interpretation, should you be presenting the film on a ward round or in an exam.

  • Tell the examiner what the study is (AXR) and the patient details (name, age etc)
  • Look at adequacy (is the whole abdomen included?)
  • Look for any lines, tubes, metalwork, stents etc as these may be the cause of the presentation. You may see aortic and femoral stents, JJ renal stents, nephrostomies, PD catheters, hip replacements, femoral lines and the like. Make sure they are correctly positioned.

Bowel and air

Large bowel – in normality contains faeces and a little air. When obstructed you will see air throughout distended bowel loops. Measurement is not an exact science but about 6cm for large bowel and 9cm for caecum is a cut off.

The large bowel is peripheral, and the ascending and descending colon are fixed as they are retroperitoneal. The transverse colon and sigmoid colon are variable in position as they are intraperitoneal. The large bowel has haustra which do not traverse the entire colon; they only go part of the way across it.

Sigmoid and caecal volvulus are specific forms of large bowel obstruction with distinctive appearances

Small bowel – again in the normal state small gas bubbles are seen within the small bowel. When obstructed you first see columns of air, which eventually fill with fluid.

Greater than 3 cm is pathological. The small bowel loops are generally central when obstructed (also intraperitoneal) and have valvulae conniventes which go the whole way across the bowel.

If you see obstruction, look for:

Evidence of free air within the abdomen to suggest perforation
Air in the hernial orifices to suggest a causative hernia (SBO)
Air in the biliary tree to suggest gallstone ileus (SBO)

Look at wall thickness – thick walled bowel is pathological and can be caused by ischaemia, infection (pseudomembranous colitis in the colon), inflammation (inflammatory bowel disease), and tumour (lymphoma) amongst other causes

Thumb printing refers to thickening of the colonic wall accompanying wall thickening and looks like thumb prints along the bowel wall.

Dilated bowel can also be caused by:

Toxic megacolon – feared sequelae look for mucosal thickening accompanying this and look for signs of perforation

Pseudo-obstruction – which will radiologically look like mechanical obstruction and you will need to correlate clinically.

Air in the wall of any organ is usually a bad sign. Air in the bowel wall (often accompanying obstruction) – pneumatosis intestinalis – implies impending ischaemia.
Air in the bladder or kidney wall or gallbladder (emphysematous cystitis/pyelonephritis/cholecystitis) is rare but life threatening infections.

Soft Tissues

The liver, spleen, kidneys and bladder can be seen on an AXR, particularly with practice. They may be enlarged. They are generally better assessed on US or CT

The psoas shadow is seen bilaterally adjacent to the spine. If the shadow is lost it may indicate retroperitoneal pathology, and in the presence of an abdominal aortic aneurysm, may represent a leak. However, in a small proportion of patients the psoas shadow may not be seen as a normal finding.

Calcifications and bones

Look for stones in the line of the ureter or projected over the kidneys. 90% of urinary tract stones are radio-opaque

Look for gallstones, although only 10% are radio-opaque and ultrasound is better for assessment.

Look for a calcified aortic rim in an aortic aneurysm. An AXR should not be requested to look for an aneurysm but you should be able to recognise an AAA in patients in whom you have requested an AXR but did not suspect an AAA. It is usually seen as a convexity to the left of the lumbar spine. The psoas shadow may be obliterated – this may be a sign of rupture.

A rounded calcified appendicolith in the RIF is a sensitive finding for appendicitis.

Check the bones for metastases and degenerative changes, as well as fractures. The hips may be well seen on an AXR.

Thick-walled large bowel( mucosal oedema)

  1. Signs to look for include increased separation of the bowel loops (their walls are of soft tissue density) and thickening of the wall,
  2. Thumbprinting refers to thickened haustrae, giving the impression of thumb prints indenting the bowel wall.
  3. Areas of active inflammation cause bowel wall thickening, in either the large or small bowel. It can be seen in inflammatory bowel disease.Infective colitis may cause an appearance indistinguishable from inflammatory bowel disease; a history of antibiotic usage should be sought, and stool cultures sent if this diagnosis is suspected.
  4. Lymphoma is a cause of bowel wall thickening and thumbprinting;
  5. Two other important causes of this appearance are trauma and ischaemic colitis.

SBO (Small bowel obstruction)

  1. There are dilated loops of bowel which are central in location
  2. Valvulae conniventes go across the whole bowel loop in the small bowel like stacks of coins ( in the large bowel, haustrae do not go all the way across)
  3. Loops will  exceed 3 cm in diameter.
  4. Normally only  short segment of small bowel is seen because of lack of luminal gas
  5. String of pearls sign- After SI is distended with gas next fluid will start to accumulate if obstruction persists. When loops are almost filled with fluid, remaining bubbles of gas show up trapped behind the valvulae or against the mucosal surfaces as string of pearls. Finally when it is all filled up with fluid SBO becomes less identifiable.
  6. AXRs are about 50% sensitive for small-bowel obstruction. Confirm with CT.
  7. Post surgical adhesions are the most frequent cause of small-bowel obstruction in the UK. Hernias (gas over hernial orifice) and tumours are other common causes. Rarely strictures (Crohn’s disease, post radiotherapy)  or gall stone ileus ( when large gall stone passes from biliary tree in to ileo caecal valve and causes obstruction- normally perforating from gall bladder or CBD) can cause SBO
  8. So clues for SBO would be to look for surgical clips, gas over hernial orifice, gallstones in right illiac fossa.
  9. Branching low density pattern can be seen in the right upper quadrant is seen in Gall stone ileus- pneumobilia. It can also be seen after biliary stent placement
  10. If ileocaecal valve is incompetent SBO may follow LBO thus reducing chance of caecal perforation.

LBO (Large bowel obstruction)

  1. Loops are peripheral in  position within the abdomen (the ascending and descending colon are fixed and retroperitoneal)
  2. Haustrae which do not cross the entire bowel loop
  3. Transverse colon more than 5.5cm and caecum more than 9cm signifies imminent risk of perforation
  4. Will have solid faeces
  5. Numbers of loops are less than in SBO
  6. LBO can be functional (pseudo-obstruction) or mechanical
  7. Commonest mechanical  cause of LBO is colorectal cancer followed by diverticular stricture.
  8. Plain films does not distinguish between functional and mechanical obstruction- water soluble enema can show the level of obstruction. Ba enema is contraindicated as it is toxic to the peritoneal cavity in event of perforation.
  9. Pseudo-obstruction carries a high mortality rate and there is no specific treatment. It usually caused by electrolyte disturbances, although other causes include scleroderma, myxoedema, diabetes and drugs.
  10. Toxic megacolon: LBO in association with mucosal oedema with transverse colon more than 5.5cm classically seen in acute severe colitis. It can also be seen in ischaemic colitis or infectious colitis.
  11. Sigmoid volvulus: Hugely  dilated loop of bowel extending from pelvis in the left iliac fossa, with characteristic inverted U loop or coffee bean appearance.
  12. Cecal volvulus: Markedly dilated air-filled viscus in the central abdomen which is separate from the stomach. It does not follow the anatomic location of transverse colon along with absence of cecum in the right iliac fossa. Far less common than sigmoid volvulus since cecum is usually a retroperitoneal structure. If diameter of cecum exceeds 9 cm there is imminent risk of perforation.


  • Rigler’s sign: Normally, in the absence of free intraperitoneal air the air in intraluminal. Presence of extraluminal air results in thin pencilled line of bowel wall with gas on either side, best seen in the loops in the left upper quadrant
  • Lucent liver sign: Air overlies the liver and reduces its apparent density
  • Air outlining  the falciform ligament: This structure is not seen in normal AXR
  • Triangles of air in the abdomen are abnormal – consider that these may be lying between bowel loops
  • Lastly but not the least free gas under the diaphgram in erect CXR
  • Free gas is seen after bowel perforation, laparatomy ( can last up to seven days) or even after prolonged endoscopy.

Abdominal aneurysm

  1. Calcified aorta should be sought in every abdominal film  in patients with abdominal pain particularly with haemodynamic comproise or low haemoglobin
  2. If an aneurysm is present it will be seen on 75% of abdominal films.
  3. The psoas shadow may be obliterated in a ruptured aneurysm, and may be lost in any retroperitoneal pathology- a soft sign as  in a small proportion of patients the psoas shadow is not seen anyway.
  4. CT with intravenous contrast is the best test, if the patient is stable,
  5. Ultrasound can be used in the emergency setting to exclude or confirm the presence of an aneurysm

Renal calculi:

  1. Ureteric calculi are radio-opaque in 90% of cases. In contrast, gallstones are only seen in 10%.
  2. Only 90% present with haematuria.
  3. CT without IV contrast (CT KUB) is the gold standard for the diagnosis of ureterolithiasis and is even more sensitive than IVU.
  4. The vast majority do not require intravenous contrast, which is used for problem solving in complicated cases.


  1. Uniform increased density extending across the abdomen and particularly in both flanks
  2. Gas-filled loops of bowel floating into central position with paucity of bowel in both flanks

Soft tissue mass:

  1. Outlines of the liver and spleen may be seen as soft tissue displacing loops of bowel from the right and left upper quadrants respectively.
  2. The renal outlines can also be seen in the retroperitoneal position outlined by fat but it does  not usually cause bowel displacement.
  3. Within the pelvis the bladder may also be seen as a uniform area of fluid or soft tissue density
  4. If an apparent soft tissue mass is seen to contain bubbles of gas then it is likely to represent an abscess containing gas forming organisms or indicate a enteric fistula.

Learning resources:

Abdominal x-ray Interpretation – Iain Au-Yong. An online learning module for members. This site is free to join. Look for the module in the radiology section of the education area.

On Call X-Rays made easy Iain Au-Yong, Amy Au-Yong, Nigel Broderick, Elsevier Publishing – comprehensively covers all abnormal AXR appearances (as well as emergency CXR, paediatric and bone pathologies). Available via the following link.

Edited by Dr Iain Au-Yong

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