Abdominal X-rays (AXRs) are a frequently performed radiological investigation that you’ll be expected to be capable of interpreting. Therefore it’s essential that you develop the ability to interpret abdominal x-rays and recognise pathology.
Always begin by checking the following:
- Patient details (name / DOB)
- Date and time the film was taken
- Any previous imaging (useful for comparison)
Assess image type and quality
Projection of image
- Anterior-posterior (AP) – either supine or erect – most common
Exposure of image
- Ensure the whole abdomen is visible from diaphragm to pelvis.
- Quality of the image is often poor, with overlying bowel obscuring more posterior structures.
- If bowel perforation is being considered, you don’t usually require an abdominal film, instead you need an erect chest x-ray, as this allows free gas under the diaphragm to be identified (the patient needs to have sat upright for at least 15-20 minutes prior to the x-ray to allow time for air to rise).
Abdominal XR interpretation (BBC approach)
It’s important to have a systematic approach to interpreting abdominal X-rays as this reduces the risk of missing pathology.
In this guide we use the BBC approach:
- Bowel and other organs
- Calcification and artefact
Bowel and other organs
Small and large bowel
Differentiating between the small and large bowel on an AXR is not always straightforward but there are a number of clues that can help you:
- The small bowel usually lies more centrally, with the large bowel framing it around the periphery.
- The small bowel’s mucosal folds are called valvulae conniventes and are seen across the full width of the bowel.
- The large bowel wall features pouches or sacculation that protrude into the lumen that are known as haustra. In between the haustra are spaces known as plicae semilunaris. The haustra are thicker than the valvulae conniventes of the small bowel. They also commonly do not appear to completely traverse the bowel. This distinction is unfortunately unreliable as dilated large bowel can have a haustral pattern that does in fact traverse the bowel.
- Faeces have a mottled appearance and are most often seen in the colon, due to trapped gas within solid faeces.
- There is considerable normal variation in the distribution of bowel gas.
The normal diameter of the intestines on an AXR do not usually exceed:
- 3 cm for small bowel
- 6 cm for colon (large bowel)
- 9 cm for caecum
This is often referred to as the ‘3/6/9 rule’
Small bowel obstruction
- Small bowel obstruction can be visualised on an AXR as dilatation of the small bowel (>3cm).
- The valvulae conniventes are much more visible and have what is referred to as a “coiled spring appearance”.
- The most common cause (75%) of small bowel obstruction in the developed world is adhesions (mostly relating to previous abdominal surgery). Some other causes include abdominal hernias (10%) and either intrinsic or extrinsic compression by neoplastic masses. 10
- You should inspect the inguinal regions on the x-ray if considering a hernia as a cause of small bowel obstruction, as they are often fairly obvious even on plain abdominal x-rays
Large bowel obstruction
- The most common causes of large bowel obstruction are colorectal carcinoma and diverticular strictures. Less common causes are hernias or volvulus.
- Volvulus is a twisting of the bowel on its mesentery and most commonly occurs at the sigmoid colon or caecum. Patients with volvulus are at high risk of bowel perforation and/or bowel ischaemia secondary to vascular compromise.
- Sigmoid volvulus has a characteristic ‘coffee bean’ appearance
- Caecal volvulus is often described as having a fetal appearance
Rigler’s (double wall) sign
- Normally only the inner wall of the bowel is visible on an AXR.
- Pneumoperitoneum may cause both sides of the bowel wall to be visible.
- Causes of pneumoperitoneum include a perforated abdominal viscus (e.g. perforated bowel, perforated duodenal ulcer) and recent abdominal surgery.
- You should look closely for air under the diaphragm on an erect CXR if you suspect pneumoperitoneum.
Features of inflammatory bowel disease on AXR
- Thumb-printing – mucosal thickening of the haustra due to inflammation and oedema causing them to appear like thumb prints projecting into the lumen
- Lead-pipe (featureless) colon – loss of normal haustral markings secondary to chronic colitis
- Toxic megacolon – colonic dilatation without obstruction associated with colitis
Other organs and structures
Although AXR isn’t well suited to imaging these structures, it’s useful to recognise them to help orientate yourself and spot relevant pathology.
- Lungs – check the lung bases if visible for pathology (e.g. consolidation) as abdominal pain can sometimes be caused by basal pneumonia
- Liver – large right upper quadrant (RUQ) structure
- Gallbladder – rarely seen, look for calcified gallstones and cholecystectomy clips
- Stomach – left upper quadrant (LUQ) to midline structure, containing a variable amount of air
- Psoas muscles – lateral edge marked by a relatively straight line either side of the lumbar vertebrae and sacrum
- Kidneys – often visible, right lower than left due to the liver
- Spleen – LUQ, superior to left kidney
- Bladder – variable appearance depending on fullness
Lots of bones are visible on an AXR and it’s important that you can identify each and screen for any pathology (which may be expected or unexpected). In addition, bones on the AXR provide useful landmarks for where you might expect to see a soft tissue structure (e.g. ischial spines are the usual level of the vesico-ureteric junction).
Bones commonly visible on AXR include:
- Lumbar vertebrae
- Proximal femurs
A wide range of bony pathologies can be identified on abdominal x-rays including fractures, osteoarthritis, Paget’s disease and bony metastases.
Calcification and artefact
Various high density (white) areas of calcification or artefact may be seen.
- Calcified gallstones in the RUQ
- Renal stones/staghorn calculi
- Pancreatic calcification
- Vascular calcification
- Costochondral calcification
- Contrast (e.g. following a barium meal)
- Surgical clips
- Naval jewellery artefact over the approximate location of the umbilicus
Presenting an abdominal x-ray
Having a structured approach to summarising your findings is key to ensuring you communicate the salient points. Below is an example of a comprehensive summary, however feel free to find a structure that suits you.
“This is a supine AP abdominal radiograph of Jayne Lister, date of birth 11/4/1970. The film is of good quality with appropriate exposure. No prior imaging is available for comparison. Both the small and large bowel appear within normal limits. Other abdominal viscera appear normal within the limits of this projection. No obvious bony pathology is identified. No abnormal calcification is seen. In summary this is a normal plain radiograph of the abdomen.”
Dr Kunal Patel – ST2 Radiology
- SBO – http://www.ganfyd.org/index.php?title=File:Gallstone_ileus.jpg#filelinks
- LBO – https://commons.wikimedia.org/wiki/File%3ALargeBowelObsFlat2008.jpg
- Rigler’s sign – https://en.wikipedia.org/wiki/Rigler%27s_sign#/media/File:Double_wall_sign.jpg
- Prostate metastases – https://commons.wikimedia.org/wiki/File%3AOsteoplastic_bone_metastasis_prostate_cancer_01.jpg
- Staghorn calculus – https://commons.wikimedia.org/wiki/File%3AMedical_X-Ray_imaging_UEI07_nevit.jpg
- Small vs Large bowel – http://www.wikiradiography.net/page/The+Abdominal+Plain+Film-++Differentiating+Large+and+Small+Bowel
- Toxic megacolon https://upload.wikimedia.org/wikipedia/commons/d/d3/Toxisches_Megacolon_bei_Colitis_ulcerosa.jpg
- Constipation – https://commons.wikimedia.org/wiki/File%3AConstipation.JPG
- Volvulus – https://commons.wikimedia.org/wiki/File%3AVoluculus.PNG
- Fitzgerald, J. Edward F. (2010). Small Bowel Obstruction. Oxford: Wiley-Blackwell. pp. 74–79. doi:10.1002/9781444315172.ch14. ISBN 9781405170253.