Abdominal X-ray

Indication/Technique

There are no clear guidelines when or when not to request an abdominal X-ray (AXR). The frequency of this examination differs among hospitals and physicians. Seeing AXRs have their limitations, some doctors tend to opt for abdominal CT as first choice.
Nevertheless, an AXR can quickly provide useful information using only minimal radiation.  
Common questions accompanying requests for an abdominal X-ray (AXR):

  • Intraperitoneal free air?
  • Widened intestinal loops?
  • Kidney stones/ureteral stones/bladder stones?
  • Foreign body?
  • Position of gastric tube/duodenal tube?

An AXR can be made in standing, supine and lateral positions (fig. 1-3).

Figure 1. Technique for supine AP (anterior-posterior) image.

Figure 2. Technique for standing AP (anterior-posterior) image.

Figure 3. Technique for lateral image lying on the left side. 

In each position, the X rays will pass through the body from front to back (anterior-posterior). Basal lung fields up to and including the pubic bone should be imaged.

Normal anatomy

Examine an AXR as if you are standing in front of the patient; so left is right and vice versa.
In a normal AXR, the contours of the psoas muscles are visible. The liver, kidneys, spleen and bladder can in some cases also be identified. The density (= whiteness) difference at the level of these organs is created by the (retro)peritoneal fat tissue. By way of repetition: fat is more lucent (= blacker) on X-rays than soft tissues, in this case the abdominal organs (fig. 4/5).

Figure 4. X-ray densities (= whiteness).

 Figure 5. Normal anatomy on a supine AP image. Note the (subtle) organ contours.

Unfortunately, the liver/kidney/spleen contours cannot always be traced optimally in practice. In most cases this is caused by overprojecting intestinal loops containing air and fecal matter.
Below is a summary of ossal structures on an AP image (fig. 6).

 Figure 6. Normal ossal structures on a supine AP image. Abdominal musculature and peritoneal fat are easily identified on this image.

In a number of cases, a lateral position will be opted for. The liver is on the right side of the image (fig. 7).

 Figure 7. Left lateral image. Normal anatomy. 

Because of the difference in X-ray absorption by air and soft tissues, the intestinal structures (intestinal air) can be differentiated from their surroundings.
The stomach is in the left upper quadrant and is visible when it is filled with air. 
The ascending colon and descending colon are at the right and left side of the abdomen respectively. Both structures are retroperitoneal and have relatively fixed positions. The transverse colon has more variation because of its intraperitoneal position. 
The colon can be recognized by the haustra (has a ‘block-shaped’ configuration) and the feces (fig. 8).

 Figure 8. Standing AP image. Normal configuration of the colon with characteristic haustra.

The small intestinal loops are predominantly central in the abdomen and can be recognized by their circular mucosal folds, also known as Kerckring folds / circular folds (fig. 9).

 Figure 9. Supine AP image. An air-filled small intestinal loop with characteristic Kerckring folds.

Checklist

The following points may be used as a guide to assess an AXR.

  1. Technique: is this a standing or supine image?
  2. Are the psoas muscle contours visible? (If not, caution: pathology) 
  3. Try to trace the liver/kidney/spleen contours.
  4. Are there calcifications or radio-opaque structures?
  5. Determine the position of the stomach, small intestinal loops and colonic loops.  Is the distribution of intestinal gas normal? Dilated intestinal loops? 
  6. Evidence of free air? 
  7. Examine the skeletal system. Are there fractures, cortex interruptions, ossal lesions? 
  8. Changes versus previous examinations?

Pathology

  • Ileus

  • Volvulus

  • Free air

  • Calcifications

  • Other calcifications/densities

 

Ileus

Intestinal obstruction (= ileus) is a common problem. Mechanical/obstructive ileus (causes include adhesion, SOL, invagination, volvulus) and paralytic/non-obstructive ileus (causes include trauma, surgery and infection) may be distinguished. In paralytic ileus, all intestinal loops are generally dilated and there is no clear point of obstruction.
Be aware that the diagnosis of ileus is always a clinical diagnosis.
The sensitivity of an AXR in small intestinal ileus is 50% - 66%. Despite its low sensitivity, an AXR may provide an indication of the location of the obstruction.
In an ileus, the intestines will dilate proximal of the point of obstruction. The 3/6/9 rule is a good rule of thumb for pathologically widened intestinal loops. The upper limit of 3 cm applies to the small intestinal loops, 6 cm for the colon and 9 cm for the cecum (blind intestine).

Signs of a small intestinal ileus:

  • Dilated small intestinal loops (> 2.5 cm).
  • Air-fluid levels: the mucosa stimulates the secretion of fluid.
  • Colonic vs small intestinal ileus: central position of the intestines & Kerckring folds suggest small intestines.
  • String of pearls sign (the dilatation will trap air bubbles between the Kerckring folds).
  • Little/no gas or feces in the colon. 
  • Excessive air pocket in the small intestine. 

Comment: when evaluating the air-fluid level, always realize which technique was used to create the image. Due to gravity, you will NOT see the air-fluid level on a supine position.

 Figure 10. Supine AP image (a) and a lateral image (b) of a small intestinal ileus. Pathologically widened small intestinal loops with characteristic Kerckring folds. The air-fluid levels are visible on the lateral image only (cause of ileus: adhesions following appendectomy).

Colonic ileus accounts for only 20% of all intestinal obstructions and is therefore much less common than small intestinal ileus. 

Common causes of colonic obstruction:

  • Malignancy
  • Diverticulitis
  • Sigmoid volvulus (3-8%)
  • Cecal volvulus (1-3%)

Signs of colonic ileus (fig. 11):

  • Gas-dilated loops proximal of obstruction 
  • Collapsed colon distal from obstruction
  • Dilated small intestinal loops and incompetent Bauhin's valve 

 Figure 11. Supine AP image (a) and a lateral image (b) of a colonic ileus. Pathologic dilatation of virtually the entire colon. Acute stop (?) in the pelvis minor. 
Eventually it was discovered this is an obstructive ileus secondary to a sigmoid tumor.

Volvulus

In a sigmoid volvulus, the sigmoid is twisted around its own mesenterial axis. The result is a closed loop (fig. 12).

 Figure 12. Sigmoid volvulus; the sigmoid loop is twisted around its own mesenterial axis.

The coffee bean sign is a classic sign of sigmoid volvulus (fig. 13). When the occluded intestinal loops are dilated with air (closed loop), the medial walls will touch and the inner line will create a coffee bean configuration. The lateral walls of the twisted intestinal loops form the outer contours of the coffee bean.

 Figure 13. Coffee bean sign in a sigmoid volvulus. An MRI examination was performed (see image lower left).

The torsion point is generally located in the pelvis minor. The obstructed sigmoid extends to the left or right upper quadrant and can take up almost the entire abdomen. Absence of air in the rectum supports the diagnosis of sigmoid volvulus. 
Cecal volvulus is somewhat less common. The cecum twists (together with part of the ascending colon) around its own axis (fig. 14). A cecal volvulus is associated with intestinal malrotation. Also the absence or an abnormally long mesenterium of the terminal ileum/cecum/ascending colon may luxate a cecal volvulus. The mesenterium normally keeps the intestinal loops in place. Mesenterial abnormalities may therefore create additional intestinal mobility and increase the risk of torsion.

Figure 14. Cecal volvulus; the cecum is twisted around its own axis with part of the ascending colon.

Compared to a sigmoid volvulus, the cecal volvulus has a less fixed position, especially when there is marked distension. Depending on the initial position and the length of the (mobile) right colon, the distended cecum may expand in each direction. Classic: basis lower right with expansion towards upper left (fig. 15).

  Figure 15. Cecal volvulus (a) with visible torsion (‘whirlpool’) on abdominal CT (b).

It can be difficult to differentiate a cecal volvulus from a sigmoid volvulus.

Tips:

  • In a cecal volvulus, intestinal haustration is frequently preserved 
  • Two air-fluid levels in the distended colon suggests a sigmoid volvulus (Note: the cecal volvulus consists of only one intestinal structure and therefore has no more than one air-fluid level).

 

Free air

A standing AXR is the most sensitive for detection of abdominal free air. Air always moves to the highest point; in a standing image this is under the diaphragm. However, patients in these conditions are frequently immobile or sick, which is why a decubitus image is frequently opted for. A left lateral decubitus image is preferred. In this way the liver is at the cranial side, creating (compared with the air-containing intestines) a better density difference with the free air.

Signs of free air (fig. 16 - 18):

  • Subdiaphragmal air (air rises!).
  • Rigler’s sign/double wall sign; when there is a large amount of free air, both the inside and outside of the intestinal wall are visible. 

 Figure 16. Subdiaphragmal free air (a) and Rigler's sign (b).

 Figure 17. Ileus and free air in perforated diverticulitis in the sigmoid. 

 Figure 18. Left lateral image. Significant amount of free air perihepatic / subdiaphragmal right.

Mimics of free air:

  • Gastric air bubble; a round/oval-shaped air configuration under the left diaphragm.  Free air is more rind-shaped. On a normal AXR, a thick dense (= white) wall can be seen at the cranial side of the gastric air bubble. As opposed to free air, there is a much thinner border between the lungs and the abdomen.
  • Chilaiditi's sign; interposition of intestinal loops between the liver and diaphragm. This may be confused with subdiaphragmal free air. 

 

Calcifications

Kidneys:

  • Nephrolithiasis/urolithiasis: concrements in the renal collecting system (nephrolithiasis) and the ureters (urolithiasis). Not all concrements are large enough or contain sufficient calcium to be visible on an AXR. About half of the concrements are invisible on X-rays. 
    Kidney stones are usually small but can sometimes fill an entire renal pelvis (= coral stone/struvite stone). 
    Most symptomatic ureteral stones are located at the level of the pyeloureteral junction (= transition of kidney-ureter), the iliac vessels and vesicoureteral junction (= transition of ureter-bladder). See figure 19.

 Figure 19. Outline of the ureters. Concrements in the right pyelum/proximal ureter. Predilection sites of symptomatic concrements (*).

  • Nephrocalcinosis: calcium salt deposits in the renal parenchyma. It occurs in patients with hyperparathyroidism and medullary sponge kidneys. Diffuse, partly fine, parenchymal calcifications can be seen on the AXR (fig. 20).

 Figure 20. Extensive bilateral nephrocalcinosis in a patient with medullary sponge kidneys.

In practice, the above concrements often remain hidden by overprojecting kidneys.

 

Other calcifications/densities

Below is a list of common calcifications and densities that may be found on an AXR.

  • Vascular calcification: particularly vascular calcifications of the lienal or splenic artery (medial of the spleen), aorta and iliac vessels. An unusual finding on an abdominal X-ray is an aortic aneurysm (fig. 21). Note: confirmation of an aneurysm is no indication for an AXR.

  Figure 21. Subtle vascular calcifications on an abdominal X-ray in a person with AAAA (a). The CT examination reveals extensive free fluid around the aorta (b).

  • Costochondral calcifications (fig. 22); irregular calcifications in the anterior cartilage of the lower ribs (generally no clinical implications).
  • Bile stones (fig. 22).
  • Surgical clips (fig. 22); they may be located anywhere (depending on the type of surgery).

 Figure 22. At left a patient who has undergone cholecystectomy. At right another patient with multiple costochondral calcifications and a large bile stone.

  • Calcified myomas (fig. 23); spherical popcorn-like calcifications.
  • Calcified lymph nodes (fig. 23); large uniform calcifications.
  • Phlebolites (fig. 23); small round calcifications in the veins.  They are generally located in the pelvis minor, often peripheral from the bladder. Unfortunately, it is not always possible to differentiate a phlebolite from a ureteral stone.
  • Bladder stone (fig. 23).

 Figure 23. Calcified myomas (upper left), calcified lymph nodes (upper right), phlebolites (lower left) and a bladder stone (lower right).

  • Foreign body (fig 24); only radio-opaque structures are visible on an AXR.

 Figure 24. A psychiatric patient who swallowed several nails and clamps.

Sources

  • P.C. Nevitt String of pearls sign. Radiology 2000.
  • Khurana et al: Bowel Obstruction Revealed by Multidetector CT. AJR 2002.
  • A.C. Silva et al: Small Bowel Obstruction: What to Look For. RadioGraphics 2009.
  • Dahnert. Radiology Review Manual 7th Edition.
  • N.C. Dalrymple et al; Problem Solving in abdominal imaging. 2009 
  • D. Feldman. The Coffee Bean Sign. Radiology 2000
  • J.Q. Ly; The Rigler Sign. Radiology 2003
  • Emilio Quaia: Diagnostic Imaging; Radiological Imaging of the kidney. 2011.
  • Fred A. Mettler, Jr; Essentials of Radiology 3rd edition (2005)

Author

  • Annelies van der Plas, MSK radiologist Maastricht UMC+

24/04/2014 (translated 14/11/2016)

Copyright
All the work (text, illustrations, visual elements) seen on this website is copyright by Annelies van der Plas.
It may not be used without written permission of Annelies van der Plas.

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