The document provides information about interpreting plain abdominal x-rays, including: - Common findings such as the intestinal gas pattern, bowel dilatation, pneumoperitoneum, ascites, and abdominal calcifications. - Details on evaluating signs of small vs. large bowel obstruction, and causes of bowel dilatation other than mechanical obstruction. - Examples of abnormal findings including pneumoperitoneum, gas in abscesses or the bile duct, and dilated loops of bowel in small bowel obstruction. - Assessing the liver and spleen size and positioning of other abdominal organs on a plain film.

1. L8: Abdominal Plain
Important things about plain abdomen

2. Plain Abdomen
The standard plain film of the abdomen is a supine anteroposterior
(AP) view.
Erect films offer little further diagnostic information at the expense of
increased radiation exposure to the patient. Free intraperitoneal air can
be seen on an erect chest radiograph (CXR), which is usually
performed at the time of the abdominal x-ray.
Rarely, in patients who are unable to sit or stand, a lateral decubitus
view (i.e. an AP film with the patient lying on his or her side) is
performed using a horizontal x-ray beam as a means of detecting free
intraperitoneal air.

3. There are a number of points to be considered when looking at a
plain abdominal film:
Analyze the intestinal gas pattern and identify any dilated portion of the
gastrointestinal tract.
Look for gas outside the lumen of the bowel.
Look for ascites and soft tissue masses in the abdomen and pelvis.
If there are any calcifications, try to locate exactly where they lie.
Assess the size of the liver and spleen.

4. Fig.: Normal plain abdominal film.
(a) Normal abdomen. The arrows point
to the lateral borders of the psoas
muscles. The renal outlines are
obscured by the overlying colon.
(b) Normal extraperitoneal fat stripe.
Part of the right flank showing the
layer of extraperitoneal fat (arrows),
which indicates the position of the
peritoneum.

5. Intestinal gas pattern
Relatively large amounts of gas are usually present in the stomach and
colon in a normal patient.
The stomach can be readily identified by its location above the
transverse colon, by the band-like shadows of the gastric rugae in
the supine view.
The duodenum often contains air and there may be some gas in the
normal small bowel, but it is rarely sufficient to outline the whole of a
loop. If the bowel is dilated it is important to try and decide which
portion is involved.

6. Dilatation of the bowel
The initial diagnosis of intestinal obstruction is usually made on clinical examination
with the help of plain abdominal films.
Dilatation of the bowel is the cardinal plain film sign of intestinal obstruction, and
the pattern of dilatation is the key to the radiological distinction between small and
large bowel obstruction. In small bowel obstruction, the small intestine is dilated
down to the point of obstruction and the bowel beyond this point is either empty or of
reduced calibre. In large bowel obstruction, the large bowel is dilated down to the
level of obstruction. If the ileocaecal valve, where the ileum joins the colon, is
incompetent, there will also be small bowel dilatation.
Making the distinction between large and small bowel obstruction depends on the
ability to recognize which portions of bowel are dilated.
Dilated small bowel usually lies in the centre of the abdomen within the ‘frame’ of the
large bowel (but the sigmoid and transverse colon may be redundant and may also
lie in the centre of the abdomen, particularly when dilated). When the proximal and
mid small intestine are dilated, the valvulae conniventes (plica circulares) can be
identified.

7. The valvulae conniventes are always closer together and cross the width of the
bowel (the colonic haustra do not), often giving rise to an appearance known as a
‘stack of coins’.
The distal small intestine has a relatively smooth outline and it may be difficult to
distinguish the lower ileum and the sigmoid colon because both may be smooth in
outline.
The radius of curvature of the loops is sometimes helpful: the tighter the curve,
the more likely the loop is to be dilated small bowel.
The colon is recognized by its haustra, which usually form incomplete bands
across the colonic gas shadows. Haustra are always present in the ascending and
transverse colon, but may be absent distal to the splenic flexure. The presence
of solid faeces is a useful and reliable indication of the position of the colon.
The number of dilated loops is another valuable distinguishing feature between
small and large bowel dilatation, because even with a very redundant colon the
numerous layered loops that are so often seen with small bowel dilatation are not
present.

8. If the cause or site of the obstruction is not evident from plain films, and immediate
exploratory surgery is not indicated, then computed tomography (CT) or a
contrast study (either a follow-through for small bowel obstruction or instant
enema for large bowel obstruction) is helpful.
CT can demonstrate the site of obstruction by showing the location of the
transition from dilated to collapsed bowel, and can confirm or exclude a mass at
the site of obstruction. With the increasing availability of CT, contrast follow-
throughs are performed less frequently.
Dilatation of the bowel occurs in a number of conditions. In remembering causes
of mechanical bowel obstruction it is often easiest to think of conditions that: (i)
obstruct the lumen (e.g. gall stone ileus); (ii) affect the bowel wall and cause a
narrowing (e.g. Crohn’s disesase; or (iii) cause extrinsic compression of the
bowel (e.g. adhesions).
Bowel dilatation, however, occurs in conditions other than mechanical bowel
obstruction, notably: paralytic ileus, acute ischaemia and inflammatory bowel
disease. The radiological diagnosis of these phenomena depends mainly on the
pattern of distribution of the dilated loops.

10. Fig.: Small bowel obstruction due to
adhesions.
(a) The jejunal loops are markedly
dilated and show air–fluid levels in
the erect film. The jejunum is
recognized by the presence of
valvulae conniventes.
(b) The ‘stack of coins’ appearance is
well demonstrated in the supine
film. Note the large bowel contains
less gas than normal.

11. Fig.: Large bowel obstruction due to
carcinoma at the splenic flexure. There
is marked dilatation of the large bowel
from the caecum to the splenic flexure.

12. Fig.: Paralytic ileus. There is considerable
dilatation of the whole of the large bowel
extending well down into the pelvis. Small
bowel dilatation is also seen.

13. Fig.: Volvulus of the caecum.
The twisted obstructed caecum and ascending
colon now lie on the left side of the abdomen
and appear as a large gas shadow. There is
also extensive small bowel dilatation from
obstruction by the volvulus.

14. Fig.: Toxic dilatation of the large bowel from
ulcerative colitis.
The dilatation is maximal in the transverse
colon. Note the loss of haustra and islands of
hypertrophied mucosa. Two of these
pseudopolyps are arrowed

15. Pneumoperitoneum
The radiological diagnosis of perforation of the gastrointestinal tract is based on
recognizing free gas in the peritoneal cavity (pneumoperitoneum).
The most common cause of spontaneous pneumoperitoneum is a perforated
peptic ulcer and two-thirds of such cases are recognizable radiologically.
The largest quantities of free gas are seen after colonic perforation, and the
smallest amounts with leakage from the small bowel.
A pneumoperitoneum is very rare in acute appendicitis even if the appendix
has perforated.
Free intraperitoneal air is a normal finding after laparotomy or laparoscopy. In
adults, all the air is usually absorbed within 7 days. In children, the air absorbs
much faster, usually within 24 hours. An increase in the amount of air on
successive films indicates continuing leakage of air.

16. Pneumoperitoneum under the right hemidiaphragm is usually easy to recognize
on an erect CXR as a curvilinear collection of gas between the line of the
diaphragm and the opacity of the liver.
Free gas under the left hemidiaphragm is more difficult to identify because of the
overlapping gas shadows of the stomach and the splenic flexure of the colon. Gas
in these organs may mimic free intraperitoneal air when none is present.

17. Fig.: Free gas in the peritoneal cavity.
On this chest radiograph, air can be seen
under the domes of both hemidiaphragms.
The curved arrow points to the left
hemidiaphragm and the arrow head to the wall
of the stomach. The two vertical arrows point
to the diaphragm and upper border of the liver.

18. Gas in an abscess
Gas in an abdominal or pelvic abscess produces a very variable pattern on
plain films. It may form either small bubbles or larger collections of air, both of
which could be confused with gas within the bowel.
Fluid levels in abscesses may be seen on a horizontal x-ray film.
As abscesses are mass lesions, they displace the adjacent structures; for
example, the diaphragm is elevated with a subphrenic abscess, and the bowel
is displaced by pericolic and pancreatic abscesses. A pleural effusion or
pulmonary collapse/consolidation are very common in association with
subphrenic abscess.
Ultrasound and CT are extensively used to evaluate abdominal abscesses.

19. Fig.: Gas in a right subphrenic abscess.
There are several collections of gas within the
abscess. The largest of these contains a fluid
level (arrowheads). The air–fluid level under
the left hemidiaphragm is normal. It is in the
stomach.

20. Gas in the wall of the bowel
Numerous spherical or oval bubbles of gas are seen in the wall of the large
bowel in adults in the benign condition known as pneumatosis coli.
Linear streaks of intramural gas have a more sinister significance as they
usually indicate infarction of the bowel wall.
Gas in the wall of the bowel in the neonatal period, whatever its shape, is
diagnostic of necrotizing enterocolitis, a disease that is fairly common in
premature babies with respiratory problems.

21. Fig.: Necrotizing enterocolitis in a neonate.
There is intramural gas throughout the colon.

22. Gas in the biliary system
Gas in the biliary system is seen on plain films following sphincterotomy or
anastomosis of the common bile duct to the bowel.
It is also seen with a fistula from erosion of a gall stone into the duodenum
or colon, or following penetration of a duodenal ulcer into the common bile
duct.
Gas may be seen, very occasionally, in the wall or lumen of the gall bladder
in acute cholecystitis from gas-forming organisms.

23. Fig.: Gas in the biliary tree.
The gall bladder (curved arrows) and the
duct system (straight arrows) have been
outlined with air. The patient had an
anastomosis of the common bile duct to
the bowel.

24. Ascites
Small amounts of ascites cannot be detected on plain films.
Larger quantities separate the loops of bowel from one another and displace the
ascending and descending colon from the fat stripes, which indicate the position
of the peritoneum along the lateral abdominal walls (see Fig. 5.1).
The loops of small bowel float to the centre of the abdomen (Fig. 5.11).
In practice, plain films are of very limited value in the diagnosis of ascites as the
signs are so difficult to interpret confidently except when a large amounts of
ascites is present. Ascites is readily recognized at ultrasound or CT.

25. Fig.: Ascites.
Note how the gas in the ascending and
descending colon (arrows) is displaced
by the fluid away from the side walls of
the abdomen.

26. Abdominal calcification
An attempt should always be made to determine the nature of any abdominal
calcification. The first essential is to localize the calcification, as once the organ of
origin is known the pattern or shape of the calcification will usually limit the
diagnosis to just one or two choices.
The most common calcifications are of little or no significance to the patient;
most are phleboliths, calcified lymph nodes, costal cartilages and arterial
calcification.

28. Calcifications in the abdomen are likely to be one of the following:
1. Pelvic vein phleboliths are very common; they may cause diagnostic confusion
in that they may be mistaken for urinary calculi and faecoliths.
2. Calcified mesenteric lymph nodes caused by old tuberculosis are important only
in that they may be difficult to differentiate from other calcifications. Their pattern is
often specific: they are irregular in outline and very dense, and because they lie in
the mesentery they are often mobile. It is usually possible to see that they are
composed of a conglomeration of smaller, rounded calcifications.
3. Vascular calcification occurs in association with atheroma, and generally has a
curvilinear appearance. There is no useful correlation with the haemodynamic
severity of the vascular disease. Calcification is frequently present in the walls of
abdominal aortic aneurysms and, if suspected, further evaluation should be
undertaken with ultrasound.
4. Uterine fibroids may contain numerous irregularly shaped, well-defined
calcifications conforming to the spherical outline of fibroids. Again, the calcification
is by itself of no significance to the patient.

29. 5. Soft tissue calcification in the buttocks may be seen following injection of
certain medicines. These shadows can at times be confused with intra-
abdominal calcifications.
6. Malignant ovarian masses occasionally contain visible calcium. The only
benign ovarian lesion that is visibly calcified is the dermoid cyst, which may
contain various calcified components, of which teeth are the commonest.
7. Adrenal calcification occurs after adrenal haemorrhage, after tuberculosis and
occasionally in adrenal tumours. However, the majority of patients with adrenal
calcification are asymptomatic healthy people in whom the cause of the
calcification is unclear. Only a minority of patients with Addison’s disease have
adrenal calcification.
8. Liver calcification occurs in hepatomas and rarely in other liver tumours.
Hydatid cysts, abscesses and tuberculosis may also calcify over time.
9. Gall stones.
10. Splenic calcification is rarely of clinical significance. It is seen in cysts,
infarcts, old haematomas and following tuberculosis.

30. 11. Pancreatic calcification occurs in chronic pancreatitis. The calcifications are
mainly small calculi within the pancreas. The position of the calcification usually
enables the diagnosis to be made without difficulty.
12. Faecoliths. Calcified faecoliths may be seen in diverticula of the colon or in the
appendix. Appendiceal faecoliths are an important radiological observation, as
the presence of an appendolith is a strong indication that the patient has
acute appendicitis, often with gangrene and perforation. However, only a small
proportion of patients with appendicitis have a radiologically visible appendolith.
13. Renal stones and other calcifications of the urinary tract.

31. Fig.: Calcified phleboliths in the pelvis.
The arrow points to one of the phleboliths.

32. Fig.: Calcified mesenteric lymph nodes from
old tuberculosis (arrows).

33. Fig.: Calcified abdominal aortic aneurysm
(arrows). The aneurysm measured 8 cm in
diameter on the lateral view.

34. Fig.: Calcification in a large uterine fibroid.

35. Fig.: Adrenal calcification (arrow).

36. Fig.: Pancreatic calcification.

37. Fig.: Appendolith.
The oval calcified shadow (arrowhead) is a
faecolith in the appendix. The patient had
perforated appendicitis. Note the dilated
loops of small bowel in the centre of the
abdomen due to peritonitis – the so-called
sentinel loops.

38. Fig.: Dermoid cyst.
Plain film of another patient showing
well-developed teeth within the cyst.

39. Liver and spleen
Substantial enlargement of the liver has to occur before it can be recognized on
a plain abdominal film.
As the liver enlarges it extends well below the costal margin, displacing the
hepatic flexure, transverse colon and right kidney downwards and displacing the
stomach to the left. The diaphragm may also be elevated.
Occasionally, there is a tongue-like extension of the right lobe into the right iliac
fossa. This is a normal variant known as a Reidl’s lobe and should not be
confused with generalized liver enlargement.
As the spleen enlarges, the tip becomes visible in the left upper quadrant below
the lower ribs. Eventually, it may fill the left side of the abdomen and even extend
across the midline into the right lower quadrant. The splenic flexure of the colon
and the left kidney are displaced downwards and medially, and the stomach is
displaced to the right.

40. Abdominal and pelvic masses
Attempting to diagnose the nature of an abdominal mass on a plain film is notoriously
difficult, and ultrasound, CT or magnetic resonance imaging (MRI) are the appropriate
imaging modalities.
The site of the mass, the displacement of adjacent structures and the presence of
calcification are important diagnostic signs but plain films are unable to distinguish
between solid and cystic masses.
An enlarged bladder can be seen as a mass arising from the pelvis, displacing loops of
bowel.
In females, uterine and ovarian enlargements also appear as masses arising from the
pelvis. Ovarian cysts can become very large, almost filling the abdomen and displacing
the bowel to the sides of the abdomen. An ovarian dermoid cyst can be readily identified
on a plain film due to its content of low attenuation fat and often the presence of other
mesenchymal structures within it, such as teeth.
Retroperitoneal tumours and lymph nodes, when large, become visible on plain films.
Renal masses, especially cysts and hydronephrosis, can become large and appear as
masses in the flank. With retroperitoneal masses, the outline of the psoas muscle may
become invisible.

41. Fig.: Mass arising out of the pelvis (arrows)
displacing bowel to the sides of the abdomen.
The mass was a large cystadenocarcinoma of
the ovary.

42. Thank You