IMAGING

IMAGING

Erect abdominal films are no longer routinely obtained and the radiological diagnosis is based on a supine abdominal film ( Figure 78.12 ). An erect film may subsequently be requested when further doubt exists. -

Figure 78.12 Gas- /f_i lled small bowel loops illustrating valvulae con

niventes; patient supine.

and remaining colon have a characteristic appearance in adults and older children that allows them to be distinguished radiologically . Summary box 78.9 Radiological features of obstruction (on plain radio graph) /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF Fluid levels seen radiologically appear later than gas shad ows as it takes time for gas and fluid to separate ( Figure 78.13 These are most prominent on an erect abdominal radiograph or cross-sectional imaging. In adults, two inconstant fluid le – one at the duodenal cap and the other in the terminal ileum – may be regarded as normal. In infants (less than 1 year old), a few fluid levels in the small bowel may be physiological. In this age group it is di ffi cult to distinguish large from small bowel in the presence of obstruction because the characteristic fea seen in adults are not present or are unreliable. During the obstructive process, fluid levels become more conspicuous and more numerous when paralysis has occurred. When fluid levels are pronounced, the obstruction is advanced. In the small bowel, the number of fluid levels is dir ectly propor tional to the degree of obstruction and to its site, the number increasing the more distal the lesion. In patients without evidence of strangulation there is a role for other imaging modalities. The appearance of contrast in the colon 4–24 hours after administration of 50–100 /uni00A0 mL of wa ter-soluble contrast agent had a sensitivity of 96% and a specificity of 98% in predicting resolution of small bowel obstruction. If contrast does not reach the colon, surgery is required in approximately 90% of patients. Administration of a water-soluble agent was also e ff ective in reducing the need for surgery and shortening the duration of hospital stay . Low colonic obstruction does not commonly give rise to small bowel fluid levels unless advanced, whereas high colonic obstruction may do so in the presence of an incompetent ileo caecal valv e. Colonic obstruction is usually associated with a large amount of gas in the caecum. A limited water-soluble enema can be undertaken to di ff erentiate large bowel obstruc tion from pseudo-obstruction. A barium f ollow-through is contraindicated in the presence of acute obstruction and may be life-threatening. - - CT scanning is now used very widely to investigate all ). forms of intestinal obstruction. It is highly accura te and its only limitations are in diagnosing ischaemia. Two features vels may be helpful when looking for intestinal ischaemia: reduced enhancement of the bo wel wall and absence of mesenteric oedema. It is important to remember that, even with the best imaging techniques, the diagnosis of strangulation remains primarily clinical. tures Summary box 78.10 CT features of strangulation /uni25CF - /uni25CF /uni25CF Impacted foreign bodies may be seen on abdominal radio - graphs. It is noteworthy that gas-filled loops and fluid levels in the small and large bowel can also be seen in established paralytic ileus and pseudo-obstruction (see Chapter 73 ). The former can, however, normally be distinguished on clinical grounds whereas the latter can be confirmed radiologically . Fluid levels may also be seen in non-obstructing conditions - such as gastroenteritis, acute pancreatitis and intra-abdominal sepsis. -

The obstructed small bowel is characterised by straight segments that are generally central and lie transversely. No/ minimal gas is seen in the colon The jejunum is characterised by its valvulae conniventes, which completely pass across the width of the bowel and are regularly spaced, giving a ‘concertina’ or ladder effect Ileum – the distal ileum is featureless A distended caecum appears as a rounded gas shadow in the right iliac fossa Large bowel, except for the caecum, shows haustral folds, which, unlike valvulae conniventes, are spaced irregularly, do not cross the whole diameter of the bowel and do not have indentations placed opposite one another Small bowel /f_l uid levels may be seen on an erect abdominal radiograph Figure 78.13 Fluid levels with gas above; ‘stepladder pattern’. Ileal obstruction caused by adhesions (erect abdominal radiograph). Reduced bowel wall enhancement on CT increases the probability of strangulation Absence of mesenteric /f_l uid on CT decreases the probability of strangulation The clinical reliability of other CT signs is doubtful for predicting strangulation