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Figure 5.23. Small-bowel obstruction with bowel necrosis. CT scan of the upper abdomen reveals a loop of small bowel with a thickened, edematous wall. Two foci of intramural air represent air within necrotic bowel or in mesen-teric veins. Air is also seen in larger mesenteric veins at the splenoportal confluence.

Figure 5.23. Small-bowel obstruction with bowel necrosis. CT scan of the upper abdomen reveals a loop of small bowel with a thickened, edematous wall. Two foci of intramural air represent air within necrotic bowel or in mesen-teric veins. Air is also seen in larger mesenteric veins at the splenoportal confluence.

mechanical small-bowel obstruction with resultant small-bowel dilatation proximal to the involved loop. Then, the loop becomes torted, changing from a simple to a closed-loop obstruction. Finally venous congestion develops within the loop [49]. As the venous return is compromised, arterial inflow is not. This accounts for the presence of distended blood vessels in the mesentery noted on CT examinations (Fig. 5.24). Ultimately, frank hemorrhage into the wall or lumen develops. A transudate or exudate that crosses the serosa accounts for the development of peritoneal fluid [50] (Fig. 5.25).

Various authors have used some of the same and some differing CT findings to differentiate ischemic or necrotic bowel from viable tissue [30,47,48,51]. Ha found the highest specificity for decreased contrast enhancement and the serrated "beak" sign [48]. Unfortunately, both radiographic signs are low in sensitivity. Of the signs with relatively high specificity and sensitivity were any unusual course of the mesen-teric vessels, mesenteric vascular engorgement, a large amount of ascites, and a hazy mesentery [48]. Somewhat similarly, Frager et al. found that abnormal small-bowel wall enhancement and the local presence of mesenteric edema or fluid were the best indicators of ischemia [30]. The analysis of Makita et al. revealed that the combined presence of increased mesenteric attenuation (due to hemorrhage or edema), a radial distribution of mesenteric vessels and bowel loops, and ascites were the best discriminators between necrotic and nonnecrotic bowel [51]. When they were present, the CT imaging was both sensitive and specific.

Figure 5.24. Closed-loop obstruction with prominent mesenteric vessels. CT

scan through the midabdomen shows dilated small bowel from an obstruction. Very prominent mesenteric vessels represent the slow flow from a closed-loop obstruction.

Figure 5.24. Closed-loop obstruction with prominent mesenteric vessels. CT

scan through the midabdomen shows dilated small bowel from an obstruction. Very prominent mesenteric vessels represent the slow flow from a closed-loop obstruction.

Figure 5.25. Ischemic bowel with mesenteric and intraperitoneal fluid. CT

scan (of the patient seen later, in Fig. 5.40) reveals free intraperitoneal fluid, interloop or mesenteric fluid, and infiltration of the adjacent omentum in the left midabdomen. Also note the "target" or "halo" appearance of the small-bowel loops due to ischemia.

Figure 5.25. Ischemic bowel with mesenteric and intraperitoneal fluid. CT

scan (of the patient seen later, in Fig. 5.40) reveals free intraperitoneal fluid, interloop or mesenteric fluid, and infiltration of the adjacent omentum in the left midabdomen. Also note the "target" or "halo" appearance of the small-bowel loops due to ischemia.

The importance of correlating imaging findings with appropriate clinical information is exemplified in the evaluation of the postoperative abdomen. Small-bowel obstructions, which occur in the immediate postoperative period, do not usually develop ischemic changes and can be treated conservatively [52,53].

More recent studies have suggested some utility for magnetic resonance (MR) imaging of the small bowel. HASTE imaging, utilizing a Half-Fourier Acquisition Single-shot Turbo spin Echo, allows subsecond imaging per section, resulting in complete examinations in a 20-second breath hold [54]. Normal patients showed a wide range of small-bowel fluid content, from none in 8% of patients to less than 25% of loops filled in 40% of patients to 25 to 50% of loops filled in another 40% of patients. Only 12% of patients had more than half their bowel loops filled with fluid [54]. The jejunum either had more fluid than the ileum (32%) or the same amount of fluid (60%), reflecting the increasing water resorption that occurs as one progresses downstream in the small bowel. The small-bowel diameter in the jejunum ranged from 1.5 to 2.7cm (mean 2.1 cm), and the ileum somewhat less, ranging from 1.3 to 2.5 cm (mean 1.9 cm). The high signal intensity intestinal contents contrasted well with the medium signal intensity wall, even allowing visualization of the valvulae conniventes [54].

A study of the efficacy of HASTE imaging in patients with suspected small-bowel obstruction revealed that the presence of a small-bowel obstruction could be identified correctly by MR images in 26 of 29 cases (90%) [55]. The actual level was correctly identified in 19 of 26 patients (73%). HASTE imaging was much less successful in identifying the cause of the obstruction, being correct in only half these patients [55]. However, when a subgroup of patients with postoperative complications were selected and analyzed separately, the results varied somewhat. The rate of detection of the obstruction remained high (13 of 15, or 87%), while successful determination of the level of the obstruction dropped off to slightly less than half (7 of 15). The detection of the actual cause of the obstruction decreased to only 27% (4 of 15) [55]. Even more recent investigations are centered upon combining MR imaging with enteroclysis, but the results are far too preliminary to report at this time.

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