1036 AJR:185, October 2005AJR 2005;185:103610440361803X/05/18541036 American Roentgen Ray SocietyNicolaou et al.Imaging of Acute Small-Bowel ObstructionGas t roi nt es t i nal I magi ng Pi ct or i al Es s ayImaging of Acute Small-Bowel ObstructionSavvas Nicolaou1Brian Kai2Stephen Ho3Jenny Su4Karim Ahamed5Nicolaou S, Kai B, Ho S, Su J, Ahamed KDOI:10.2214/AJR.04.0815Received May 24, 2004; accepted after revision November 19, 2004.1Department of Radiology, Vancouver General Hospital, 899 W 12th Ave., Vancouver, BC, V5Z 1M9, Canada. Address correspondence to S. Nicolaou (snicolao@vanhosp.bc.ca).2University of British Columbia, Vancouver, BC, Canada.3Department of Radiology, Gastrointestinal Radiology, Vancouver Hospital & Health Sciences Centre, Vancouver, BC, Canada.4Department of Internal Medicine, University of British Columbia, Vancouver, BC, Canada.5Department of Diagnostic Radiology, University of Alberta, Edmonton, AB, Canada.OBJECTIVE. The objective of this pictorial essay is to review the different imaging tech-niques used for diagnosing small-bowel obstruction.CONCLUSION. Small-bowel obstruction is a common presentation, for which safe andeffective management depends on a rapid and accurate diagnosis. Conventional radiographs re-mainthefirstlineofimaging.CTisusedincreasinglymorebecauseitprovides essentialdiagnostic information not apparent from radiographs. MRI may play a role in the future astechnology improves and it becomes more readily available.he morbidity and mortality associ-atedwithacutesmall-bowelob-structioncontinuetobesignifi-cant. It accounts for 1216% of allsurgical admissions in patients with acute ab-dominal conditions [1]. Small-bowel obstruc-tioniscausedbypostoperativeadhesionsin70% of all cases [2]. Other common causes in-clude hernias, neoplasms, and Crohns disease[1, 2]. The important question in small-bowelobstructionmanagementliesindeterminingwhetherearlylaparotomyisrequiredorwhetheratrialofnonoperativemanagementshould be instituted [1]. Clinical examinationfindings and laboratory values are often non-specificandunreliableatdifferentiatingsim-plemechanicalobstructionfromstrangulatedbowel. Imaging in the acute setting plays a keyrole.Itcanindicatethelocation,degree,andcause of an obstruction and assess for the pres-ence of ischemia [3]. This pictorial essay aimsto review the various imaging techniques usedinestablishingthediagnosisofacutesmall-bowel obstruction.Conventional RadiographyAbdominal radiography in conjunction withtheclinicalexaminationisdiagnosticinonly5060% of cases [1]. Radiographs have beenshowntobesensitiveforhigh-gradebutnotlow-gradeobstructions[1].Signsofsmall-bowel obstruction on radiographs include dis-tended loops of bowel greater than 3 cm, col-lapsed colon, differential airfluid levels, andthickenedbowelwall(Fig. 1).Thestring-of-pearls sign may also be identified (Fig. 1). It iscausedbyslowresorptionofintraluminalairleavingsmallbubblestrappedbetweenthefoldsofthevalvulaeconniventes.Exceptforinguinalhernias[3]andgallstoneileus(Fig. 2), the cause of obstruction is often indis-cernible on radiographs. Strangulation may beindicated by edematous folds, pneumatosis in-testinalis (Fig. 3A), and gas in the portal vein(Fig. 3B), but these features are rarely seen. Ifa high clinical suspicion of obstruction exists,additionalimagingisrequiredevenifradio-graphsare reported toshow normal findings.Despite its limitations, conventional radiogra-phy continues to be the initial imaging exami-nation for patients with suspected small-bowelobstruction because of its sensitivity in reveal-ing high-grade obstruction [1], wide availabil-ity, and relatively low cost.Contrast StudiesOral contrast studies such as a small-bowelfollow-throughcanofferadditionalinforma-tion regarding the degree of obstruction. Find-ingssuggestiveofobstructionincludedilatedloops of small bowel and a delayed transit timeof barium through a transition point [3]. Limi-tations of small-bowel follow-through includethe length of time required to perform the study,dilutionofbariumbecauseofexcessresidualintraluminal fluid, and the inability of patientsto drink the barium in an acute setting [3].Enteroclysisallowsareasthatarenondis-tensible or fixed to be more easily identified[4].Enteroclysisisperformedbyintubatingthesmallbowelandinfusingcontrastmate-rial, essentially bypassing the stomach. In theTImaging of Acute Small-Bowel ObstructionAJR:185, October 2005 1037Fig. 1Small-bowel obstruction on radiography.A, Supine abdominal radiograph in 45-year-old woman with adhesional small-bowel obstruction shows multiple dilated loops of small bowel. Valvulae conniventes appear prominent. In appropriate clinical context, this would be diagnostic of small-bowel obstruction.B, Upright abdominal radiograph in 56-year-old woman with adhesional small-bowel obstruction shows multiple airfluid levels (arrows) and string-of-pearls sign (arrowhead).A BAFig. 248-year-old woman presenting with gallstone ileus.A, Upright abdominal radiograph shows multiple airfluid levels. Pneumobilia (arrow) is present, as is string-of-pearls sign (arrowheads).B, CT scan through upper abdomen shows air in gallbladder (arrow) and proximal cystic duct.C, CT scan obtained inferior to B shows calcified impacted gallstone (arrow) in distal jejunum with proximal dilated loops of bowel.B CNicolaou et al.1038 AJR:185, October 2005subacute setting, enteroclysis is very accuratein diagnosing low-grade and intermittent ob-structions [4] and can serve as an adjunct toCTifmoreinformation,suchashowmuchcontrastmaterialismakingitswaythroughthe obstruction, is required [4] (Fig. 4).SonographyOn sonography, small-bowel obstruction issuspected if multiple dilated (> 3 cm), fluid-filled loops are seen (Fig. 5). The obstructingcause can occasionally be visualized if it is atumor or hernia. The presence of aperistalsis,fluid-filled bowel distention, and wall thick-eningsupportsinfarctionintheappropriateclinical context [5]. Bowel wall perfusion canalso be assessed by Doppler sonography.Sonographyhasbeenreportedtohaveasensitivityof89%comparedwith71%forconventional abdominal radiography in diag-nosingsmall-bowelobstructionandissupe-rior in its ability to identify features of stran-gulation and to predict the location and causeofobstruction[5].Althoughnotroutinelyused,sonographymaybeindicatedincriti-callyillpatientsbecausetransferofthepa-tienttotheexaminationtablemaybetime-consuming and difficult [5].CTIf an acute obstruction is suspected, CT isthetechniqueofchoiceforseveralreasons.First, it does not require oral contrast mate-rialbecausetheretainedintraluminalfluidservesasanaturalnegativecontrastagent.Second,whencomparedwithenteroclysis,CTisrapid,noninvasive,andreadilyavail-able [3]. Finally, it also allows extramural ar-eas that would not be visible on contrast stud-ies to be assessed.Thediagnosisofsmall-bowelobstructiononCTinvolvesidentifyingdilatedloopsofbowelproximallywithnormal-caliberorA BFig. 3Strangulation.A, Supine abdominal radiograph in 46-year-old woman with ischemic colitis shows linear radiolucency (arrows) along wall of bowel, which is consistent with pneumatosis intestinalis. Dilated loops of small bowel are also present.B, Right-side-up decubitus abdominal radiograph in 69-year-old woman shows multiple branching radiolucencies (arrows) in periphery of liver shadow, which is indicative of portal venous gas. Dilated loops of small bowel are also present, which is consistent with small-bowel obstruction.Fig. 4Enteroclysis. 54-year-old woman with adhesional small-bowel obstruction. Spot film from enteroclysis shows small-bowel loop narrowing (arrow) due to postoperative adhesion.Imaging of Acute Small-Bowel ObstructionAJR:185, October 2005 1039A BFig. 5Sonography features of small-bowel obstruction. Both cases are due to postoperative adhesions.A, Abdominal sonogram in 40-year-old woman shows dilated, fluid-filled loop of small bowel with prominent valvulae conniventes (arrows).B, Abdominal sonogram in 62-year-old man shows thickened small-bowel wall (arrows). Real-time scanning showed small bowel to be hyperperistaltic.A BCFig. 6Small-bowel obstruction secondary to adhesions.A, Axial CT scan through lower abdomen in 54-year-old woman with small-bowel obstruction secondary to adhesions shows multiple fluid-filled loops of small bowel (arrows).B, CT scan obtained inferior to A shows transition point (arrows) with dilated bowel proximally and collapsed bowel distally. No pathologic process is visualized at transition point, and transition is smooth. This obstruction was found to be adhesional in nature.C, Axial contrast-enhanced CT scan through mid abdomen of 55-year-old man with small-bowel obstruction secondary to adhesions shows multiple fluid-filled loops with tapering transition point (arrows), otherwise known as beak sign.Nicolaou et al.1040 AJR:185, October 2005A BFig. 7Small-bowel obstruction secondary to Crohns disease.A, Axial CT scan through lower abdomen of 44-year-old woman with small-bowel obstruction secondary to Crohns disease shows multiple fluid-filled loops of small bowel (arrows) and CT equivalent of string-of-pearls sign on radiography.B, Axial CT scan through lower abdomen in 28-year-old woman with Crohns disease shows partially solid material intermixed with air within distal small bowel (arrows), similar in appearance to feces in colon; this finding is called the small-bowel feces sign.A BFig. 858-year-old woman with small-bowel obstruction secondary to adhesions.A, Axial CT scan through lower abdomen shows dilated proximal loop (arrow) and collapsed distal loop (arrowhead).B, CT scan obtained inferior to A shows narrowing of involved loop of bowel (arrows). Adhesion is inferred to be causing narrowing given history of previous abdominal surgery and given neither masses nor extrinsic processes are seen to result in narrowing. Multiple dilated loops of small bowel are also seen.Fig. 926-year-old woman with vasculitis and small-bowel obstruction. Axial contrast-enhanced CT scan through mid abdomen shows thickened loops of small bowel and target sign (arrows). Free fluid (arrowhead) is also seen.Imaging of Acute Small-Bowel ObstructionAJR:185, October 2005 1041collapsed loops distally. A small-bowel cali-berofgreaterthan2.5cmisconsidereddi-lated [6]. If a transition point is detected, thediagnosisismorecertain[6].Thetransitionpoint often resembles a beak and is describedasthebeaksign(Fig. 6).Thisfindinghasbeenshowntobepresentin60%ofsimplesmall-bowel obstruction cases [7]. Other reli-able features include the string-of-pearls signA BCFig. 1066-year-old woman with diagnosis of ischemic bowel.A, Axial contrast-enhanced CT scan through mid abdomen shows multiple dilated air- and fluid-filled loops of small bowel. There is evidence of pneumatosis intestinalis and lack of bowel wall enhancement (thin arrow) as compared with normally enhancing loop (thick arrow). Also seen is intraperitoneal free fluid (arrowhead). Round radiodensity seen in one loop of small bowel is surgical drain.B, CT scan obtained inferior to A shows air in mesentery (arrowhead), and lack of bowel wall enhancement (arrows) is again seen.C, CT scan obtained superior to A shows air in intrahepatic portal venous vasculature (arrow).Fig. 1157-year-old woman with small-bowel volvulus. Axial CT scan through upper pelvis shows whirl sign (arrow) signifying volvulus. Volvulus can result if loop of bowel is able to rotate around its mesentery. If loop sits in axial plane, it will appear as or shape. If orientation of loop is at right angle to axial plane, appearance will vary depending on slice.Nicolaou et al.1042 AJR:185, October 2005(Fig. 7A)andthesmall-bowelfecessign(Fig. 7B). The small-bowel feces sign is a re-sult of stasis and mixing of small-bowel con-tents and is present in 82% of cases of small-bowel obstruction [2, 3]. Occasionally, visu-alization of an adhesional band is possible, al-though it is rare to be able to do so (Fig. 8).CT has a sensitivity of 8194% and a spec-ificity of 96% for diagnosing high-grade ob-structions[2].However,whenallgradesofsmall-bowelobstructionsaretakenintoac-count, the reliability of CT decreases dramat-ically(sensitivityof64%andspecificityof79%) [2]. Therefore, CT is not the ideal tech-nique for diagnosis of low-grade or subacuteobstructions and should be complemented bya contrast study, ideally enteroclysis [6].ThemostimportantinformationthatCTcan provide the surgeon is whether there is anassociatedstrangulation.Thesensitivityofcontrast-enhanced CT for intestinal ischemiahasbeenreportedtobeashighas90%[1].There are various signs that have been associ-ated with ischemia [3, 7], although their use-fulnessisdebatable.Theseinclude,first,thickened bowel wall (Fig. 9); second, ascites(Fig. 9); third, the target sign, a trilaminar ap-pearance of the bowel wall resulting from IVcontrastenhancementofthemucosalandmuscularislayers,plussubmucosaledema(Fig. 9); fourth, poor or absent enhancementof bowel wall on IV contrast-enhanced scans(Figs. 10Aand10B);fifth,pneumatosisin-testinalis and gas in mesenteric or portal veins(Fig. 10C); sixth, the whirl sign, a twisting ofthemesentericvasculaturesignifyingavol-vulus(Fig. 11);seventh,tortuousengorgedmesenteric vessels (Fig. 12A); eighth, mesen-terichemorrhage;and,finally,increasedat-tenuation of bowel wall on noncontrast scans.Althoughthesesignsareindividuallyin-sufficientlysensitive,theyarequitesugges-tive of ischemia when used together [7]. CT isalso useful in differentiating small-bowel ob-structionfromileusanddeterminingthecause of obstruction (Figs. 11 and 12).Multiplanarreformationsarenowbeingused in difficult cases. Multiplanar views mayhelp identify the site, level, and cause of ob-structionwhenaxialfindingsareindetermi-nate [7] (Figs. 13 and 14). CT enteroclysis, arelativelynewinvestigationaltoolfordiag-nosingsmall-bowelobstruction,canalsobeusedwithmultiplanarreconstructionstoovercome the unreliability of CT for diagnos-inglow-gradeobstructions.CTenteroclysishas a greater sensitivity and specificity (89%and 100%, respectively) than CT alone (50%and 94%, respectively) [1]. At the same time,the 3D imaging provides precise localizationof the pathology [1].MRIMRI provides rapid, accurate identificationofsmall-bowelobstruction[8]andassistsinthedeterminationofcausewithoutexposingthepatienttoradiation.MRIalsoutilizesin-traluminal air as a natural contrast agent and isnot limited by previous administration of bar-ium. The diagnosis of small-bowel obstructionon MRI is similar to CT and involves identify-ing dilated loops of bowel proximal to the ob-struction,adistincttransitionpoint,andnor-mal-caliberorcollapsedboweldistally.Multiplanar capabilities of MRI allow visual-ization of the cause of small-bowel obstruction(Fig. 15). Rapid scanning with MRI using theHASTE sequence can, within seconds, evalu-ate small-bowel obstruction with a high degreeA BFig. 1264-year-old man with small-bowel obstruction secondary to incarcerated right inguinal hernia.A, CT scan shows incarcerated right inguinal hernia resulting in small-bowel obstruction. Left and right arrows point to dilated loop of small bowel with engorged mesentery (middle arrow).B, Inferior transverse CT image obtained at level of symphysis pubis reveals incarcerated thick wall loop of small bowel within right inguinal canal (arrow).Fig. 1380-year-old man with small-bowel obstruction secondary to adenocarcinoma of large bowel. Coronal reformatted 2-mm-thick CT view of abdomen reveals small-bowel obstruction is caused by thick annular constricting mass lesion involving hepatic flexure of large colon (thin arrows) resulting in proximal dilata-tion of cecum (thick arrow) and small bowel (arrow-heads). Pathology revealed colonic adenocarcinoma.Imaging of Acute Small-Bowel ObstructionAJR:185, October 2005 1043A BFig. 1449-year-old man with incarcerated abdominal hernia.A, Axial CT scan shows defect in lower abdominal wall (arrow) that has incarcerated lower abdominal hernia within it (arrowhead).B, Sagittal reformatted MDCT view depicts defect in lower abdomi-nal wall (long thick arrow) and incarcerated lower abdominal hernia. Within hernia sac, thick wall loop of small bowel and free fluid (arrow-head) are noted with dilated loops of small bowel proximal (short thick arrow) to incarcer-ated small-bowel loop. Free fluid (star and thin arrow) is also present in abdomen, which is an associated finding in small-bowel obstruction.ABCFig. 1580-year-old man with small-bowel obstruction secondary to adenocarcinoma of large bowel.A, Transverse gadolinium-enhanced T1-weighted image (TR/TE, 400/10) obtained with fat saturation shows narrowing of large bowel (arrow) caused by mass (small arrowhead) with resultant proximal small-bowel obstruction (large arrowhead).B, Coronal single-shot fast spin-echo T2-weighted image (1,800/103) reveals same constricting mass seen in A but with intermediate signal (thin arrows). Resultant proximal dilatation of large (thick arrow) and small (arrowheads) bowel is visualized.C, Subsequent coronal image reveals numerous proximal dilated loops of small bowel (arrows), which is consistent with diagnosis of small-bowel obstruction.Nicolaou et al.1044 AJR:185, October 2005of accuracy [8]. Furthermore, it avoids artifactsrelatedtoperistalsisandbreathingthathavelimited the diagnostic yield of MRI in small-bowel obstruction in the past [8]. MRI is un-likelytoreplaceCTforevaluatingsmall-bowel obstruction because of longer scanningtimeandinferiorresolution[8];however,asavailability and technology in MRI continuesto improve, it has the potential to be an excel-lentdiagnosticmethodforevaluatingsmall-bowel obstruction.ConclusionSmall-bowel obstruction is a common pre-sentation,forwhichsafeandeffectiveman-agement depends on rapid and accurate diag-nosis. Imaging can be diagnostic and helpful inguiding management. Abdominal radiographyremains the first-line imaging examination be-cause it is readily available, is cheap, and canbe done serially to follow clinical progression.CT can provide additional information such asconfirmation of an obstruction, degree and siteofanobstruction,presenceofischemia,andthe cause of the obstruction. 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