Applied Radiology

With its unparalleled ability to depict abnormalities in the bowel wall,mesentery, and abdominal and pelvic viscera, computed tomography (CT) of thesmall bowel has provided extremely useful information for management of smallbowel disorders. Spiral CT provides several defined advantages over standarddynamic CT and has been shown to be of value in the evaluation of majorabdominal organs.1-3 However, there has not been much mention of the role of CTin the detection and staging of pathology of the small bowel.4 This reviewillustrates the clinical value of spiral CT in small bowel evaluation.

Imaging technique

We performed spiral CT of the small bowel using scan parameters of 10 mmslice thickness, 10 mm/s table speed (pitch of 1), 5 mm intervalreconstruction, 120 kVP and 210 mAS. At times when a higher image resolutionwas needed, additional scanning was done with a narrower collimation (5 mm) andslower speed (5 mm/s). A careful technique with oral contrast is required inorder to avoid false wall thickening caused by suboptimal distention.5 Patientswho were not suspected of having a high degree obstruction were given oralcontrast (3% solution of flavored hypaque) in two doses: 500 to 1000 ccapproximately 30 to 45 min prior to the study, and an additional 250 cc at thetime of the study. In patients who were suspected of having partialobstruction, sequential scanning performed immediately after the ingestion oforal contrast was found to be helpful.6,7 Intravenous contrast (100 to 120 ccof Omnipaque® 300 or 350) was injected at a rate of 2 to 3 cc/sec, if itwas not contraindicated. Scanning began within 10 seconds of completion of theinjection, and data acquisition started approximately 50 to 60 seconds afterinitiation of contrast injection.

Normal findings and interpretation of small-bowelCT

The normal small bowel, when imaged axially on CT scans, should have a wallthickness of less than 4 mm.8 The wall should be symmetric and have ahomogeneous attenuation. Valvulae conniventes are commonly seen in the jejunumand are usually visualized in the ileum. The surrounding mensentery should havea fat density (excluding lymph nodes and blood vessels) measuring less than 75Hounsfield units (HU).9

Mural thickening on a CT scan is the hallmark of small-bowel disease.Neoplastic, inflammatory, and vascular disorders of the small bowel arerecognized on CT scans by thickening of the bowel wall. It is important tocharacterize the lesion as to its location and to determine the degree of muralthickening, symmetry of involvement, pattern of contrast enhancement, andsmooth versus irregular or lobulated inner or outer contour. To further narrowthe differential diagnosis, associated findings such as abscess,lymphadenopathy, metastases, and adjacent inflammatory response in themesentery should be sought. Transition of luminal diameter and pattern of foldsalso are helpful for detecting bowel pathology.

The diseased small bowel

Benign diseases manifesting bowel thickening-Most benign intestinal lesionscause circumferential and symmetric mural thickening, usually of less than 1cm. The bowel wall will show either a homogeneous soft-tissue density oralternate rings of high and low density, known as the "double halo"or target sign (figure 1). These different densities are secondary tosubmucosal edema and/or fat deposition and are best appreciated during thearterial phase of enhancement using spiral CT. They can be found in somephysiologic and diseased states such as Crohn's disease, ischemic enteritis,infectious enteritis, radiation enteritis, eosinophilic gastroenteritis,Henoch-Schönlein purpura, and bowel edema associated with portalhypertension. In mural hemorrhage, the wall is thickened with areas of highdensity on the precontrast image.10

In benign disease, the involvement of the small bowel is usually segmentaland the adjacent mesenteric fat is often thickened, with a streaky,higher-density appearance. With progressive disease, the bowel wall may becomethicker (1 to 2 cm), but the symmetric, circumferential involvement andsegmental distribution are maintained.11

Inflammatory disease-Inflammatory diseases of the gut can cause full muralthickening when the disease has spread beyond the mucosa, manifesting itself onCT. However, this finding is nonspecific because it is also common inneoplastic and ischemic disorders. When inflammatory disease is suspected, thefat of the adjacent mesentery should be carefully examined. The involved fatusually will show a hazy density and linear stranding. Additionally, phlegmonsor frank abscesses may develop (figure 2).11-13

The extramucosal complications of Crohn's disease, such as "creepingfat" of the mesentery, phlegmon, abscess, and fistula, are well depictedon CT scans (figure 3). Inflammatory strictures of the small intestine cancomplicate peptic ulceration, Crohn's disease, potassium chloride toxicity,eosinophilic enteritis, radiation injury, ischemia, surgical anastomoses,celiac disease, and NSAID enteropathy. These strictures are usually short,tapered, and few in number.14

Infections of the small bowel can lead to increased secretions, muralthickening, lumen dilatation, and fold thickening. Clinical history andancillary findings such as ascites, solid organ disease, adenopathy, andmesenteric changes may help clarify the diagnosis.11,15

Ischemic disease-Mesenteric ischemia is a devastating disease which demandsa prompt diagnosis and quick decisions for treatment. Ischemic bowel diseasetypically produces mild (5 to 10 mm), circumferential, and symmetric muralthickening with segmental distribution. The wall may have a homogeneous ordouble halo density (figure 1).11 Mesenteric and intraperitoneal blood andsmall bowel dilatation with congestive changes in the mesentery also may bepresent. Using proper intravenous contrast techniques and rapid scanning withspiral CT may add some valuable information in detecting small bowelischemia.16 With contrast enhancement, thrombus may be visible in the superiormesenteric and portal vein and, rarely, superior mesenteric artery. Also, whendealing with infarction, pneumatosis and portal venous gas can be identifiedwith greater accuracy on contrast-enhanced CT than on plain films becauseoverlap of other bowel loops is not a problem and because CT is sensitive inidentifying gas density.

CT scans are increasingly used as screening tests in patients with acuteabdominal pain. However, whether it is rational to image patients with aconstellation of symptoms suggesting mesenteric ischemia using the CT scanremains unanswered. In previous reports, the incidence of the specific signs ofmesenteric ischemia demonstrated by CT ranges from 26 to 77%.17,18 Predictingthe reversibility of the process in mesenteric ischemia is becoming important,as conservative therapy using intraarterial urokinase or papaverine infusionsis preferable in a relatively stable patient. Bowel dilatation and abnormal gasin the bowel wall or the portal system are clues for predicting irreversibilityof bowel ischemia. If bowel dilatation is extensive, or if it exceeds 35 mm inthe maximum diameter, the chance of having reversible ischemia lessens.Therefore, it should be emphasized that patients with these findings arecandidates for surgery.18

Obstruction-The diagnosis of bowel obstruction is traditionally made on thebasis of clinical findings, history, plain films of the abdomen, and contraststudies of the gut. Dilatation of the small bowel is a common finding inpatients with various systemic or regional processes. CT can be useful forevaluating small-bowel dilatation and can aid in both the diagnosis ofsmall-bowel obstruction and the confident differentiation from other conditionsresulting in small-bowel dilatation. Megibow and colleagues found that CT had a94% sensitivity, 96% specificity, and 95% accuracy in diagnosing bowelobstruction. Furthermore, CT definitively identified the cause of obstructionin 73% of patients in their series.19

In clinically equivocal small bowel obstruction, both CT and small bowelenema (SBE) can be used. While SBE is more accurate in identifying the presenceand location of obstruction, CT is superior for detection of the cause of smallbowel obstruction and also of the presence of strangulation (figure 4).20Furthermore, spiral CT volume sets allow for a detailed display of data, whichcan enhance the ability to determine the cause of small bowel obstructiveprocesses.4,21

CT findings of obstruction vary with the cause of the obstruction.Intussusception is almost invariably associated with either acute intestinalobstruction or partial and recurrent obstruction, air-fluid levels, andproximal bowel distention. The mesenteric arcade associated with anintussuscepted loop may show tractions as it accompanies this eccentricallyplaced region of mesentery. Closed loop obstruction is indicated on CT scans bya characteristic "u" shaped configuration of a distended loop ofbowel, with collapsed bowel distal to the obstruction; mural edema andhemorrhage also may be present.

When the afferent loop of a Billroth II gastrojejunostomy becomes obstructed(afferent loop syndrome), it can appear as a cystic mass in the right upperquadrant and epigastrium. Additionally, hernias are a major cause of mechanicalsmall bowel obstruction, and CT is useful in depicting the precise site andtype of hernia and its contents.11

Neoplasms-The hallmarks of a neoplastic small bowel lesion are eccentric orasymmetric mural thickening, a lobulated inner and outer contour, and/or afocal soft-tissue mass exceeding 2 cm from the lumen to the serosal surface(figure 5). The lumen is narrowed, the outer contour of the mass is oftenspiculated, and there is abrupt transition between normal and abnormal gutwall. The presence of mesenteric, retroperitoneal, and liver metastases;regional adenopathy; and/or malignant-appearing ascites confirms the presenceof a malignant neoplasm.11,22

Small bowel tumors often show relatively specific morphologic features onCT. Adenocarcinomas typically manifest as solitary soft-tissue masses whichcause lumen narrowing and obstruction. Leiomyomas and leiomyosarcomas share acharacteristic pattern of a bulky lesion that grows eccentrically and sometimescalcifies (figure 6). When larger that 4 cm, these lesions may have alow-attenuation center. Carcinoids present with radiating soft-tissue strandsin the mesentery, along with displacement of small bowel loops and a smallmesenteric mass (figure 7). Lymphomas present with homogeneous mural thickeningof greater than 2 cm and frequently are associated with a normal-sized orenlarged lumen (figure 5). Lipoma appears on CT as a well-circumscribed,intraluminal homogeneous mass with fat attenuation.11

CT has a detection rate of 80% for small bowel tumors and has been found toprovide accurate preoperative staging in 61% of cases.23 With the increased useof spiral CT for the evaluation of small bowel pathology, it has becomeaccepted that the ability to scan at the optimal vascular enhancement of spiralCT may be important in detecting and defining the extent of tumor involvement,thus resulting in more accurate staging.4

Blunt bowel and mesenteric injuries-CT can detect small bowel and mesentericinjuries, manifesting mural and/or mesenteric hemorrhage, intraperitoneal fluidand/or free air, and associated organ injury (figure 2). CT also is widelyaccepted now as the method of choice for assessment of solid visceral injuries,but no clear consensus exists in the surgical or radiological literatureregarding the relative merit of CT versus other methods for detection of boweland mesenteric injuries. The principle disadvantage of CT is the time requiredfor an examination, which may be critical in an unstable trauma patient. Insuch patients, abdominal ultrasonography may be used for detectingintraperitoneal fluid.24

While spiral CT has been shown to be superior to conventional axial CT in avariety of applications in the abdomen, its use has not been validated in theassessment of bowel and mesenteric injuries. In their report, Janzen andcolleagues indicate that spiral CT provides a reasonable level of accuracy fordetection of bowel and mesenteric injuries, with accuracies of 84% for bowelinjuries and 77% for mesenteric injuries;24 however, its ability to determinethe degree of injury severity and need for immediate surgical intervention wasless accurate. It can therefore be surmised that the severity of injury as seenon spiral CT should be used in conjunction with clinical parameters(hemodynamic stability, sepsis, peritonitis, etc.) as the basis for decisionsregarding operative versus non-operative treatment.24-26

Comprehensive imaging of the small bowel

The small intestine is almost entirely the province of the radiologist;only the extreme ends of the small bowel are amenable to endoscopicexamination. In the absence of a high degree of suspicion of true abnormality,the most economic means to reassess the gastrointestinal tract would be abarium study. Endoscopy, however, allows immediate access to tissue for biopsy,allowing definitive diagnosis.

Sonography can be valuable in the evaluation of intestinal disease. The useof sonography to detect bowel wall thickening has been established for variousdisorders, such as inflammatory bowel disease and neoplasms.

By virtue of its lack of radiation, noninvasiveness, and cost-effectiveness,sonography frequently is the first examination performed in the evaluation ofpatients with abdominal complaints. Real-time visualization of bowel movementenhances the detection of obstruction and subsequent strangulation. Also,ultrasound is useful for image-guided interventional procedures, such as biopsyand drainage. But the accuracy of ultrasound is operator-dependent, and bowelgas or body fat may deteriorate the proper evaluation of small bowelpathology.27

CT is able to accurately visualize the entire bowel wall, as well as localextra- enteric and distant changes. In certain clinical situations, (e.g.,ischemia, abscess, obstruction, Crohn's disease), CT should be the initialdiagnostic modality. A suspected finding on the CT scan would then requirefurther evaluation with a small bowel barium examination. Beyond that,intervention either with laparotomy, laparoscopy, or imaging-guidedpercutaneous biopsy would be the next option.

The recent advance of MR techniques, such as single-shot fast spin echoT2-weighted sequences (HASTE), makes routine imaging of the gastrointestinaltract feasible. These techniques arrest bowel motion, expand the dynamic rangeof abdominal tissue signal intensities, and distinguish between intraluminalbowel contents and the bowel wall without oral contrast ingestion.28

Although MR imaging is unlikely to replace radiography and CT for evaluatingsmall-bowel diseases, it may have a future role in this area as the technologyimproves and increases in availability.

Conclusion

In summary, small bowel diseases are relatively common entities that havesome characteristic CT features, though they may share findings with manyentities involving the intestine. With its unparalleled ability to detect theprocess of bowel itself and its surrounding structures, CT of the small bowelprovides extremely useful information in management of small bowel disorders.

Spiral CT provides several defined advantages over standard dynamic CT. Itwould appear that spiral CT volume sets will allow for a detailed display oflesions, and the ability to scan at optimal vascular enhancement enables spiralCT to play a vital role in detecting and defining small bowel pathology. AR

Conclusion

Noncontrast helical CT has quickly become the imaging study of choice inevaluating patients with acute flank pain. This exam can identify both stonesize and location. When renal stones are absent, careful inspection for otherCT findings, such as ipsilateral hydroureter, perinephric edema,hydronephrosis, and renal swelling can provide evi-dence of a recently passedstone. Other pathology mimicking renal colic such as cholecystitis,appendicitis, PID, pancreatitis, and diverticulitis can be identified withoutadditional imaging. Due to its speed, safety, and diagnostic accuracy, NCHCThas rapidly gained acceptance from our radiologists, technologists, clinicians,and administrators. AR

References