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ReviewBowel Wall Thickening on Transabdominal SonographyHans Peter Ledermann 1, Norbert Brner 2, Holger Strunk 3, Georg Bongartz 1, Christoph Zollikofer 4, Gerd Stuckmann 4he potential value of transabdominal sonography in the diagnosis of bowel diseases is often not sufciently appreciated and is even underestimated. Bowel gas artifacts and the somewhat confusing sonographic appearance of the gastrointestinal tract may render orientation and interpretation of sonographic structures difcult. Bowel wall thickening, the main sonographic correlate of bowel disorders, seems at rst glance a very nonspecic sign, which may explain why inexperienced investigators do not feel condent in the sonographic evaluation of the gastrointestinal tract. However, it has been shown that transabdominal sonography achieves good to excellent results as a directed tool for evaluating potential bowel disorders: appendicitis can be diagnosed with a sensitivity ranging from 80% to 93% and a specicity between 94% and 100% [1, 2]. Reported sensitivity rates for evaluating inammatory bowel disease range between 67% and 96%, with specicities of 7997% [3, 4]. Equal diagnostic accuracy of 84% was found for CT and for sonography in the workup of diverticulitis, with sensitivities of 91% and 85%, respectively, and specicities of 84% and 77%, respectively [5]. Although the sonographic appearance of bowel wall thickening of different diseases sometimes overlaps, careful examination of the thickened1 2 3 4

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bowel segment in context with the clinical information often leads to a limited differential diagnosis or to the correct diagnosis. The differential diagnoses of smallbowel wall thickening and of colonic wall thickening are shown in Appendixes 1 and 2, respectively. This article provides a systematic overview of diseases that may cause bowel wall thickening. Typical sonographic features of these disorders are discussed and compared. A review of the literature further summarizes the reported diagnostic potential of sonography and its limitations.

Technique

Examination of the intestinal tract usually begins with a systematic standardized survey using a curvilinear 3.55-MHz transducer. In patients with localized abdominal pain, however, it may be helpful and timesaving to let patients indicate the position of maximum pain with their ngers on the abdominal wall and begin the examination there. In case of diffuse abdominal pain, the frame of the colon is identied by its strong gas artifacts and is screened from the cecum to the sigmoid colon. The rest of the abdomen is examined in an individual standardized fashion to assure complete coverage of the entire

gastrointestinal tract. If intestinal wall thickening is found, detailed evaluation of the diseased segment is performed with a linear or curved high-frequency (7.513 MHz) transducer. When the affected bowel segment is far from the abdominal surface and when the patient is obese, a fair amount of pressure must be applied to the transducer to get acceptable images. For optimal results, it may be necessary to change the patients position several times during the examination. Only careful methodic examination of the entire abdomen leads to acceptable results; the accuracy of the examination depends largely on the radiologists experience and patience [5]. Ideally, patients fast overnight before the examination, but at least 45 hr of fasting are needed to avoid excessive gas in the intestinal lumen.

Normal Sonographic Bowel Wall Anatomy

The typical sonographic appearance of the normal bowel wall consists of ve concentric, alternately echogenic and hypoechoic layers that we describe from the lumen outward (Fig. 1). First, a small echogenic layer is seen that reects the supercial mucosal interface. The deep mucosa, including the muscularis mucosa, is seen as a second hyper-

Received January 27, 1999; accepted after revision June 7, 1999. Department of Radiology, University Hospital of Basel, Petersgraben 4, CH-4031 Basel, Switzerland. Address correspondence to H. P. Ledermann. Gastroenterologische Gemeinschaftspraxis, Parcusstr. 8, 55116 Mainz, Germany. Department of Radiology, Friedrich-Wilhelms-Universitt Bonn, Sigmund-Freud-str. 25, 53105 Bonn, Germany. Department of Radiology, Kantonsspital Winterthur, Brauerstr. 15, 8400 Winterthur, Switzerland.

AJR 2000;174:107117 0361803X/00/1741107 American Roentgen Ray Society

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Ledermann et al. echoic layer. A third hyperechoic layer is produced by the submucosa and the muscularis propria interface. The muscularis propria is seen as a fourth hypoechoic layer. Finally, the marginal interface to the serosa is seen as the fth small hyperechoic layer. The average thickness of the normal gut wall is 24 mm [6].Inammatory Bowel Disease

The classic sonographic feature of Crohns disease is the target sign (Fig. 2) on transverse images, which means a strong echogenic center surrounded by a relatively sonolucent rim of more than 5 mm. This transmural inammation or brosis can lead to complete circumferential loss of the typical gut wall layFig. 1.4-year-old girl with gastroenteritis. Sagittal sonogram shows normal gut wall layering of rectum (RE) from lumen outward. Note small echogenic layer in lumen that reects supercial mucosal interface (short thin arrow ). Deep mucosa, including muscularis mucosa, is seen as second hypoechoic layer (long thin arrow ). Third broad hyperechoic layer is produced by submucosa and muscularis propria interface (open arrow ). Muscularis propria is seen as fourth hypoechoic layer (short thick arrow ). Marginal interface to serosa is seen as small fth hyperechoic layer (curved arrow ). ASC = ascites in retrovesical space, B = bladder.

ers, which results in a thick hypoechoic rim on axial images. Strictures are shown as marked thickening of the gut wall with a xed hyperechoic narrowed lumen (Fig. 3A), dilatation, and hyperperistalsis of the proximal gut. Periintestinal inammation leads to the creeping fat sign, which appears as a uniform hyperechoic mass typically seen around the ileum and cecum. Mesenteric lymphadenopathy is seen as multiple oval hypoechoic masses, usually in the right lower quadrant. In contrast to other forms of colitis, Crohns disease is suggested by skip areas and involvement of the distal ileum [7]. Possible complications of Crohns disease comprise stulas, abscess formation, mechanical bowel obstruction, and perforation [8]. Abscesses are seen as poorly dened, mostly hypoechoic focal masses that can contain hyperechoic gas (Fig. 3B). Fistulas are a hallmark of Crohns disease and are seen in as many as one third of patients with advanced disease as hypoechoic tracts with gas inclusions connecting bowel loops or adjacent

A

B

Fig. 2.25-year-old woman with Crohns disease who presented with new onset of crampy abdominal pain. A, Transverse sonogram shows concentric echolucent wall thickening producing typical target sign. B, Close-up longitudinal sonogram of same segment as A shows circular hypoechoic wall thickening and loss of stratication that, together with clinical information, led to diagnosis of Crohns disease. C, Small-bowel contrast-enhanced enema shows segmental bowel wall edema (arrow ) with thumbprinting and narrowing of jejunal lumen in left lower abdomen. Diagnosis of Crohns disease was later clinically conrmed.

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Sonography of Bowel Wall

A

B

C

Fig. 3.Complications of Crohns disease. A, Stricture with obstruction in 52-year-old man. Transverse sonogram of ileum (arrows) shows severe narrowing of small hyperechoic central lumen caused by excessively echolucent wall thickening and loss of stratication, indicating scarring of entire bowel wall. B, Hypoechoic ileal abscess (A) in highly hypertrophic and inamed hyperechoic fat of mesentery of 25-year-old woman. C, Sonogram obtained at time of suspected relapse of 31-year-old woman shows hypoechoic stula with small hyperechoic gas inclusion (arrow ).

structures (bladder, abdominal wall, vagina, psoas muscle) (Fig. 3C). Detection of gas bubbles in abnormal locations raises the possibility of stulous communication. In expert hands, the distribution of frank lesions of inammatory bowel disease can be determined with a sensitivity of 7387% on sonography [3, 9, 10]. However, mild lesions that produce less bowel wall thickening are frequently not diagnosed, and the sensitivity for these lesions drops to 52% [3]. These results indicate that sonography cannot replace a contrast-enhanced examination or endoscopy when highly accurate

assessment of the extent of the inammatory lesion is requested [3]. Determination of disease activity by sonography is controversial. Whereas some investigators showed correlation with disease activity [4, 10, 11], others found only a loose correlation between bowel wall thickening and disease activity [9]. The ranges of reported sensitivities and specicities in the diagnosis of Crohns disease are 6796% and 7997%, respectively [4, 1214]. The relatively wide range in the values of sensitivity and specicity may be explained by the use of low-frequency transducers (3.5 MHz) in older studies and the use of high-reso-

lution equipment using 510-MHz broadband linear transducers. In ulcerative colitis, sensitivity reaches 89% and specicity reaches 100% [11]. Differentiation between Crohns disease and ulcerative colitis based on sonographic ndings includes the location of the disease, the presence of skip lesions, and the presence of pericolic abscesses [14]. Bowel wall thickening is usually less marked in ulcerative colitis with preserved stratication [15] (Fig. 4). However, denite differential diagnosis is difcult on transabdominal sonography [4, 16].

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