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Signs in Imaging |
1 From the Department of Radiology, Centro Radiológico Luis Méndez Collado, Muñecas 444, San Miguel de Tucumán, Tucumán 4000, Argentina. Received June 28, 2003; revision requested September 9; revision received September 23; accepted October 21. Address correspondence to the author (e-mail: joahualli@sinectis.com.ar).
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The lower attenuation of the middle layer is believed to result from edema (thought to be the dominant component of this layer) and is assumed to be located in the submucosa (2). The target sign indicates hyperemia in the mucosa and the muscularis propria, serosa, or both with submucosal edema or inflammation (4–6). It is uncertain, however, whether either layer represents the true histologic boundaries of those structures, particularly the mucosa (2).
The different layer attenuations are better appreciated during the late arterial and early portal venous phases of intravenous contrast material enhancement. This sign may be totally absent during an examination performed without intravenous contrast material, with slower and/or lower volume injections, or with delayed (>2 minutes) intravenous contrast-enhanced CT (1,4).
When submucosal edema is severe, the target sign may be demonstrated at nonenhanced CT (4). The high attenuation of the mucosal layer is best demonstrated when the bowel is distended with water-attenuation contrast material (2).
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The target sign was reported in patients with Crohn disease, but it has since been observed in patients with other benign entities (1,7). Some diseases with which this sign has been associated include ischemic bowel disease (ischemic enteritis and ischemic colitis), intramural intestinal hemorrhage, idiopathic inflammatory bowel diseases (Crohn disease and ulcerative colitis), vascular disorders (Henoch-Schönlein purpura), infectious diseases (infectious colitis and pseudomembranous colitis), radiation damage (enteritis and radiation colitis), and bowel edema associated with portal hypertension (1,2,7–9).
The target sign does not allow a specific diagnosis, but it does allow one to predict that, since the sign uncommonly occurs with malignancy, the thickened bowel wall is most likely caused by inflammatory disease as opposed to neoplasm (1,2,6). A notable exception to this general rule is the occurrence of the target sign in cases of infiltrating scirrhous carcinoma of the rectum (4).
The most common CT finding in bowel ischemia is bowel wall thickening, which is sometimes associated with the target sign (10–12). This sign has been well described as an early and nonspecific CT finding of intestinal ischemia (1,9,10,12). In a study by Macari et al (9), it was present in nine of 19 patients (47%) with intestinal ischemia.
Ischemic colitis is considered a form of nonoclusive ischemic disease that is usually seen in older patients and that manifests without a marked sex predilection. Ischemic colitis occurs when the blood flow to the colon is disrupted, usually as a result of hypoperfusion and vasospasm of the sphanchnic arteries (7). This condition is generally suspected on the basis of clinical and CT findings. In a study by Balthazar el al (13), the target sign was present in the affected colonic wall in 13 of 54 patients (24%) with ischemic colitis.
Intramural intestinal hemorrhage is usually diagnosed when CT depicts circumferential and symmetric bowel wall thickening in patients who are undergoing anticoagulation therapy or who have an underlying bleeding diathesis (4). However, the target sign may also be seen in patients with intramural hemorrhage. In another study (9), the target sign was present in seven of 18 bowel segments (39%) with intramural hemorrhage.
At CT, the most frequent finding in inflammatory bowel disease (Crohn disease and ulcerative colitis) is bowel wall thickening (7). During the acute, noncicatrizing phase of Crohn disease, the small bowel and colon show mural stratification and often have a target appearance. Because the target sign represents submucosal edema, the pathophysiologic basis of this sign is similar in Crohn disease and other entities (6).
Inflamed mucosa and serosa may demonstrate substantial contrast enhancement after the intravenous administration of contrast material, and the intensity of enhancement seems to correlate with the clinical activity of the disease (6,14). When patients with long-standing Crohn disease (chronic phase) develop transmural fibrosis, the target sign is no longer present at intravenous contrast-enhanced CT (4,14,15).
A low-attenuation layer in the bowel wall caused by submucosal fat deposits (known as the fat halo sign) is seen in patients with either ulcerative colitis and Crohn disease (4,7). The lower attenuation of intramural fat can usually be distinguished from the grayer appearance of the target sign (2).
In conclusion, the target sign can be seen at transverse CT of the abdomen after the intravenous administration of contrast material. While nonspecific, the target sign does allow one to predict that the presence of a thickened bowel wall is almost always the result of inflammatory disease.
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This article has been cited by other articles:
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  M. d'Almeida, J. Jose, J. Oneto, and R. Restrepo Bowel Wall Thickening in Children: CT Findings RadioGraphics, May 1, 2008; 28(3): 727 - 746. [Abstract] [Full Text] [PDF]
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 J. Ahualli The Fat Halo Sign Radiology, March 1, 2007; 242(3): 945 - 946. [Full Text] [PDF]
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