Wednesday, January 20, 2010

Target and Fat Halo Signs of Bowel

Target Sign

The target sign refers to the enhanced CT appearance of edematous bowel, which demonstrates three layers:
  • Inner: Contrast-enhanced inner mucosa
  • Middle: Submocosal edema
  • Outer: Muscularis propria and/or serosa
The target sign may also be see on nonenhanced CT when the edema is severe.

Differential considerations include:
  • Inflammatory bowel disease (Crohn disease and ulcerative colitis)
  • Ischemic bowel disease
  • Intramural intestinal hemorrhage
  • Vasculitis (Henoch-Schönlein purpura)
  • Infection (infectious and pseudomembranous colitis)
  • Radiation damage (radiation enteritis colitis)
  • Portal hypertension

Fat Halo Sign

A similar sign is the fat halo sign, in which there are also three layers:
  • Inner: Inner mucosa
  • Middle: Fatty infiltration of the submucosa with Housnfield units lower than -10.
  • Outer: Muscularis propria and/or serosa
The inner mucosal layer can be appreciated even without intravenous contrast due to the contrast between the soft tissue of the mucosa and the underlying low-attenuation fat.

Differential considerations include:
  • Chronic inflammatory bowel disease (Crohn disease and ulcerative colitis)
  • Cytoreductive therapy.
  • Graft vs host disease.
  • Normal appearance of underdistended bowel.


  • Ahualli J. The target sign: bowel wall. Radiology. 2005 Feb;234(2):549-50.
  • Ahualli J. The fat halo sign. Radiology. 2007 Mar;242(3):945-6.

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