The fat halo sign refers to a feature seen on CT examination of the abdomen, and represents infiltration of the submucosa with fat, between the muscularis propria and the mucosa. It is characterised by an inner (mucosa) and outer (muscularis propria and serosa) ring of enhancing bowel wall along with a non-enhancing middle layer (submucosa).
The submucosal fat halo used to be considered nearly pathognomonic of inflammatory bowel disease (Crohn disease and ulcerative colitis) or less commonly due to cytoreductive therapy and graft versus host disease.
However, a retrospective review of 100 consecutive CT exams performed for reasons other than gastrointestinal disease found the fat halo sign in 21% of subjects, leading to the conclusion that "in the absence of clinical or radiologic evidence of inflammatory bowel disease, the presence of the fat halo sign may represent a normal finding that is possibly related to obesity." 1
- the fat halo sign is separate from target sign which is due to submucosal oedema and should be readily differentiable in most cases
- on occasion it may be necessary to measure attenuation of the halo which should be around -10 HU for fat halo sign and in the positive range in cases of target sign
- fat ring sign: in mesenteric panniculitis
- 1. Harisinghani MG, Wittenberg J, Lee W et-al. Bowel wall fat halo sign in patients without intestinal disease. AJR Am J Roentgenol. 2003;181 (3): 781-4. AJR Am J Roentgenol (full text) - Pubmed citation
- 2. Ahualli J. The fat halo sign. Radiology. 2007;242 (3): 945-6. doi:10.1148/radiol.2423041600 - Pubmed citation