Fat halo sign of inflammatory bowel disease

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 and the mucosa.

It should not be confused with the fat ring sign of mesenteric panniculitis (which is also sometimes known as the fat halo sign). 

The submucosal fat halo is nearly pathognomonic of inflammatory bowel disease (Crohn disease and ulcerative colitis) although it has been reported in cytoreductive therapy and graft versus host disease.

As long as the latter two conditions are excluded on history, presence of a fat halo of small bowel (as in this case - see the anterior loop of bowel) and be not only considered equivalent to a diagnosis of Crohn's disease, but also indicates chronicity.

Rectal fat halo suggests chronic ulcerative colitis.

Occasionally a very thin layer of intramural fat can be seen both within the colon and terminal ileum, as of course at the iliocaecal valve, as a variant of normal (see this case).

Differential diagnosis

• target sign :
  • the fat halo sign is separate from target sign which is due to submucosal edema and should be readily differentiable in most cases.
  • on occasion it may be necessary to measure attenuation of the halo which should be around -10HU for fat halo sign and in the positive range in cases of target sign.

See also

• fat ring sign - in mesenteric panniculitis