Fat Halo Sign

The “halo” represents infiltration of the submucosa with fat, between the muscularis and the mucosa. It is nearly pathognomonic of inflammatory bowel disease ( Crohn disease and ulcerative colitis) although it has been reported in cytoreductive therapy and graft vs 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 disease, but also indicates chronicity.
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.
For another image visit Radiopaedia.org here.
References:
1. Mukesh G. Harisinghani “Bowel Wall Fat Halo Sign in Patients Without Intestinal Disease” AJR 2003; 181:781-784
2. J Ahualli “Fat halo sign” Radiology 2007; 242:945–946
Credit: Dr Frank Gaillard
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