The fat halo sign is seen in various diseases of the bowel in which fatty infiltration of the submucosa is present. The sign has been described as typically appearing in patients with chronic inflammatory bowel disease (Crohn disease and ulcerative colitis). Although the fat halo sign can also be seen in a patient undergoing cytoreductive therapy and in graft versus host disease, the observation of this sign in the small intestine is highly diagnostic of Crohn disease and by itself is a sign of a chronic phase. When found in the colon, this sign is associated with the same diseases as those occurring in the small intestine (eg, cytoreductive therapy, graft vs host disease, and Crohn disease). Nonetheless, ulcerative colitis should be included in the differential diagnosis.

The fat halo sign is seen on computed tomographic (CT) scans of the abdomen as a thickened bowel wall demonstrating three layers: an inner and an outer layer of soft-tissue attenuation, between which lies a third layer of fatty attenuation