Fat halo sign | Radiology Signs

Answer:

The fat halo sign is a ring of fat attenuation, typically between minus 10 and minus 100 HU, within the wall of the bowel, lying between the enhancing mucosa and the muscularis propria, so that the wall shows three layers on CT. It represents submucosal fat deposition and indicates a chronic process rather than acute inflammation. It is described in chronic Crohn disease and ulcerative colitis, where it is a marker of long-standing disease and often persists in quiescent segments, and it also occurs in graft-versus-host disease, after radiation, in cytoreductive states and, importantly, as an incidental finding in obese individuals, particularly in the rectum and terminal ileum, where it has no pathological significance. It must be distinguished from the target or double halo sign of acute bowel wall stratification, in which the intermediate layer is of water attenuation because it is oedematous submucosa rather than fat; that appearance is a sign of active inflammation, ischaemia or infection and carries a very different meaning. Correct interpretation therefore depends on measuring the attenuation of the intermediate layer rather than judging it by eye. In the assessment of inflammatory bowel disease, active disease is indicated by mural hyperenhancement, wall thickening, the comb sign of mesenteric vascular engorgement, fibrofatty proliferation and mesenteric lymphadenopathy.

Why is it called so?

The layer of fat forms a halo of low attenuation encircling the bowel lumen within the wall.

Pathophysiology

Repeated cycles of inflammation and repair in chronic inflammatory bowel disease alter the composition of the submucosa, with deposition of adipose tissue in place of the normal loose connective tissue. The mechanism is not fully explained but is related to chronic inflammation, altered adipocyte signalling in the mesentery, corticosteroid exposure and obesity, and the same mesenteric adipose response produces the creeping fat of Crohn disease. Because the mucosa enhances and the muscularis propria is of soft tissue attenuation, the interposed fat layer is displayed as a distinct low-attenuation ring. In acute disease, by contrast, the submucosa is expanded by oedema and inflammatory fluid, which is of water attenuation, giving the stratified target appearance instead.

Alternative names: Fat halo; submucosal fat deposition

Other associated named signs: Target sign of acute mural stratification, the comb sign and creeping fat of Crohn disease

References

1. Wittenberg J, Harisinghani MG, Jhaveri K, Varghese J, Mueller PR. Algorithmic approach to CT diagnosis of the abnormal bowel wall. Radiographics. 2002;22(5):1093-107; discussion 1107-9. PMID: 12235339.