The comb sign refers to the hypervascular appearance of the mesentery in active Crohn disease.  Fibrofatty proliferation and perivascular inflammatory infiltration outline the distended intestinal arcades. This forms linear densities on the mesenteric side of the affected segments of small bowel, which give the appearance of the teeth of a comb. 

The sign may be seen on both CT and MRI.  It is not pathognomic of Crohn disease; however in a known case of Crohn disease would suggest that the disease was extensive advanced and active, however comb sign can be seen in other acute inflammatory conditions of bowel and lupus mesentric vasculitis. 

The comb sign may be used to differentiate active inflammatory condition from lymphoma and metastases, which tend to be hypovascular.

Credits: Vijayakumar Sadhanandham Sir