The comb sign refers to the hypervascular appearance of the mesentery in active Crohn's 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's Disease; however in a known case of Crohns disease would suggest that the disease was extensive advanced and active.
Comb sign may also be seen in lupus mesentric vasculitis.