Figure 8b. Crohn disease in a 26-year-old woman with RLQ pain and fever. (a) Axial T2-weighted fast SE image of the lower abdomen shows marked mural thickening in the distal ileum (arrows) with a layer of increased signal intensity suggestive of submucosal edema. There is prominence, dilatation, and tortuosity of the vasa recta coursing through proliferative mesenteric fat, the comb sign (arrowheads). (b) Axial gadolinium-enhanced 3D fat-saturated T1-weighted GRE image obtained at the same level shows marked mural enhancement (arrows) in the abnormal loop of distal ileum. Linear areas of low signal intensity in the wall correspond to the edema in the bowel wall. The comb sign is better appreciated with gadolinium contrast material (arrowheads). These findings are consistent with an acute flare of Crohn disease.