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This is the second part of a 3-part article by Drs. Baldwin, Genant, Braver, and Mortele. Part 3 will appear in the March 2012 issue.
The names of many classic radiologic signs derive from analogues to objects commonly encountered in everyday life; eg, the "comb" sign. The use of familiar objects to describe visual findings enables radiologists both to arrive at a correct diagnosis and to effectively convey such diagnostic findings to clinicians.
Comb sign
The "comb sign" is seen in the presence of Crohn's disease. This sign is observed on contrast-enhanced CT or magnetic resonance imaging (MRI) scans. The teeth of the comb in this instance represent engorged small arteries, the vasa recta, perfusing the small bowel (Figure 14).37
The vasa recta of the small bowel seen in Crohn's disease become tortuous and enlarged. They appear as prominent opacities on the mesenteric side of the small bowel. These small arteries become engorged due to increased blood flow to the inflamed small bowel and are accentuated due to the fibrofatty proliferation in the mesentery. The presence of the comb sign suggests an acute exacerbation of Crohn's disease.37
Target sign
The "target sign" represents an enhancement pattern of the bowel wall seen in various disease processes on contrast-enhanced CT or MRI scans. The target-like appearance is formed when a thickened bowel wall demonstrates alternating degrees of attenuation, with an inner and outer layer of higher attenuation and a middle layer of lower attenuation (Figure 15).38
The higher-attenuation inner and outer layers represent the mucosa and muscularis propria, respectively. The high attenuation of these layers is believed to be secondary to contrast enhancement from inflammation. The lower attenuation middle layer is thought to be due to submucosal bowel wall edema.39 Submucosal edema can be seen even on noncontrast-enhanced CT if severe.38,40
The target sign is classically seen in patients with Crohn's disease. It can, however, be seen in many other conditions. This pattern of bowel wall enhancement has been observed in patients with radiation enteritis, GVHD, ischemic bowel, intramural hemorrhage, vasculitides such as Henoch-Schonlein purpura, and pseudomembranous colitis.41-44
Coiled spring sign
In 1929, Artner was the first to describe concentric ring shadows in a barium-filled cecum with a central filling defect. He found that this appearance...