Figure 4a. Colocolic (double-tracking) fistula. (a) Frontal radiograph from air-contrast barium enema examination in a 50-year-old man 1 month after an episode of acute diverticulitis shows a long-segment narrowing (arrowheads) involving the sigmoid colon. At the distal aspect of the stricture, a second channel (arrow) parallels the colonic lumen, the so-called double-tracking sign. Note additional scattered diverticula. (b) Transverse contrast-enhanced CT scan obtained 1 month earlier than a during an acute episode shows pericolonic inflammatory changes and a small peridiverticular abscess (arrow). Adjacent large diverticulum (arrowhead) may represent the point of eventual fistula reentry. Perforation with a localized fistula was confirmed at surgery and pathologic examination.