Crohn’s Disease is an inflammatory bowel disease of unknown etiology that is characterized by transmural inflammation of any part of the gastrointestinal tract. The small bowel is the most common site of involvement with the terminal ileum the most common location of small bowel involvement. The transmural inflammation can lead to fibrosis, fistulae, structuring, and microperforations as well as obstructive clinical symptoms. Typical involvement of the GI tract is as follows:
•   80% of patients have small bowel involvement, most commonly at the terminal ileum.
•   30% of patients have exclusively ileitis.
•   50% of patients have ileocolitis
•   20% of patients the affected areas are limited to colon

A small bowel follow through study is typically performed when there is suspected disease that cannot be evaluated by colonoscopy. In a small bowel follow through, signs to look for in the small bowel that are consistent with Crohn’s disease are:
•   Luminal narrowing or structuring with nodularity and ulceration (which can also be seen with Crohn's colitis)
•   A "string" sign, which is when the lumen has been severely narrowed and the barium in the lumen appears as a string (typically in advanced luminal narrowing or spasm).
•   cobblestone appearance
•   fistulae and abscess formation
•   separation of bowel loops (secondary to transmural inflammation with associated bowel wall thickening

Circumferential thickening of the bowel wall is a hallmark of Crohn’s Disease, with skip lesions typically 1-2 cm long with areas of normal bowel separating the diseased portions of the small bowel.