General

• The small bowel, derived from the fetal foregut and midgut, is comprised of duodenum, jejunum, ileum.  The small bowel generally lies centrally within the abdomen, “framed” by the large bowel.  On imaging, the small bowel can be differentiated from the large bowel based on the presence of plicae circulares (circular folds), which traverse the entire diameter of the lumen. The large bowel does not possess these circular folds, but rather has saccular dividers called haustra. The normal dimensions of the small bowel can be remembered by the “Rule of 3’s”–

•           Bowel wall < 3mm thick

•           Bowel folds < 3mm thick

•           Bowel diameter < 3cm wide

•           No more than 3 air-fluid levels should be present

• The duodenum begins at the pyloris, which lies to the right of midline on frontal plain films, and terminates at the Ligament of Treitz, which lies to the left of midline. Comprised of four portions, the duodenum forms a  “C” shape. The 1^st and 4^th parts are intraperitoneal, while the 2^nd and 3^rd parts are retroperitoneal. The main pancreatic duct (duct of Wirsung) opens at the greater papilla in 2^nd part of duodenum, with the accessory duct (duct of Santorini) opening at the lesser papilla (2 cm above greater papilla). The duodenum is vascularized by foregut and midgut arteries, the superior pancreaticoduodenal artery (celiac axis) and inferior panceaticoduodeal artery (SMA).

• The majority of the small bowel is comprised of the jejunum and ileum.  Both segments derive their blood supply from branches of the SMA and have very similar appearances. Some guidelines, however, when applied to film interpretation, can aid in differentiating these two segments of bowel.  The jejunum begins at Ligament of Treitz and resides predominantly in the left upper quadrant.  It possesses a feathery mucosal pattern (appreciated best on fluoroscopy studies) and is usually larger In caliber than the ileum. Coversely, the ileum resides predominantly in the right lower quadrant, terminating at the ileocecal junction. It is usually larger in caliber than the jejunum, with fewer folds and a smoother (more featureless) mucosal pattern.

Normal small bowel follow-through demonstrates the normal anatomy of the small bowel.