Small Bowel Anatomy and Radiographic Features

Small Bowel

• Divided into three sections: duodenum, jejunum, and ileum
• Average length of the jejunum and ileum is 6 meters, ranging from 3 to 10 meters
• Attached to mesentery; the root of the mesentery extends from the duodeno-jejunal junction (left of L2 vertebrae) to the right sacroiliac joint
• Mucosal folds called valvulae conniventes are present, largest in the jejunum and decrease in size distally in the ileum, disappearing in the distal ileal loops.

Radiographic Features (Abdominal X-ray)

• Air/fluid levels are often seen, up to five on an erect abdominal X-ray
• The small bowel generally lies centrally
• Valvulae conniventes are thin, circular folds of mucosa that can be seen passing across the full width of the lumen on an X-ray

Jejunum (Comparison with Ileum)

• Shorter, occupying the proximal 2/5 of the small intestine
• Wider (3-3.5 cm)
• Thicker walls
• Located in the left upper abdomen
• Thicker and more prominent valvulae conniventes

Ileum (Comparison with Jejunum)

• Longer, occupying the distal 3/5 of the small intestine
• Narrower (2.5 cm)
• Thinner walls
• Located in the right iliac fossa
• Thinner and less prominent valvulae conniventes

Barium Studies of Small Bowel Loops

• Small Bowel Follow-Through (SBFT): Performed after an upper GI fluoroscopic study; contrast column moves from stomach and duodenum into the small bowel. Can be done without upper GI first. This shows mucosal detail of Jejunum has high density, with density reduction in the proximal ileum.
• Small Bowel Enema: Naso-jejunal tube is passed to the duodenojejunal flexure and barium is passed directly into the small intestine; good for double contrast images.

Large Bowel

• Approximately 1.5 meters long
• Outer longitudinal muscle layers (taeniae coli) are shorter than the bowel, causing sacculations (haustra)
• Divided into cecum, appendix, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon, and rectum
• Cecum: The starting point, a blind pouch of the ascending colon
• Appendix: A thin tube attached to the cecum
• Ascending Colon: Runs superiorly on the right side of the abdomen from the right iliac fossa to the right lobe of the liver
• Transverse Colon: Most mobile and longest part; found between the right and left colic flexures
• Descending Colon: Extends from the splenic flexure to the sigmoid colon
• Sigmoid Colon: S-shaped loop linking the descending colon to the rectum
• Rectum: From the sigmoid colon to the anal canal, occupying the sacrococcygeal curvature
• Anal Canal: A narrow muscular canal, directed almost perpendicular to the rectum

Radiographic Features (Abdominal X-ray - Large Bowel)

• Retroperitoneal structures of the colon (ascending, descending, and rectum) are relatively constant in position
• The cecum is often the widest part and can vary in position
• Haustra (sacculations) are characteristic of the colon and are not present in the rectum
• Normal large bowel appearance has peripheral position, presence of haustra, and presence of feces.