Small Bowel Anatomy and Radiographic Features

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Study Notes

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.

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

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.

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

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.