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Duodenal bulb

The duodenal bulb, also called the duodenal cap, is the first part of the duodenum, extending from the pylorus of the stomach to the superior duodenal flexure. It is approximately 2–3 cm long and is the only intraperitoneal portion of the duodenum.

It plays a critical role in regulating the passage of gastric contents into the small intestine and is the most common site of duodenal peptic ulcers. Its location at the junction between the stomach and small bowel makes it a key landmark in gastrointestinal imaging and endoscopy.

Synonyms

  • Duodenal cap

  • First part of the duodenum

  • Superior duodenum

Structure

  • Length: 2–3 cm

  • Shape: Rounded and smooth, often dilated (bulb-like) on imaging

  • Wall layers: Same as other parts of small intestine — mucosa, submucosa, muscularis propria, and serosa

  • Lumen: Receives chyme directly from the stomach through the pylorus

Relations

  • Anteriorly: Gallbladder, liver (quadrate lobe)

  • Posteriorly: Gastroduodenal artery, portal vein, bile duct, pancreas head

  • Superiorly: Liver and gallbladder

  • Inferiorly: Head of pancreas, superior duodenal flexure

Function

  • Acts as a reservoir for gastric chyme entering from the pylorus

  • Neutralizes acidic gastric contents with secretions from pancreas and bile ducts (downstream in the duodenum)

  • Coordinates gastric emptying with intestinal peristalsis

  • Common site for absorption of iron and other nutrients in early digestion

Clinical Significance

  • Duodenal ulcers: Most common in the bulb; may perforate posteriorly into the gastroduodenal artery → massive hemorrhage

  • Perforation: Causes peritonitis and free air under diaphragm

  • Malignancy: Rare but possible, can mimic ulcer disease

  • Imaging relevance: Key site assessed in barium studies, endoscopy, CT, and MRI for ulcer disease, tumors, or inflammation

  • Surgical relevance: Important in gastric bypass, Whipple procedure, and ulcer surgery

MRI Appearance

T1-weighted images:

  • Normal bulb: low-to-intermediate signal intensity wall, lumen may contain variable signal depending on contents (air = void, fluid = low signal, chyme may be intermediate)

  • Fat planes around bulb: bright, aiding localization

  • Ulcer crater: focal contour irregularity with wall thickening

T2-weighted images:

  • Wall: intermediate signal intensity

  • Lumen fluid: bright signal

  • Edema or inflammation: focal or diffuse wall thickening with increased signal intensity

  • Perforation/abscess: bright fluid around the duodenum

STIR (Short Tau Inversion Recovery):

  • Normal wall: low-to-intermediate signal

  • Inflammation/edema: bright signal intensity

  • Periduodenal fat suppressed → highlights edema or abscess

T1 Fat-Sat Post-Contrast:

  • Normal bulb: mild homogeneous enhancement of mucosal lining

  • Ulcer or inflammation: focal or circumferential wall thickening with hyperenhancement

  • Malignancy: irregular, nodular enhancement

  • Abscess: rim enhancement with non-enhancing central fluid

CT Appearance

Non-Contrast CT:

  • Bulb appears as a rounded soft tissue structure anterior to pancreatic head

  • Wall normally thin; lumen may contain air or fluid

  • Ulcer or inflammation: wall thickening ± surrounding fat stranding

  • Perforation: extraluminal air anteriorly or posteriorly

Post-Contrast CT:

  • Normal mucosa enhances uniformly

  • Ulcer: focal mucosal defect, wall thickening, possible contrast extravasation in bleeding

  • Inflammation: circumferential enhancement, periduodenal fat stranding

  • Tumor: irregular asymmetric wall thickening with heterogeneous enhancement

  • Abscess: rim-enhancing fluid collection adjacent to duodenum

MRI image

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CT image

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CT image

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CT image

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