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Duodenum – Horizontal part (D3)

The horizontal part of the duodenum (D3), also known as the third part of the duodenum, is a short, approximately 10 cm long segment of the small intestine. It extends from the right of the midline at the L3 vertebral level, passing horizontally to the left, crossing anterior to the inferior vena cava (IVC) and abdominal aorta, and posterior to the superior mesenteric artery (SMA) and vein. It continues into the ascending part (D4) of the duodenum at the left side of the aorta.

This anatomical relationship makes D3 clinically important in conditions such as superior mesenteric artery syndrome (Wilkie’s syndrome), where the duodenum is compressed between the SMA and aorta, leading to obstruction. The D3 segment is retroperitoneal and fixed, making it less mobile compared to other segments.

Synonyms

  • Third part of duodenum

  • Horizontal duodenum

  • Transverse duodenum

Function

  • Continues the passage of chyme from the descending duodenum to the ascending duodenum

  • Participates in digestion and absorption, receiving bile and pancreatic secretions upstream

  • Maintains continuity of duodenojejunal motility

Nerve Supply

  • Parasympathetic: Vagus nerve (via celiac and superior mesenteric plexuses)

  • Sympathetic: Greater and lesser splanchnic nerves via superior mesenteric plexus

Arterial Supply

  • Inferior pancreaticoduodenal artery (branch of SMA)

  • Anastomoses with superior pancreaticoduodenal artery (branch of gastroduodenal artery)

Venous Drainage

  • Venous return via inferior pancreaticoduodenal vein into the superior mesenteric vein → portal vein

MRI Appearance

T1-weighted images:

  • Duodenal wall shows intermediate signal intensity

  • Lumen content (fluid/chyme) variable; fat planes around duodenum clearly visible

T2-weighted images:

  • Fluid within the lumen appears bright (hyperintense)

  • Duodenal wall is intermediate-to-low signal; wall thickening indicates pathology (inflammation, tumor, or compression)

STIR:

  • Suppresses fat, highlighting edema, wall inflammation, or periduodenal pathology

  • Useful for identifying changes in SMA syndrome, Crohn’s disease, or neoplasia

T1 Fat-Saturated (Pre-contrast):

  • Wall shows intermediate signal intensity, standing out against suppressed mesenteric fat

  • Helps differentiate duodenal wall from surrounding vessels and fat

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Duodenal wall enhances uniformly in normal cases

  • Heterogeneous or irregular enhancement indicates pathology (duodenitis, tumors, ischemia)

  • Enhances detection of vascular compression or mass invasion

MRI Non-Contrast 3D Imaging:

  • Provides 3D visualization of duodenum in relation to SMA, aorta, pancreas, and IVC

  • Critical in diagnosing SMA syndrome and congenital anomalies

CT Appearance

CT Pre-Contrast:

  • Duodenum appears as a soft tissue tube crossing anterior to aorta and IVC

  • Can detect calcifications, mural thickening, or gross obstruction

CT Post-Contrast:

  • Wall enhances uniformly, lumen may opacify with oral contrast

  • Demonstrates relationships with SMA, aorta, pancreas, and mesentery

  • Useful for detecting tumors, duodenitis, or external compression

CT Angiography (Abdomen with venous phase):

  • Excellent for visualizing SMA–aorta relationship and identifying SMA syndrome

  • 3D reconstructions show duodenum compressed between SMA and aorta

  • Detects vascular anomalies, tumors, or periduodenal pathology

CT images

Duodenum – Horizontal part (D3) CT axiall anatomy image -img-00000-00000

CT images

Duodenum – Horizontal part (D3) CT axiall anatomy image -img-00000-00000_00001

CT images

Duodenum – Horizontal part (D3) CT coronal image

MRI image

Duodenum – Horizontal part (D3) MRI coronal anatomy image -img-00000-00000