What is the anatomy of the portal vein?

Portal Vein Anatomy

Formation and Location

The portal vein is formed by the confluence of the superior mesenteric vein and splenic vein behind the neck of the pancreas at the level of the second lumbar vertebra. 1, 2, 3

• The portal vein, along with the hepatic artery and common bile duct, comprises the porta hepatis 1
• In a transverse ultrasound view of the porta hepatis, the common bile duct and hepatic artery are typically seen anterior to the portal vein 1
• The common bile duct is usually positioned more lateral (to the left on the screen) than the hepatic artery 1

Anatomical Dimensions

• The mean length of the right portal vein is approximately 2.1 cm in males and 1.7 cm in females 3
• The mean length of the left portal vein is approximately 3.5 cm in males and 3.1 cm in females 3
• The average total length of the main portal vein trunk is approximately 50.6 mm 4
• Males have statistically significantly longer right and left portal veins compared to females (p=0.010) 3

Anatomical Variants

Thomson Classification (Most Common)

Type I (most common, 47-90% of cases): Portal vein formed by the simple confluence of superior mesenteric and splenic veins 3, 4, 5

Type II (18.6% of cases): Portal vein formed by the confluence of superior mesenteric, splenic, and inferior mesenteric veins 4, 5

Type III (27.8% of cases): The inferior mesenteric vein joins the splenic vein before the splenic vein joins the superior mesenteric vein 5

Rare Variants

• Nine additional rare variants have been documented, collectively accounting for less than 7% of cases 5
• Extremely rare cases of portal vein duplication with three separate main portal vein trunks entering the liver have been reported 6
• The portal vein is derived embryologically from the vitelline veins, a component of the extraembryonic venous system, and developmental anomalies can occur during this process 2

Clinical Imaging Considerations

Ultrasound Identification

• The portal vein can be located sonographically by tracking the hepatic artery from the celiac axis, tracking the portal vein from the confluence of the splenic and superior mesenteric veins, or following the portal vessels in the liver to the hepatic hilum 1
• The portal vein can be distinguished from adjacent structures by its absence of color-flow Doppler signal when this modality is employed 1

CT Imaging Technique

• Portal phase CT imaging is mandatory to prevent diagnostic pitfalls 1
• Images acquired during the late arterial phase are not optimal for portal vein evaluation 1
• In cases of low portal vein flow, delayed contrast arrival can create the false appearance of a filling defect, mimicking thrombosis 1

Surgical and Interventional Relevance

Knowledge of portal vein anatomical variations is essential for liver transplantation, segmentectomy, pancreaticoduodenal surgeries, preoperative portal vein embolization, and catheter-based interventional procedures. 3, 4, 5

• The portal vein provides approximately three-fourths of the liver's blood supply 3
• Understanding normal portal vein anatomy and its variants prevents surgical adverse events during abdominal operations 5
• Radiologists must be aware of these variations when interpreting abdominal angiographs and performing interventional procedures 3, 4