Normal Liver Segments Anatomy on Ultrasound
The liver is divided into eight functionally independent segments according to the Couinaud classification, each with its own vascular inflow, outflow, and biliary drainage. Accurate identification of these segments on ultrasound is fundamental for precise lesion localization, guiding biopsy, and planning hepatic resection or ablation. Radiologists and clinicians must be familiar with the anatomical landmarks that define segment boundaries on cross-sectional and real-time imaging.
Normal Reference Values
| Location | Measurement |
|---|---|
| Segment I (Caudate Lobe) | Located posteriorly, tucked between the IVC and the ligamentum venosum. |
| Segment II | The superior part of the far-left lateral liver. |
| Segment III | The inferior part of the left lateral liver, below Segment II. |
| Segment IV | The Quadrate lobe, divided into IVA (superior) and IVB (inferior), located between the left and middle hepatic veins. |
| Segment V | The inferior part of the right lobe, often found near the gallbladder. |
| Segment VI | The inferior part of the right lobe, located more posteriorly/laterally than Segment V. |
| Segment VII | The superior part of the posterior right lobe. |
| Segment VIII | The superior part of the anterior right lobe, sitting "on top" of Segment V. |
Clinical Significance
Correct segment assignment directly influences surgical and interventional decision-making. A lesion reported simply as "right lobe" is far less actionable than one localized to Segment VI or Segment VIII. Hepatic resections are planned along segmental boundaries to preserve functional parenchyma, and ablation margins are calculated relative to segmental vascular anatomy.
Key anatomical landmarks to internalize include: the middle hepatic vein separating left and right lobes, the left hepatic vein dividing Segments II and III from IV, the right hepatic vein separating anterior (V, VIII) from posterior (VI, VII) right-lobe segments, and the main portal vein bifurcation defining superior from inferior segments. The caudate lobe (Segment I) is unique in draining directly into the IVC, which is why it may be spared or preferentially involved in certain conditions such as Budd-Chiari syndrome.
- Budd-Chiari syndrome — caudate lobe (Segment I) hypertrophy due to independent venous drainage
- Hepatocellular carcinoma — precise segmental localization required for resectability assessment
- Colorectal metastases — segment-level reporting guides surgical planning
- Focal nodular hyperplasia / hemangioma — localization aids follow-up and intervention planning
- Biliary obstruction — segmental involvement informs biliary drainage strategy
Reference: No reference specified.
Imaging Notes
On ultrasound, hepatic veins and portal venous branches serve as the primary landmarks for segmental localization. The three hepatic veins are best visualized in a high transverse subcostal view and define the intersegmental planes. Portal vein branches run centrally within each segment, while hepatic veins course along segmental borders. Color Doppler helps distinguish portal from hepatic venous flow when landmarks are uncertain.
Practical tips: begin with a transverse sweep from the hepatic veins superiorly down to the porta hepatis to mentally map the segments. The gallbladder fossa reliably marks the boundary between Segment V (anteriorly) and Segment IVB (medially). The ligamentum venosum separates the caudate lobe (Segment I) from Segment II, and is identifiable as a hyperechoic line posterior to the left lobe. In obese patients or those with limited acoustic windows, supplemental CT or MRI correlation may be necessary for confident segment assignment.