Segments of liver US anatomy

Couinaud Segmental Anatomy (8 Segments)

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The anatomical landmarks for liver segments are based on the Couinaud classification, which divides the liver into eight functionally independent segments. These segments are defined by the distribution of the portal vein, hepatic veins, and bile ducts, and are organized around the central axis of the hepatic and portal structures.

Segment	Location	Main Landmarks
I	Caudate lobe	Posterior to portal vein, near IVC
II	Left superior lateral	Above left portal vein
III	Left inferior lateral	Below left portal vein
IVa/b	Left medial (superior/inferior)	Medial to falciform ligament
V	Right anterior inferior	Below right portal vein, anterior
VI	Right posterior inferior	Below right portal vein, posterior
VII	Right posterior superior	Above right portal vein, posterior
VIII	Right anterior superior	Above right portal vein, anterior

Segment -1

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Segment I, or the Caudate Lobe, is a posterior and functionally independent liver segment in the Couinaud classification. It lies behind the porta hepatis, between the inferior vena cava (IVC) and the ligamentum venosum, with a unique dual blood supply and separate venous drainage.

Anatomical Location
Posterior and superior to the porta hepatis
Anterior to the IVC
Medial to the right lobe, posterior to the left lobe
Extends between the fissure for ligamentum venosum (medially) and caudate process (laterally)
Vascular Characteristics

Vascular Feature	Details
Arterial Supply	From both right and left hepatic arteries
Portal Supply	From both right and left portal veins
Venous Drainage	Directly into IVC via caudate veins (independent of right, middle, or left hepatic veins)

Sonographic Appearance
Appears as a triangular or oval hypoechoic structure between:

Left lobe and IVC

Posterior to ligamentum venosum

Superior to the main portal vein

In cases of caudate hypertrophy, it may compress the IVC (seen in Budd–Chiari syndrome)
Clinical Significance
Frequently hypertrophied in:

Cirrhosis

Budd–Chiari syndrome

Portal hypertension

Important in segmental liver resections
May remain functional in global liver disease due to independent venous drainage
Ultrasound Probe Placement to Visualize Segment-I
Approach 1: Subcostal/Oblique
Patient Position: Supine or slight left posterior oblique
Probe: Curvilinear (2–5 MHz)
Placement:

Just below the xiphoid process, angled posterior-superiorly toward the left shoulder

Rotate slightly to align with the ligamentum venosum–IVC axis

Approach 2: Intercostal Approach
Patient Position: Supine or left lateral decubitus
Probe: Curvilinear
Placement:
In right anterior intercostal spaces

In right anterior intercostal spaces

Angle medially and posteriorly toward IVC and left lobe

Look for caudate between IVC and portal vein bifurcation

Segment -2

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Segment II is the superior portion of the left lateral sector of the liver in the Couinaud classification. It lies above the left portal vein, in the left lobe, and is closely related to the diaphragm and anterior abdominal wall in thin individuals.

Anatomical Location
Located in the left lobe
Lateral to the falciform ligament
Superior to the left portal vein
Contacts the diaphragm posteriorly and the stomach anteriorly
Vascular Characteristics

Vascular Feature	Details
Arterial Supply	From the left hepatic artery (superior branch)
Portal Supply	From the left portal vein (superior branch)
Venous Drainage	Drains via the left hepatic vein into the IVC

Sonographic Appearance
Segment II appears as a wedge-shaped area superior and lateral to the left portal vein.

Visualized above the portal vein in transverse or coronal scan

Bordered medially by the falciform ligament

Often seen near the diaphragm in subcostal views

Clinical Significance
Accessible for percutaneous liver biopsy
Common location for subcapsular lesions such as metastases
Involved in left lateral segmentectomy (Segment II & III resection)
Important in pre-surgical and interventional planning
Ultrasound Probe Placement to Visualize Segment-II
Approach 1: Subcostal View
Patient Position: Supine
Probe: Curvilinear (2–5 MHz)
Placement:

Just below the left costal margin angled superiorly toward the left hemidiaphragm

Look lateral to falciform ligament and above left portal vein

Approach 2: Intercostal View
Patient Position: Supine or slight right lateral decubitus
Probe: Curvilinear
Placement:

Left anterior intercostal space

Angle posteriorly and medially toward diaphragm and left lobe

Segment II lies superior to left portal vein and lateral to falciform

Segment -3

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Segment III is the inferior portion of the left lateral sector of the liver in the Couinaud classification. It lies below the left portal vein, in the anterior part of the left lobe, and is in close proximity to the anterior abdominal wall and stomach.

Anatomical Location
Located in the left lobe
Lateral to the falciform ligament
Inferior to the left portal vein
Adjacent to the anterior abdominal wall and stomach
Vascular Characteristics

Vascular Feature	Details
Arterial Supply	From the left hepatic artery (inferior branch)
Portal Supply	From the left portal vein (inferior branch)
Venous Drainage	Drains via the left hepatic vein into the IVC

Sonographic Appearance
Segment III appears as a triangular or wedge-shaped area below and lateral to the left portal vein.

Visualized inferior to the portal vein in transverse or sagittal views

Medially bordered by the falciform ligament

Frequently visible near the anterior abdominal wall

Clinical Significance
Frequently targeted in percutaneous liver biopsy due to superficial location
Commonly involved in left lateral segmentectomy (Segment II & III resection)
Important for surgical resections and tumor localization in preoperative planning
Ultrasound Probe Placement to Visualize Segment-III
Approach 1: Subcostal View
Patient Position: Supine
Probe: Curvilinear (2–5 MHz)
Placement:

Below the left costal margin directed inferomedially

Visualize inferior to the left portal vein and lateral to the falciform ligament

Approach 2: Intercostal View
Patient Position: Supine or right lateral decubitus
Probe: Curvilinear
Placement:

Left anterior intercostal space

Angle inferiorly and medially toward the anterior liver surface

Segment III lies beneath the left portal vein and anterior to stomach

Segment -4

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Segment IV is the medial segment of the left lobe in the Couinaud classification. It lies between the gallbladder fossa and the falciform ligament and is subdivided into IVa (superior) and IVb (inferior). It is central to liver function and surgical resections.

Anatomical Location
Located in the left lobe
Medial to the falciform ligament
Between the gallbladder fossa and the ligamentum teres
IVa is superior to the left portal vein; IVb is inferior
Vascular Characteristics

Vascular Feature	Details
Arterial Supply	From the left hepatic artery (medial branch)
Portal Supply	From the left portal vein (medial branch)
Venous Drainage	Primarily via the middle hepatic vein

Sonographic Appearance
Segment IV appears centrally, anterior to the portal vein bifurcation and between the ligamentum teres and gallbladder fossa.

Visualized in sagittal or oblique subcostal scans

IVa lies above the portal vein; IVb lies below it

Located between left and right lobes on imaging

Clinical Significance
Frequently involved in central hepatectomy
Important in identifying liver mass proximity to gallbladder
Includes segments often resected in left trisegmentectomy
Central to bile duct and vascular mapping in liver surgeries
Ultrasound Probe Placement to Visualize Segment-IV
Approach 1: Subcostal Oblique View
Patient Position: Supine
Probe: Curvilinear (2–5 MHz)
Placement:

Place probe below right costal margin angling medially

Visualize area between gallbladder and falciform ligament

Look for portal vein bifurcation and ligamentum teres

Approach 2: Intercostal View
Patient Position: Supine or left lateral decubitus
Probe: Curvilinear
Placement:

Right anterior intercostal space

Angle medially toward midline and portal bifurcation

Identify middle hepatic vein and adjacent medial segment

Segment -5

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Segment V is part of the anterior sector of the right lobe in the Couinaud classification. It lies inferior to the portal vein and anterior to the inferior vena cava and gallbladder fossa. It plays a key role in gallbladder-related interventions and anterior liver surgeries.

Anatomical Location
Located in the right lobe
Anterior to the right kidney and IVC
Inferior to the right portal vein
Adjacent to the gallbladder fossa
Vascular Characteristics

Vascular Feature	Details
Arterial Supply	From the right hepatic artery (inferior branch)
Portal Supply	From the right portal vein (anterior inferior branch)
Venous Drainage	Primarily via the middle hepatic vein

Sonographic Appearance
Segment V appears anterior and inferior in relation to the portal vein bifurcation.

Visualized in subcostal and sagittal views below the main portal vein

Located near gallbladder; easily seen during cholecystectomy planning

Often seen anterior to IVC in transverse scans

Clinical Significance
Common site for gallbladder bed lesions and surgical resection
Frequently involved in right anterior sectorectomy
Critical for procedures like transhepatic interventions and gallbladder removal
Close proximity to biliary tree increases importance in surgical navigation
Ultrasound Probe Placement to Visualize Segment-V
Approach 1: Subcostal Longitudinal View
Patient Position: Supine
Probe: Curvilinear (2–5 MHz)
Placement:

Probe below right costal margin angled superiorly

Focus on area just inferior to portal vein and medial to gallbladder

Look for IVC and anterior surface of the right lobe

Approach 2: Intercostal View
Patient Position: Supine or right lateral decubitus
Probe: Curvilinear
Placement:

Right lower intercostal space

Angle medially and slightly superior to visualize anterior right lobe

Identify gallbladder and trace liver parenchyma anterior to it

Segment -6

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Segment VI is part of the posterior sector of the right lobe in the Couinaud classification. It lies inferior to the portal vein and posterior to the gallbladder and kidney. It is a key segment for posterior liver surgeries and commonly visualized in intercostal ultrasound views.

Anatomical Location
Located in the right lobe
Posterior to the gallbladder and anterior to the posterior abdominal wall
Inferior to the right portal vein
Adjacent to the right kidney
Vascular Characteristics

Vascular Feature	Details
Arterial Supply	From the right hepatic artery (posterior inferior branch)
Portal Supply	From the right portal vein (posterior inferior branch)
Venous Drainage	Primarily via the right hepatic vein

Sonographic Appearance
Segment VI appears posterior and inferior to the main portal vein and right kidney.

Visualized best using right posterior or intercostal approach

Posterior to the gallbladder and below segment VII

Identified near Morrison's pouch in transverse or coronal views

Clinical Significance
Important for right posterior sectorectomy and trauma assessment
May be involved in posterior hepatic tumors or metastases
Accessible through posterior and lateral percutaneous approaches
Adjacent to kidney—relevant for renal-hepatic pathology correlation
Ultrasound Probe Placement to Visualize Segment-VI
Approach 1: Posterior Intercostal View
Patient Position: Left lateral decubitus or prone
Probe: Curvilinear (2–5 MHz)
Placement:

Right posterior intercostal space

Angled anteriorly and slightly medially

Segment VI appears posterior to right kidney and gallbladder fossa

Approach 2: Subcostal Oblique View
Patient Position: Supine or right lateral decubitus
Probe: Curvilinear
Placement:

Probe placed just below right costal margin

Sweep laterally and posteriorly toward the flank

Visualize liver tissue posterior to gallbladder and kidney

Segment -7

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Segment VII is part of the posterior sector of the right lobe in the Couinaud classification. It lies superior to Segment VI and is positioned posteriorly and superiorly, close to the diaphragm. It is one of the most difficult segments to access due to its deep and posterior location.

Anatomical Location
Located in the right lobe
Posterior and superior to Segment VI
Adjacent to the diaphragm
Posterior to the right hepatic vein and right kidney
Vascular Characteristics

Vascular Feature	Details
Arterial Supply	From the right hepatic artery (posterior superior branch)
Portal Supply	From the right portal vein (posterior superior branch)
Venous Drainage	Primarily via the right hepatic vein

Sonographic Appearance
Segment VII appears in the posterior and superior part of the right lobe, beneath the diaphragm.

Best visualized using intercostal or coronal approach

Located posterior to right hepatic vein and upper pole of right kidney

Diaphragmatic motion can assist in localization during scanning

Clinical Significance
Often involved in subdiaphragmatic abscesses or metastases
Challenging location for surgical or percutaneous interventions
Target for interventional radiology in case of deep posterior lesions
Important in planning right posterior sectorectomy
Ultrasound Probe Placement to Visualize Segment-VII
Approach 1: Right Intercostal View
Patient Position: Left lateral decubitus or upright
Probe: Curvilinear (2–5 MHz)
Placement:

Placed in the right upper intercostal space

Angle posteriorly and superiorly toward the diaphragm

Segment VII appears superior to segment VI and deep to the rib cage

Approach 2: Coronal Oblique View
Patient Position: Right lateral decubitus
Probe: Curvilinear
Placement:

Right flank with longitudinal orientation

Angle slightly superior and posterior

Scan through diaphragm to view segment VII deep in the posterior liver

Segment -8

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Segment VIII is the superior portion of the anterior sector of the right lobe in the Couinaud classification. It lies superior to Segment V and is bordered by the middle hepatic vein. It is centrally located and closely related to the hepatic dome, making it essential in central liver procedures.

Anatomical Location
Located in the right lobe
Superior to Segment V
Anterior to the right hepatic vein
Close to the hepatic dome and diaphragm
Vascular Characteristics

Vascular Feature	Details
Arterial Supply	From the right hepatic artery (anterior superior branch)
Portal Supply	From the right portal vein (anterior superior branch)
Venous Drainage	Primarily via the middle and right hepatic veins

Sonographic Appearance
Segment VIII appears in the superior central region of the right lobe, anterior to the right hepatic vein and beneath the diaphragm.

Visualized best using subcostal or intercostal approach

Located above segment V and below the diaphragm

Can be challenging to visualize due to rib shadowing and diaphragm motion

Clinical Significance
Frequently involved in central hepatic resections
Important in evaluation of centrally located hepatic lesions
Diaphragmatic proximity may result in atypical presentation of subcapsular pathologies
Segment VIII resections often require precise preoperative vascular mapping
Ultrasound Probe Placement to Visualize Segment-VIII
Approach 1: Subcostal Oblique View
Patient Position: Supine
Probe: Curvilinear (2–5 MHz)
Placement:

Just below right costal margin in the midclavicular line

Angled superiorly and medially toward the diaphragm

Segment VIII visualized anterior to right hepatic vein and below diaphragm

Approach 2: Right Intercostal View
Patient Position: Left lateral decubitus
Probe: Curvilinear
Placement:

Right upper intercostal space

Angle medially and slightly superiorly

Segment VIII appears near hepatic dome, anterior to the right hepatic vein