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Liver Segment III – Left lateral inferior segment

Segment III of the liver (left lateral inferior segment) is defined by Couinaud’s classification as the inferior portion of the left lateral sector. It lies on the anteroinferior surface of the left lobe, extending to the left hepatic margin and adjacent to the falciform ligament medially. Segment III lies inferior to Segment II (left lateral superior segment) and anterior to the gastric impression.

It is supplied by the left portal vein and left hepatic artery branches and drains via tributaries of the left hepatic vein. Segment III is surgically important as part of the left lateral sectionectomy (removal of Segments II and III), frequently performed in living donor liver transplantation and resection for localized tumors.

Synonyms

  • Couinaud Segment III

  • Left lateral inferior segment

  • Inferior part of the left lateral sector

Function

  • Contributes to detoxification, metabolism, and bile production as part of the liver

  • Serves as an important functional unit for resection and transplantation

  • Frequently involved in surgical resections due to its accessible peripheral location

Nerve Supply

  • Autonomic innervation via the hepatic plexus

    • Parasympathetic: vagus nerve fibers

    • Sympathetic: celiac plexus fibers

Arterial Supply

  • Left hepatic artery, specifically its inferior segmental branch

Venous Drainage

  • Portal inflow via left portal vein, inferior branch

  • Hepatic outflow primarily via left hepatic vein into the IVC

MRI Appearance and Signal

T1-weighted images:

  • Parenchyma shows intermediate signal, iso- to slightly hyperintense compared to rest of liver

  • Fat planes along falciform ligament enhance border definition

T2-weighted images:

  • Normal parenchyma shows intermediate to mildly hyperintense signal

  • Lesions such as cysts and hemangiomas appear bright hyperintense

STIR:

  • Suppresses fat, making lesions, edema, or inflammatory changes conspicuous

  • Useful for characterizing infiltrative disease

T1 Fat-Saturated (Pre-contrast):

  • Segment shows intermediate parenchymal signal against suppressed perihepatic fat

T1 Fat-Saturated Post-Contrast (Dynamic Gadolinium):

  • Arterial phase: uniform enhancement in normal parenchyma; hypervascular tumors show bright uptake

  • Portal venous phase: homogeneous enhancement matching background liver

  • Delayed phase: iso- to mildly hyperintense unless fibrosis or tumor present

MRI Non-Contrast 3D Imaging:

  • 3D reconstructions clearly show segment III’s boundaries relative to falciform ligament and stomach

  • Useful in segmentectomy and transplant donor evaluation

Triple-Phase MRI:

  • Arterial phase: brisk, homogeneous enhancement in normal tissue

  • Portal venous phase: uniform enhancement, delineating hypovascular lesions

  • Delayed phase: washout in malignancies, persistent enhancement in fibrotic tissue

CT Appearance

CT Pre-Contrast:

  • Homogeneous soft tissue attenuation, isoattenuating to liver parenchyma

  • Borders visible along falciform ligament and stomach

CT Post-Contrast (Single Phase):

  • Enhances homogeneously with the rest of the left lobe

  • Pathology stands out as hypo- or hyperdense relative to parenchyma

Triple-Phase CT (Liver Protocol):

  • Arterial phase: uniform enhancement in normal parenchyma; hypervascular lesions (HCC, NET metastases) become conspicuous

  • Portal venous phase: homogeneous parenchymal enhancement; hypovascular tumors (mets, cholangiocarcinoma) stand out

  • Delayed phase: uniform density in normal liver; fibrosis, scar, or cholangiocarcinoma may show persistent enhancement

CT images

Left lateral inferior segment — Segment III CT axial anatomy image-img-00000-00000

CT images

Left lateral inferior segment — Segment III CT axial anatomy image-img-00000-00000_00001

MRI image

Left lateral inferior segment — Segment III MRI axial anatomy image-img-00000-00000