Liver- Gross and Imaging Anatomy

The liver is the largest gland and largest internal organ (average weight: 1,500 grams). It Processes all nutrients (except fats) absorbed from the GI tract; conveyed via the portal vein, Stores glycogen, secretes bile.

Transverse scan of the liver

Gross Anatomy of Liver

Relations

• The Anterior and superior surfaces are smooth and convex, Posterior and inferior surfaces are indented by the colon, stomach, right kidney, duodenum, inferior vena cava(IVC), gallbladder.

Covered by peritoneum except along gallbladder fossa, porta hepatis, and bare area.

• Bare area: Nonperitoneal posterior superior surface where liver abuts diaphragm. 
• Porta hepatis: Portal vein, hepatic artery, and bile duct are located within the hepatoduodenal ligament.

Falciform ligament: Extends from the liver to anterior abdominal wall

• Separates right and left subphrenic peritoneal recesses (between liver and diaphragm).
• Marks plane separating medial and lateral segments of the left hepatic lobe.
• Carries round ligament (ligamentum teres), a fibrous remnant of the umbilical vein.

Four unique dual afferent blood supply Vascular anatomy. 

1. Portal vein
2. Hepatic artery
3. Hepatic veins
4. Portal triad

1. Portal vein

It Carries nutrients from the gut and hepatotrophic hormones from the pancreas to the liver along with oxygen (contains 40% more oxygen than systemic venous blood).
The portal vein delivered 75-80% of blood supply to the liver

2. Hepatic Artery

It supplies 20-25% of blood to the liver, Liver is less dependent than the biliary tree on hepatic arterial blood supply. It usually arises from the celiac artery. Variations are common, including arteries arising from the superior mesenteric arteries.

3. Hepatic Veins

It Collects blood from the liver and returns it to IVC at the confluence of hepatic veins just below the diaphragm and entrance of IVC into the right atrium, Usually 3 (right, middle, and left), many variations and accessory veins.

4.  Portal triad

At all levels of size and subdivision, branches of the hepatic artery, portal vein, and bile ducts travel together, Blood flows into hepatic sinusoids from interlobular branches of hepatic artery and portal vein → hepatocytes (detoxify the blood and produce bile) → bile collects into ducts, blood collects into central veins → hepatic veins.

Segmental anatomy of liver

In the Liver, there are 8 hepatic segments [divided in left (lobe) and right (lobe) ]

Each receives a secondary or tertiary branch of hepatic artery and portal vein, Each is drained by its own bile duct (intrahepatic) and hepatic vein branch.

 Caudate lobe- (segment 1)

- It Has independent portal triads and hepatic venous drainage to IVC

(Left lobe)

• Lateral superior- (segment 2)
• Lateral inferior- (segment 3)
• Medial superior- (segment 4A)
• Medial inferior- (segment 4B)

(Right lobe)

• Anterior inferior- (segment 5)
• Posterior inferior- (segment 6)
• Posterior superior- (segment 7)
• Anterior superior- (segment 8)

Liver Internal Imaging Anatomy

Left lobe

• It Contains segments 2, 3, 4A, and 4B
• In longitudinal scan, the liver is Triangular in shape, Rounded Upper surface, sharp inferior border.
• In the transverse scan wedge-shaped tapering to left, liver parenchyma interrupted by vessels.

Right lobe

• It Contains segments 5, 6, 7, and 8
• In the Liver, parenchymal echoes similar to the left lobe & Sections of the right lobe show the same basic shape, though the right lobe is usually larger than the left.

Caudate lobe

• In the Longitudinal scan seen Almond-shaped structure posterior to the left lobe.
• In Transverse scan Seen as an extension of the right lobe.

Portal vein & Hepatic vein

Portal Vein has thicker reflective walls than hepatic veins. portal veins have fibromuscular walls, Wall reflectivity also depends on the angle of interrogation portal veins cut at a more oblique angle may have a less apparent wall. It Can be traced back towards porta hepatis, Normal portal flow is hepatorenal on color Doppler; absence or reversal of flow may be seen in portal hypertension. Its Normal velocity 13-55 cm/s, the Portal waveform has an undulating appearance due to variations with cardiac activity and respiration, Branches run in the transverse plane, Hepatic portal vein anatomy is variable.

Hepatic artery

In the hepatic artery, the Flow pattern has low-resistance characteristics with a large amount of continuous forward flow throughout the diastole. The Normal velocity 30-70 cm/s, Resistive index ranges 0.5-0.8, increases after meal Common hepatic artery usually arises from celiac axis Classic configuration: 72%.

– Celiac axis → common hepatic artery → gastroduodenal artery and proper hepatic artery → latter gives rise to the right and left hepatic artery.

○ Variations from the classic configuration

– Common hepatic artery arising from SMA (replaced

hepatic artery): 4%

– Right hepatic artery arising from SMA (replaced right

hepatic artery): 11%

– Left hepatic artery arising from the left gastric artery

(replaced left hepatic artery): 10%

Bile ducts

• Normal peripheral intrahepatic bile ducts are too small to be demonstrated
• Normal right and left hepatic ducts measuring a few millimeters are usually visible
• Normal common duct-Most visible in its proximal portion just caudal to porta hepatis: Less than 5 mm – Distal common duct should typically measure < 6-7 mm
• In the elderly, generalized loss of tissue elasticity with advancing age leads to an increase in bile duct diameter: < 8 mm (somewhat controversial)