Liver: Largest gland and largest internal organ (average weight: 1,500 g)

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  Functions

  
  
  
  
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    Processes all nutrients (except fats) absorbed from gastrointestinal (GI) tract; conveyed via portal vein

    
    
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    Stores glycogen, secretes bile

  
  
  
  
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  Relations

  
  
  
  
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    Anterior and superior surfaces smooth and convex

    
    
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    Posterior and inferior surfaces indented by colon, stomach, right kidney, duodenum, inferior vena cava (IVC), and gallbladder

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

  
  
  
  
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    Bare area : Nonperitoneal posterior superior surface where liver abuts diaphragm

    
    
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    Porta hepatis : Portal vein, hepatic artery, and bile duct located within hepatoduodenal ligament

  
  
  
  
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  Falciform ligament

  
  
  
  
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    Extends from liver to anterior abdominal wall

    
    
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    Separates right and left subphrenic peritoneal recesses (between liver and diaphragm)

    
    
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    Marks plane separating medial and lateral segments of left hepatic lobe

    
    
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    Carries round ligament (ligamentum teres), fibrous remnant of umbilical vein

  
  
  
  
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  Ligamentum venosum

  
  
  
  
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    Remnant of ductus venosus

    
    
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    Separates caudate from left hepatic lobe

  
  

Vascular anatomy (unique dual afferent blood supply)

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  Portal vein

  
  
  
  
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    Carries nutrients from gut and hepatotrophic hormones from pancreas to liver along with oxygen

    
    
    
    
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      Contains 40% more oxygen than systemic venous blood

    
    
    
    
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    75-80% of blood supply to liver

  
  
  
  
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  Hepatic artery

  
  
  
  
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    Supplies 20-25% of blood

    
    
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    Liver less dependent than biliary tree on hepatic arterial blood supply

    
    
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    Usually arises from celiac artery

    
    
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    Variations common, including arteries arising from superior mesenteric artery

  
  
  
  
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  Hepatic veins

  
  
  
  
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    Usually 3 (right, middle, and left)

    
    
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    Many variations and accessory veins

    
    
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    Collect blood from liver and return it to IVC

    
    
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    Confluence of hepatic veins just below diaphragm and entrance of IVC into right atrium

  
  
  
  
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  Portal triad

  
  
  
  
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    At all levels of size and subdivision, branches of hepatic artery, portal vein, and bile ducts travel together

    
    
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    Blood flows into hepatic sinusoids from interlobular branches of hepatic artery and portal vein → hepatocytes, which detoxify blood and produce bile

    
    
    
    
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      Blood collects into central veins → hepatic veins

      
      
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      Bile collects into ducts → stored in gallbladder and excreted into duodenum

    
    

  
  

Segmental anatomy

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  8 hepatic segments

  
  
  
  
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    Each receives secondary or tertiary branch of hepatic artery and portal vein

    
    
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    Each drained by its own bile duct (intrahepatic) and hepatic vein branch

  
  
  
  
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  Caudate lobe = segment 1

  
  
  
  
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    Has independent portal triads and hepatic venous drainage to IVC

  
  
  
  
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  Left lobe

  
  
  
  
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    Lateral superior = segment 2

    
    
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    Lateral inferior = segment 3

    
    
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    Medial superior = segment 4a

    
    
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    Medial inferior = segment 4b

  
  
  
  
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  Right lobe

  
  
  
  
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    Anterior inferior = segment 5

    
    
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    Posterior inferior = segment 6

    
    
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    Posterior superior = segment 7

    
    
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    Anterior superior = segment 8

  
  

Capsule

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  Reflective Glisson capsule making borders of liver well defined

Left lobe

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  Contains segments 2, 3, 4a, and 4b

  
  
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  Longitudinal scan

  
  
  
  
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    Triangular in shape

    
    
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    Rounded upper surface

    
    
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    Sharp inferior border

  
  
  
  
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  Transverse scan

  
  
  
  
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    Wedge-shaped tapering to left

  
  
  
  
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  Liver parenchyma echoes are midgray with uniform, sponge-like pattern interrupted by vessels

Right lobe

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  Contains segments 5, 6, 7, and 8

  
  
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  Liver parenchymal echoes similar to left lobe

  
  
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  Sections of right lobe show same basic shape, though right lobe usually larger than left

Caudate lobe

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  Longitudinal scan

  
  
  
  
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    Almond-shaped structure posterior to left lobe

  
  
  
  
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  Transverse scan

  
  
  
  
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    Seen as extension of right lobe

  
  

Portal veins

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  Have thicker reflective walls than hepatic veins; portal veins have fibromuscular walls

  
  
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  Wall reflectivity also depends on angle of interrogation; portal veins cut at more oblique angle, may have less apparent wall

  
  
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  Can be traced back toward porta hepatis

  
  
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  Normal portal flow is hepatopetal on color Doppler; absent or reversal of flow may be seen in portal hypertension

  
  
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  Normal velocity: 13-55 cm/s

  
  
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  Normal diameter: < 13 mm

  
  
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  Portal waveform has undulating appearance due to variations with cardiac activity and respiration

  
  
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  Branches run in transverse plane

  
  
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  Hepatic portal vein anatomy is variable

Hepatic veins

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  Appear as echolucent tubular structures within liver parenchyma with no reflective wall: Large sinusoids with thin or absent wall

  
  
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  Branches enlarge and can be traced toward IVC

  
  
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  Flow pattern has triphasic waveform

  
  
  
  
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    Resulting from transmission of right atrial pulsations into veins

    
    
    
    
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      A wave: Atrial contraction

      
      
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      S wave: Systole (tricuspid valve moves toward apex)

      
      
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      D wave: Diastole

    
    

  
  
  
  
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  Right hepatic vein

  
  
  
  
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    Runs in coronal plane between anterior and posterior segments of right hepatic lobe

  
  
  
  
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  Middle hepatic vein

  
  
  
  
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    Lies in sagittal or parasagittal plane between right and left hepatic lobe

  
  
  
  
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  Left hepatic vein

  
  
  
  
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    Runs between medial and lateral segments of left hepatic lobe

    
    
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    Frequently duplicated

  
  
  
  
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  1 of 3 major branches of hepatic veins may be absent

  
  
  
  
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    Absent right hepatic vein: ~ 6%

    
    
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    Less commonly middle and left hepatic vein

  
  

Hepatic artery

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  Flow pattern has low-resistance characteristics with large amount of continuous forward flow throughout diastole

  
  
  
  
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    Normal velocity of proper hepatic artery: 40-80 cm/s

    
    
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    Resistive index ranges 0.5-0.8, increases after meal

  
  
  
  
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  Common hepatic artery usually arises from celiac axis

  
  
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  Classic configuration: 72%

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

  
  
  
  
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  Variations from classic configuration

  
  
  
  
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    Common hepatic artery arising from superior mesenteric artery (replaced hepatic artery): 4%

    
    
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    Right hepatic artery arising from superior mesenteric artery (replaced right hepatic artery): 11%

    
    
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    Left hepatic artery arising from left gastric artery (replaced left hepatic artery): 10%

  
  

Bile ducts

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  Normal peripheral intrahepatic bile ducts too small to be demonstrated

  
  
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  Normal right and left hepatic ducts measuring few millimeters usually visible

  
  
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  Normal common duct

  
  
  
  
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    Most visible in its proximal portion just caudal to porta hepatis: < 5 mm

    
    
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    Distal common duct should typically measure < 6-7 mm

    
    
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    In elderly, generalized loss of tissue elasticity with advancing age leads to increase in bile duct diameter: < 8 mm (somewhat controversial)

  
  

Designating and remembering hepatic segments

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  Portal triads are intrasegmental, hepatic veins are intersegmental

  
  
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  Separating right from left lobe

  
  
  
  
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    Plane extending vertically through gallbladder fossa and middle hepatic vein

  
  
  
  
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  Separating right anterior from posterior segments

  
  
  
  
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    Vertical plane through right hepatic vein

  
  
  
  
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  Separating left lateral from medial segments

  
  
  
  
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    Plane of falciform ligament

  
  
  
  
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  Separating superior from inferior segments

  
  
  
  
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    Plane of main right and left portal veins

  
  
  
  
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  Segments numbered in clockwise order, as if looking at anterior surface of liver

Transducer

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  2.5- to 6.0-MHz curvilinear or vector transducer generally most suitable

  
  
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  Higher frequency linear transducer (i.e., 7-12 MHz) useful for evaluation of liver capsule and superficial portions of liver

Left lobe

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  Subcostal window with full inspiration generally most suitable

Right lobe

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  Subcostal window

  
  
  
  
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    Cranial and rightward angulation useful for visualization of right lobe below dome of hemidiaphragm

    
    
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    Can sometimes be obscured by bowel gas

  
  
  
  
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  Intercostal window

  
  
  
  
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    Usually gives better resolution for parenchyma without influence from bowel gas

    
    
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    Right lobe just below hemidiaphragm may not be visible due to obscuration from lung bases

    
    
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    Important to tilt transducer parallel to intercostal space to minimize shadowing from ribs

  
  

Because of variations of vascular and biliary branching within liver (common), frequently impossible to designate precise boundaries between hepatic segments on imaging studies

Liver ultrasound often 1st-line imaging modality in evaluation for elevated liver enzymes

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  Diffuse liver disease, such as hepatic steatosis, cirrhosis, hepatomegaly, hepatitis, and biliary ductal dilatation, well visualized on ultrasound

  
  
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  Documentation of patency of portal vein, hepatic vein waveforms, and hepatic arterial velocities helpful in evaluation for etiologies of elevated liver function tests

Liver metastases common

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  Primary carcinomas of colon, pancreas, and stomach commonly metastasize to liver

  
  
  
  
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    Portal venous drainage usually results in liver being initial site of metastatic spread from these tumors

  
  
  
  
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  Metastases from other non-GI primaries (breast, lung, etc.) commonly spread to liver hematogenously

Primary hepatocellular carcinoma

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  Common worldwide

  
  
  
  
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    Risk factors include cirrhosis of any etiology and chronic viral hepatitis B in certain populations

    
    
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    Chronic hepatitis C with stage 3 fibrosis and nonalcoholic steatohepatitis may also have increased risk of hepatocellular carcinoma

    
    
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    Ultrasound commonly used for screening and surveillance in patients at risk for development of hepatocellular carcinoma typically at 6-month intervals