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GROSS ANATOMY

Overview

•Liver is largest gland and largest internal organ (average weight: 1,500 grams)

○Functions

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

–Stores glycogen, secretes bile

○Relations

–Anterior and superior surfaces are smooth and convex

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

○Covered by peritoneum

– Nonperitoneal posterior superior surface

–Porta hepatis: Portal vein, hepaticartery, and bile duct are located within hepatoduodenal ligament

○ Extends from liver to anterior abdominal wall

– Separates right and left

–

–Carries (ligamentum teres),

○Ligamentum Remnant of ductus venosus

–

•Vascular anatomy (unique dual afferent blood supply)

○Portal vein

–Carries from gut and from pancreas to liver along with oxygen (contains 40% more oxygen than systemic venous blood)

– to liver ○ Hepatic artery

– Supplies 20-25% of blood

–

–Variations are common, including arteries arising from

○Hepatic veins

–Usually 3 (right, middle, and left)

–Many variations and accessory veins

–Collect blood from liver and return it to IVC at confluence of hepatic veins just

○

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

–Blood flows into from of hepatic artery and portal vein → (detoxify blood and produce bile) → bile collects into ducts, blood collects into central veins → hepatic veins

•Segmental anatomy of liver

–Each receives 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 =

–Has

○Left lobe

–Lateral superior = segment 2

–

○Right lobe

–Anterior inferior = segment 5

–Posterior inferior = segment 6

–Posterior superior = segment 7

–Anterior superior = segment 8

IMAGING ANATOMY

Internal Contents

•Capsule

○ well defined

•

○ Contains segments 2, 3, 4A, and 4B ○

–

○

–

○Liver parenchyma echoes are

•Right lobe

○Contains segments 5, 6, 7, and 8

○Liver parenchymal echoes similar to left lobe

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

•Caudate lobe

○Longitudinal scan

–Almond-shaped structure posterior to left lobe

○Transverse scan

–Seen as extension of right lobe

•Portal veins

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

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

○Can be traced back towards porta hepatis

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

○Normal velocity 13-55 cm/s

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

○Branches run in transverse plane

○Hepatic portal vein anatomy is variable

•Hepatic veins

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

○Branches enlarge and can be traced towards IVC

○Flow pattern has a triphasic waveform

–Resulting from transmission of right atrial pulsations into veins

□A wave: Atrial contraction

□S wave: Systole (tricuspid valve moves toward apex)

□D wave: Diastole

○Right hepatic vein

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

○Middle hepatic vein

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

○Left hepatic vein

–Runs between medial and lateral segments of left hepatic lobe

–Frequently duplicated

○1 of 3 major branches of hepatic veins may be absent

–Absent right hepatic vein ~ 6%

–Less commonly middle and left hepatic vein

•Hepatic artery

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

–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 right and left hepatic artery

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

ANATOMY IMAGING ISSUES

Imaging Recommendations

•Transducer

○2.5-5 MHz curvilinear or vector transducer is generally most suitable

○Higher frequency linear transducer (i.e., 7-9 MHz) useful for evaluation of liver capsule and superficial portions of liver

•Left lobe

○Subcostal window with full inspiration generally most suitable

•Right lobe

○Subcostal window

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

–Can sometimes be obscured by bowel gas

○Intercostal window

–Usually gives better resolution for parenchyma without influence from bowel gas

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

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

Imaging Pitfalls

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

CLINICAL IMPLICATIONS

Clinical Importance

•Liver ultrasound often first-line imaging modality in evaluation for elevated liver enzymes

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

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

•Liver metastases are common

○Primary carcinomas of colon, pancreas, and stomach are common

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

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

•Primary hepatocellular carcinoma

○Common worldwide

–Risk factors include chronic viral hepatitis B or C, alcoholic cirrhosis, or nonalcoholic steatohepatitis

–Ultrasound commonly used for screening and surveillance in patients at risk for development of hepatocellular carcinoma (HCC) typically at 6 month intervals

SELECTED REFERENCES

1.Heller MT et al: The role of ultrasonography in the evaluation of diffuse liver disease. Radiol Clin North Am.52(6):1163-75, 2014

2.McNaughton DAet al:Doppler US of the liver made simple. Radiographics. 31(1):161-88, 2011

3.Kruskal JB et al: Optimizing Doppler and color flow US: application to hepaticsonography.Radiographics. 24(3):657-75, 2004

Coronary ligament

Right triangular ligament

Gallbladder

Gallbladder

Porta hepatis

Right renal impression

Bare area

Diaphragm

Left triangular ligament

Falciform ligament

Ligamentum teres

Falciform ligament

Gastric impression

Fissure for ligamentum venosum

Inferior vena cava

Coronary ligament

Adrenal gland

Right triangular ligament

Falciform ligament

Left triangular ligament

Ligamentum venosum

Lateral segment (left lobe)

Falciform ligament

Medial segment (left lobe)

HEPATIC ATTACHMENTS AND RELATIONS

Falciform ligament

Left triangular ligament

Lesser omentum

Coronary ligament

Sulcus for IVC

Right triangular ligament

Right lobe

Rectus abdominis muscle

Left portal vein

Middle hepatic vein

IVC

RIght hepatic vein

Rectus abdominis muscle

Left portal vein

IVC

Rectus abdominis

Left portal vein

Middle hepatic vein Right portal vein

Right hepatic vein

IVC

LEFT LOBE OF LIVER: LEFT PORTAL VEIN

Left lobe lateral segment

Ligamentum venosum

Caudate lobe

Spectral tracing of left portal vein

Right rectus abdominis muscle

Middle hepatic vein

Right hepatic vein

Segment 8

Segment 7

Middle hepatic vein

IVC

Middle hepatic vein

Right hepatic vein

Left rectus abdominous muscle

Left hepatic vein

Left rectus abdominis muscle

Segment 2

Left hepatic vein

Left rectus abdominis muscle

Left hepatic vein

A wave

D wave

S wave

Abdominal muscle

Diaphragm

Heart

Heart

Aorta

Left portal vein

Heart

Left hepatic vein

Junction of IVC and right atrium

LEFT LOBE OF LIVER: LONGITUDINAL VIEW

Left lateral liver

Stomach

Superior mesenteric artery

Celiac artery

Portal vein

Hepatic artery

Falciform ligament