Liver Vascular Anatomy and Doppler Ultrasound - Vocabulary Flashcards

Portal Triad and Mickey Mouse Sign

• Portal triad consists of the common bile duct (CBD), the hepatic artery, and the portal vein. These three structures form the Mickey Mouse sign when visualized in the porta hepatis.
• The Mickey Mouse sign is a key landmark you’ll identify in ultrasound exams of the liver.
• The area around the porta hepatis is more echogenic; the transcript notes discussion about why that area appears brighter (reference to peritoneum/colonic sheath in the dialogue).

Liver Blood Supply and Drainage

• Portal vein supplies about 70\%\text{ to }80\% of the liver’s blood; the hepatic artery supplies the remaining about 20\%\text{ to }30\%.
• Blood exiting the liver leaves via the hepatic veins, which drain into the IVC.
• Distinguish vessels with similar names:
  • Portal veins carry blood into the liver (venous inflow).
  • Hepatic veins drain blood away from the liver into the IVC (venous outflow).
• Important memory aid: portal veins (veins bringing blood in) vs hepatic veins (veins taking blood out).

The Aorta, Celiac Axis, and the Hepatic Arterial System

• The aorta gives off anterior branches; one of the first is the celiac axis (also called celiac trunk).
• The celiac axis trifurcates into three vessels:
  • Common hepatic artery (toward the liver).
  • Splenic artery (toward the spleen).
  • Left gastric artery (not typically visualized in ultrasound).
• The gastroduodenal artery (GDA) branches off the common hepatic artery and feeds the stomach/duodenum; it continues toward the liver and becomes the proper hepatic artery.
• The common hepatic artery bifurcates (within the liver) into the left hepatic artery and the right hepatic artery.
• Variations: the hepatic artery can show variability; in about 45% of people the hepatic artery bifurcates and courses differently, so color Doppler is used to confirm which is which.
• “Seagull sign” is a classic ultrasound appearance of the celiac axis and its branches near the aorta.

Portal Venous System and Confluence

• Portal vein origin: formed by the confluence of the Superior Mesenteric Vein (SMV) and the Splenic Vein (SV) behind the pancreas.
• The SMV drains midgut structures; the Splenic Vein drains the spleen and portions of the GI tract; the Inferior Mesenteric Vein (IMV) drains parts of the colon and rectum and dumps into the Splenic Vein.
• The portal confluence (where SMV + SV join) is just before entering the liver. The IMV joins the Splenic Vein; the confluence then becomes the portal vein as it travels toward the liver.
• After passing the pancreas, the portal vein enters the liver and typically bifurcates into the right portal vein and the left portal vein within the liver.
• The right portal vein further divides into the right anterior portal vein and the right posterior portal vein, feeding the right lobe's anterior and posterior segments, respectively.
• The left portal vein supplies the left lobe with its own branches.
• Within Couinaud’s framework, the portal system delivers inflow to liver segments; the hepatic veins drain those segments into the IVC.

Hepatic and Portal Vein Segmentation of the Liver

• The right portal vein supplies the right lobe; it bifurcates into anterior and posterior branches.
• The left portal vein supplies the left lobe; it has multiple branches.
• The hepatic veins (right, middle, left) drain blood from the liver segments into the IVC.
• The portal tree runs within the liver segments (intra-segmental vessels), while hepatic veins run between segments (intersegmental veins).
• The portal triad and its orientation: the portal vein is typically seen at the top of long-axis views, with the hepatic artery and CBD nearby.

Signs Used in Ultrasound to Describe Vascular Anatomy

• Mickey Mouse sign: portal triad at the porta hepatis (CBD, hepatic artery, portal vein).
• Seagull sign: depiction on the celiac axis with its branches.
• Bunny sign (midline transverse view): shows IVC with left, middle, and left hepatic veins converging into it.
• Moose sign: when angling to the patient’s right to reveal the right, middle, and left hepatic veins in another orientation.
• Doppler hepatic veins: may show pulsatile, triphasic venous flow due to proximity to the right atrium; this is normal anatomy for hepatic venous Doppler.

Doppler and Color Flow Imaging Principles

• Portal vein color should be red toward the transducer (hepatopetal flow) when in the correct window.
• If color is inverted, you may mistake flow direction and pathology; do not invert the color on portal vein Doppler in standard exams unless you know what you’re doing.
• The hepatic artery is usually anterior to the portal vein in the porta hepatis and has a distinct arterial waveform with systolic and diastolic components.
• The CBD (bile duct) does not show blood flow on color Doppler; use color to distinguish arteries (will light up) from ducts (no flow).
• When Doppler is used, document the direction and quality of flow (e.g., hepatopetal flow in the portal vein; arterial waveform in hepatic artery).

Portal Vein Measurement and Normal Values

• Portal vein width is typically measured where it crosses the IVC.
• Normal portal vein diameter is up to 13\ \text{mm} (i.e., 1.3\ \text{cm}).
• If the portal vein diameter exceeds 13\ \text{mm}, investigate for portal hypertension or other pathology.
• Liver length normal range (cranial-caudal measurement) is about 15.5\ \text{cm}; some sources vary (e.g., up to ~17\ \text{cm} in older norms), but the standard taught here is 15.5\ \text{cm}.
• Various measurement methods exist (cranial-caudal; portal vein-based method), and institutions may have slightly different normal values.

Liver Anatomy in Ultrasound Images

• Hepatic veins (right, middle, left) drain into the IVC and are intersegmental; their walls are less echogenic than portal veins because hepatic veins have less collagen.
• Portal veins have thicker, brighter walls and run horizontally within the liver parenchyma; hepatic veins run more vertically.
• The IVC and hepatic veins are often visualized together in transverse views; the hepatic veins lie near the top of the liver in such images and drain into the IVC.
• In transverse liver views, the middle hepatic vein often appears as a vertical structure entering the IVC from the liver; the right hepatic vein is seen in the right intersegmental fissure, and the left hepatic vein appears in the left hepatic fissure.
• Segmental anatomy (Couinaud): 8 segments (commonly enumerated 2–8 with 4 split into 4a and 4b).
  • Segments 2, 3, 4a, 4b, 5, 6, 7, 8 form the functional units.
  • Cantlie’s line (along with the falciform ligament and middle hepatic vein) divides the liver into right and left hemilivers; the gallbladder and middle hepatic vein help define segments within these halves.
• A practical mnemonic is the hand model: the knuckles mark the hepatic veins (e.g., right, middle, left hepatic veins) and the portal vein runs across the top (portal triad across the porta hepatis).

Couinaud Segments and Vascular Borders

• The right lobe contains segments 5–8; the left lobe contains segments 2–4 (with 4a and 4b being the anterior/posterior divisions of the medial segment).
• The right portal vein feeds the right lobe and bifurcates into anterior and posterior branches for segments 5/8 (anterior) and 6/7 (posterior).
• The left portal vein feeds segments 2 and 3 (lateral and medial divisions of the left lobe).
• The right hepatic vein divides segments 7 and 6 from segments 8 and 5 via the right intersegmental fissure; the left hepatic vein lies within the left fissure and divides segments 4a/4b and 2/3.
• Eight functional units (segments): 2,3,4a,4b,5,6,7,8; Segments 4 can be further subdivided into 4a and 4b.

Practical Scanning Protocols and Workflow

• Patient prep: NPO for at least 6–8 hours (often 8 hours recommended); NPO means nothing by mouth (including gum, smoking, mints, and even certain beverages).
• Equipment and technique:
  • Use a curvilinear (mid- to high-frequency) transducer suitable for RUQ imaging.
  • Adjust Time-Gain Compensation (TGC), depth, and focal zones to optimize the image; ensure the diaphragm is included in the window and not cut off.
  • Obtain a series of views to identify the porta hepatis, hepatic artery, portal vein, CBD, and hepatic veins.
  • Obtain Doppler studies for vessels (portal vein, hepatic artery, hepatic veins as appropriate). If needed, power Doppler can help detect low-flow states.
• Documentation and reporting:
  • In the right upper quadrant ultrasound, verify landmarks: porta hepatis (Mickey Mouse sign), Morrison’s pouch for free fluid, and comparison with the kidney for echogenicity.
  • Note any signs suggestive of pathology (e.g., altered echogenicity, nodules, ductal dilation, or fluid in Morrison’s pouch).
  • Include prior imaging and relevant history (surgeries, liver disease, elevated liver enzymes, etc.).
• Practical tips:
  • Doppler the portal vein in the correct orientation to show hepatopetal flow (toward the liver).
  • If color flow is inverted, revert to the standard orientation to avoid misinterpretation of flow direction.
  • The hepatic artery often lies anterior to the portal vein; identify arterial waveforms with systolic/diastolic components.
  • When measuring the CBD vs hepatic duct, identify flow characteristics to distinguish vessels from ducts (arteries light up on color; ducts do not).
• Common laboratory correlations (hepatic function tests):
  • AST and ALT are aminotransferases that rise with hepatocellular injury.
  • Alkaline phosphatase (ALP) and bilirubin rise with biliary obstruction or cholestasis.
  • Albumin reflects synthetic function of the liver (longer-term marker).
  • Overall, abnormalities in AST/ALT/ALP/bilirubin can indicate liver disease or obstruction; correlate with symptoms and imaging.

Variants, Pathology, and Practical Implications

• Variants and anomalies: agenesis (absence of liver tissue), situs inversus (liver located on the left), accessory fissures, and vascular variations (e.g., nonstandard hepatic artery courses).
• Portal hypertension: high pressure in the portal venous system can cause pathophysiologic changes; potentially reverses flow, leads to collateral formation, and requires careful evaluation.
• Vascular assessment for disease: a dedicated liver Doppler study examines the portal vein, hepatic artery, and hepatic veins to assess patency, velocity, waveforms, and direction of flow.

Liver Function and Pathophysiology (High-Level)

• The liver processes blood, metabolizes drugs, produces clotting factors, destroys old red blood cells, and generates bile.
• Diseases can produce measurable changes in ultrasound echogenicity and texture (e.g., coarse or heterogeneous echotexture in cirrhosis or fatty liver).
• Chronic liver conditions may alter liver size, echogenicity, and the architecture of ligaments and fissures; ultrasound is sensitive to these changes and Morrison’s pouch is checked for fluid.

Quick Reference: Key Numbers and Names (LaTeX-formatted)

• Portal vein contribution to liver blood: 70\%-80\%\,, hepatic artery contribution: 20\%-30\%\,.
• Portal vein width normal: \le 13\ \text{mm}\, (i.e., 1.3\ \text{cm}).
• Normal liver cranial-caudal length: \approx 15.5\ \text{cm}\,.
• Portal triad components visible at porta hepatis (Mickey Mouse sign): CBD, hepatic artery, portal vein.
• Segments: 2,3,4\,\text{(with }4a/4b),5,6,7,8 (Couinaud eight-segment model).
• Portal signal naming: hepatopetal (toward the liver); hepatofugal (away from the liver).
• Hepatic veins drain to the IVC and are blue on Doppler when the transducer is positioned to visualize flow away from the liver toward the heart.

Final Notes and Exam Readiness

• Be able to identify the porta hepatis and the Mickey Mouse sign on RUQ ultrasound.
• Distinguish portal vs hepatic arteries using color Doppler and waveform characteristics.
• Identify portal vein, SMV, splenic vein, IMV, portal confluence, and the bifurcation into left and right portal veins; further branch into anterior/posterior within the right lobe.
• Recognize signs (bunny/moose) and know when to document which hepatic veins drain into the IVC.
• Be familiar with Couinaud segments and Cantlie’s line for surgical planning and biopsy targeting (e.g., segment 4a vs 4b, 2 vs 3, 7 vs 6).
• Understand the steps of RUQ scanning protocol: patient prep, image acquisition views, Doppler evaluation, and reporting essentials (size, echotexture, vascular flow, ducts, lesions, and Morrison’s pouch).