Liver Anatomy: Spaces, Ligaments, Vessels, and Segmental Division
Liver Anatomy Overview
The liver is described as a large, smooth organ with characteristic sharp inferior margins.
Venous Systems:
Portal Venous System: This system enters the liver at the porta hepatis, carrying nutrient-rich blood from the gastrointestinal tract.
Systemic Venous System: This system is responsible for draining blood from the liver via the hepatic veins, which subsequently empty into the Inferior Vena Cava (IVC).
Portal Triad: This critical anatomical unit consists of the bile ducts, hepatic arteries, and portal veins, which all run in close proximity and are encased within connective tissue.
Glisson's Capsule: This is a fibrous capsule that encloses the entire liver. It extends internally from the porta hepatis, accompanying and enveloping the portal triad throughout the liver parenchyma.
Extrahepatic Spaces and Peritoneum
Sagittal Right Upper Quadrant View:
Diaphragm: Positioned superior to the dome of the liver, presenting a characteristic dome shape.
Right Kidney: Located posteriorly and inferiorly relative to the liver, situated within the abdominal cavity.
Posterior Abdominal Wall: Lies posterior to the right kidney, comprising ribs and muscle tissue, which can be visualized with deeper penetration during imaging.
Peritoneal Cavity: A potential space lined by the peritoneum.
Parietal Peritoneum: A lining that reflects from the abdominal wall.
Visceral Peritoneum: The parietal peritoneum extends to cover the liver, at which point it becomes the visceral peritoneum.
The peritoneum continues posteriorly, reflecting again; notably, the kidneys are described as retroperitoneal, meaning they lie behind the peritoneum.
Anterior Abdominal Wall: Features the paired rectus abdominis muscles (right and left).
Preperitoneal (Extraperitoneal) Fat:
A small adipose tissue pad located deep to the rectus muscles, typically found in the midline at the Linea Alba.
This fat can sometimes extend into and penetrate the falciform ligament, which is an important anatomical consideration in imaging.
Falciform Ligament and Ligamentum Teres
Ligamentum Teres (Lig Teres):
Often depicted in diagrams as a folded 'U' shape, sometimes metaphorically referred to as a "crepe," "tortilla," or "taco."
It is a significant remnant of the fetal umbilical vein.
Falciform Ligament:
A double fold of peritoneum that encases the Ligamentum Teres.
It serves to anchor the liver to the anterior abdominal wall.
In a transverse cadaveric cross-section, the arrangement is typically anterior parietal peritoneum, followed by the falciform ligament wrapping around Lig Teres, and then another layer of anterior parietal peritoneum.
Visualization of the falciform ligament with ultrasound is often challenging and typically only achieved when conditions such as gross ascites are present.
Relationship with Preperitoneal Fat:
The preperitoneal fat, which sometimes appears as an "eye of raw" or lens shape in transverse views at the level of the diaphragm, can communicate with or extend between the folds of the falciform ligament.
If a structure is observed anterior to Lig Teres that is not liver tissue, it is a strong possibility that it represents an extension of this preperitoneal fat.
Ultrasound and CT Visualization:
On ultrasound, Lig Teres usually appears as a hyperechoic structure often accompanied by posterior shadowing. The falciform ligament itself is rarely seen unless gross ascites distends the peritoneal cavity, outlines it. Gross ascites can provide an outline of the falciform ligament, making it visible.
On CT scans, when imaged inferior to the liver, Lig Teres can be identified extending anteriorly and lying within radiolucent preperitoneal fat. Lig Teres typically presents as a brighter (hyperattenuating) structure compared to the darker appearance of fat.
Lig Teres extends caudally all the way to the umbilicus and can be tracked continuously on CT imaging.
Ligamentum Venosum
Classification: This is categorized as an intrahepatic ligament.
Visualization: It is not directly visualized on imaging; typically, only the fissure (groove) within which it resides is seen.
Fetal Remnant: It is a fibrous remnant of the fetal ductus venosus.
Fetal Pathway Context: During fetal development, the umbilical vein (which later becomes Lig Teres) connected to the anterior turn of the left portal vein. From there, the ductus venosus formed a direct shunting pathway to either the hepatic veins or the IVC, effectively bypassing the hepatic parenchyma.
Structure: The ligamentum venosum is a thin, fibrous cord, distinguishing it from the thicker, rope-like structure of Lig Teres.
Location within Fissure: The true ligamentum venosum is located only within the superior aspect of the fissure for ligamentum venosum. The inferior aspect of this fissure is associated with a different, often unnamed, extrahepatic ligament.
Cadaver and Imaging Views:
The fissure for ligamentum venosum serves as an anatomical separator between the caudate lobe and the main body of the liver.
In transverse views, it is identifiable superior to the anterior turn of the left portal vein, which marks the approximate origin point of the ductus venosus in the fetus.
Fissure Definition: A fissure is defined as a narrow, thin, deep groove or cleft, likened to an "earthquake fissure" opening in the ground.
Anatomical Position: It presents as a groove located in the posterior aspect of the liver, extending from right to left.
Contents: The fissure contains the lesser omentum, which extends inferiorly to connect with the stomach.
Dr. Roy Filly, a renowned radiologist, is noted to use an alternative term for this anatomical feature.
Hepatic Arterial and Portal Venous Systems
Arterial Supply (from Celiac Axis):
The celiac artery branches into the common hepatic artery.
The common hepatic artery then becomes the proper hepatic artery.
The proper hepatic artery subsequently divides into the right hepatic artery and the left hepatic artery.
A less frequently observed branch, the middle hepatic artery, may originate from the right hepatic artery within the liver parenchyma.
These arterial vessels progressively maneuver closer to and ultimately penetrate deeper into the liver tissue.
It is important to note that anatomical variations of these vessels are common and will be addressed later in the curriculum.
Blood Flow to Liver:
Both the main portal vein and the hepatic arteries (e.g., proper or right hepatic artery) are responsible for transporting blood into the liver.
Radiological Terminology: This inward direction of blood flow is clinically described as antegrade flow. While the term "hepatopedal" (meaning "towards the liver") can be used, radiologists typically prefer "antegrade flow" to ensure clarity and avoid confusion.
The right and left hepatic arteries commonly bifurcate in close proximity to the main portal or right portal vein.
Fate of Blood Supply: Ultimately, both arterial and portal venous blood converge into the hepatic sinusoids, thereby supplying the same hepatic lobules. They do not terminate in distinct anatomical locations within the liver.
Doppler Flow Characteristics
Portal Venous Flow: Characteristically exhibits low pressure and low velocity.
Arterial Flow (Hepatic Arteries): Marked by high pressure and high velocity.
Doppler Settings:
To effectively differentiate arteries from veins, particularly when both display antegrade flow, adjustments to scan parameters are crucial.
Given that arteries exhibit faster flow, a reduction in the Doppler scale (Pulse Repetition Frequency - PRF) can induce aliasing in the arterial waveform. This phenomenon causes the artery to appear to "pop off the screen," facilitating its distinction from the comparatively slower portal venous flow.
Flow Directions:
Hepatopedal (antegrade): Denotes blood flow into the liver (e.g., normal portal vein and hepatic artery flow).
Hepatofugal (retrograde): Denotes blood flow away from the liver. This directional flow within the main portal vein is considered an abnormal finding, indicative of reversed flow. This implies underlying pathological conditions where blood is described as "fleeing the liver."
Pulse Doppler Waveforms:
Arterial flow waveforms typically present with a swift upstroke. Ideally, with proper angle correction, pulse Doppler should demonstrate low resistance flow, reflecting the liver's substantial demand for a consistent and ample blood supply.
A blue color observed below the baseline on a spectral Doppler waveform signifies a negative shift, generally indicating flow away from the transducer. In specific clinical contexts (such as an example of hepatofugal portal vein flow), this would correspond to flow away from the liver.
Mickey Mouse Sign (Porta Hepatis)
Description: This is a classic sonographic landmark visualized in a transverse view, typically located just inferior to the liver at the porta hepatis.
Components: It comprises three distinct circular anatomical structures that, together, resemble the head of the cartoon character Mickey Mouse:
Main Portal Vein: This is the largest of the three circles, situated posteriorly, representing Mickey's head.
Common Bile Duct: A smaller circle, located anteromedially, forming one of Mickey's ears.
Proper Hepatic Artery: Another small circle, positioned anterolaterally, forming the other "ear."
Clinical Significance: This characteristic configuration is invaluable for quickly identifying and delineating these crucial structures based on their distinct sonographic appearance and spatial relationships.
Anatomical Context: These three structures are typically enveloped within the right margin of the lesser omentum, also known as the hepatoduodenal ligament.
Cheat Sheet (without Color Doppler): In situations where color Doppler is unavailable to differentiate between the artery and the duct, a helpful mnemonic is that the proper hepatic artery is generally situated closer to the aorta (to the patient's anatomical left), while the common bile duct is positioned to the patient's anatomical right. This aids in identifying each "ear."
Diaphragmatic Slips (Accessory Fissures)
Nature: These are muscle bundles originating from the diaphragm that can indent or press into the liver parenchyma, creating grooves or fissures.
Mimicry: Their clinical importance stems from their ability to mimic the appearance of masses, tumors, or neoplasms on various imaging modalities, particularly ultrasound.
Appearance on Imaging:
They can create deep "bites" or indentations in the liver contour.
On sonography, they may appear heterogeneous, shadowy, or could even be mistaken for a complex fluid collection.
A still ultrasound image can deceptively portray them as a genuine, concerning mass.
Identification Clues:
Clinical literature suggests that diaphragmatic slips typically align with the ribs, a crucial diagnostic cue that can assist in their identification on CT and coronal reconstructions.
During dynamic scanning, they should exhibit some discernible length and can be elongated by rotating the transducer, which helps distinguish them from focal lesions.
On dynamic imaging (cine loops), their direct contiguity and connection to the diaphragm may become evident, providing a clear distinction from true liver masses.
Clinical Incidents: There have been documented instances where sonographers or even attending radiologists initially misidentified extensive diaphragmatic slips as pathological lesions, necessitating further diagnostic imaging (e.g., a CT scan) to arrive at the correct diagnosis of diaphragmatic indentations. This underscores the critical importance of recognizing these pseudo-lesions to prevent misdiagnosis and unnecessary interventions.
Historical Context: The renowned anatomist Netter meticulously illustrated numerous diaphragmatic grooves within the liver, demonstrating their common occurrence and varied presentations. Modern CT examples consistently corroborate these observations, frequently depicting multiple diaphragmatic slips that perfectly match Netter's classical anatomical depictions.
Segmental Liver Anatomy
Overall Goal: The primary objective is to systematically divide the liver into standardized anatomical and functional segments. This is essential for precisely localizing pathology, as the liver's large size precludes vague descriptions like merely "right liver" or "left liver."
Learning Resources:
A 3D interactive liver website, accessible via a link on "Learn." This resource historically required the Ruffle extension due to its Flash-based platform.
An alternative interactive resource is anatomylearning.com, which also provides comprehensive 3D anatomical views of the liver's segmental anatomy.
Three Main Methods of Liver Division:
Old Traditional Method:
This method is based on gross, naked-eye observation of an excised liver.
It divides the liver into 4 lobes: the Right lobe, Left lobe, Caudate lobe, and Quadrate lobe.
While this classification is not utilized for detailed functional segmental anatomy in modern imaging, understanding its historical significance and the origin of terms like "Caudate lobe" is important. Netter's anatomical illustrations often reflect this older, descriptive paradigm.
Simple Segmental Method:
This method provides an excellent framework for learning the fundamental liver landmarks and basic segments.
It is based on the internal vascular and biliary divisions of the liver.
Couinaud Segmental Anatomy:
This is the predominant system currently employed in clinical practice for liver segmentation.
It builds upon the principles of the simple segmental method but offers a more detailed and refined division of the liver.
The Couinaud system divides the liver into superior and inferior segments, primarily based on the level of the portal veins.
Main Lobar Fissure (MLF)
Function: The Main Lobar Fissure is an anatomical plane that functionally divides the liver into its anatomical right and left lobes. It serves as a crucial landmark for sonographic and sectional orientation.
Key Sonographic Landmarks (Filly's Line): To reliably identify the MLF, sonographers look for the precise alignment of the following structures within a single imaging plane:
The middle hepatic vein, frequently described as being remarkably long.
The proximal right portal vein.
Fat within the main lobar fissure, famously known as Filly's line.
The gallbladder.
Visualization in Sagittal Plane:
When the transducer is perfectly aligned with the MLF, the entire image displayed on the screen can represent the fissure itself. This can be a conceptual challenge or "mind warp" for those new to sonography.
In this specific view, liver tissue appearing towards the transducer is considered part of the right lobe (lying out of the immediate imaging plane), while tissue appearing away from the transducer constitutes part of the left lobe.
Visualization in Transverse Plane:
In transverse views, the MLF is often depicted passing posteriorly through the IVC. The internal hepatic fissures, in this orientation, can appear to emanate like "spokes on a wheel" from the IVC.
Identifying Lobes Adjacent to the MLF: When scanning directly within the plane of the MLF, to visualize the right lobe of the liver, the transducer must be angled or tilted towards the patient's anatomical right.
Clinical Importance: Even if not all 4 of the classical landmarks are perfectly visualized, the consistent presence and alignment of structures such as the proximal right portal vein, Filly's line, and the gallbladder are usually sufficient to confirm the correct identification of the MLF.
Historical Context: Dr. Roy Filly, whose influential article was published in 1979 (co-authored with Callan, a noted author of an OB/GYN textbook), is credited with describing "Filly's line," a seminal contribution to the field of sonographic liver anatomy.
Caudate Lobe (Future Discussion)
Future lectures will delve into the Caudate lobe, including its anatomical variants and specific "processes" (small, dangling structures often observed off its inferior aspect).
This detailed discussion will serve to integrate and reinforce the principles of Couinaud segmental anatomy.