Anomalies and Anatomic Variants of the Liver

Abstract

Knowledge of the anomalies and anatomical variants of the liver is often of great importance because these may be misinterpreted as pathologic conditions. While congenital anomalies of the liver are rare, anatomic variants are relatively common. The congenital anomalies of the liver include agenesis or hypoplasia of the hepatic lobes or segments, Riedel’s lobe, and other accessory hepatic lobe. Anatomic variants contain accessory fissures and diaphragmatic slips, sliver of the liver, and variants related to papillary process of the caudate lobe.

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Knowledge of the anomalies and anatomical variants of the liver is often of great importance because these may be misinterpreted as pathologic conditions. While congenital anomalies of the liver are rare, anatomic variants are relatively common. The congenital anomalies of the liver include agenesis or hypoplasia of the hepatic lobes or segments, Riedel’s lobe, and other accessory hepatic lobe. Anatomic variants contain accessory fissures and diaphragmatic slips, sliver of the liver, and variants related to papillary process of the caudate lobe.

1 Agenesis and Hypoplasia of the Hepatic Lobe or Segment

Agenesis and hypoplasia of hepatic lobes or segments are uncommon developmental anomalies. Agenesis is a condition of complete absence of hepatic lobes or segments, whereas hypoplasia is a condition which the size of a hepatic lobe or segment is relatively small but otherwise normal. The most commonly involved segments are the anterior segment of the right lobe and the medial segment (segment IV). Before making a diagnosis of agenesis or hypoplasia, it is important to exclude other causes of acquired atrophy of the hepatic lobes or segments, caused by cirrhosis, biliary obstruction, postsurgical resection, or rarely vascular compromise. In cases of agenesis, hepatic parenchyma as well as corresponding lobar or segmental vessels is absent, whereas at least one of those structures is detected in cases of acquired atrophy. Agenesis or hypoplasia of the hepatic lobes and segments may alter the normal topography of the upper abdomen, that is, change in position of adjacent organs such as the stomach, colon, or kidney.

2 Riedel’s Lobe

Riedel’s lobe is a downward, tongue-like projection from the anterior aspect of the right hepatic lobe. It is controversial whether Riedel’s lobe is congenital or acquired in origin. Awareness of Riedel’s lobe is clinically important since it is one of the causes of right-sided abdominal palpable masses on physical examination. However, correct diagnosis can be easily achieved by demonstrating of its connection with right hepatic lobe on imaging modalities. Riedel’s lobe is usually asymptomatic. However, it can give rise to symptoms such as acute or intermittent abdominal pain if it is complicated by torsion.

3 Accessory Hepatic Lobe

Accessory hepatic lobe is a rare congenital anomaly and occurs from an error in the formation of the endodermal caudal foregut in the third gestational week and segmentation of the hepatic bud. It is composed of normal hepatic tissue and contains its own hepatic vessels and bile ducts. Most cases of accessory lobes are attached to the inferior surface of the liver by either a normal hepatic parenchyma or a mesentery. Occasionally, they have been found around gallbladder fossa, gastrohepatic ligament, umbilicus, adrenal gland, pancreas, esophagus, and rarely the thoracic cavity. Accessory hepatic lobes can mimic soft tissue masses or lymph nodes, but they can be differentiated from other pathologic condition by means of identification of their continuity with main liver on imaging modalities using multiplanar reconstruction. Although most accessory lobes are usually asymptomatic and found incidentally, some pedunculated ones may undergo torsion of their vascular pedicles.

4 Accessory Fissure and Diaphragmatic Slip

In addition to the major hepatic fissures such as fissures for falciform ligament and ligamentum venosum, the liver may contain accessory and pseudoaccessory fissures. True accessory fissures result from inward folding of the peritoneum; therefore, ascites may extend into these fissures or peritoneal pathology can be appeared. Accessory fissures are rare but relatively common in the undersurface of the liver. The most common one is the inferior accessory fissure, which is located in the surface of posterior segment of the right lobe. Invagination of the diaphragmatic muscle fibers results in pseudoaccessory fissures, usually along the superior surface of the liver. They are common anatomic variants and more frequently seen in the right hepatic lobe. These diaphragmatic slips can mimic hepatic nodules.

Differentiation of hepatic accessory or pseudoaccessory fissures from pathologic lesions may be achieved by a careful analysis of contours of the liver and diaphragm and by knowing the various findings of these variants. On CT scan, they can appear as hypoattenuated nodules in the peripheral portion of the liver. On ultrasound, they may be seen as echogenic nodular lesions in one plane. However, when scanning in the orthogonal plane, echogenic linear morphology may be revealed along the hepatic dome.

5 Sliver of the Liver

Leftward extension of the left lateral segment of the liver is referred to as “sliver of the liver.” It is a common anatomic variant and appears as a crescent density which wraps around the spleen in the left upper quadrant abdomen. Knowledge of imaging features of this variant is important to not to confuse this portion of the liver for a pathologic condition originating from the stomach or spleen. This potential misdiagnosis can be avoided by demonstration of continuity between the “sliver of the liver” and the remainder of the left hepatic lobe.

6 Papillary Process of the Caudate Lobe

The caudate lobe is a medial extension of the right hepatic lobe between inferior vena cava and the fissure for ligamentum venosum. Occasionally, it is divided inferiorly into two processes: papillary and caudate processes. The papillary process extends medially and to the left in the region of the lesser sac, while the caudate process extends posteriorly. Papillary process of the caudate lobe can appear separate from the liver on some sections of the axial images; therefore, it can mimic an enlarged periportal lymph node or a soft tissue mass near the pancreas head. Serial axial images as well as multiplanar reformatted images are occasionally helpful for differentiating the papillary process form extrahepatic lesions.

7 Summary

• 1. Knowledge of the imaging features of anomalies and anatomic variants of the liver is important not to misinterpret these as pathologic conditions. Multiplanar reformatted images would be occasionally helpful to make correct diagnosis.

• 2. Riedel’s lobe is one of the possible causes of abdominal palpable mass, and it can undergo torsion.

• 3. Accessory fissure of the liver is most commonly found in the surface of posterior segment of the right lobe.

• 4. Diaphragmatic slips are common pseudoaccessory fissures in the hepatic dome which result from invagination of the diaphragm.

• 5. Leftward extension of the left lateral segment of the liver (sliver of the liver) is a common anatomic variant which may mimic perisplenic mass.

• 6. Papillary process of the caudate lobe can mimic an enlarged lymph node or a soft tissue mass on axial images.

8 Illustrations: Anomalies and Anatomic Variants of the Liver

8.1 Illustrations of Normal Segmental Anatomy of the Liver

Fig. 1.1

Illustrations of normal segmental anatomy of the liver on cross-sectional images. (a, b) Segmental anatomy of the liver is usually determined by the portal vein branches and hepatic veins. Congenital anomalies such as agenesis and hypoplasia of hepatic lobes or segments alter the normal morphology which can be appeared as absence and relatively small size of lobes or segments. RP right posterior segment, RA right anterior segment, LM left medial segment, LL left lateral segment, C caudate lobe

8.2 Agenesis of the Right Lobe of the Liver

Fig. 1.2

Agenesis of the right lobe of the liver. (a) Axial CT image at the level of left portal vein shows bifurcation of the segmental portal branches from the umbilical portion (arrowhead) of the left portal vein. Note the absence of the right hepatic lobe and hypertrophy of the caudate lobe (asterisk). (b) CT image which is more caudal to (a) demonstrates the origination of the left portal vein (arrowhead); however, there is no vascular structure corresponding to right portal vein. Note the accessory fissure (arrow) in the left lateral segment

8.3 Agenesis of the Left Lateral Segment of the Liver

Fig. 1.3

Agenesis of the left lateral segment of the liver. (a) Axial contrast-enhanced CT reveals absence of left portal vein while right portal vein is present (arrow). (b) On the more cranial image, the left lateral segment is absent while the medial segment is supplied by middle hepatic artery (arrow) and caudate lobe (asterisk) is present. (c) Topogram shows transverse and high-positioned stomach (arrows)

8.4 Hypoplasia of the Left Lateral Segment of the Liver

Fig. 1.4

Hypoplasia of the left lateral segment of the liver. (a) Axial CT image shows the hypoplastic liver tissue of left lateral segment (arrow) which mimics an enhancing soft tissue mass or lymphadenopathy of the perigastric space. Notice the presence of left portal vein (arrowhead). (b) More superior image and (c) coronal reformatted image show the small size of left lateral segment. (b) Notice the left hepatic vein (arrowhead)

8.5 Hypoplasia of the Left Medial Segment of the Liver

Fig. 1.5

Hypoplasia of the left medial segment of the liver. (a) CT shows the small medial segment (arrowhead) lying between the fissure for falciform ligament (arrow) and gallbladder (black asterisk). (a, b) CT and topogram demonstrate that hepatic flexure colon (white asterisk) and omental fat fill the gap created by hypoplasia of the medial segment of the liver

8.6 Riedel’s Lobe: Ultrasound Findings

Fig. 1.6

Riedel’s lobe: ultrasound findings. (a) Ultrasound image which is scanned in an oblique coronal plane shows soft tissue (arrows) anterior to the right kidney. (b) Ultrasound image on a different plane in the same individual reveals elongation of the right lobe (Riedel’s lobe) (arrow) which shows same echogenic texture with the other part of the liver

8.7 Riedel’s Lobe: CT Findings

Fig. 1.7

Riedel’s lobe: CT findings. (a, b) Axial CT images in a 28-year-old female show the inferior extension of the liver parenchyma (arrows) from the anterior portion of the right lobe which is located anteriorly to the right kidney and along the right paracolic gutter. (c) Coronal reformatted CT image demonstrates the tongue-like downward elongation of the hepatic parenchyma (arrow), which is consistent with Riedel’s lobe

8.8 Accessory Lobe of the Liver with Mass

Fig. 1.8

Accessory lobe of the liver with mass. (a) Arterial and (b) portal phase CT images of a 78-year-old female show hepatocellular carcinoma (asterisks) in the accessory hepatic lobe (arrows) which is attached to the left lateral superior segment (segment II). Portovenous shunt is detected in the segment II (arrowhead)

8.9 Diaphragmatic Slips: CT Findings

Fig. 1.9

Diaphragmatic slips: CT findings. (a) Axial and (b) coronal CT images in a 60-year-old female demonstrate the diaphragmatic invaginations (arrow and arrowheads) in the superior aspect of the liver which result in pseudoaccessory fissures, “diaphragmatic slips.” Diaphragmatic slips can be seen as nodular density (arrow) in some sections of CT images; therefore, it may mimic hepatic nodule

8.10 Diaphragmatic Slips: Radiography and Ultrasound Findings

Fig. 1.10

Diaphragmatic slips: radiography and ultrasound findings. (a) Diaphragmatic invagination in the liver causes lobulated soft tissue densities (arrowheads) along the right diaphragm on the chest radiography. These findings can be mistaken for soft tissue masses in the lung or pleura. (b) Ultrasound image in the same patient shows lobulation of the liver surface as well as echogenic lines (arrows) which are consistent with diaphragmatic slips

8.11 Accessory Fissure of the Liver

Fig. 1.11

Accessory fissure of the liver. (a) Axial and (b) coronal CT images in a 37-year-old female show incidentally detected accessory fissure (arrows) of the liver which is located in the inferior surface of the right hepatic lobe

8.12 Accessory Fissure of the Liver with Loculated Fluid Collection

Fig. 1.12

Accessory fissure of the liver with loculated fluid collection. (a) Preoperative CT image revealed slit-like accessory fissure (arrow) in the right hepatic lobe. (b) CT scan after pylorus-preserving pancreaticoduodenectomy in the same patient shows loculated ascites (arrow) in the accessory fissure of the liver

8.13 Sliver of the Liver

Fig. 1.13

Sliver of the liver. (a) Axial and (b) coronal CT images show the leftward extension of the left lobe of the liver (white asterisks) wrapping the spleen (black asterisks), which is a common anatomic variation, “sliver of the liver.” (c) Ultrasound image in a coronal plane shows the sliver of the liver (white asterisk) around the spleen (black asterisk) which may be mistaken for a perisplenic pathologic condition

8.14 Papillary Process of the Caudate Lobe

Fig. 1.14

Papillary process of the caudate lobe. (a) Axial CT scan at the level of main portal vein shows protruded papillary process of the caudate lobe (arrow) which mimics a periportal lymph node or an exophytic pancreatic mass. (b) Coronal reformatted CT image reveals the inferior and medial extension of the papillary process of the caudate lobe (arrow)