The Liver



The Liver


M. Robert Dejong

Jeanine Rybyinski















The liver is an organ that is essential for life. It is the largest gland and solid organ in the body and is considered an accessory organ of digestion. It is important for the sonographer to understand the anatomy, physiology, and the various pathologies of the liver. The liver can be easily evaluated with ultrasound, and the use of new technologies—such as contrast and elastography—can help provide a lot of information about the liver for the physician. When a disease process of the liver is suspected, a sonography examination is commonly the first imaging test ordered because it can be quickly scheduled, has high patient acceptance, and does not use nephrotoxic contrast agents. Ultrasound can be used to measure the size of the liver, evaluate it for various masses and cysts, evaluate and stage parenchymal disease, and provide guidance for biopsies. This chapter will not cover biliary pathology because it is covered in another chapter. Liver transplants are covered in another chapter as well as in another book in this series.


ANATOMY


Embryology

Early in the fourth week of fetal development, the liver, gallbladder, and bile duct system develop from the endoderm. The hepatic diverticulum, also called the liver bud, develops from the ventral aspect of the foregut of the endoderm and gives rise to the parenchyma of the liver as well as the gallbladder and bile ducts. During week 5, the diverticulum differentiates into the origin of the cystic duct and the gallbladder in the caudal portion and the two endodermal cellular buds begin forming the right and the left hepatic lobes.1, 2 and 3 These solid cell buds grow into columns or cylinders that branch and form networks, which will envelope the vitelline and umbilical veins and become the liver sinusoids. The columns of endodermal cells in the liver parenchyma grow into the surrounding mesoderm. The mesoderm provides the hemopoietic tissue, which forms blood cells, until the bone marrow and spleen take over the process in late fetal life, and the connective tissue for the portal tracts and the fibrous liver capsule that becomes Glisson capsule.1,3 As the terminal branches of the right and left hepatic lobes canalize, the bile duct system is formed. The development of the Kupffer cells starts in the yolk sac and they will eventually migrate to the fetal liver.

Both lobes are equal in size until the beginning of the sixth week, at which time the right lobe (RL) becomes larger, with the caudate and quadrate lobes developing from the RL. The left lobe actually undergoes some degeneration. At week 6, the liver fills most of the abdominal cavity and, relative to other organ development, the liver becomes less active.1,2

Hemopoiesis takes place in the liver at week 6, peaks at 12 to 24 weeks, and ceases at birth.1,4 At week 10, lymphocyte formation occurs in the liver, which also ceases at birth. Coagulation factors are manufactured at 10 to 12 weeks and bile is produced by 13 to 16 weeks,1 but the fetal liver does not take part in digestion until after birth.5

Oxygenated blood and nutrients are delivered to the fetus through the umbilical vein, which ascends and divides into two branches.6,7 The left branch joins the portal vein and enters the liver and the right branch, the ductus venosus, flows directly into the inferior vena cava (IVC), bypassing the liver5,8 (Fig. 7-1). Normally, both of these vessels deteriorate into fibrous cords sometime after birth. The left umbilical vein becomes the ligamentum teres, or round ligament, and the ductus venosus becomes the ligamentum venosum. Both the ligamentum teres and the ligamentum venosum can become recanalized as collateral vessels in patients with portal hypertension.8


Location and Size

The liver fills the right hypochondrium, epigastric region, and the left hypochondrium as far as the mammillary line. Most of the RL and usually the entire left lobe are protected by the ribs; therefore, intercostal scanning may be needed to visualize the entire liver. The liver can provide an acoustic window through which the upper abdomen and retroperitoneum may be imaged (Fig. 7-2A, B). The liver is wedge or triangular in shape with its base to the right and its apex to the left. Sonographically, the liver displays a medium-level homogeneous echo pattern owing to the nonspecular reflections from the hepatocytes. Interspersed within the parenchyma are tubular, fluid-filled structures representing the branches of the portal and hepatic veins. The liver is usually more echogenic, or it can also appear to be isoechoic, to the normal renal cortex (Fig. 7-3A, B). The liver is less echogenic than the normal pancreas and spleen. Disease processes can change these relationships. An example is when a normal right kidney appears to be
more echogenic than the liver in a patient with hepatitis, whose inflamed liver is abnormally hypoechoic.






The liver varies somewhat in shape and size, depending on the patient’s body type, the size of the left lobe, and the length of the RL. The weight of a normal liver varies but usually ranges from about 1,200 g in an adult female to about 1,600 g in an adult male.7 The weight of the liver is approximately 1/36th of the total body weight for an adult as compared to approximately 1/18th of the total body weight for an infant.9 The normal transverse measurement of the liver can range from 20 to 22.5 cm, the anteroposterior (AP) measurement from 10 to 12.5 cm, and the length
from 13 to 15.5 cm, although some references go as high as 17 cm.11 An AP measurement can be obtained on the same image as the length if needed. The length of the liver is obtained by measuring the liver from the diaphragm to the tip of the RL at the right midclavicular level, which is an imaginary vertical line that goes through the middle of the clavicle.10,11 The liver’s size can increase with increased height and body surface area and decrease with age.10 Liver length is needed to determine if hepatomegaly is present.











Kratzer and coworkers did an extensive sonographic liver measurement study on 2,080 subjects. They found that the average liver length at the midclavicular line was 14.0 ± 1.7 cm. The average length for male subjects was 14.5 ± 1.6 cm and for female subjects 13.5 ± 1.7 cm.12 Their technique allows for easy assessment and accurate follow-up examinations. They also demonstrated that body mass index, height, sex, age, and frequent alcohol consumption in men influenced liver size.12


Perihepatic Relationships

The liver is an intraperitoneal organ that is covered by a capsule that is composed of two adherent layers. One layer is an outer serous layer that is derived from the visceral peritoneum and covers the liver except at the bare area near the diaphragm, the porta hepatis, and the area where the gallbladder is attached to the liver. The inner layer is a dense, fibroelastic connective tissue called Glisson capsule, which is named after the British physician and anatomist Francis Glisson, who was the first person to describe the
layer of connective tissue that covers the entire liver and each lobule. Glisson capsule contains blood, lymphatic vessels, and nerves and is highly echogenic by sonography.10,13,14 Distention of the capsule from liver disease or swelling can cause pain, and lymphatics may ooze fluid into the peritoneal space.13 Glisson capsule also ensheaths the portal triad, which consists of a branch of the hepatic artery, portal vein, and bile duct, which explains why the portal triad has echogenic borders.






The external surfaces of the liver are described as the diaphragmatic and visceral surfaces. The diaphragmatic surface is the anterosuperior surface of the liver and is smooth and convex, fitting beneath the curvature of the diaphragm. The posterior aspect of the diaphragmatic surface is not covered by the visceral peritoneum and is called the bare area. The bare area is a small triangular area where the liver connects to the diaphragm, and although it is not covered by the visceral peritoneum, it is covered by Glisson capsule. The bare area lies between the anterior and posterior folds of the coronary ligament and is clinically important because it represents an area where disease, such as an abscess or tumor, can spread from the abdominal cavity to the thoracic cavity because the barrier in the form of the visceral peritoneum is absent9,15 (Fig. 7-4 A-D).

The visceral surface of the liver faces inferiorly and posteriorly with its shape determined by the surrounding organs. It is uneven and concave and is covered by the peritoneum except in the fossa of the gallbladder, the
porta hepatis, and the IVC groove. It is in contact with the esophagus, the stomach including the pylorus, the first part of the duodenum, the right hepatic flexure, the transverse colon, the lesser omentum, the gallbladder, the right kidney, and the right adrenal gland.


Ligaments

The liver is tethered to the undersurface of the diaphragm, the anterior wall of the abdomen, the lesser curvature of the stomach, and the retroperitoneum by eight ligaments, seven of which are either parietal or visceral peritoneal folds and one of which is a round, fibrous cord. These ligaments are as follows: the coronary, falciform, ligamentum teres (round ligament), ligamentum venosum, right and left triangular, gastrohepatic, and hepatoduodenal ligaments.15 The largest of these ligaments is the coronary ligament. They are described in Table 7-1 and can be identified on Figures 7-4 and 7-5. Understanding the location of these various ligaments can help in identifying the location of various pathologies and fluid collections.


Lobar Anatomy

The lobes of the liver can be described based on their anatomic or external landmarks or their functional anatomy, which is based on the portal and hepatic veins. Because the author may not specify which method is being used, the literature can be confusing and may appear to be contradictory. The next section presents each method separately. The sonographic images are presented in the segmental division because sonographers evaluate and document the liver using internal landmarks.









Anatomic Division

The anatomic division of the liver is based on external markings that divide the liver into right, left, caudate, and quadrate lobes. This method uses the falciform ligament on the anterior surface to divide the liver into its left and right lobes and considers the caudate and quadrate lobes as part of the RL. A simulated “H” configuration on the visceral surface has the ligamentum venosum and the ligamentum teres separate the caudate and the quadrate lobes from the left lobe, the porta hepatis separates the caudate from the quadrate, and the main lobar fissure (MLF) separates the caudate and the quadrate from the RL (Fig. 7-6).


Right Lobe

The RL occupies the right hypochondrium and is six times larger than the left lobe with this method.5 It is separated from the left lobe by the falciform ligament on its anterior surface and by the left intersegmental fissure on its visceral
surface.16 It is somewhat quadrilateral. The posterior surface is marked by three fossae: the porta hepatis, the gallbladder, and the IVC.17












Caudate Lobe

The caudate lobe (CL) is anatomically distinct from the left and right lobes and is situated on the posterosuperior surface of the RL opposite the 10th and 11th thoracic vertebrae. It is located between the IVC posteriorly and the ligamentum venosum anteriorly.18,19 The ligamentum venosum separates the caudate from the left lobe. The left margin of the caudate forms the hepatic boundary of the superior recess of the lesser sac. The caudate process is a small elevation of hepatic tissue that extends obliquely and laterally, from the lower aspect of the CL to the undersurface of the RL. It is situated behind the porta hepatis and separates the gallbladder fossa from the commencement of the fossa for the IVC. The papillary process is an anteromedial extension of the CL, which may appear separate from the CL and mimic lymph nodes.10,15 Physiologically, the CL is considered independent from the liver vasculature because it has its own arterial supply and venous drainage. Blood drains from the CL directly into the vena cava via the small caudate veins.9


Quadrate Lobe

Anatomists describe the quadrate lobe as a distinct lobe, but it is physiologically and sonographically the same as the medial segment of the left lobe. The quadrate lobe is located on the undersurface of the liver between the middle and the left hepatic vein. Situated on the visceral surface, it is bounded posteriorly by the porta hepatis, anteriorly by the inferior margin of the liver, and laterally by the gallbladder fossa on the right and the fissure for the ligamentum teres on the left.15


Left Lobe

The left lobe is situated in the epigastric and left hypochondriac regions. Anatomically, it is separated from the RL by the falciform ligament on its anterior surface. On the visceral surface, the fissure for the ligamentum teres separates it from the quadrate lobe, and the fissure for the ligamentum venosum separates it from the CL.15 The left lobe is flatter and smaller than the right and can vary in size. Its superior surface is slightly convex, is molded to the diaphragm, and tapers off to the left at about the left mammary line.9,15


Sonographic Segmental Division

Ultrasound can easily divide the liver into three lobes and four segments using the main lobar fissure, the hepatic
veins, and the ligamentum venosum and ligamentum teres (Fig. 7-7A). The lobes and segments are the RL, with an anterior and a posterior segment, the left lobe, with a medial and a lateral segment, and the CL.15 A combination of scanning planes that includes transverse, oblique, and parasagittal is needed to delineate the landmarks to identify the lobes and segments.


Right and Left Lobes

The MLF is a line that connects the gallbladder and the IVC and contains the middle hepatic vein (MHV). It is used to separate the liver into right and left lobes (Fig. 7-7B). It is seen sonographically as an echogenic line that runs obliquely between the neck of the gallbladder and right portal vein (RPV) (Fig. 7-7C).

The right intersegmental fissure divides the RL into its anterior and posterior segments. The sonographic landmark for the right intersegmental fissure is the right hepatic vein (RHV) (Fig. 7-7D-F).

The left intersegmental fissure divides the left lobe into medial and lateral segments. The sonographic landmarks used to define this fissure are the left hepatic vein (LHV); the ascending branch of the left portal vein (LPV); and more inferiorly, the hyperechoic ligamentum teres (Fig. 7-7G-J). In patients with ascites, the externally located falciform ligament may be a visible landmark for separation of the medial and lateral segments of the left lobe (Fig. 7-7K).

The sonographer should understand that the hepatic veins are intersegmental—that is, they run between the segments—and that the portal veins are intrasegmental, because they will travel within the segments, except for the ascending portion of the LPV, which runs in the left intersegmental fissure (Fig. 7-7L). The RPV divides into an anterior segmental branch and a posterior segmental branch, supplying blood to each respective segment (Fig. 7-7M). The LPV divides into medial and lateral segment branches supplying blood to their respected segments (Fig. 7-7N). Both portal veins and hepatic veins have distinguishing features by sonography that enable differentiation between them (Table 7-2).10,17,26


Caudate Lobe

The CL is located in the posterior aspect of the liver and corresponds to the anatomic CL. Sonographically, three anatomic landmarks identify the CL. The anterior border of the CL is the fissure for the ligament venosum, which appears as a hyperechoic line that separates the CL from the left lobe, the IVC runs along the posterior aspect of the CL and the main portal vein (MPV) is located at the inferior aspect of the CL, separating it from the head of the pancreas (Fig. 7-7O, P).15,16,19

The CL is functionally distinct from the right and left lobes because it has its own bile ducts and receives its blood supply from both right and left portal and hepatic arterial branches.10,19 It is drained by short venous channels that extend from its posterior aspect and drain directly into the IVC (Fig. 7-7Q).19 This independent blood supply has significant clinical implications that will produce changes in the size of the lobe with certain pathologies.20,21 The portal veins and hepatic arteries of the CL have a short intrahepatic course, which is not as readily affected by hepatic fibrosis. CL enlargement and RL shrinkage are seen in patients with cirrhosis and may be attributed to the discrepancy between blood perfusion of the two lobes.22 CL enlargement is also seen in patients with Budd-Chiari syndrome, which is thrombosis of the hepatic veins, cavernous transformation of the portal vein, and end-stage primary sclerosing cholangitis, which is a long-term inflammation and scarring of the bile ducts.


Couinaud’s Hepatic Segment Classification

In 1957, Claude Couinaud (pronounced kwee-NO), a French surgeon, described a detailed method of liver segmentation, which has become the universal nomenclature for hepatic lesion localization.10 It is currently the most widely used system to describe functional liver anatomy. The Couinaud classification system creates eight functionally separate liver segments, counted in a clockwise fashion. Traditionally, the hepatic segments were numbered using Roman numerals I to VIII, but the Arabic numerals 1 to 8 are now preferred (Fig. 7-8A-F). The CL is segment 1, the left lobe makes up segments 2 to 4, and the RL makes up segments 5 to 8 (Table 7-3).10,23,24 The division is based on the hepatic and portal vein branches. Each of the eight segments has its own branch of a portal vein, hepatic artery, hepatic vein, and bile duct. This allows the surgeon to resect a segment of a hepatic lobe, allowing the vascular supply to the remaining lobes to be left intact. The hepatic veins provide the boundaries of each segment. In Couinaud’s segmental anatomy, the three planes of the right, middle, and left hepatic veins divide the liver vertically. The MHV lies in the MLF and divides the liver into right and left lobes. This vertical plane courses from the IVC to the gallbladder fossa and is also known as the Cantlie line. The portal plane is a horizontal plane where the portal vein bifurcates and further divides the liver into superior and inferior segments. These three vertical planes and one horizontal plane divide the liver into the eight segments as summarized in Table 7-3.10,23,24 These segmental divisions do not coincide completely with the lobar anatomy divisions because the quadrate lobe is now the medial segment of the left lobe, the anatomic left lobe is now the lateral segment of the left lobe, and the CL is considered a separate lobe.

The Couinaud classification system of eight independent functional segments is important for the sonographer to understand because computed tomography (CT) and magnetic resonance imaging (MRI) will report the location of liver masses based on these eight segments. If the CT report states that the mass is in segment V, the sonographer will know to concentrate on the RL between the RHV and the MHV and posterior to the RPV. If the CT report states that the mass is in segment III, the sonographer will know to look in the left lobe lateral to the LHV. When a liver mass is initially discovered on an ultrasound examination, the sonographer should try and determine which segment contains the mass.10,23 This can be difficult with ultrasound, and the sonographer may only be able to narrow the location down to the left medial or lateral lobe using the LHV or ligamentum teres, the right anterior or posterior segment using the RHV or the CL.


Anatomic Variations

Variations of the liver include variations in shape, variations in lobe size, thinning of the left lobe, congenital absence of the left lobe, diaphragmatic indentations called pseudofissures,



high posterior hepatodiaphragmatic interposition of the colon, and situs inversus (Fig. 7-9A-C).9,20,25 A common variant that can mimic hepatomegaly is called a Riedel lobe, which is a downward projection of the RL. (Note that the proper name is Riedel lobe and not Riedel lobe.) It is more common in women than in men. Sonographically, a Riedel lobe is identified as a finger-like or a tongue-like projection of the RL that extends past the ribs and may reach as far as the iliac crest25 (Fig. 7-10A, B). A Riedel lobe can be felt clinically, and the patient may be referred for a liver ultrasound to rule out a liver mass or hepatomegaly. To differentiate it from a mass, the sonographer must observe consistency of the echotexture between the Riedel lobe and the rest of the RL. To differentiate it from hepatomegaly, the sonographer should note that the rest of the liver’s size appears normal, and that the lobe comes to a point, whereas with hepatomegaly, the lobe has a more rounded edge (also see Fig. 7-16F, G). In hepatomegaly, the RL of the liver can displace the right kidney downward and up toward the diaphragm whereas a Riedel lobe “skims” across the top of the kidney.
























Vascular System

The liver is a unique organ in that it receives a blood supply from two different sources. The hepatic artery supplies about 20% to 30% of the blood supply to the liver and 40% to 50% of oxygenated blood. The remaining 70% to 80% is supplied by the portal venous blood, which is nutrient rich and contains oxygenated blood, supplying 50% to 60% of the oxygenated blood owing to its greater volume. The portal vein and the hepatic artery mix their blood in the liver sinusoids, which are drained by the central hepatic vein (Fig. 7-11A-C).


Hepatic Arteries

The common hepatic artery supplies oxygenated blood to the liver, pylorus of the stomach, duodenum, pancreas, and gallbladder. It is the largest branch of the celiac axis or trunk and the only branch that courses to the right across the epigastric region of the abdomen. The common hepatic artery passes anterior to the pancreas, and then inferiorly to the right toward the first part of the duodenum. It gives off the right gastric artery, which will anastomose with the left gastric artery. The common hepatic artery then travels upward, lying to the left of the common bile duct and anterior to the portal vein. As it courses toward the porta hepatis, it gives off the gastroduodenal artery and then becomes the proper hepatic artery, terminating into the right and left hepatic arteries that will supply blood to the right and left lobes, respectively. The cystic artery arises off of the right branch and the middle hepatic artery usually arises from the left branch (Fig. 7-11D-H).


Portal Veins

The MPV measures approximately 8 cm in length and has a maximum diameter of 13 mm. It originates just to the right of the midline at the junction of the splenic vein and superior mesenteric vein (SMV) anterior to the IVC and posterior to the neck of the pancreas. It carries blood containing nutrients and toxins from the gastrointestinal (GI) tract, except for the lower section of the rectum, the pancreas, gallbladder, and the spleen. The MPV courses upward and toward the porta hepatis within the hepatoduodenal ligament and is posterior to the hepatic artery and the common bile duct. Before entering the liver, the MPV divides into a smaller LPV and into a larger RPV with each branch entering the liver separately (Fig. 7-11I).10,26 The LPV lies more anterior and superior to the RPV. The LPV can be divided into transverse and umbilical portions. The LPV, prior to its bifurcation, gives off a branch that supplies the medial segment of the left lobe10 and a branch for the lateral segment of the left lobe (Fig. 7-11J). Both are intrasegmental in their course.26 Prior to the bifurcation is the umbilical portion of the LPV, named because in utero, the umbilical vein is attached at this point.9 The umbilical portion of the LPV provides blood to the CL through small branches that are not visualized sonographically. The size of the LPV and its angle of bifurcation with the MPV vary, depending largely on the size and configuration of the left lobe.17 The main branches of the LPV originate from the umbilical portion and supply liver segments 2, 3, and 4. The RPV bifurcates at various distances from the MPV to






supply the right anterior and right posterior intrahepatic lobar segments (Fig. 7-11K-N).9 The RPV divides into an anterior branch that supplies blood to segments 5 and 8, and a posterior branch that supplies blood to segments 6 and 7. Segment 1, the CL, is not considered to be part of either the right or left lobe and receives blood from multiple branches from both the left and right portal veins. The hepatic artery, portal vein, and intrahepatic duct course parallel to each other with hepatic and portal blood flowing into the liver and bile flowing in the opposite direction out of the liver (Fig. 7-11O). Intrahepatic ducts are not visualized routinely. If biliary radicles and portal venous radicles are imaged simultaneously side by side, the appearance, referred to as the parallel-channel sign, is used to diagnose biliary obstruction (Fig. 7-11P). Color or power Doppler imaging can distinguish the vessel from the bile duct. The biliary system is discussed in more detail in a later chapter.












































Hepatic Veins

The hepatic veins are best visualized on transverse scans by angling the transducer toward the patient’s head from a subcostal approach. An intercostal approach from the right can also be helpful to evaluate them. The hepatic veins drain directly into the superior aspect of the IVC and all three veins, the right, middle, and left, should routinely be demonstrated emptying into the IVC (Fig. 7-11Q-S).27

The RHV is the largest of the three hepatic veins and is located in the right intersegmental fissure and is used to divide the right hepatic lobe into anterior segments 5 and 8 and posterior segments 6 and 7. The RHV drains segments 5, 6, 7, and 8. A common anatomical variant is an accessory inferior right hepatic vein (IRHV). There is controversy regarding the incidence based on autopsies ranging from 61.4% to 88%,28 and based on sonography, color Doppler, and CT imaging; the incidence has been reported from 10% to 28.33%.28 When identified, the IRHV is visualized on a transverse section below the RHV (Fig. 7-11A and S).10,29 Attempting to identify an IRHV variant can be clinically important for several reasons: (1) The entire main RHV is resected during a hepatectomy, and the RL can be preserved along with the hypertrophic IRHV. (2) Thrombus has been identified in the IRHV in patients with hepatocellular carcinoma (HCC). (3) Finally, the RL’s main drainage vein becomes the IRHV in patients with Budd-Chiari syndrome.28,29 The MHV is located in the MLF, separates the right and left lobes, and drains segments 4 of the left lobe and segments 5 and 8 of the RL.10 The smaller LHV is located in the cephalic portion of the left intersegmental fissure and divides the left lobe into lateral and medial segments27 and drains segments 2, 3, and 4. A common variant is for the MHV and the LHV to join and become a common trunk before emptying into the IVC.


Sonographic Distinction between Portal and Hepatic Veins

It is important to be able to differentiate between the portal and hepatic veins sonographically. The five criteria used to distinguish hepatic veins and portal veins are described in Table 7-2. The easiest way to differentiate between the two vessels is by spectral Doppler imaging because each vessel has its own distinct waveform.


Porta Hepatis

The porta (gate) hepatis (liver) is a fissure where the portal vein and hepatic artery enter the liver and the bile duct exits the liver.9 In the normal relationship of these three structures within the hepatoduodenal ligament, the bile duct is ventral and lateral, the hepatic artery is ventral and medial, and the portal vein is dorsal (see Fig. 7-11B, C).15

It is important to evaluate these structures and measure the bile duct in either a transverse or longitudinal section. To do this, the splenic vein should be located on a transverse section and followed to the right, where it is joined by the SMV and becomes the portal-splenic confluence (Fig. 7-12A). From the confluence, the MPV will course toward the liver and appear round. The transducer should be positioned at an oblique angle, approximately 45 degrees (from right shoulder to left hip), until the long axis of the MPV, bile duct, and hepatic artery are identified. A measurement of the bile duct should be made by placing the calipers along the inner wall to the opposing inner wall of the duct. The published values for the normal internal diameter of the bile duct vary from 4 to 8 mm. A transverse section through the portal triad creates what is called the Mickey Mouse sign (Fig. 7-12B). This is just a quick overview, and in-depth knowledge of the biliary system can be acquired from Chapter 8.

The bile duct and hepatic artery may be confused because of their proximity, their similar internal diameter, and common anatomic variations of these structures. Color Doppler imaging can help in distinguishing the bile duct from the hepatic artery owing to the presence of flow in the artery and absence of flow in the bile duct (Fig. 7-12C). In addition, Berland and colleagues29 found several reliable sonographic signs to differentiate the bile duct from the hepatic artery, including evaluating the porta hepatis with Doppler techniques:



  • Only pulsations should be exhibited by an artery or a vein.


  • An artery can indent the wall of a duct or a vein, but the reverse is not true. This is probably because of the lower venous and ductal pressures and the thicker, less easily deformed arterial wall.


  • The duct can occasionally decrease several millimeters in caliber during an examination and can have various calibers along its course, whereas arteries are uniform in caliber.


  • The artery may not parallel the portal vein or may do so only for a short distance, whereas the duct parallels the portal vein closely.


  • Arteries may be tortuous and loop in and out of the scanning plane.


  • Arteries produce pulsatile Doppler signals, veins produce continuous Doppler signals, and ducts produce no signal.








Microscopic Structures

Hepatic lobes are made up of the basic functional unit of the liver, which is the liver lobule.5,6 The liver parenchyma is made up of 50,000 to 100,000 individual lobules (Fig. 7-13A, B).4 Within each lobule, small bile canaliculi lie adjacent to the cellular plates and receive the bile produced by the hepatocytes to carry it toward the bile duct branches in the triad regions (Fig. 7-13C).4,6,13 These branches open into the interlobular bile ducts accompanying the hepatic artery and portal vein, except that bile flows in the direction opposite to that of blood in these vessels.4,6 Bile ducts join other bile ducts and eventually form two main trunks, the right and left hepatic ducts, which eventually join to become the common hepatic duct.6,7








PHYSIOLOGY

The liver is an organ essential to life and performs more than 500 separate functions.4,13 A single liver cell is so diversified in its activities that it is analogous to a factory for many chemical compounds, a warehouse with short- and long-term storage capabilities, a power plant producing heat, a waste disposal plant excreting waste, and to a chemistry lab regenerating tissue that has not been too severely damaged. These functions are carried out by three types of cells in the parenchyma: the hepatocyte, which carries out most metabolic functions; the biliary epithelial cells, which line the biliary system, bile ducts, canaliculi, and gallbladder; and the Kupffer cells, which are phagocytic and belong to the reticuloendothelial system.4

It is not necessary to know these liver functions in great detail to obtain quality sonograms; however, because hepatic diseases alter these functions and produce identifiable clinical manifestations, it is important to have a basic understanding of some normal functions (Table 7-4).2,4, 5, 6, 7, 8 and 9,13











LIVER FUNCTION TESTS

Liver function tests, commonly referred to as LFTs, are a group of blood tests that can tell the physician how the liver is performing under normal and diseased conditions. This group of tests is also referred to as a liver panel and includes total bilirubin, alanine aminotransferase (ALT), aspartate aminotransferase (AST), AST/ALT ratio, alkaline phosphatase (ALP), gamma-glutamyl phosphatase (GGT), and albumin. AST may also be referred to as aspartate transaminase or the older serum glutamic-oxaloacetic transaminase (SGOT), which is no longer used. ALT may also be referred to as alanine transaminase or the older serum glutamic-pyruvic transaminase (SGPT), which is also no longer used. Patients undergoing certain invasive procedures or surgery will have their bleeding times evaluated, especially if they have liver disease because one of the functions of the liver is to make clotting factors. These tests include prothrombin time (PT) and partial thromboplastin time (PTT). To evaluate for dilated biliary ducts is the main reason an ultrasound is ordered on a patient with increased LFTs. The presence of dilated ducts will mean that the patient will need to be treated surgically to relieve the obstruction. If the ducts are normal, then the reason for the increased values will be a medical condition such as hepatitis, fatty liver disease, overuse of over-the-counter drugs such as acetaminophen, prescription drugs such as statin drugs, alcohol abuse, and primary or metastatic disease. (The most common cause of acute liver failure in the United States is an overdose of acetaminophen, usually as a suicide attempt.) The most common laboratory tests are listed in Table 7-5.26,31, 32 and 33 Normal reference ranges are not provided because they can vary by gender and age, and values can be updated periodically. The range of normal values used will be on the patient’s report next to their current value, with abnormal values indicated and clearly marked as high or low.








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Dec 10, 2022 | Posted by in ULTRASONOGRAPHY | Comments Off on The Liver
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