Localization of tumors to segments of the liver is critical in the planning of surgical resection. The international Couinaud (pronounced “kwee-NO”) system of hepatic nomenclature is currently utilized [4,5,6]. This system is based on the distribution of portal and hepatic veins. The right and left lobes are divided by the main hepatic fissure defined by the middle hepatic vein in the superior portion of the liver and by a line connecting the GB with the IVC in the inferior portion of the liver. Each segment has a branch PV at its center and a hepatic vein at its periphery. Segments are numbered clockwise starting with the caudate lobe (segment 1), left lobe (segments 2-4), and right lobe (segments 5-8) (Table 2.1, Fig. 2.1). The caudate lobe is anatomically distinct, extending between the IVC and the left lobe and separated from the left lobe by the fissure of the ligamentum venosum (Fig. 2.2).

Blood supply to the liver is provided by both the PV (~70%) and the hepatic artery (~30%). This dual blood supply makes infarction rare in the liver. Both enter the liver at the porta hepatis and divide into right and left lobe branches. Doppler documents blood flow direction in both vessels as into the liver (hepatopetal). Spectral Doppler shows a low-resistance, arterial flow pattern for the hepatic artery with forward flow throughout the cardiac
cycle. Diastolic flow velocity increases and resistance index decreases after eating. The PV shows continuous antegrade venous flow with small pulsations that mirror the cardiac cycle. Mean velocity in the PV is 15-18 cm/sec.

Venous drainage of the liver is by three major hepatic veins that enter the IVC just below the diaphragm (Fig. 2.3). The right hepatic vein divides the anterior and posterior segments of the right lobe and enters the IVC separately. The middle and left hepatic veins may join just before entering the IVC. The middle hepatic vein separates the right and left lobe while the left hepatic vein divides the medial and lateral segments of the left lobe. Hepatic veins have no valves and their blood flow reflects the triphasic pulsatility of the IVC and right atrium. Spectral Doppler shows prominent antegrade flow toward the heart during systole reflecting movement of the tricuspid valve toward the cardiac apex. A second antegrade peak is produced in early diastole by opening of the tricuspid valve. In late diastole flow is reversed in the hepatic vein owing to atrial contraction. The caudate lobe drains directly into the IVC via small venous channels.

The liver parenchyma has homogeneous echogenicity equal to or slightly greater than the echogenicity of renal parenchyma (Fig. 2.4). Liver echogenicity is slightly less than that of the spleen. Portal triads are seen as echogenic foci well out into the periphery of the liver
(Fig. 2.4). Fissures and ligaments are usually invested in fat and are highly echogenic (Fig. 2.2). The fissure of the ligamentum teres in the left lobe must not be mistaken for an echogenic mass (Fig. 2.5).

Intrahepatic bile ducts (IHBD) course in the portal triads with the PVs and hepatic arteries. In the portal triads the relationship of the three structures to each other is not constant. The bile ducts may be anterior to, posterior to, or wrap around the PV [7]. Small bile ducts progressively anastomose to form the right and left lobe bile ducts that join in the porta hepatis to form the common hepatic duct. The common hepatic duct becomes the CBD approximately 3 cm distally at the junction of the cystic duct. The common hepatic duct and CBD are approximately 10 cm in length (Figs. 2.6, 2.7). Since they course outside of the liver parenchyma both are considered extrahepatic bile ducts (EHBD). The CBD joins with the main PV and the proper hepatic artery to cross the foramen of Winslow in the hepatoduodenal ligament. In this region the three structures maintain a constant relationship with the larger PV posterior and the smaller CBD and hepatic artery anterior. In cross section the three structures resemble Mickey Mouse with the vein being his face, the CBD his right ear, and the hepatic artery his left ear (Fig. 2.8). Dilatation of the CBD enlarges Mickey’s right ear. The distal CBD passes behind the duodenum to run in a groove formed by the posterior pancreatic head and the medial aspect of the duodenum (see Fig. 2.14). The CBD enters the duodenum opposite the uncinate process of the pancreas.

The GB is a pear-shaped, bile-filled sac nestled in a concave fossa on the visceral surface of the liver (Figs. 2.6, 2.9). The fundus usually projects beyond the edge of the liver while the body and neck extend dorsally toward the porta hepatis. The hepatic surface of the GB is attached to the liver by blood vessels and connective tissue whereas the inferior surface and fundus of the GB are covered by visceral peritoneum. Occasionally the GB is suspended on a mesentery and is not closely applied to the liver. The GB wall is made up of three layers of tissue. The inner mucosa is redundant and loosely connected to the fibromuscular layer. The mucosa is mucin-secreting columnar epithelium and is continuous with the epithelium of the cystic duct and biliary tree. The fibromuscular layer provides the framework of the sac with interlaced fibrous tissue and smooth muscle. The external coat is the covering peritoneum. The cystic duct is 4 cm long extending from the GB neck to the hepatic duct which it joins to form the CBD. The mucosal lining of the cystic duct forms 5-12 folds that constitute the spiral valve of Heister. These folds may cast acoustic shadows and mimic small stones lodged in the cystic dust. After a fast of 4 or more hours the normal GB is well distended and easily visualized by US as a gourd-shaped sac of fluid. Normal bile is echo-free. The normal GB does not exceed 5 cm in transverse diameter. In 4% of patients the fundus
folds back on itself forming a “Phrygian cap.” Additional normal folds may be seen near the neck of the GB. The GB wall is visualized as a thin echogenic line at the GB interface with the liver. Wall thickness is measured between the liver parenchyma and the GB lumen. This measurement, which includes the liver capsule, the entire GB wall, and intervening tissue, is normally less than 3 mm.

The pancreas is identified on US by recognition of the blood vessels within and around the pancreatic parenchyma (Figs. 2.10, 2.11, 2.12, 2.13 and 2.14). The neck, body, and tail of the pancreas course anterior and parallel to the SV. The total length of the pancreas from head to tail is 12-15 cm. The neck of the pancreas is anterior to the confluence of the SV and the SMV that forms the PV. The SV is of uniform diameter (<10 mm) until its junction with the SMV where the combined veins form a teardrop-shaped dilatation (Figs. 2.12, 2.13 and 2.14). The SMV courses in sagittal plane just to the right and usually slightly anterior to the superior mesenteric artery (SMA). The uncinate process of the pancreas extends leftward beneath the SMV to form a
tapered projection. Blunting of the normally tapered uncinate process is a sensitive sign of pancreatic enlargement or tumor. The SMA arises anteriorly from the aorta at or near the level of the crossing SV (Fig. 2.10). The SMA is surrounded by a collar of echogenic fat and appears, on transverse section, as the hole in a doughnut (Figs. 2.12, 2.14). The left renal vein and the transverse portion of the duodenum course underneath the SMV and SMA caudal to the level of the pancreas. The celiac axis arises from the aorta just cephalad to the pancreas (Fig. 2.10). In transverse plane the bifurcation into the hepatic and splenic arteries resembles a seagull in flight. The left gastric artery origin off the celiac axis may be seen in longitudinal plane.

The CBD extends caudally in the posterior aspect of the pancreatic head where it is commonly visualized as a tubular structure that ends at the major papilla entering the descending duodenum (Fig. 2.14). The pancreatic duct courses centrally within the pancreatic parenchyma from the tail to the head (Fig. 2.15). It joins the CBD to drain into the major papilla in 80% of individuals. In the remaining 20% the pancreatic duct enters the duodenum separately at the minor papilla. The hypoechoic muscular wall of the stomach should not be mistaken for the pancreatic duct (Fig. 2.16). The gastroduodenal artery courses anterior and parallel to the CBD. Doppler US confirms its identification.

The normal echogenicity of the pancreatic parenchyma depends upon the amount of fatty infiltration. The pancreas has no distinct capsule and, with aging, fat infiltrates between the lobules of parenchyma and increases the echogenicity. In children and younger adults the pancreas resembles a slab of meat and has well-defined margins with echogenicity

approximately equal to liver parenchyma. In older patients the pancreas resembles a dust mop and has poorly defined margins with echogenicity just slightly less than fat.

The spleen is normally visualized in the left upper quadrant of the abdomen between the diaphragm and the fundus of the stomach (Fig. 2.17). The spleen is soft and pliable allowing it to conform to the shape of the structures around it. Its diaphragmatic surface is smooth and convex, matching the concavity of the diaphragm. Its visceral surface is rounded and smooth with concavities for the stomach, left kidney, and the splenic flexure of the colon. Usual dimensions in adults are 12 cm in length, 7 cm in breadth, and 3-4 cm in thickness.

The spleen parenchyma is a network of lymphatic follicles (the white pulp) surrounded by vascular lakes filled with blood (the red pulp). On US the splenic parenchyma is extremely homogeneous with mid-to-low-level echogenicity slightly greater than that of the liver. The SV runs a relatively straight course rightward from the splenic hilum to the commencement of the PV dorsal to the neck of the pancreas. The SV receives the small inferior mesenteric vein in the region of the distal body of the pancreas and joins with the larger SMV to form the PV. The splenic artery is tortuous as it courses from the celiac axis to the splenic hilum. As the splenic artery enters the hilum it divides into six or more branches which ramify throughout the parenchyma. The capsule of the spleen is covered by closely applied visceral peritoneum except at the hilum and a small “bare area” at the posterior dome of the diaphragm. The spleen is anchored by phrenicolienal, lienorenal, and gastrolienal ligaments that converge at the hilum.

The peritoneal cavity is that portion of the abdominal cavity that is bounded by the parietal peritoneum. It consists of numerous recesses formed by organs, ligaments, and peritoneal reflections [8]. The lesser sac is the large potential space behind the stomach. It communicates with the remainder of the peritoneal cavity only by the small opening of the foramen of Winslow. Fluid in the lesser sac usually occurs only as a result of disease in structures bordering the lesser sac [9]. The major recesses of the greater peritoneal cavity are the right
and left subdiaphragmatic spaces, the hepatorenal recess (Morison’s pouch) (Fig. 2.18), the paracolic gutters, and the pelvic cul-de-sac. These recesses are most apparent when fluid is present in the peritoneal cavity.