CT of the Hepatobiliary System and Pancreas Kelly Pollak, MS3
Module Outline • Part I: Liver Parenchyma • Part II: Biliary Tract • Part III: Gallbladder • Part IV: Pancreas
Part I: Liver Parenchyma
CT of the Liver Normal Anatomy (as seen on un-enhanced CT): • Hepatic parenchyma high density (liver > spleen > muscle) • Homogenous appearance of parenchyma • Hepatic veins and portal veins branch through parenchyma as lower density structures
Hepatic Anatomy – Segments • The liver is composed of right and left lobes (separated anatomically by a vertical plane through the IVC, gallbladder fossa, and middle hepatic vein), and a total of 8 segments, which are divided by main hepatic veins and portal veins (inferiorly) • Each segment has its own vascular supply and biliary drainage • The segments are numbered clockwise when the liver is viewed ventrally • It is useful to learn the individual segment locations on CT in order to localize masses
Hepatic Segments as seen on CT • Superior liver: Left, middle, and right hepatic veins (arrows) can be used to demark segments II, IV, VII, and VIII, and the IVC can be used to locate I (which lies next to it) : • Inferior liver: Fissure for falciform ligament appears (block arrow) and the left, middle, and right hepatic veins (black arrows) now can be used to demark segments III, IV, V, VI:
Role of intravenous contrast in liver CT • Increases the density of normal liver parenchyma • Emphasizes difference between parenchyma and poorly enhancing lesions • Scans at different time intervals after contrast administration allow visualization of different phases of opacification, enabling distinction of lesions such as hemangiomas and neoplasms
IV Contrast Distribution Over Time • Three phases of hepatic enhancement post-contrast injection: • Vascular: Rapid rise in aortic enhancement and gradual hepatic enhancement • Redistribution: Contrast diffuses from central blood compartment to extravascular liver compartment (increase in hepatic enhancement and decrease in aortic) • Equilibrium: Aortic and hepatic enhancement gradually decline as contrast diffuses back into central vascular compartment and to muscle and fat compartments
Normal liver, unenhanced CTNote the areas of hypodensity (arrows), which are normal hepatic and portal veins coursing through the liver. Photo, Armstrong et al, 2004
Normal liver CT, enhancedNote the increased density of the hepatic and portal veins. Also note the adjacent stomach, which is filled with contrast. Photo, Armstrong et al. 2004
Systematic Approach to Examining Liver Parenchyma • Observe for: • Overall shape • Should have smooth edges • cirrhosis • Homogeneity of parenchyma • Parenchyma should be homogenous. This helps in determining: • Liver metastases • Primary tumors • Abscesses • Cysts • Trauma
Shape • Normal liver edges should be smooth: • In Cirrhosis, liver edges have a nodular contour: Photo Lee et al, 1998 L=liver, C=caudate lobe
Homogeneity: Primary Benign Liver Masses • Contrast enhancement helps determine presence of hemangiomas: • In early vascular phase, hemangiomas are lower density than surrounding parenchyma • During later phases, hemangiomas appear higher density than surrounding parenchyma
CT Detection of Hemangioma Early arterial phase Later (redistribution) phase Photos, Armstrong et al, 2004
Homogeneity – Hepatic Neoplasms Contrast enhancement also helps identify hepatic neoplasms: Neoplasms, both metastases and primary neoplasms, can be hyper- or hypovascular. Hypervascular enhance brightly during early arterial phase, whereas hypovascular are hypodense in the early arterial phase (but enhance during the redistribution phase).
Homogeneity – Hepatic Neoplasms • Knowing which lesions are hypervascular and which are hypovascular can help identify the type of neoplasm, but the key thing is that they are of a different density than the surrounding liver parenchyma. • Hypervascular examples: carcinoid tumor mets, hepatocellular carcinoma • Hypovascular examples: colon cancer mets, cholangiocarcinoma • Most mets, as opposed to primary tumors, are rounded and well demarcated from surrounding parenchyma on enhanced scans.
Appearance of various liver neoplasms during early arterial phase Hypovascular metastasis due to colon cancer Hypovascular primary cholangiocarcinoma Carcinoid tumor metastasis is hypervascular Primary hepatocellular carcinoma is hypervascular (hypodense area is necrosis)
Homogeneity – Cysts and Abscesses Contrast also helps identify cysts and abscesses, which contain collections of fluid • Cysts: Have well-defined margins and are low density (attenuation similar to water), unenhancing lesions • Note: cysts below ~ 1cm in size cannot be reliably distinguished from neoplasms • Abscesses: appear similar to cysts, but usually their walls are thicker (due to surrounding edema) and more irregular • May not be able to distinguish from a necrotic tumor
Hepatic Cyst vs. Abscess Photo, Novelline et al, 2004 Photo Lee et al, 1998 Left, hepatic cyst; right, hepatic abscess. Note the thickened wall of the abscess.
Homogeneity – Liver Trauma • Trauma can cause hepatic parenchymal lacerations, subcapsular and intrahepatic hematomas • All are low-density areas relative to contrast-enhanced parenchyma • Leakage of contrast = active bleeding
Hepatic Laceration Photo, www.e-radiography.net
Quiz time Identify and localize the following liver abnormalities on CT
Answer There is hepatocellular carcinoma in the second segment of the liver. Notice how it enhances here during an early arterial phase scan (hypervascular) and is less well defined than metastases would appear.
Answer This represents cirrhosis of the liver. Note the nodular appearance of the liver, instead of the usual smooth edges characteristic of a normal, healthy liver.
What is the abnormality, and in what segment is it located? Image, www.learningradiology.com
Answer • There is a laceration from a traumatic injury to the liver, located in segment VII.
Part II: Biliary Tract CT
Normal Anatomy • Bile (green tract in image) flows thru biliary tree from periphery of liver to duodenum • Biliary tree: intrahepatic ducts, common hepatic duct (CHD), and common bile duct (CBD) • Intrahepatic ducts course from periphery centrally to hepatic hilum • Join to form centrally located main left and right hepatic ducts • Portal triad: intrahepatic ducts are located adjacent to portal veins and hepatic arteries • Left and right hepatic ducts join to form common hepatic duct near liver margin • Porta Hepatis – CHD runs with portal vein and hepatic artery • CHD joins cystic duct to form CBD inferior to the liver
Appearance on CT • Superior slices: With contrast, intrahepatic ducts appear as hypodense areas in the periphery of the parenchyma (look very closely to see); they appear near portal veins and hepatic arteries, which enhance. • More inferior slices: As move inferiorly, right and left hepatic ducts appear centrally (hypodensities, arrows), adjacent to the right and left portal veins (brightly enhancing, block arrows).
Appearance on CT, cont’d. Common hepatic duct forming • Further inferiorly: The left and right main hepatic ducts fuse to form the common hepatic duct, and the left and right portal veins fuse to form the portal vein. • Even more inferior: Common hepatic duct (and porta hepatis) appears. Portal vein forming CHD Hepatic artery Portal vein
Appearance on CT, cont’d. • Most inferior: Gallbladder appears, left lobe of liver starts to disappear
Click through the following slides to familiarize yourself with the progression of the biliary system superior-to-inferior within the liver
Systematic Approach to Examining Biliary Tract on CT • Things to look at: • Bile duct size • Peripheral ducts: mean diameter=1.8mm • Central ducts: mean diameter=2mm • Common hepatic duct: mean diameter=2.8mm • Bile duct wall • Wall enhances to varying degrees with IV contrast (insensitive indicator of pathology) • Thickness important; normal 1-1.5mm • Density • Normal is near water density
Abnormalities – Biliary Dilatation • Dilated biliary ducts are a feature of biliary obstruction, common causes of which include: • Impacted stone in CBD • Carcinoma in head of pancreas • Carcinoma in ampulla of Vater Note the greatly enlarged intrahepatic bile ducts. As expected, they are hypodense compared to the liver parenchyma. Photo Armstrong et al, 2004
Part III Gallbladder
Gallbladder Anatomy • Gallbladder is a storage organ • It is located within the gallbladder fossa of the liver, which separates the right and left lobes of the liver • Its wall is normally thin, and it is usually filled with bile
Gallbladder Appearance on CT • Sits in fossa between right and left lobes of liver • Density: fluid density, free of particulate debris • Usually distended with bile
Systematic Approach to Observing Gallbladder on CT Observe for three things: size, density, and surroundings: • Size: • Overall size: Diameter 2-5cm • Wall size: 3mm thickness • Density: Homogenous, fluid density • Surroundings: No surrounding edema should be present