Imaging Tests of the Liver and Gallbladder
Imaging tests of the liver, gallbladder, and biliary tract include ultrasonography, radionuclide scanning, computed tomography (CT), magnetic resonance imaging (MRI), endoscopic retrograde cholangiopancreatography (ERCP), percutaneous transhepatic cholangiography, operative cholangiography, and simple x-rays.
(See also Overview of the Liver and Gallbladder.)
Ultrasonography uses sound waves to provide images of the liver, gallbladder, and bile ducts. Transabdominal ultrasonography is better for detecting structural abnormalities affecting just certain parts of the liver, such as tumors, than for abnormalities that affect the entire liver uniformly, such as cirrhosis (severe scarring of the liver) or fatty liver (excess fat in the liver). It is the least expensive and safest technique for creating images of the gallbladder and bile ducts.
Using ultrasonography, a doctor can readily detect gallstones in the gallbladder. Ultrasonography of the abdomen can distinguish whether jaundice (a yellowish discoloration of the skin and the whites of the eyes) is caused by obstructed bile ducts or by malfunctioning liver cells. If ultrasonography shows bile ducts that are dilated (widened), the cause is typically obstruction. Ultrasonography also provides guidance when doctors insert a needle to obtain a tissue sample for biopsy of the liver.
A type of ultrasonography called Doppler ultrasonography can show blood flowing through the blood vessels of the liver. Doppler ultrasonography can detect blockages in the liver's arteries and veins, particularly the portal vein, which brings blood from the intestines to the liver. Doppler ultrasonography can also detect the effects of high blood pressure within the portal vein (a condition called portal hypertension). Endoscopic ultrasonography uses a tiny probe on the tip of an endoscope that is passed through the mouth into the stomach and the first segment of the small intestine (duodenum), bringing the probe closer to the liver and its surrounding organs.
Radionuclide (radioisotope) scanning uses a substance containing a radioactive tracer that, when injected intravenously, collects in a particular organ. The radioactivity is detected by a gamma-ray camera, which is positioned over the upper abdomen and is attached to a computer that generates an image. A liver scan uses a radioactive substance that collects in liver cells.
Cholescintigraphy (hepatobiliary scintigraphy or scan), another type of radionuclide imaging, follows the movement of a radioactive substance as it is secreted from the liver and passes into the gallbladder and through the bile ducts into the duodenum (the first segment of the small intestine). This technique can detect a blocked cystic duct (the tube that joins the gallbladder to the major bile duct—see figure View of the Liver and Gallbladder). Such a blockage indicates acute inflammation of the gallbladder (cholecystitis).
Magnetic resonance imaging (MRI) can detect certain liver disorders, such as hepatitis, hemochromatosis, and fatty liver disease, that affect all areas of the liver uniformly. MRI shows blood flow, providing information about blood vessel disorders.
MRI technology can also provide images of the bile ducts and nearby structures, using a technique called magnetic resonance cholangiopancreatography (MRCP). The images produced are as good as those produced by more invasive tests, in which a contrast agent is directly injected into the biliary and pancreatic ducts. Unlike CT, MRI tests do not involve exposure to x-rays, though they are more expensive than CT and take longer.
Endoscopic retrograde cholangiopancreatography (ERCP) involves passing an endoscope (a flexible viewing tube) through the mouth, esophagus, and stomach into the duodenum. A thin tube is then inserted through the endoscope into the biliary tract. Doctors inject a radiopaque contrast agent through the tube into the biliary tract, and, at the same time, x-rays are taken of the biliary tract and pancreatic duct.
ERCP is occasionally used simply to see the biliary tract structures, although doctors usually prefer MRCP when available because it is just as good and is safer. However, unlike other diagnostic tests, ERCP allows doctors to do biopsies and certain treatments because an endoscope is used during the procedure. For example, with the endoscope, a stone in a bile duct can be removed, or a tube (stent) can be inserted to bypass a bile duct blockage caused by cancer. With ERCP, complications (such as inflammation of the pancreas [pancreatitis] or bleeding) occur about 1% of the time. If a treatment is done during ERCP, such complications can occur more often.
Understanding Endoscopic Retrograde Cholangiopancreatography
Percutaneous transhepatic cholangiography involves inserting a long needle through the skin into the liver and then injecting a radiopaque contrast agent into a bile duct in the liver, using ultrasonography for guidance. The x-rays clearly reveal the biliary tract, particularly any blockage within the bile ducts. Like endoscopic retrograde cholangiopancreatography (ERCP), percutaneous transhepatic cholangiography is used more often for treatment or biopsy than to obtain images of the biliary tract. Complications of percutaneous transhepatic cholangiography, such as bleeding and internal damage, make it a less desirable method than ERCP, except in special circumstances.
Operative cholangiography involves the injection of a radiopaque contrast agent directly into the ducts of the biliary tract during gallbladder surgery. X-rays then reveal clear images of the biliary tract. This test is used only occasionally, when other, less invasive tests do not provide enough information.