ULTRASONOGRAPHY
Ultrasonography is a noninvasive method of reliably diagnosing portosystemic shunts and is the imaging modality most widely
used to diagnose portosystemic shunts. It does not require general anesthesia and allows visualization of the entire abdomen
so that concurrent abnormalities may be detected. The accuracy of ultrasonography, however, largely depends on the operator's
skill and experience.
Evaluating portosystemic shunts
Ultrasonography allows direct visualization of shunting vessels, differentiation between intrahepatic and extrahepatic portosystemic
shunts, and categorization of the subtype of portosystemic shunts. The use of Doppler color-flow ultrasonography allows detection
of turbulent blood flow and measurement of portal blood flow velocity. This method has enhanced the sensitivity of ultrasonography
in diagnosing portosystemic shunts in recent reports.18
A reduced ratio of portal vein diameter to aortic diameter (PV:Ao), as measured by ultrasonography, has been shown to predict
the presence of extrahepatic portosystemic shunts in one recent study.19 A right intercostal approach, usually between the 11th and 12th and ribs, is used to visualized both the portal vein and
aorta in cross-section. A PV:Ao ≤ 0.65 is predictive of an extrahepatic portosystemic shunt. A PV:Ao ≥ 0.8 excludes the presence
of an extrahepatic portosystemic shunt.19 A similar right intercostal approach is used to visualize and compare the portal vein and caudal vena cava. A PV:CVC of
≥ 0.75 has been shown to exclude the presence of an extrahepatic portosystemic shunt.19 Ultrasound may also be used to scan the portal vein for aberrant connections.18,19
Figure 3. A Doppler ultrasonogram of a dog with an intrahepatic portosystemic shunt. This image depicts a shunting vessel
within the right lateral liver lobe, connecting the portal vein and caudal vena cava. (Figure 3 courtesy of Philip Steyn,
BVSc, MS, DACVR, director of professional services and chief radiologist for Antech Imaging Services.)
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Intrahepatic portosystemic shunts are generally easier to visualize (Figure 3) than extrahepatic portosystemic shunts are because of interference by the gastrointestinal tract with visualization of extrahepatic
portosystemic shunts. A method of ultrasound-guided percutaneous splenic injection of agitated saline solution and heparinized
blood has been described to identify portosystemic shunting through visualization of microbubbles in the portal vein, caudal
vena cava, and right atrium in different patterns.20 The sensitivity and specificity have not yet been determined for this method.
Additional ultrasonographic findings that may be detected in cases of portosystemic shunts include microhepatica, decreased
numbers of hepatic vessels, renomegaly, and urolithiasis.18 The sensitivity and specificity for detection of intrahepatic portosystemic shunts is 95% to 100% and 100%, respectively,
compared with the more variable sensitivity and specificity values reported for extrahepatic portosystemic shunts of 81% to
90% and 67% to 97%, respectively.3,18,19 The overall sensitivity and specificity for the ultrasonographic diagnosis of all types of portosystemic shunts is 92% to
95% and 98% as reported in recent studies.18,19
Evaluating other hepatic disorders
Acquired extrahepatic portosystemic shunts often occur secondary to portal hypertension, as may be created by conditions such
as primary hypoplasia of the portal vein, hepatic arteriovenous malformation, and hepatic cirrhosis. Shunt vessels are generally
small, multiple, and visualized most commonly near the kidneys or spleen.3,21
Hepatic arteriovenous malformation is diagnosed through ultrasonography by detecting an enlarged portal vein, dilated and
tortuous intrahepatic portal branches, and hepatofugal flow.21 Portal blood flow is normally nonpulsatile, but in cases of hepatic arteriovenous malformation, Doppler pulse wave examination
reveals a pulsatile pattern within the portal venous system similar to that within the hepatic artery.22
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