Ultrasounds are not reliable for diagnosing liver shunts. They can be as low as 60% accurate according to Dr. Sharon Center.
Do you have a vet school near you? You can waste a lot of time and money with your regular vet. Most vets don't see enough liver disease in their practice to know how to diagnose and treat it.
The protocol has changed dramatically in the past few years. A bile acids test should have been done first, then a Protein C test should be done. Depending on the results, a scintigraphy is the next step.
Here is some information for you from the country's top two experts in liver disease:
Portosystemic Shunts FAQ Hepatic Vascular Disorders - WSAVA 2006 Congress