As far as imaging goes, scintigraphy is preferred over ultrasound for diagnosing shunts, but you don't have to jump into either right now. As I said, a Protein C test would your next step. Can you speak to the vets at Banfield and see if they can collect the sample to send to Cornell?
Here is a little more information for you on the differnet diagnostics:
Hepatic Vascular Disorders - WSAVA 2006 Congress
BTW, Dr. Sharon Center at Cornell and Dr. Karen Tobias at UT are considered the top experts on liver disease. Dr. Center is currently working on trying to find a genetic marker for shunts with a grant from the AKC.