Probably talking about a radiograph which is the best way to positively identify a liver shunt without surgery. It's possible the liver shunt is within the liver itself and can't be seen with an ultrasound.
Taken from a very informative site on liver shunts:
http://www.geocities.com/class_act_k...ver_shunt.html
Liver Imaging: RADIOGRAPHY - Invasive - A dye is injected into the spleen and the dye is tracked through Radiograph through the blood vessels. A blood vessel bypassing the liver can easily be seen on the image. This is the only universally accepted method of confirming a shunt, short of major surgery, it however is extremely invasive and is a poor choice for "screening" purposes.
I believe the sentence at the end is only if the bile acid and blood tests do not indicate a problem since the site says ultrasound is unreliable and this is the only way to tell for sure, short of major surgery.