Dogs livers are very sensitive to steroids. It causes a benign liver enlargement called steroid hepatopathy. The liver will enlarge and the enzymes will elevate(Especially the Alkaline phosphatase and the ALT). The liver enlargement will cause a large belly and somtimes panting. If the dog has sarcoptic mites, an insectidical dipping in either amitraz or phosmet will kill the mites. Westies are known for their allergic skin disease. An alternative to Prednisone is Atopica(cyclosporine). Also treat the skin or any secondary bacterial or yeast infection, as they both contribute to the itchiness.
As far as the ultrasound, if the dog is off steroids for several months, the liver enzymes should decrease. If they do, then the ultrasound may not be necessary. If the liver enzymes remain elevated then an ultrasound is probably indicated to see if liver disease is the reason for the elevation.