Ultrasounds are very misleading. I have heard many stories of Ultrasounds being misread. Ask Julia CHang, she has some misleading ultrasound stories to tell.
CT Scans are magnetic and show greater detail. In fact a CT Scan can even tell stone density, you will know if it is calcified or not after the CT scan.
My dr ordered both at my insistence. I have both scans on the same day the 25th. Cant wait to compare the reports.