I did some searching on-line and I haven't seen anything saying that cholesterol gallstones(which are over 70% of what people have) can't be found with ultrasound. It looks like they are almost always detected.
What report did you find that says that they can't be detected with ultrasound? Could you post the link to that article?
This first report says that 90-95% of Gallstones are detectable.
And this one too:
"HOW ARE Gallstones AND GALLBLADDER DISEASE DIAGNOSED?
The diagnostic challenge posed by Gallstones is to be sure that abdominal pain is caused by stones and not by some other condition. Ultrasound or other imaging techniques usually detect gallstones readily. Nevertheless, because gallstones are common and most cause no symptoms, simply finding stones does not necessarily explain a patient's pain, which may be caused by numerous other conditions. " http://www.reutershealth.com/wellconnected/doc10.html http://www.umm.edu/patiented/doc10full.html