What are the non shadowing gallstones?

The ghost type: When gallstones are not heavily calcified, they are tough to see on x rays.

Sludge v. Stones: The preferred mode of identification of gallstones is ultrasound; stones typically are solid and cast a shadow on ultrasound. Some stones are too small to cast a shadow or may represent "sludge" within the gallbladder. When i see this result on ultrasound, it tells me that there may not be stones; therefore, i would only recommend surgery if the rest of the clinical picture was crystal-clear.