Gall bladder ejection fractions greater than 35% are generally considered normal. Most often, values lower than 35% are due to gall stones, or gall bladder muscle dysfunction NOT attributable to obstruction from gall stones.
See the following reference for elaboration http://jnm.snmjournals.org/cgi/reprint/32/9/1695?ijkey=04710f50335540ad9c2c23277417b2ab42a16df0