SelectBvalueData[{data,vals},sel]

selects the volumes and bvalues based on sel. The value of sel can be a number, for which al higher bvalues are selected, or a list {min, max}.

SelectBvalueData[{data,vals,grads},sel]

the same but also including the gradients. Output is {data, vals} or {data, vals, gradsgrads}}.