How to extract a dataset from a bigger dataset?

Hi everyone,

I have an urgent problem to solve, any help rendered is greatly appreciated!!

What I have now is a BIG collection of nodes, representing points in space, which in collection describes a bone segment in the body.

what I need to do now is to extract a dataset from this bigger domain for further analysis, which serves to improve computation time as i do not need to analyse the whole bone but only the area of interest within the bone. Some parameters i thought of for extraction may be one of the following:

1) A given node(x,y,z) at the surface of this bone, with a radius(or a sphere), of which nodes within this segment is extracted.
2) Given coordinates at the surface of the bone, with a radius(similar to the above)

Do you guys have any other great ideas to extract a dataset from another dataset based on a given node?

THANKS IN ADVANCE!!
Dan