***Not a teacher***
OK - this is a bit of a guess, but assuming this is related to your other post recent post....
I think a wire contour may be something like the following:
So the snippet of the article you are quoting is talking about how a patient is represented as a model on a computer as an electron density matrix. If the density is set to 0, there is an absence of body matter, and if it is set to 1, this indicates the presence of body matter. Therefore it is necessary to set these densities according to whether they are inside or outside the wire contour (as this represents the surface of the body, where there is matter inside, but not outside).
As I said, it's just a guess.