Find diameter of a spanning tree

hello, I have a weighted undirected minimum spanning tree and I want to find the find the diameter of this. I know that diameter is the longest shortest path but I haven't understood what parameters do i have to fill in the function graphshortestpath() in order to find the shortest path. Let's assume that my spanning tree have 23 indices. So do I need to fill these parameters ...

        [dist,path,pred]=graphshortestpath(ST1,1,23,'Directed',false)

ST1 is a sparse matrix that represents the minimum spanning tree.And after the calculations do I have to retrieve the maximum dist in order to find the longest shortest path?

thanks in advance