  We study in this paper time-reversal $\delta$-impurity scattering effects in
the bulk of topological insulators (TI) in two and three dimensions.
Specifically we consider how impurity scattering strength is affected by the
bulk band structure of topological insulators. An interesting band inversion
effect associated with the change of the system from ordinary to topological
insulator is pointed out. Experimental consequences of our findings are
discussed.
