This invention provides treatment compositions as well as systems and methods of determining and administering an effective amount of treatment for a neurological disorder. The treatment composition can contain a labeled interfering RNA (iRNA) agent capable of decreasing expression of a target RNA associated with the neurological disorder. The methods of the invention include determining an effective amount of a therapeutic composition by introducing a solution containing a tracer into the brain of a mammal. The tracing solution is monitored until a target volume of distribution at steady state distribution is substantially achieved, and the rate of delivery of the therapeutic composition is determined. The therapeutic composition can then be administered at the rate determined by use of the tracing solution.