Question: What is the name of the geographical region that early Islamic usage often regarded everywhere east of western Persia and where Vistaham helped Khosrow regain his throne after the rebellion of Bahram Chobin?  
Sentence (from "Greater Khorasan"): Khorasan (Middle Persian: , Persian: خراسان‎ ‎ "Xorâsân"    ) is a historical region lying in the northeast of Persia.
Relevant evidence: Yes