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 "Vistahm"): Bistam or Vistahm (also transliterated Wistaxm, Persian: بیستام‎ ‎ ), was a Parthian dynast of the Ispahbudhan house, and maternal uncle of the Sasanian "shah" of Persia, Khosrow II (reigned 591–628).
Relevant evidence: No