Calculation program software of the four rules of arithmetic in computed tomography (CT) has been introduced for clinical purposes since 1990. Evaluation of hemodynamic reserve and cerebral autoregulation can be calculated using this software in ischemic cerebrovascular diseases and delayed cerebral vasospasm after ruptured intracranial aneurysms. Eight hemodynamic maps can be entered and calculation based on the four rules of arithmetic can be performed between them and shown visually. For example, delta CBF [CBF after administration of acetazolamide (B)-CBF before its administration(A)] and % CBF [(B - A)/A x 100] maps are useful in evaluation of the capacity of hemodynamic reserve in ischemic cerebrovascular diseases. And delta CBF [CBF before induced hypertension using intravenous dopamine administration (B)-CBF during induced hypertension (A)] and % CBF [(B - A)/A x 100] also can be used in evaluation of cerebral autoregulation and decision concerning indication of the need for induced hypertension during the period of delayed cerebral vasospasm after subarachnoid hemorrhage.