1. An image processing apparatus comprising: processing circuitry configured to: acquire volume data in time phases including a blood vessel image; position the volume data in time phases; perform a blood flow analysis on a basis of the volume data in time phases after the positioning thereby generating volume data of time to peak; perform a dominance region analysis on a blood vessel on a basis of the volume data of time to peak, thereby generating volume data indicating a dominance region; and generate three-dimensional image data indicating a blood vessel image of a region corresponding to a predetermined dominance region of dominance regions included in the volume data indicating the dominance region.