@Mapleowo emperor title is better and can be used universal, to make sure she is a ruler, not wife of an emperor. Like the Chinese, Japanese call the female reigning empress is (female) emperor, like Wu Zetian was Empress consort of Emperor Gaozong, later are Empress Dowage of her children emperors, but ultimately she took the throne with absolute power for herself and rule as Emperor (Empress Regnant). If keep using Empress, it's will confuse she is a ruler or just wife of Emperor.
Meanwhile Prince Philip, husband of Queen Elizabeth is not called "king", just prince (not even prince consort) because he not hold the throne of United Kingdom.