It may be impossible for the U.S. Federal Reserve to raise interest rates until the rest of the world economy improves, Fed board member Lael Brainard said on Tuesday, in the most direct acknowledgement yet of how weak global markets could handcuff the U.S. central bank. Brainard, who is hyper-attentive to the impacts of globalization given her prior role as head of international affairs at the U.S. Treasury, sketched out a world in which a strong dollar and weak overseas demand were holding back the U.S. recovery and slowing the Fed's progress toward more normal monetary policy.