Interest in transition radiation was renewed when Garibian showed that the radiation should also appear in the x-ray region for ultrarelativistic particles. His theory predicted some remarkable features for transition radiation in the x-ray region. In 1959 Garibian showed theoretically that energy losses of an ultrarelativistic particle, when emitting TR while passing the boundary between media and vacuum, were directly proportional to the Lorentz factor of the particle. Theoretical discovery of x-ray transition radiation, which was directly proportional to the Lorentz factor, made possible further use of TR in high-energy physics.