A step up transformer has an inert core, primary windings concentrically surrounding the inert core, and secondary windings concentrically surrounding the primary windings, the ratio of secondary windings to primary windings of up to 1,000,000:1. The step-up transformer is particularly useful in an x-ray catheter having an x-ray source in a vacuum tube and a power source. The power source can deliver a lower voltage which is stepped up at by the transformer to produce a voltage sufficient to stimulate production of x-rays by the x-ray emitter source. Methods for preparing the step-up transformer and x-ray catheter and methods of administering x-rays to subjects with the x-ray catheter of the invention are also disclosed.