A miniature x-ray tube capable of intra vascular use, has a micro cathode preferably formed by MEMS techniques. The very fine wire of the cathode filament is formed on a semiconductor base and draws a current sufficiently low that lead wires in a cathode heater circuit, passing through a probe line connected to the x-ray tube, can be very small wires, which helps maintain sufficient dielectric spacing in the high voltage circuit handled by the same probe line. In a preferred embodiment the probe line comprises a glass fiber, held at a small diameter to allow flexibility for navigating small-radius turns within the vessels. In a preferred embodiment the fiber is overcoated with a high-dielectric polymer to significantly increase the dielectric strength of the overall cable, without adding significantly to stiffness. The high voltage ground conductor is a coaxial sheath on the outside of the polymer. Exterior to the ground conductor is a further flexible layer having paths for coolant.