X-rays were a high energy type of electromagnetic radiation. X-ray photons were used for medical and dental imaging, as well as weapons systems. They are also produced naturally by a variety of stellar phenomena, including pulsars and quasars.
While working on a way to separate Tuvok and Neelix following their transporter accident, The Doctor referred to the use of barium as a contrast agent for the "primitive imaging technology called X-rays." (VOY: "Tuvix")