Where are x-rays emitted from?

The energy to generate X-rays for medical imaging comes from high voltage. High voltage is applied to an X-ray tube to accelerate electrons across the space between the anode and cathode in the tube. The higher the voltage, the more the electrons are accelerated. The electrons slam into the cathode, which is a metal target. This "knocks loose" some electrons in the metallic crystal structure of the metal of the cathode, and when the electrons that have been slammed into give up energy to return to a "relaxed" state, they give off very high energy electromagnetic radiation - X-rays.