X-ray imaging encompasses a number of “modalities” that all use X-rays generated in an X-ray tube to form an image based on the transmission of X-rays through the body. All diagnostic X-ray imaging systems generate X-rays using an X-ray tube. The X-ray tube uses a high voltage to accelerate electrons produced by thermionic emission across a vacuum tube. X-rays tubes are used in different ways to obtain the information required to diagnose and treat patients with the lowest possible dose. Although X-ray film is now rarely used, X-ray imaging continues to be referred to as plain film, planar or two-dimensional (2D) projection radiography. Fluoroscopy enables X-ray imaging to be performed in real time. X-ray Computerised Tomography scanners are used to map regions of the body with differing attenuation, rather than providing a simple 2D projection. Narrow beams of X-rays with relatively high kV values are generated.