X-ray Production and the X-ray Tube

Freeing the Electrons

X-ray production starts at the cathode, the negative side of the tube. The cathode holds one or two small tungsten wire coils called filaments; a dual-filament tube uses the small filament for the small focal spot and the large filament for the large focal spot. When filament current heats the wire, outer-shell electrons boil off its surface, a process called thermionic emission. The freed electrons hover around the filament as a cloud called the space charge, and at low kVp and high mA this cloud repels new electrons and limits emission, which is the space charge effect. Selecting a higher mA heats the filament more, so more electrons are released and tube current rises.