Introduction
This topic covers the design of the conventional X-ray tube, the physical principles by which it produces X-rays, and how an X-ray image is formed from the radiation that passes through the patient. It also covers the qualities that determine how useful a radiograph is for diagnosis, and which technical factors control those qualities.
Basic components of the X-ray tube
The X-ray tube is a vacuum diode: electrons are emitted from a heated cathode, accelerated by a high voltage, and stopped at a metal target (anode). The tube is built so that the electron stream is well focused and the heat produced at the target can be removed.
- Glass envelope (or metal-ceramic envelope) kept under a high vacuum, so that electrons reach the anode without colliding with air molecules and the filament does not oxidise.
- Cathode, made of a coiled tungsten filament heated by a low current, which releases electrons by thermionic emission, and a focusing cup (a negatively charged nickel-coated molybdenum part) that shapes the electron beam into a small area on the target.
- Anode, a tungsten target (often a rhenium-tungsten alloy) set at an angle, usually 7 to 17 degrees, on a backing of molybdenum or graphite. In high-output tubes the anode is a rotating disc driven by a stator and rotor (induction motor) to spread the heat over a larger track.
- Focal spot: the area of the target struck by the electrons. The anode angle makes the effective focal spot seen from the film or detector smaller than the actual focal spot (line-focus principle).
- Housing filled with insulating oil that absorbs heat and insulates the high-voltage parts, and which also contains the cooling system.
- Window and filters: the window lets the useful beam out; inherent and added aluminium filters remove low-energy photons that would only add dose to the skin.