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Imaging in medicine
Imaging in medicine

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2.2 Components of an X-ray unit

2.2.1 X-ray source

X-rays are produced when energetic electrons strike a metal target. The X-ray source consists of an evacuated tube containing a cathode, from which the electrons are emitted, and an anode, which supports the target material where the X-rays are produced. Only about 1 per cent of the energy used is emitted as X-rays – the remainder is dissipated as heat in the anode. In most systems the anode is rotated so that the electrons strike only a small portion at any one time and the rest of the anode can cool. The X-rays are emitted from the tube via a radio-translucent exit window.

Figure 4
Figure 4: Schematic representation of a rotating-anode diagnostic X-ray tube

Some of the X-rays are given off at an energy that depends on the nature of the target material. These are known as characteristic radiation (see Figure 5). There is also a broad spectrum of radiation known as bremsstrahlung (braking radiation); it is this radiation that is used in most diagnostic procedures (see Figure 5). However, both types contribute to the radiation dose given to the patient.

Figure 5: A typical X-ray spectrum produced by a tube with a tungsten target

The maximum photon energy is determined by the voltage applied to the tube (known as kVp). The peak of the X-ray spectrum is at about half of this energy. kVp values vary between about 20 kV for very thin body parts and 120 kV for a pelvic X-ray. Filters can be placed in front of the exit window to eliminate low-energy X-rays that do not contribute to the final image.