X-ray generator and X-ray tube

X-rays are obtained from an X-ray tube, to which an X-ray generator feeds energy in the form of kinetic energy of electrons (Fig. 8). The primary function of a generator is to accelerate electrons to a constant energy value during the whole period of exposure by generating constant high voltage (kVp) between the glow cathode and rotary anode of the tube. There are also other requirements:
1. The system should tolerate continuous lasting use.
2. The intensity of radiation should be sufficiently high to enable short exposure times (diminishes motion artefacts) at a focus-film distance of approximately 1 m (diminishes distortions and dose as well as proves resolution).
3. The size of the focus should be as small as possible (between 0.1 x 0.1 and 2x2 mm2, improves resolution), and the radiation field must be limited to the immediate vicinity of the organ in question (radiation protection, reduces scattered radiation).

The generator is an electric transformer which converts 220 V (1-phase) or 380 V (3-phase) alternating voltage to high voltage somewhere between 20-150 kV. High voltage is rectified with a diode bridge. Depending on the electric coupling in the transformer, 6 or l2-pulse (during the period of 20 ms) high voltage is generated, with corresponding pulsatile variations in the radiation yield (both in intensity and in the ability to penetrate tissues). However, so-called medium or high frequency generators have progressively come into use; they use modem small scale electronics to generate constant (as opposed to pulsed) high voltage. In the past, 2-pulse generators were used especially in dental radiography which meant that voltage (and X -ray output also) went down to zero between peaks of pulses.

Filtration of X-ray spectrum

There is often both a small and a big focus in the cathode (dual-focus). Electrons emitted from the cathode wire hit the anode in an oblong area, the surface of which in the direction of the patient is a square of normally 0.3x0.3, 0.6x0.6 or 1.2x1.2 mm2. Both braking radiation and characteristic radiation (see Interactions of electron; X-ray spectrum from X-ray tube) are generated in the anode. X-rays leave the focal point in all directions from the focus, but they are utilized for imaging purposes only in the direction of the patient by using a multileaf collimator.

In an X-ray tube there is also an aluminium (1-5 mm) and/or a copper (0.1-0.5 mm) filter and a light source with a mirror to simulate the radiation field on the skin. The filtration of the X-ray spectrum diminishes particularly the number of low energy quanta and therefore raises the average energy of the beam. The patient's body diminishes further the total intensity (area) of the spectrum and simultaneously hardens the average energy. Spectra at different phases in the imaging chain are shown in Fig. 8.

The lower curve of Fig. 5 (and the curve labelled "leaves tube" in Fig. 8) shows that the glass or metal envelope and filter have removed all very low energy photons, which have only a small probability of penetrating the patient. In the braking radiation spectrum in Fig. 8 one can see the peaks of characteristic radiation (59 and 67 keV from tungsten anode), if the energy of incoming electrons is sufficient to excite electrons in the K-shell (Fig. 1). The area underneath these peaks is only a small percentage of the total area of the spectrum.

Aaro Kiuru