Electrical safety is designed around four basic principles:

The tube shield is connected to earth via the outer braiding of the high-tension cables (Fig. 30.2) and so the casing will always remain at earth potential. If a live wire within the casing becomes disconnected and touches the shield, then the current will readily flow to earth and the casing will present minimal electrical hazard to someone touching it at the time.

Figure 30.2 (A) Structure of a high-tension cable. Note the thickness of the insulation surrounding the central conductors. LT, low tension. (B) The X-ray tube shield is made electrically safe by connecting the copper braiding of the high-tension (HT) cables to both the shield and the casing of the high-tension transformer tank. The latter is securely earthed.

The live components in the X-ray tube are secured inside the tube shield and the ends and the high-tension cable connectors are securely fixed. This means that, under normal circumstances, the operator has no easy access to the live components. In addition, the circuits may be isolated from the mains by switches, fuses or circuit breakers.

Radiation safety is important for the operators of X-ray equipment and for patients and others who may be in the vicinity at the time of making an X-ray exposure. The lead lining of the tube housing (Fig. 30.1) limits the radiation leakage from the tube and so provides protection to both the operator and the patient. It should be borne in mind that X-rays are emitted in all directions from the focus on the anode, but only those which pass through the tube port are allowed to leave the housing. The anode itself has a high absorption and so the lead lining at the anode end of the shield is often absent or thinner than at the cathode end.

The radiation leakage rate from the tube is normally measured at a distance of 1 metre from the housing and should not exceed an air kerma of 1.00 milligray per hour, measured at a distance of 1 metre from the focus (for definitions of the kerma and the gray, see Ch. 27). Any break in the lead lining will result in radiation leakage from the tube, so the leakage levels should be checked at regular intervals as part of the quality checks on the X-ray unit.

Radiation safety means exposing the patient only to the minimum dose necessary to produce a radiograph of acceptable quality. As we saw in Chapter 21, the spectrum of X-rays produced at the target is a continuous spectrum containing a mix of low-, medium- and high-energy photons. Some of the photons have very low energy and will not be able to pass through the patient to reach the image receptor. These photons would contribute to the patient dose but not to the image production. Some of these are removed by the glass envelope of the insert and the oil as the beam passes to the tube port. This is known as the inherent filtration of the tube. Further filtration takes place through the aluminium filters placed at the tube port (Fig. 30.1). These are known as added filtration. Thus, the total filtration of the beam is determined by the inherent filtration plus the added filtration. Details of this information are frequently marked on the end of the shield.

Scattered radiation contributes to the patient dose and the operator dose and causes degradation in the radiographic image quality. As we have seen in Chapter 23, the amount of scatter produced at a given set of exposure factors is dependent on the volume of tissue irradiated. The volume can be reduced by reducing the area irradiated using a light-beam diaphragm (Fig. 30.1). This also means that parts of the body not required on the radiograph can be protected from radiation. In this way, adequate collimation using a light-beam diaphragm (or cones) will reduce the patient dose and the operator dose and produce an improvement in the image quality.

X-ray production is at its most efficient when a vacuum exists between the cathode and the anode of the X-ray tube, so these structures are enclosed within an evacuated metal or heatproof glass envelope, which must be sufficiently strong to preserve this vacuum. Where glass envelope is used, it is joined to the anode spindle at one end and to the nickel cathode support at the other end by re-entrant seals – so called because the glass is shaped to point inwards at the area of contact. Slightly different glass seals are used at each end so that the thermal expansion of the glass is similar to that of the metal used in the construction of the anode spindle and cathode. This reduces the stress on the glass when the insert is hot and so limits the chance of cracking. The glass must be a good electrical insulator or a substantial current will flow through it when the high potential difference is applied between the anode and the cathode. However, electrical charges are built up on the inside of the glass during operation and so the glass must have sufficient electrical conductivity to allow these to leak away, usually between exposures, avoiding the build up of high amounts of static charge. The glass is gently rounded so that there are no sharp corners, which would allow the build up of high amounts of static charge (see Ch. 6).

During operation of the X-ray tube, a thin film of tungsten will be deposited on the inside of the envelope as a result of the release of tungsten vapour from the filament and the target. This film acts as a filter to emergent radiation, and where a glass envelope is used, may eventually cause electrical breakdown within the tube as it can act as an electrical conductor around the inside wall of the glass envelope. If this happens, the tube is classed as ‘gassy’ and is of no further use. The rate of tungsten vaporization can be reduced by not keeping the tube in the ‘prep’ mode (see Sect. 29.2) any longer than is required and by keeping exposures well within the rating of the tube (see Ch. 31).

As shown in Figure 30.3

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