The aim of this chapter is to enable the reader to understand the concept of the rating of X-ray tubes and why it is monitored. The chapter will discuss factors which affect tube rating and the effects of single and multiple exposures on rating of the X-ray tube. An example of how a microprocessor is used to monitor these effects and prevent damage to the X-ray tube is illustrated.

The general term rating is used to describe the practical limits that are inherent in any device. An example of this is that a fuse rated at 5 amperes will tolerate currents up to 5 amperes. If a current above 5 amperes passes through the fuse, then the fuse will melt, causing a break in the circuit. A high-tension transformer is a more complicated device than a fuse and so has a more complicated set of rating conditions (see Sect. 14.9). Similarly, the rating of an X-ray unit – the X-ray tube and the associated equipment – depends both on how it is constructed and how it is being used. The rating may be defined as follows:

All radiographic exposures cause slight wear and tear on the X-ray tube, the anode becomes more slightly pitted and the filament becomes slightly thinner as a result of any exposure. However, in this context, ‘unacceptable damage’ means damage that would seriously impair the performance of the unit for further exposures or might indeed make it inoperative. In this context, a single short exposure where the mA was above the rating of the tube might damage the anode by melting the focal track. Alternatively, a long exposure at low mA may again damage the anode if the total amount of heat generated in the anode caused a sufficient temperature rise to melt the tungsten. It is also true that multiple exposures, each of which is individually within the tube rating, might damage the tube because of the total heat accumulated in the anode by the exposure series. As the anode takes a finite time to cool after an exposure, the closer the exposures follow each other, the more likely the anode is to suffer thermal damage. For this reason, the rating for single and multiple exposures will be considered separately in this chapter.

The rating for a single exposure is affected by a number of different factors; some of these are under the control of the operator while others are not, as shown in Table 31.1 (see page 232).

Table 31.1 Factors affecting the rating of a particular X-ray tube

In any particular unit, the non-selectable factors are fixed, so a rating must be used which is applicable to that set of circumstances, e.g. a rating appropriate to an X-ray tube connected to a two-pulse unit. The rating can be plotted as a graph showing the effect on the rating of varying the quantities in the selectable group. A simplified form of such a graph is shown in Figure 31.1. Here the rating curve corresponds to a 1.2-mm focus selection and a kVp selection of 80 kVp. The curve indicates the upper limit for all combinations of mA and time for this value of kVp. The points below the line are safe, whereas points above the line are unsafe because they would result in unacceptable damage to the X-ray tube. If we consider an exposure of 80 kVp, 100 mA and 0.2 seconds, then we can see that this exposure may be safely made, as the point P is below the 80-kVp line. Note that because of the wide range of exposure times possible, it is usual to use a logarithmic scale on the x-axis.

Figure 31.1 Graph of rating at a single kVp.

It can also be seen from the graph that higher values of mA will be tolerated for short exposure times – 150 mA at a time of 0.1 second is within the rating – whereas, at long exposure times, only lower values of mA are within the rating – for an exposure time of 2 seconds, the maximum mA permissible within the rating is 50 mA. In each case, the limiting factor is the anode temperature. For longer exposure times, there can be significant cooling of the anode during the exposure. This is shown by the flattening-off of the curve at longer exposure times.

In clinical practice, a range of kVp values is applied to the X-ray tube. Rating calculations take this and the focus size into account, producing separate rating graphs for broad and fine focus. Examples of such graphs are shown in Figure 31.2. Figure 31.2A

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