The inherent filtration of x-ray tubes.

Abstract

As many recommendations on radiation protection mention the total filtration demanded in diagnostic radiology, there is a growing interest in a relatively simple method for evaluating the inherent filtration of the x-ray tube. With this knowledge, it is possible to add extra filters to arrive at the prescribed total filtration. When considering the inherent filtration equivalent of x-ray tubes, one must bear in mind the fact that this inherent filtration depends on the tube voltage and the wave form. Further, one can think of the inherent filtration equivalent with respect to the absorption of the x-rays (1) or with respect to their resulting quality, characterized by the half-value layer expressed in terms of the equivalent filter material. Trout and his associates (2) published a simple experimental method for determining the inherent filtration of diagnostic x-ray tubes. A satisfactory value, they stated, can be obtained by determining the half-value layer of the unfiltered beam and relating this to the thickness of aluminum, for instance, which must be added to a beryllium-window tube to produce the same half-value layer under similar conditions. These workers published a number of curves with the half-value layer plotted against inherent filtration aluminum equivalent, with the tube voltage (kvp) as a parameter. No mention was made, however, of the wave form. Actually, according to a personal communication from Mr. Trout, the wave form used was “full-wave” rectified and as close to a sine-wave as one ever sees on diagnostic equipment. We made some similar measurements, for which we used a constant potential wave form. Our curves are shown in Figure 1, along with the curves published by Trout et al. (broken lines) for corresponding tube voltages and full-wave rectified wave form. To check the above method we measured the quality of the radiation from a diagnostic x-ray tube for two wave forms, viz. constant potential and full-wave rectified. The results are given in Table I. With the aid of a cut-away tube unit we also measured the inherent filtration equivalent with respect to the absorbing properties as a function of the tube voltage for constant potential. These results are given in Table II. Conclusions 1. For a given voltage the radiation quality of an x-ray beam, expressed in mm. Al h.v.l., depends strongly on the wave form. 2. There is relatively a great difference in inherent filtration equivalent depending on whether one considers this with respect to adsorption of the x-rays or to radiation quality. 3. For the determination of the inherent filtration equivalent with respect to the radiation quality, one can make use of the set of curves as shown in Figure 1.