A Study of the Inherent Filtration of Diagnostic X-Ray Tubes

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The use of added filtration in diagnostic technics and the variation of inherent filtration in different types of diagnostic tubes has brought about the need for a simple procedure for determining the aluminum equivalent of the inherent filtration of the x-ray tube. Once the inherent filtration equivalent is determined, additional filter can be added to arrive at any desired total filtration. Since there will be differences in the inherent filtration of tubes of the same type and variation in the thickness of the added filters due to commercial tolerances, the accuracy of the measurement of the inherent filtration need not be of a high order. A measurement of the half-value layer of the beam from the x-ray tube without added filter would seem to be a useful and sufficiently accurate means of determining the inherent filtration. With the aid of better low-energy instrumentation than previously available in our laboratory, and of beryllium window tubes, measurements have been made to relate, inherent filtration to half-value layer. For further relating the experimental data to the actual components going to make up the inherent filtration (glass, oil, Bakelite), a cutaway tube unit of typical component thicknesses was constructed. An x-ray beam having low filtration (2 mm. Be) was directed through each of the elements and through the assembly to determine their influence on the resulting total filtration. Glass was found to have a transmission of 6 per cent at 80 kvp, which was reduced to 5 per cent by the addition of oil and Bakelite. From this it is evident that the glass window of the tube accounts for most of the inherent filtration. When the beam from the 2-mm. beryllium window was filtered by directing it through the cutaway tube assembly, the half-value layer and the aluminum filtration necessary to obtain the same half-value layer at 80 kvp were: h.v.l. 0.90 mm. Al, aluminum equivalent 0.87 mm. Al. There was no significant change in the aluminum equivalent of the cutaway tube unit from 50 to 100 kvp. All this leads to the assumption that a satisfactory value for the inherent filtration of a diagnostic tube unit can be obtained by determining the half-value layer of the unfiltered beam from the tube unit and relating this to the thickness of aluminum which must be added to a beryllium window to produce the same half-value layer under similar conditions.