Measurements and comparison of focal spot sizes of two types of x-ray tubes installed in simulators

Full text: Measurements of the effective focal spot size of two types of X-ray tubes installed in radiotherapeutic simulators (TOSHIBA and VARIAN-Ximatron) are presented. As recommended in ACPSEM Quality Assurance Protocols for Diagnostic X-ray Equipment [Heggie and Petty, APESM, Vol.8 No.1 1985], the effective focal spot size of X-ray tube is vital in determining the maximum resolution of radiographic and fluoroscopic images. Hence, the quality of clinical performance of diagnostic equipment is depended to a large extent upon the accuracy in measuring and maintaining the focal spot size of the X-ray tube and regular QA program. In determining the shape and the dimension of the effective focal spot that is located within the X-ray tube and can not be measured directly, a method developed by Spiegler and Breckinridge [Spiegler and Breckinrige, Radiation Physics, Vol. 102 Mar. 1972] was used. By irradiating a lead star pattern which is placed in a divert X-ray beam between the tube and a negative film, a magnified radiographic image of the test pattern can be obtained from a standard X-ray film. The actual shape of the effective focus is determined by tracing the locus of the image intensity inversion of the radial pattern where that is occurring when only a certain condition of spatial variation from a central ray-line is met. Then, the dimension of the focal spot can be calculated. In this measurements, three different test patterns (Typ 9/1, 5 deg Vertex Angle 0.03 mm Pb, Typ 9-2 deg-360 0.05 mmPb and Typ 9 Teilung 2 deg 0.05 mm Pb) with same diameter of 45 mm are used. Results from a series measurements which were carried out repetitively under same working condition of both simulators indicate that the effective focal spots of these two tubes are of similar rectangular shape. However, the dimensions of the focal spots are about 1.297 X 0.848 mm for the TOSHIBA tube and 0.692 X 0.532 mm for the Ximatron tube. The orientation of the focal spot for TOSHIBA tube is of such that there is 15 deg angle between the anode-cathode axis and the long side of rectangle, while for Ximatron tube that angle is 45 deg. A comparison of both shape and dimension of the focal spot of the tube are made between these two simulators of similar working load. After considering the difference in the initial manufacturer's specification of the focal spot size, it is clear that there is a aging effect on the effective focal spot of the X-ray tubes. However, the relationship between the size of focal spot and the age of the X-ray tube is complex and non-linear. Limitations of a average Life-time of these two types of X-ray tubes are derived respectively according to the requirements on spatial resolution of X-ray diagnostic equipment set by the national standards