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PostPosted: 02.12.2019, 15:21 
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New measurement methods of focal spot size and shape of X-ray tubes in digital radiological applications in comparison to current standards.
Klaus Bavendiek, Uwe Heike, Uwe Zscherpel, Uwe Ewert and Adrian Riedo, WCNDT 2012, Durban, South Africa
Abstract:
The exact focal spot size is required for optimization of image quality in radiological testing. The current standards (EN12543 and ASTM E 1165) describe different measurement methods, which do not provide identical values. In practice the operator needs to achieve a sufficient image quality, which depends on the total image unsharpness too. Both, the inherent detector unsharpness and the geometric unsharpness contribute to the total unsharpness. The geometric unsharpness is determined by the focal spot intensity distribution and the magnification used. Therefore, the shape of the focal spot cannot be neglected, even if the shape distribution is not required by any standard practice. To determine the effective focal spot size, thousands of images with different tubes, energies, magnifications and IQIs from different EN and ASTM standards were taken to evaluate the influence of the unsharpness in the digital image due to the focal spot. This leads to a median effective focal spot size for the individual tubes. These results are compared with the values of EN12543 and they deviate significantly. Several modified methods for evaluation of the pin hole camera images according to EN12543-2 were tested, and an update of standards for digital radiology is proposed. With the proposed method for the determination of the effective focal spot size, the computed unsharpness, using this value as an entry into the formulas of ASTM E2033 and E2698 or ISO 17636-2, and the measured unsharpness, will agree much better than with the current method.

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