23 March 2010 Characterization of focal spots of x-ray tubes in CT systems: method development and examples
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Abstract
Characterizations of x-ray based imaging systems often focus on a detailed examination of the detector properties while the x-ray tube and its properties usually are investigated to a limited extent. Here we present a method to measure the size of x-ray focal spots, using the measurement system provided by the CT system itself. The method is based on the measurement of the intensity profile of a highly absorbing plate placed within the x-ray geometry of the imaging system. A beam blocking plate yields an intensity step function. For the case of an ideal pointlike focal spot, the response function would be a theta (step) function. For real focal spot sizes with finite spatial extension, the step function will be smeared out accordingly. The derivative of the intensity step function yields the intensity profile of the focal spot. Knowledge of the system geometry - i.e. the focus-detector distance and the position of the absorbing edge - allows calculating the intensity profile with an absolute spatial scale at the tube anode. In our experimental realization the edge is made of tungsten. The edge of the 2 mm plate is machined precisely to limit the effects of the transition zone between air and high absorption. Since we are using the detector of the imaging system, we can evaluate the focal spot performance in all modes available at the scanner. This allows e.g. measuring the dependence of the x-ray intensity profile on kVp and mA settings or even the dynamic behavior in time.
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M. Grasruck, M. Grasruck, U. Kühn, U. Kühn, S. Müller, S. Müller, K. Stierstorfer, K. Stierstorfer, T. Flohr, T. Flohr, } "Characterization of focal spots of x-ray tubes in CT systems: method development and examples", Proc. SPIE 7622, Medical Imaging 2010: Physics of Medical Imaging, 76224V (23 March 2010); doi: 10.1117/12.843666; https://doi.org/10.1117/12.843666
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