The imaging characteristics of diagnostic x-ray systems are dependent, in part, upon the size, shape, and intensity distribution of the x-ray source. These properties, however, vary with such parameters as kVp (tube mA (tube current), and, because of the steep anode face, field position. We have investigated the voltage), characterization of the focal spot using the image of a random array of small radiopaque objects. The Fourier transform of this image is, to a good approximation, the modulation transfer function (MTF) of the focal spot. We have compared the accuracy and reliability of this technique with both pinhole and star imaging Focal spot variations with kVp, mA, and field position have been measured, and the use of this techniques. technique with microfocus x-ray sources has been demonstrated.
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