X-ray tube focal spot sizes: comprehensive studies of their measurement and effect of measured size in angiography.

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Abstract
Thirty-two focal spot sizes of 4 X-ray tubes were measured by the pinhole, star pattern, slit and root-mean-square (RMS) methods under various exposure conditions [for angiographic use in humans]. The modulation transfer functions (MTF) and line spread functions (LSF) were also determined. The star pattern focal spot sizes agreed with the effective sizes calculated from the frequencies at the 1st minimum of the MTF within 0.04 mm for large focal spots and within 0.01 mm for small focal spots. The focal spot size determined by the slit method was approximately equal to the width of the LSF at the cutoff level of 0.15 .+-. 0.06 of the peak value. The RMS method provided the best correlation between the measured focal spot sizes and the corresponding image distributions of blood vessels. The pinhole and slit methods tended to overestimate the focal spot size, but the star pattern method tended to underestimate it. For .apprx. 90% of the focal spots, the average of the star and slit (or pinhole) focal spot sizes agreed with the RMS focal spot size within .+-. 0.1 mm.