Focal-spot measurement: comparison of slit, pinhole, and star resolution pattern techniques.

The National Electrical Manufacturers Association (NEMA) has established a new standard for x-ray tube focal-spot measurements using a focal-spot camera with a slit instead of a pinhole. Measurements from the slit focal-spot camera are compared and star resolution pattern techniques for 50 focal spots. In addition, measurements were made using direct-exposure film as required by the NEMA standard and an extremity screen-film system, which minimizes x-ray tube loading. The slit, pinhole, and star resolution techniques provided similar results that were within the accuracy required for clinical focal-spot measurements. The focal-spot measurements were also similar when a direct-exposure film or an extremity screen-film system was used. A description of the use of the slit focal-spot camera and difficulties with implementing the new NEMA standards in the clinical setting are also presented.