Poster — Wed Eve—11: Pinhole Image Characterization of X‐Ray Tube Focal Spots using Storage Phosphor Imaging Technology

Abstract

The x‐ray tube focal spot is a key determinant of the resolution of an imaging system. Although system resolution can be assessed using bar pattern test objects, a pinhole image provides the point‐spread function due to the focal spot and complete information on the radiation source distribution. The NEMA standard protocol specifies no‐screen film for pinhole imaging, but in the digital age this is problematic. We investigated use of a medium‐resolution storage phosphor imaging system, similar to a hospital CR system. An x‐ray tube with 15° target angle and nominal 1.0 and 2.0 mm focal spots was assessed using a 0.075 mm diameter commercial pinhole and a storage phosphor system. The screen resolves nearly . Its blur of about 0.2 mm is comparable to the pinhole aperture size projected onto the image receptor by the radiographic magnification, , used. A simple expression is obtained which corrects a distance measured in the digital image of the focal spot for both pinhole blur and screen blur to yield the true focal spot dimension. For the x‐ray tube assessed, focal spot size was independent of current, and decreased very slightly with increased potential. A second tube, mounted for research purposes to foreshorten the focal spot to an effective angle of 2.2°, was also assessed. In both tubes, the two focal spots superimposed but were not centered on each other. A medium‐resolution storage phosphor imaging system can adequately replace no‐screen film in pinhole imaging and is much more convenient.