Postmortem examination of an x-ray tube

During rapid sequence radiographics in a special procedure room, unexplained random loss of contrast was observed. The effective focal-spot size, output, and the half-value layer of the tube was measured. The focal-spot size was within the +50 percent established by the National Electrical Manufacturer's Association (NEMA). The half-value layer was within the limits suggested in Table 6 of the National Council on Radiation Protection and Measurement (NCRP) Report Number 33, but the output was half the suggested value.