Manufacturers' designation of diagnostic x-ray tube focal spot size: a time for candor.

Focal spot sizes may now be characterized by any of three different designations: “nominal” (manufacturer's) size, “pinhole image” size, and “effective” size deduced from a resolution test pattern such as the Siemens star. Of these, the nominal size is certified by the manufacturer at loading conditions which do not approach those of clinical use and hence is seldom if ever achieved in actual practice. Pinhole images provide a better guide to actual focal spot size but are still too small. Despite stated objections to the use of resolution test patterns in designating the focal spot size, the author feels that this is still the best method and should be standardized.