The spectrum and intensity of extra-focal (off-focus) radiation

Conflicting results have been obtained by other workers for the quality of radiation originating outside the area of the focal spot on X-ray tubes. This extra-focal radiation results in poorer picture quality, especially in cases where grids are not being used. As part of an investigation to assess the importance in diagnostic radiology of the various factors affecting the output from X-ray tubes, the photon spectrum has been examined using a Ge(Li) detector. The measured spectra have been adjusted for the effect of the detector characteristics. Results show that the extra-focal radiation is softer than that from the focal spot and, for the X-ray generator measured, constitutes approximately 8% of the exposure at 100 kV with a total filtration equivalent to 2 mm of aluminium. The possibility of further collimation to reduce this component, thus improving picture quality, is considered.