An evaluation of correction factors applied to dose-area product meter readings for the use of sinus cones.

As a result of the refurbishment of a dedicated skull radiography unit (Siemens Orbix) a dose-area product (DAP) meter was fitted into the light beam diaphragm/X-ray collimating system. Following calibration of the DAP system for X-ray beams defined by the unit's integral rectangular and octagonal iris diaphragms, DAP correction factors were obtained for the clinical use of externally fitted sinus cones. Correction factors can be derived from a simple ratio of coned to unconed beam areas, and these agree well with measured correction factors. The simple ratio of areas factor may be used in clinical practice to reduce potential errors.

Technical note: a survey of the illumination from diagnostic X-ray light-beam diaphragm systems.

UK radiation protection legislation requires, where applicable, the fitting of light-beam diaphragm systems to diagnostic radiographic X-ray equipment. Measurements performed as part of a quality assurance programme showed that the level of illumination from equipment varied considerably. We decided to include a measurement of the illumination of light-beam systems as a routine part of our programme. The standard method used involved measuring the average illumination at 100 cm focus to field distance for a 25 x 25 cm light field. The results of initial measurements for 73 individual systems (44 fixed, 29 mobile) demonstrate a considerable range in levels with the ratio of maximum to minimum values being approximately 16. Mean values are 130 lux (all systems), 123 lux (fixed systems) and 141 lux (mobile systems). Comparisons are made with the limiting values specified by the Department of Health for mammographic X-ray equipment for breast screening and for new General Electric equipment. A local limiting value of 100 lux is proposed for routine testing purposes.