Is the 25% rule used in film radiography still valid for digital radiography systems?

Abstract

Purpose: To investigate the validity or otherwise of the 25% rule in digital radiography systems. Research Design: The study was conducted using a prospective experimental design, utilizing direct observation for data collection. Using a water phantom, images were taken with an x-ray tube potential range of 50kV to 120kV, for different depths of water, under grid and no grid conditions. Different depths of water represented different patient thicknesses. At a given kV, increments of water were added until a depth of 25cm was reached and the mAs necessary for maintaining constant Mean Pixel Values as proposed by the 25% rule was applied. Actual Mean Pixel Values were read and then compared to values predicted by the 25% rule. Results: The 25% rule was only found to be applicable for voltages of up to 80kV with grid, and 60kV without a grid, and only for thicknesses up to 15cm. Conclusions: On the basis of this study, the researcher concluded that the 25% rule is in general not valid for digital systems. In fact, strict adherence to the rule would result in an unnecessary dose to the patient and would therefore contravene the optimisation principle.