The relationships between kV, mAs and thickness in film-based radiography: 25% and 15% rules. OK?

Abstract

Purpose: To investigate the validity of the 25% and 15% rules in manual exposure selection. Methods: The mAs required to produce a constant film density of 1.5 was determined over the X-ray tube potential range 50–150 kV for different thicknesses of water, under grid and no grid conditions. Results: The mAs change is an exponential function with depth and is dependent upon the attenuation coefficient. The 25% rule assumes a constant attenuation coefficient with a half value layer (HVL) equal to 3 cm. The results indicate that for non-grid work at low X-ray tube potentials and for grid work at higher tube potentials the 25% rule is applicable. The 15% rule implies an inverse power relationship between mAs and tube potential, the power term being 5. The results indicate that for both grid and no grid conditions the 15% rule is obeyed only over the tube potential range 50–100 kV and for thicknesses up to 15 cm. Discussion: The 25% and 15% rules have provided useful practical guidelines for exposure factor selection. It is shown that they are applicable to some but not all radiographic situations.