Muscle and bone dose in paediatric limb digital radiography: a Monte Carlo evaluation

The proliferation of digital radiography (DR) has led to a re-evaluation of exposure parameters and image quality. Currently, there is a move towards reducing X-ray tube voltage (kVp) in paediatric exposures down to 40 kVp to achieve better images. However, the effect on patient dose of these modifications is uncertain. The main aims of this phantom study were to evaluate the effect of reducing the kVp in paediatric limb DR exposures on contrast-to-noise ratio (CNR) and patient dose. For this purpose, Monte Carlo simulations of radiographic exposures on a paediatric limb phantom were performed. The phantom included muscle tissue and bone segments of five different densities in the range of 1.12 to 1.48 g/cm³. The overall thickness of the phantom varied between 1 and 12 cm. Dependence of the CNR at constant limb phantom muscle and bone doses and dependence of the CNR per unit of muscle and bone dose at constant detector dose on radiographic exposure factors and limb thickness were calculated. X-ray tube current-time product (mAs) values required to achieve equal detector dose versus limb thickness for different kVp were calculated, as well as muscle and bone doses for the limb phantom of varying thickness. Present work has shown that reducing the kVp in paediatric radiography of the extremities can result in a significant increase in radiation dose, particularly for thicker limbs. Low kVp radiography requires justification for use on the extremities.