Monte Carlo analysis of the degradation of the spectrum produced by an X-ray tube in conventional radiography

Medical imaging detectors used in diagnostic radiology are very sensitive to the spectra of the X-ray beams used. In this work, we use Monte Carlo simulation to analyse the degradation of various X-ray spectra at different stages of their travel from the X-tube focus to the detector. Special attention is paid to the antiscatter grid that is normally used in diagnostic radiology. For the present study, we have considered various measured spectra corresponding to accelerating voltages of 60, 80, 90 and 125 kV. We have assumed that the X-ray beams are parallel beams going successively through an aluminium filter, the air gap between the focus and the patient, a patient-equivalent water phantom and a parallel cross antiscatter grid. The Monte Carlo simulation code PENELOPE has been used to find out the X-ray spectrum at three planes: the patient, the grid, and the detector entrance. Additional simulations with monoenergetic beams of 30, 40, 55 and 100keV have been carried out for comparison. We have investigated in detail the dependence of the attenuation of the primary beam with the initial energy, the percentage of the initial X-rays reaching the grid and the ratio of scatter to primary photons reaching the grid. The role played by the grid has been studied in depth. In particular, we have analysed the modification of the incident spectrum that it produces and its efficiency in removing the scattered photons. (C) 2007 Elsevier B.V. All rights reserved.