Simulação e validação de perfis de dose e seu uso para estimativa de grandezas dosimétricas para tomografia computadorizada

Abstract

Computed Tomography (CT) has become a reference medical imaging technique with number of procedures growing over the years. This fact is mainly due to the rapid evolution of CT technology that means the utilization of the devices in a number of applications. Since a CT procedure implies dose delivered to the patient, it is a concern to track the radiation emission by a CT device. Efforts aiming to fill the gap about the capability of the traditional Computed Tomography Dose Index (CTDI) methodology to estimate the radiation emission characteristics of a current CT device have been performed. An analytical approach about the description of the dose distribution in cylindrical phantoms subdued to clinical CT scanning has been developed. This analytical approach helps to have a better understanding of the physical processes governing the dose deposition in a cylindrical phantom. The analytic approach predicts with sufficient accuracy the dose profile and the cumulated dose profile from single and multiple CT scans, respectively, along the central and peripheral axes of a cylindrical phantom. Moreover, from the single or multiple dose profiles, it is possible to derive more accurate dose descriptors for the current CT examinations than the traditional 100 metric. In this work, a Monte Carlo (MC) modelling of the helical CT scan mode was performed aiming to compute single and cumulated dose profiles. The radiation emission characteristics of the multiple-detector CT (MDCT) scanners, GE Discovery CT750HD, and GE Lightspeed Ultra were modelled and used to assess dose deposition in PMMA cylindrical phantoms of 16 and 32 cm diameters. To model the helical procedure and the radiation transport of photons and electrons in the phantom material, the PENELOPE/penEasy package was used. The target self-attenuation, focal spot angle, and fan-beam geometry are incorporated. Body helical protocol with pitch values of 0.516, 0.984, and 1.375, nominal beam width, =40 mm, and the head helical protocol with pitch values of 0.531, 0.969, and 1.375, and nominal beam width =20 mm were chosen to be modelled for X-ray spectra corresponding to 80, 100, 120, and 140 kV. The analytical formulation for the single dose profiles and experimental measurements of single and cumulated dose profiles were used to validate the MC modelling. The experimental dose profiles were measured using Optically Stimulated Luminescence (OSL) dosimeters in the form of ribbons. Also, the experimental values of the 100 were used to calibrate the simulated single and cumulative profiles. The match of the simulated dose profiles with the reference data reflects the correct modelling of the target self-attenuation and the radiation transport in the phantom material reflected in the tails of the dose profiles. From the calibrated cumulative dose profiles, metrics describing the CT dose characteristics were derived, such as Dose Line Integral (), Equilibrium Dose (), pitchxEquilibrium Dose product (), Approach to Equilibrium function (()), and Equilibrium Scanning Length ...