Desarrollo de un programa de simulación basado en el método de Montecarlo para el cálculo de dosis con maniquíes divididos en voxels. Aplicaciones en tomografía computarizada

by Salvadó Artells, Marçal

Abstract (Summary)
Resum en anglès; Among the different radiodiagnostic techniques, computed tomography (CT) represents the major contribution to the collective dose. In order to have available a tool to assess and ascertain the imparted dose upon CT examinations, we have developed and validated a method to calculate radiation absorbed dose in CT examinations from images of phantoms and standard patients by using a voxel-based Monte Carlo simulation method. Simulations and measurements of radiation dose were conducted in air, within cylindrical phantoms of both circular and elliptic cross sections, and in an anthropomorphic phantom, with the simulations being made on volumes based on their own CT images. The radiation beam geometry (focus-to-axis distance, field of view, collimation, and primary and beam shaper filtration) and the X-ray spectral distribution for each scanner under study were implemented for the simulation. Simulations were performed for both single axial scan and a series of contiguous scans covering the complete phantom-object volume. All the axial CT images were reduced to a 256 x 256 matrix and stacked in a volume. Patient images were segmented prior to simulation of radiation transport by using three categories (lung, muscle and bone). To test the method, values of CT specific dose quantities for CT exposure simulations were calculated and compared to those derived from measurements performed free-in-air and within cylindrical phantoms. To complete the scope of the work, series of CT scans of the head, neck, and trunk of an anthropomorphic phantom and standard patients were simulated. Using the EGS4 code, written in an extended FORTRAN called Mortran3, we have estimated first the radiation dose in the individual voxels; radiation transport was simulated by means of the EGS4 code system. The simulation of photon transport included incoherent and coherent scattering, photoelectric absorption with creation of fluorescent photons or Auger electrons. The photon interaction cross sections and the electronic density for each material or compound were prepared by the PEGS4 pre-processor. The electron interaction processes considered were inelastic collisions with atomic electrons, scattering effects by the nucleus, and bremsstrahlung losses that are negligible for the energies considered. The low energy cutoff for photon transport was 5 keV, whereas for electrons was fixed at 30 keV, with a ESTEPE (fraction of the energy lost in a step) value of 1%. The conversion of energy fluence to dose rate was made using the mass energy-absorption coefficients from Hubbell and Seltzer. The number of histories in each simulation was 3×106 per each 1 cm slice. Secondly, the dose CT indices for peripheral and central positions and the dose to bladder were estimated as well, the later being used to test organ doses in the anthropomorphic phantom. The dose measurements were performed by using a "pencil" ionisation chamber and 30 thermoluminiscent dosimeters. The comparison between simulated and measured dose data shows differences below 5% in all cases. The designed program allows contrasted results to be achieved, and might be an adequate tool for the analysis of doses imparted to individual patients with different models of CT scanner.
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Document Full Text
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Bibliographical Information:

Advisor:Hernández Oter, Ramon; Calzado Cantera, Alfonso

School:Universitat Rovira i Virgili

School Location:Spain

Source Type:Master's Thesis

Keywords:departament de ciències mèdiques bàsiques


Date of Publication:12/10/2004

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