Computed tomography x-ray characterization: A Monte Carlo study

Abstract

The Monte Carlo method is a powerful tool for investigating the characteristics of x-ray beams in diagnostic radiology. An accurate CT MC simulation typically requires a detailed description of the scanner under investigation, including specifications of the photon energy spectrum, the bowtie and inherent filtration design, and the geometry of the scanner (focal spot to isocenter distance, fan angle). Modern CT dosimetry is substantially informed by data produced using Monte Carlo radiation transport techniques. In order to achieve desired levels of accuracy, methods require a good model of the x-ray spectrum used. It has been shown that x-ray spectrum modeling can produce adequate spectra if both the tube potential and the HVL of the x-ray beam are known [1]. Therefore, for accurate CT dosimetry, characterization of the HVL of the CT scanner is necessary. In this work the Phillps® provided the spectrum of Brilliance 64 scanner. The purpose of this work is evaluating all data send by the manufacturer in the way to obtain a spectrum that can be used in Monte Carlo simulations to estimate organ dose or other dosimetric quantities. The EGSnrc MC code was used to evaluate the HVL obtained with the spectrum provided by the manufacturer. The cavity code was used for these simulations and variance reduction techniques were used. The HVL of our MC models were benchmarked against experimental data. The authors found that the HVL of the MC simulations agreed well with the manufacturer-specified data within 1–5% on average for the scanner. This study may be useful to establish a patient-specific dosimetry for the Phillips Brilliance 64 CT systems.