Better knowledge of elemental composition of patient tissues may improve the accuracy of absorbed dose delivery
in brachytherapy. Deficiencies of water-based protocols have been recognized and work is ongoing to implement
patient-specific radiation treatment protocols. A model based iterative image reconstruction algorithm DIRA
has been developed by the authors to automatically decompose patient tissues to two or three base components
via dual-energy computed tomography. Performance of an updated version of DIRA was evaluated for the
determination of prostate calcification. A computer simulation using an anthropomorphic phantom showed that
the mass fraction of calcium in the prostate tissue was determined with accuracy better than 9%. The calculated
mass fraction was little affected by the choice of the material triplet for the surrounding soft tissue. Relative
differences between true and approximated values of linear attenuation coefficient and mass energy absorption
coefficient for the prostate tissue were less than 6% for photon energies from 1 keV to 2 MeV. The results indicate
that DIRA has the potential to improve the accuracy of dose delivery in brachytherapy despite the fact that
base material triplets only approximate surrounding soft tissues.
Monte Carlo (MC) computer simulation of chest x-ray imaging systems has hitherto been performed using anthropomorphic phantoms with too large (3 mm) voxel sizes. The aim for this work was to develop and use a Monte Carlo computer program to compute projection x-ray images of a high-resolution anthropomorphic voxel phantom for visual clinical image quality evaluation and dose-optimization. An Alderson anthropomorphic chest phantom was imaged in a CT-scanner and reconstructed with isotropic voxels of 0.7 mm. The phantom was segmented and included in a Monte Carlo computer program using the collision density estimator to derive the energies imparted to the detector per unit area of each pixel by scattered photons. The image due to primary photons was calculated analytically including a pre-calculated detector response function. Attenuation and scatter of x-rays in the phantom, grid and image detector was considered. Imaging conditions (tube voltage, anti-scatter device) were varied and the images compared to a real computed radiography (Fuji FCR 9501) image. Four imaging systems were simulated (two tube voltages 81 kV and 141 kV using either a grid with ratio 10 or a 30 cm air gap). The effect of scattered radiation on the visibility of thoracic vertebrae against the heart and lungs is demonstrated. The simplicity in changing the imaging conditions will allow us not only to produce images of existing imaging systems, but also of hypothetical, future imaging systems. We conclude that the calculated images of the high-resolution voxel phantom are suitable for human detection experiments of low-contrast lesions.
Cone-beam computed tomography (CT) projections were calculated by the Monte Carlo method for two cylindrical water phantoms of different sizes and for an antropomorphic voxel phantom with and without the presence of an anti-scatter grid. The scatter-to-primary ratio (SPR) was evaluated for each projection and the dependence of the amount of scattered radiation on the phantom size, cone beam size, photon energy, and antiscatter grid was investigated. It was found that the amount of scattered radiation is a slowly varying function of position in the image plane whose values, depending on configuration parameters, may cover a range of several magnitudes. The SPR reflects changes in the amount of primary photons and may reach values around 5 for large phantoms, wide beams and 120 kV spectrum or even higher values for low energy photons.