The primary aim of the present work was to analyze the effects of varying scatter-to-primary ratios on the appearance of
simulated nodules in chest tomosynthesis section images. Monte Carlo simulations of the chest tomosynthesis system
GE Definium 8000 VolumeRAD (GE Healthcare, Chalfont St. Giles, UK) were used to investigate the variation of
scatter-to-primary ratios between different angular projections. The simulations were based on a voxel phantom created
from CT images of an anthropomorphic chest phantom. An artificial nodule was inserted at 80 different positions in the
simulated phantom images, using five different approaches for the scatter-to-primary ratios in the insertion process. One
approach included individual determination of the scatter-to primary-ratio for each projection image and nodule location,
while the other four approaches were using mean value, median value and zero degree projection value of the scatter-toprimary
ratios at each nodule position as well as using a constant scatter-to-primary ratio of 0.5 for all nodule positions.
The results indicate that the scatter-to-primary ratios vary up to a factor of 10 between the different angular
tomosynthesis projections (±15°). However, the error in the resulting nodule contrast introduced by not taking all
variations into account is in general smaller than 10 %.
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.
The aim of this work was to investigate and quantify the effects of system noise, nodule location, anatomical noise and anatomical background on the detection of lung nodules in different regions of the chest x-ray. Simulated lung nodules of diameter 10 mm but with varying detail contrast were randomly positioned in four different kinds of images: 1) clinical images collected with a 200 speed CR system, 2) images containing only system noise (including quantum noise) at the same level as the clinical images, 3) clinical images with removed anatomical noise, 4) artificial images with similar power spectrum as the clinical images but random phase spectrum. An ROC study was conducted with 5 observers. The detail contrast needed to obtain an A<sub>z</sub> of 0.80, C<sub>0.8</sub>, was used as measure of detectability. Five different regions of the chest x-ray were investigated separately. The C<sub>0.8</sub> of the system noise images ranged from only 2% (the hilar regions) to 20% (the lateral pulmonary regions) of those of the clinical images. Compared with the original clinical images, the C<sub>0.8</sub> was 16% lower for the de-noised clinical images and 71% higher for the random phase images, respectively, averaged over all five regions. In conclusion, regarding the detection of lung nodules with a diameter of 10 mm, the system noise is of minor importance at clinically relevant dose levels. The removal of anatomical noise and other noise sources uncorrelated from image to image leads to somewhat better detection, but the major component disturbing the detection is the overlapping of recognizable structures, which are, however, the main aspect of an x-ray image.