Computer simulation tools for X-ray CT are important for research efforts in developing reconstructionmethods, designing
new CT architectures, and improving X-ray source and detector technologies. In this paper, we propose a physics-based
modeling method for X-ray CT measurements with energy-integrating detectors. It accurately accounts for the dependence
characteristics on energy, depth and spatial location of the X-ray detection process, which is either ignored or over
simplified in most existing CT simulation methods. Compared with methods based on Monte Carlo simulations, it is
computationally much more efficient due to the use of a look-up table for optical collection efficiency. To model the CT
measurments, the proposed model considers five separate effects: energy- and location-dependent absorption of the incident
X-rays, conversion of the absorbed X-rays into the optical photons emitted by the scintillator, location-dependent
collection of the emitted optical photons, quantumefficiency of converting fromoptical photons to electrons, and electronic
noise. We evaluated the proposed method by comparing the noise levels in the reconstructed images from measured data
and simulations of a GE LightSpeed VCT system. Using the results of a 20 cm water phantom and a 35 cm polyethylene
(PE) disk at various X-ray tube voltages (kVp) and currents (mA), we demonstrated that the proposed method produces realistic CT simulations. The difference in noise standard deviation between measurements and simulations is approximately
2% for the water phantom and 10% for the PE phantom.
Ray-tracing based simulation methods are widely used in modeling X-ray propagation, detection and imaging. While
most of the existing simulation methods rely on analytical modeling, a novel hybrid approach comprising of statistical
modeling and analytical approaches, is proposed here.
Our hybrid simulator is a unique combination of analytical modeling for evoking the fundamentals of X-ray transport
through ray-tracing, and a look-up-table (LUT) based approach for integrating it with the Monte Carlo simulations that
model optical photon-transport within scintillator. The LUT approach for scintillation-based X-ray detection invokes
depth-dependent gain factors to account for intra-pixel absorption and light-transport, together with incident-angle
dependent effects for inter-pixel X-ray absorption (parallax effect). The model simulates the post-patient collimator for
scatter-rejection, as an X-ray shadow on scintillator, while handling its position with respect to the pixel boundary, by a
smart over-sampling strategy for high efficiency.
We have validated this simulator for computed tomography system-simulations, by using real data from GE Brivo
CT385. The level of accuracy of image noise and spatial resolution is better than 98%. We have used the simulator for
designing the post-patient collimator, and measured modulation transfer function (MTF) for different widths of the
Validation and simulation study clearly demonstrates that the hybrid simulator is an accurate, reliable, efficient tool for
realistic system-level simulations. It could be deployed for research, design and development purposes to model any
scintillator-based X-ray imaging-system (2-dimensional and 3-dimensional), while being equally applicable for medical
and industrial imaging.