Development and validation of a clinical CT system simulator

Many advanced reconstruction and image processing methods are being developed with the aim of improving image quality in CT. Development and testing of these methods is aided by the ability to simulate realistic images, to both control the acquisition process and be able to use digital phantoms with known ground truth. Therefore, in this work, we present a method to simulate realistic scanner-specific sinograms from digital phantoms. For this, a series of measurements was conducted on a clinical CT system to characterize resolution loss, noise characteristics, and the exposure-to-detector output relationship. These measurements were used to develop a simulation pipeline, which involves raytracing of a digital phantom, taking into account the focal spot size and gantry rotation, followed by the use of Lambert's Law to determine the amount of energy arriving at each detector element. The spectrum for the specific tube voltage and current was modeled using previously published spectral models. The resulting sinogram was then corrupted, by applying the measured detector Modulation Transfer Function (MTF), and adding noise based on the Noise Power Spectrum (NPS) and mean-variance relationship. Simulator results were compared to those acquired with the CT system in our clinic, showing an average difference of 2.1% in the off-center MTF magnitude, 0.048 in normalized NPS magnitude and only 6 and 5 Hounsfield Units (HU) difference in the voxel values for respectively water and air. The developed simulator seems capable of generating realistic CT images, which can help researchers develop and test their algorithms.