Simulating lower-dose scans from an available CT scan

Low-dose CT scans can be obtained by reducing the radiation dose to the patient; however, lowering the dose results in a lower signal-to-noise ratio and therefore also in a reduced image quality. In this research, we aim to develop a tool to simulate a reduced-dose scan from an existing standard-dose scan. The motivation for simulating a reduced-dose scan is to determine how much the dose can be reduced without losing the relevant information required for proton treatment planning. The method estimates the noise equivalent number of photons in the sinogram and applies a thinning to reduce that number. The method accounts for the bowtie filter, for the noise correlation between neighboring detector elements and for the fact that for the same image intensity, a harder beam has fewer photons and therefore a higher variance. The proposed model shows a close agreement between the variance in the observed and in the simulated lower-dose scans. Simulations of low-dose scans of a 21 cm and a 6 cm water phantom in a range from 300 to 20 mAs show that the noise variance of the reconstructed images matches the reconstructions from the real scans with less than 5% error.