Photon fluence and dose estimation in computed tomography using a discrete ordinates Boltzmann solver

Edward T Norris,Xin Liu

doi:10.1038/s41598-020-68320-8

Edward T Norris, Xin Liu

Open Access

https://doi.org/10.1038/s41598-020-68320-8

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Abstract

In this study, cone-beam single projection and axial CT scans are modeled with a software package—DOCTORS, which solves the linear Boltzmann equation using the discrete ordinates method. Phantoms include a uniform 35 cm diameter water cylinder and a non-uniform abdomen phantom. Series simulations were performed with different simulation parameters, including the number of quadrature angles, the order of Legendre polynomial expansions, and coarse and fine mesh grid. Monte Carlo simulations were also performed to benchmark DOCTORS simulations. A quantitative comparison was made between the simulation results obtained using DOCTORS and Monte Carlo methods. The deterministic simulation was in good agreement with the Monte Carlo simulation on dose estimation, with a root-mean-square-deviation difference of around 2.87%. It was found that the contribution of uncollided photon fluence directly from the source dominates the local absorbed dose in the diagnostic X-ray energy range. The uncollided photon fluence can be calculated accurately using a ‘ray-tracing’ algorithm. The accuracy of collided photon fluence estimation is largely affected by the pre-calculated multigroup cross-sections. The primary benefit of DOCTORS lies in its rapid computation speed. Using DOCTORS, parallel computing with GPU enables the cone-beam CT dose estimation nearly in real-time.

Highlights

In this study, cone-beam single projection and axial CT scans are modeled with a software package— Discrete Ordinate Computed TOmography and Radiography Simulator (DOCTORS), which solves the linear Boltzmann equation using the discrete ordinates method
Deterministic photon dose estimation has been widely used in the field of radiation therapy[28,29,30,31,32], its use has not been fully investigated for CT imaging
Since the uncollided photon fluence is calculated by a ray-tracing algorithm in DOCTORS which is independent of quadrature and Legendre parameters, only one set of uncollided photon fluence was calculated and compared to the MCNP6 simulations

Summary

Introduction

Cone-beam single projection and axial CT scans are modeled with a software package— DOCTORS, which solves the linear Boltzmann equation using the discrete ordinates method. Stochastic methods (e.g., Monte Carlo simulation) have been used extensively in the past[4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20], and are generally considered to be the gold standard for estimating photon distributions and CT doses They require a large number of particle histories and, a lengthy computation time is needed to reduce statistical uncertainty to an acceptable level. We examined the accuracy and runtime of DOCTORS to compute energy-resolved photon fluence and dose distribution of a cone-beam CT with uniform and non-uniform phantoms

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Discussion

Conclusion