Study of dose reduction in computed tomography using a time-of-flight scatter rejection mechanism in a 3D voxelized phantom

Objective: Scattered photons are the main source of contrast degradation in X-ray imaging when not filtered out appropriately. Measuring the time-of-flight (ToF) of X-ray photons has recently been revisited as an innovative way to discriminate between primary and scattered photons without significant loss in sensitivity in order to improve the contrast-to-noise ratio (CNR) in radiography and computed tomography (CT). The technique could allow for the removal of the anti-scatter grid (ASG) while still producing a CNR comparable to that of a fan-beam CT, allowing for potential dose reduction and more uniform axial spatial resolution when used in a cone-beam configuration. Approach: This work presents an evaluation study of the potential reduction in absorbed radiation dose in the body during a CT image acquisition when using time-correlated single photon counting CT in a cone-beam geometry. A comparative study is carried out using Monte Carlo simulations in GATE of a voxelized body of an adult female for a multi-slice fan beam CT scanner and a cone beam system using ToF measurements with various timing jitters or ASGs. Main results: For CNR equivalent torso images, simulation data in the high contrast region for ToF of 10, 50, 100, 200 ps FWHM timing jitters show a dose reduction of 35%, 28%, 15%, 5% respectively compared to cone-beam CT with an ASG. Compared to a fan-beam setup without an ASG, the cone-beam ToF CT achieved a dose reduction of 13% and 4% for 10 and 50 ps FWHM timing jitter respectively, and an increase of 13% and 26% at 100 and 200 ps timing jitter. In the low contrast region, 10 ps ToF led to a reduction in dose of 15% and 50 ps led to an increase of 5%. Resolutions above 100 ps failed to accurately reject scattered photons and led to a considerable increase in dose. Significance: Time-of-flight scatter rejection shows potential for reducing radiation dose while preserving image quality when the timing resolution is 50 ps FWHM or better, but better ToF-based scatter rejection algorithms are required to enable this method for worse timing resolutions and low contrast regions.