A Geant4 tool for edge-illumination X-ray phase-contrast imaging

Abstract

The PEPI project is developing a new experimental facility integrating a chromatic photon-counting detector within an edge-illumination (EI) phase-contrast setup. In this context, a novel Geant4-based simulation tool has been introduced with the aim of defining the optimal design of the experimental setup. The code includes a custom X-ray refraction process and allows simulating the whole EI system, comprising a polychromatic and extended source, absorbing masks, substrates, their movement during acquisition, and X-ray detection. In this paper, a realistic spectral detector model is introduced and its energy response validated against experimental data acquired with synchrotron radiation at energies between 26 and 50 keV. Moreover, refraction and transmission images of a plastic phantom are reconstructed from simulation data and successfully compared with theoretical predictions. Finally, an optimization study aiming at finding the effect of the X-ray focal spot size (i.e. spatial coherence) on image quality is presented; the results suggest that, in the considered configuration, the system can tolerate source sizes up to 30 μm, while, for a fixed exposure time, the best signal-to-noise ratio in refraction images is found for source sizes in the order of 10 to 15 μm.

Virtual grating approach for Monte Carlo simulations of edge illumination-based x-ray phase contrast imaging

“…(2) modelling the grating as a volume with bars extending along the direction of the optical axis (3D) [21,22,40,41], or (3) simulation frameworks where the effect of the grating is accounted for implicitly, without introducing the gratings as components in the simulation [16,33,31]. Two-dimensional (flat) grating models are common in, but not restricted to, wave-optics simulations and involve either an idealised binary representation of the grating [18,26,27,44] or application of a complex transmission function (projection approximation) [23,14,15,25,24,32,20].…”

“…X-rays can, for example, intersect the grating bars under an angle, impacting the effective distance traveled through the material. To incorporate these effects, grating bars are modelled as full 3D objects [21,22,40,41]. Three-dimensional MC models of membranes have been used for simulating speckle-based imaging as well [45].…”

“…XPCI simulators are often either based on wave optics calculations [13,14,15,16], Monte Carlo (MC) code [17,18,19,20,21,22], or a combination thereof [23,24,25,26,27,28]. In addition, ray tracing-based [29,30,31,32] and empirical methods [33] have been reported.…”

“…In addition, ray tracing-based [29,30,31,32] and empirical methods [33] have been reported. Some of these studies have explicitly included simulations for EI [15,18,19,30,21,22]. In earlier work [26,27], we have demonstrated the hybrid (MC and wave optics) simulation of XPCI for GBI using an extended version of GATE, a Geant4-based MC framework [34,35,36].…”

“…System design parameter studies however, where possibly hundreds or more variations of the grating geometry are tested, easily require hundreds of MC simulations. Even simply stepping the EI illumination curve (IC) already requires multiple simulations [22]. Hence, there is a need for computationally efficient MC simulation strategies in order to perform these studies in an effective way.…”