RECENT IMPROVEMENTS FOR SCATTER SIMULATION IN SINDBAD, A COUPLED PHOTON MONTE CARLO AND CAD SOFTWARE

In an X-ray radiography, scattering of photons from the inspected object, as well as backscattering or scattering from the environment, may have significant deleterious effects on image quality, reducing the relative contrast of the flaw indication. Therefore, development of computational models simulating scattering in radiographic studies is of primary interest, to correctly evaluate flaw detectability. The X-ray radiographic simulation software, Sindbad, developed for Non-Destructive Evaluation applications, models the whole radiographic set-up, with the X-ray source, the beam interaction inside the object represented by its CAD model and the imaging process in the detector. An analytical computation is used for the uncollided image whereas the scatter flux is computed with a Monte Carlo approach. Two major evolutions in Sindbad have been recently developed to provide realistic scatter images in reasonable computing time. Up to now, because of a geometrical restriction of the coupling of EGS4 Nova and BRL-CAD, we couldn’t simulate detectors inside the object. An algorithm has been recently modified to consider the whole radiographic set-up, including the environment and all the parts of the object and detector which are behind the sensitive detector layer. Therefore, both backscattering and scattering from the environment are computed. Examples of simulations on industrial parts under experimental conditions show that the contribution of backscattering can exceed half of the overall scattering flux. Moreover, as scattering is not sensitive to sharp structures of the inspected items, an object simplification algorithm has been developed to speed up the Monte Carlo computation without modifying the scatter image. This simplification is automatic and takes into account the spectrum energy, the materials and the set-up geometry. On complex industrial objects, the computations can be twenty times faster. Introduction: An X-ray radiographic image is generated by both uncollided and scattered photons. Only the uncollided photons contribute to the exploitable part of radiographs, with the sharp structures of the examined parts. On the other hand, scattered radiation generated inside an object may have significant deleterious effects on image quality [1, 2, 3]. First, the scattered radiation adds an important continuous component to the whole beam detected in the detector, with a contribution which may exceed the uncollided flux. Consequently and especially for film detectors, scattered radiation can induce problems of saturation and contrast. Moreover, depending on the equipment set-up and the examined part, the scattered radiation component can present low frequencies which disturb the radiograph. Finally, the scattered radiation can also add significant noise to the signal, thus reducing the relative contrast of the flaw indication. Many parameters of the radiographic scene may influence the shape and the contribution of the scattered radiation. Of course, the energy of the source as well as the materials of the object to be examined have an influence on the scattering phenomena (Compton, Rayleigh, Photoelectric) and on the deviation of particles. The scattered image is also largely dependant on the position, specifically on the distance between the object and the detector. Indeed, when the object is closer to the detector, more scattered radiation is detected and the associated image is a closer representation of the object. Finally, backscatter radiation coming from the environment (detector, wall, equipment set-up) can also present a considerable contribution to the overall radiation detected in the detector. In the context of a scattering study, X-ray simulation tools are of primary interest during the design stage of radiographic facilities, when they can help to choose the device parameters (X-ray tube settings such as voltage and filtration, detector type and thickness, geometry of the bench, etc.) and predict performances of the future device. Several teams involved in X-ray NDE simulation [4, 5, 6, 7] have developed their own software, based on analytical or Monte Carlo models and using ray tracing techniques, computer aided design (CAD) of the examined sample, and a graphical user interface (GUI). The X-ray radiographic simulation software, Sindbad [8, 10, 12, 13], has been developed to help the design stage of radiographic systems or to evaluate the efficiency of image processing techniques, in both medical imaging and Non-Destructive Evaluation (NDE) industrial fields. In this paper we give an overview of Sindbad and present the most recent evolutions for scattered radiation computation : simulation of complex geometries and speed up techniques based on a CAD model simplification. Results: Overview of Sindbad: The physics of the radiographic inspection process can be divided into three separate parts, namely the X-ray beam generation in the source, the beam interaction with the examined sample, and the imaging process (detection of the remaining photon flux and transformation into a measured signal) as shown on Figure 1. X-ray source Object (CAD or voxel) Image from an X-ray detector Figure 1 : Sindbad architecture. The implemented X-ray tube model, which can be used between 30 and 450 kV, simulates the physical phenomena involved in bremsstrahlung and characteristic photon production with a semi-empiric model. It takes into account the anode angle and composition, the inherent and additional filtration and the photon exit angle. Experiments performed at LETI show that the calculated and measured doses usually agree to within 20%. Detectors are modeled in two successive steps. The first step which is common to all types of detectors computes the energy deposition in the sensing part of the detector using the energy absorption attenuation coefficients. The second step, specific to each type of detector, simulates the successive physical phenomena involved in the energy to signal transformation. For instance, in the case of a scintillating screen viewed by a CCD camera, it accounts for the energy to light photon transformation, the light photon absorption in the screen and optical coupling system, and the photon to electron conversion in the CCD device. Noise and resolution effect (MTF) can be added on the resulting image. Concerning the interaction in the object, the first approach adopted was an analytical one which combines ray tracing techniques and the attenuation law (Beer Lambert) by calculating the energy dependent attenuation due to the crossed materials. The 3D analytical simulation computes an image of the uncollided flux simulation which relies on the computation of the attenuation of the incident flux, binned in narrow energy channels, by the examined sample. This is performed by tracing rays from the source point to every pixel of the detector through the sample, either a CAD model built with BRL-CAD [9] or a 3D data volume segmented in materials. This computing model shows very efficient results concerning the uncollided photon flux but fails to evaluate the scattered photon flux correctly. A Monte Carlo simulation module [10] has also been implemented in Sindbad in order to compute the radiation scattered in the examined object. It relies on a coupling of the BRL-CAD CAD and ray-tracing package [9] with EGS-NOVA [11], a program dedicated to Monte Carlo radiation transport simulation. Once incident particles are emitted from the source, their tracking and interaction in the object are managed by the EGS Nova shower simulation. The geometry interrogation function, whose goal is to compute the distance from the current particle position to the next boundary that will be crossed, uses the ray tracing functions provided by BRL-CAD. History flags are used in order to finally provide the uncollided, once scattered and several times scattered photon images. This Monte Carlo method gives good quality results either for uncollided or scattered photon flux images [12] but its use is largely limited by the execution time drawback. An original model was developed after, combining the advantages of both analytical and Monte Carlo techniques [13]. The purpose of this computing model is to provide a total synthetic radiograph by combining images obtained from two simulations, one performed with the analytical model for the uncollided photon flux and the other one with the Monte Carlo model for the scattered radiation. As presented in the scheme in Figure 2, the absorbed energy Monte Carlo scattered flux image estimated for a low dose is scaled up to the analytical dose level, and then combined to the uncollided flux image. This scaling is independently performed for both the mean scattered image and the scattered image noise. The main advantage of the combination is that it becomes possible to obtain detailed simulated images, taking into account various interaction effects such as scatter, in a feasible computation time (a few minutes when a pure Monte Carlo simulation would take a few months). extrapolation energy to signal