Philippe Duvauchelle

A computer code to simulate X-ray imaging techniques

Abstract A computer code was developed to simulate the operation of radiographic, radioscopic or tomographic devices. The simulation is based on ray-tracing techniques and on the X-ray attenuation law. The use of computer-aided drawing (CAD) models enables simulations to be carried out with complex three-dimensional (3D) objects and the geometry of every component of the imaging chain, from the source to the detector, can be defined. Geometric unsharpness, for example, can be easily taken into account, even in complex configurations. Automatic translations or rotations of the object can be performed to simulate radioscopic or tomographic image acquisition. Simulations can be carried out with monochromatic or polychromatic beam spectra. This feature enables, for example, the beam hardening phenomenon to be dealt with or dual energy imaging techniques to be studied. The simulation principle is completely deterministic and consequently the computed images present no photon noise. Nevertheless, the variance of the signal associated with each pixel of the detector can be determined, which enables contrast-to-noise ratio (CNR) maps to be computed, in order to predict quantitatively the detectability of defects in the inspected object. The CNR is a relevant indicator for optimizing the experimental parameters. This paper provides several examples of simulated images that illustrate some of the rich possibilities offered by our software. Depending on the simulation type, the computation time order of magnitude can vary from 0.1 s (simple radiographic projection) up to several hours (3D tomography) on a PC, with a 400 MHz microprocessor. Our simulation tool proves to be useful in developing new specific applications, in choosing the most suitable components when designing a new testing chain, and in saving time by reducing the number of experimental tests.

EFFECTIVE ATOMIC NUMBER IN THE RAYLEIGH TO COMPTON SCATTERING RATIO

Abstract Detection and counting X-ray photons scattered by the Rayleigh and Compton processes enable matter to be characterized locally. A theoretical relation was first established which simulates the result of a Rayleigh to Compton ratio measurement. It can thus be shown that a correct choice of scattering angle and photon energy enables a result to be obtained which is almost independent of X-ray attenuation inside the sample. With this condition, the Rayleigh to Compton scattering ratio depends only on the mixture under study and provides a local measurement of certain complicated functions of the atomic number Z and of the weight percentage of the different elements which constitute the compound. This function is usually called the “effective atomic number”, Zeff. Different methods of calculation of Zeff are found in the literature, four of them, those used most frequently, were tested. There is no unique relation between the computed Zeff and 80 experimental results performed on aqueous solutions with different concentrations of eight elements, having Z values ranging from 13 to 64. This observation led us to the conclusion that any effective atomic number is valid only for given experimental conditions. Finally, a new method of calculating Zeff was developed for the Rayleigh to Compton scattering ratio, which is applicable for any material, scattering angle or photon energy.

A 2D multiresolution image reconstruction method in X-ray computed tomography

We propose a multiresolution X-ray imaging method designed for non-destructive testing/evaluation (NDT/NDE) applications which can also be used for small animal imaging studies. Two sets of projections taken at different magnifications are combined and a multiresolution image is reconstructed. A geometrical relation is introduced in order to combine properly the two sets of data and the processing using wavelet transforms is described. The accuracy of the reconstruction procedure is verified through a comparison to the standard filtered backprojection (FBP) algorithm on simulated data.

Modelling of Radiographic Inspections

Computer modelling of non-destructive testing methods has come a long way from the beginnings in the mid 90s to today. Radiographic modelling for components with higher wall thicknesses, as they are typical for nuclear applications, must include precise predictions of scattered radiation and its impact in terms of contrast reduction. Dedicated or general purpose Monte Carlo methods with the ability to calculate higher order scattering events are the state of the art for these applications. Aerospace applications, on the other hand, have stronger requirements on the modelling code’s capabilities to import complex CAD geometries, and can benefit from faster analytical scatter models, limited to first or second order scattering events. Similar distinctions can be made for the various approaches proposed to accurately model geometrical and film unsharpness, film granularity, film responses, film/foil cartridges and photon noise. This article presents a state-of-the-art review of radiographic modelling from the perspective of two important application domains with very different requirements, nuclear and aerospace.

Rayleigh to Compton ratio computed tomography using synchrotron radiation

The detection of X-ray photons scattered through a sample by the Rayleigh and Compton processes is used to perform tomographic images. A map can thus be obtained, which emphasizes small changes in atomic number Z within the sample. In such a way to distinguish between photons scattered through Rayleigh and Compton processes, a monochromatic photon beam must be used. Choosing a 60 keV photon energy, difference between polyethylene and an aqueous solution containing a low concentration of iodine (0.5 mg.cm-3) is easily obtained. The most common experimental device involves a collimator with an unique slit. The scanning throughout the slice is performed point by point and the corresponding image can directly be drawn up. Beside the point by point method, the present paper describes a new experimental arrangement and the corresponding reconstruction method. The scanning method is similar to the one used for first generation tomographs. A standard reconstruction algorithm delivers two intermediate images, corresponding to the Compton and Rayleigh contributions. On both images artifacts are present, due to photon attenuation inside the sample. Computation of the ratio between those two images gives a Z map of the sample, free of artifacts. The experiment was performed at the European Synchrotron Radiation Facility (ESRF), in Grenoble (France), on line ID15 B. Due to the very high photon intensity, short measurement times are allowed (around five seconds by point), as well as a good spatial resolution. The voxel size is 1 mm X 1 mm in the plane of the slice, and 0.3 mm in the third direction.

Simulating Radiographic Inspections with Imaging Plates

Computed Radiography (CR) based on photostimulable imaging plates (IP) is a potential replacement technique for traditional silver film radiography. For the inspections of components with high wall thicknesses requiring higher energy sources, however, imaging plate performance suffers from a spectral response which is low for the higher energies and high in the energy range where scattered radiation is typically observed. For these applications, care must be taken to apply appropriate filtering. Simulation tools are expected to be helpful in determining optimal operating conditions. We present a computer model which combines deterministic and Monte Carlo methods to simulate the imaging chain, focusing in particular on the scanner model.

A multiresolution image reconstruction method in X-ray microCT

We propose a method for multiresolution image reconstruction in X-ray micro computed tomography (microCT). It can have a variety of applications, from material characterization to small animal imaging studies. The main idea is to recover an overall image of the sample with a coarse resolution, and with a fine resolution for a region-of-interest (ROI). In a zoo-min CT type setup, two sets of data are used, taken at different magnifications ratios. They are combined with the help of an analytical relation and the reconstruction is an extension of the filtered back-projection (FBP) algorithm. We present results with simulated data, some performance aspects and a simple noise analysis.