Valérie Kaftandjian

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.

Uncertainty modelling using Dempster-Shafer theory for improving detection of weld defects

This paper presents an approach that is based on the combined use of Dempster-Shafer (DS) theory and fuzzy sets for improving automatic detection of weld defects. It consists in modelling detection uncertainty in feature space through using the mass function weighted by membership degrees, and fusing the features of objects using DS com, bination rule. The method is demonstrated on the typical industrial application of weld inspection. The obtained results show that, by modelling detection uncertainty, a confidence level can be associated to each detected object, making the defect detection more precise and reliable.

The combined use of the evidence theory and fuzzy logic for improving multimodal nondestructive testing systems

A data fusion approach for improving weld inspection by multimodal nondestructive testing (NDT) is investigated. The method relies on the combined use of evidence theory and fuzzy logic. In this framework, X-ray and ultrasonic inspection is modeled using fuzzy sets and mass function in evidence theory, and objects corresponding to X-ray and ultrasonic testing are fused at the level of features. The results show that the fusion improves significantly the reliability of defect detection and the defect sizing.

A Comparison of the Ball, Wire, Edge, and Bar/Space Pattern Techniques for Modulation Transfer Function Measurements of Linear X-Ray Detectors

Actual assessment of the modulation transfer function (MTF) of a physical system requires objective experimental data to be obtained. This paper presents four practical methods for measuring the MTF of a linear x-ray detector. These methods are based on using the ball, wire, edge, and bar/space pattern as stimuli. The measurement procedure and experimental conditions are described in detail. The MTF results obtained with these different techniques are analyzed and compared.

Validation of a novel microradiography device for characterization of bone mineralization

In order to simplify bone mineralization measurements, a system using radiographic films has been updated with a digital detector. The objective of this paper was to validate this new device. Technologies and physical phenomena involved in both systems (radiographic films and digital detector) are different. The methodology used to compare the two systems was based on image quality and assessed on two main parameters: contrast to noise ratio and spatial resolution. Results showed that the contrast to noise ratio was similar between the two systems, provided that acquisition parameters were optimized. With regard to spatial resolution, a magnification factor of at least 4 or more was required to achieve the same resolution as films. A final validation was also shown on a real image of a bone sample. The results showed that both systems have similar image quality performances, and the system using digital detector has several advantages (easier to use than films, no consumables and faster acquisition time).

Improvement of X-ray castings inspection reliability by using Dempster-Shafer data fusion theory

The aim of this work is to improve the classification of defects in X-ray inspection by developing a new method based on Dempster-Shafer data fusion theory where measured features on the detected objects are considered as information sources. From the histogram of features values on a learning database of manually classified objects, an automatic procedure is proposed to define a set of mass functions for each feature. The spatial repartition of features is divided into regions of confidence with corresponding mass functions. A smooth transition between regions is ensured by using fuzzy membership functions. The whole process is carried out without any expert intervention. Validation takes place on a testing database. Data fusion leads to a significant improvement of classification performances with respect to the actual system.

High-resolution x-ray computed tomography using a solid-state linear detector.

In this paper we present an original low cost acquisition system for computed tomographic imaging. On the first hand, the use of a linear detector consisting of sensitive elements 0.225 mm × 0.5 mm in size allows one to obtain a resolution of about 150 μm after magnification. Image quality has been assessed in terms of spatial resolution and contrast sensitivity by imaging test objects. The images ofin vitrovertebrae acquired by this experimental system show great improvement compared to the images acquired by a conventional medical scanner. On the other hand, an even better resolution (25 μm) has been obtained by using an x-ray sensitive Vidicon camera, and an industrial application is presented with advanced composite materials.

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.

Modulation transfer function evaluation of linear solid-state x-ray-sensitive detectors using edge techniques

The exact determination of the modulation transfer function (MTF) of a physical system is a persistent problem. We present a practical method of measuring numerically the MTF of linear solid-state x-ray-sensitive detectors. The method is based on the use of edge techniques and allows us to obtain the MTF of a linear detector from its edge-spread function (ESF). ESF measurement techniques are discussed in detail, and calculation of the corresponding MTFs are shown.

Contrast transfer function measurement of X-ray solid state linear detectors using bar/space pattern methods

This paper presents a detailed analysis of bar/space pattern methods used in the contrast transfer function (CTF) calculation of linear solid-state X-ray sensitive detectors (LD). The analysis presented centres on the effect of cell aperture on LD response as well as on the measurement of the LDs CTF along its main axis. Various theoretical simulations are designed to obtain a better understanding of the behaviour of the LD. The simulation results are also compared with the experimental CTF measurements.