D. Babot

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.

Automatic determination of mass functions in Dempster-Shafer theory using fuzzy c-means and spatial neighborhood information for image segmentation

The Dempster-Shafer (DS) evidence theory is a new approach to the problem of segmenting multimodal images coming from different sources. The performance of such segmentation scheme is, however, largely conditioned by the appropriate determination of mass functions in DS evidence theory. We present a method of automatically determining the mass function for image segmentation problems. The idea is to link, at the image pixel level, the notion of mass functions to that of membership functions in fuzzy logic. The mass assigned to a pixel is obtained from both the membership degree of the current pixel and those of its neighboring pixels. The membership degree of each pixel is determined by applying fuzzy c-means (FCM) clustering to the gray levels of the image. A method is presented to determine the simple or composite classes in DS evidence theory from the obtained membership degree. Final segmentation is achieved using the DS combination rule and decision. The developed mass function determination method is illustrated with both simulations and examples of physical images. We demonstrate the value of introducing fuzzy clustering in evidence theory for image segmentation.

Mechanical Properties of Ewe Vertebral Cancellous Bone Compared With Histomorphometry and High-Resolution Computed Tomography Parameters

The goal of the present study was to determine if a high-resolution computed tomography (HRCT) system with 150 microns resolution was sufficient to predict mechanical properties in ewe lumbar vertebrae. To answer this question, we used a triangular comparison between: HRCT; biomechanics (compression and shear tests); and histomorphometry, which was the reference method for the measurements of morphometric parameters. Two dissected lumbar vertebrae (L-4 and L-5) from 32 ewes were used. Both compressive and shear properties correlated significantly with amount of bone and structural parameters evaluated by histomorphometry (bone volume/tissue volume, trabecular thickness, trabecular separation), but no significant correlation was found with the trabecular number. With our shear test involving the trabecular architecture itself more significant correlations were found with the node-strut analysis parameters than from the compressive test. Significant correlations were also found between HRCT and histological parameters (bone volume/tissue volume, bone surface/bone volume, trabecular separation, trabecular number, total strut length, number of nodes, and number of termini). Correlations between HRCT structural parameters and mechanical properties on L-4 were of the same magnitude as the correlations between the histomorphometric structural parameters and mechanical results on L-5 but with the remarkable advantage the HRCT is a noninvasive method. In spite of the resolution (150 microns) of our HRCT system, which entailed mainly an enlargement of the thinnest trabeculae or their loss during the segmentation process, we obtained coherent relationships between mechanical and tomographic parameters. The thinnest trabeculae probably had little effect on the mechanical strength. Also, this type of resolution allows us to consider the possibility of perfecting an in vivo HRCT system. However, physical density and bone mineral density correlated much better with strength than either classical histomorphometric or tomographic parameters. The current conclusion is fairly negative with respect to the ability of HRCT to assess mechanical properties nondestructively as compared with dual-energy X-ray absorptiometry. But, the noninvasive nature of the imaging modality and the capacity for three-dimensional imaging at arbitrary orientation make HRCT a promising tool in the quantitative assessment of cancellous architecture.

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.

X-ray Compton backscattering techniques for process tomography: imaging and characterization of materials

Non-destructive evaluation by Compton scattering using an industrial x-ray tube allows three-dimensional (3D) imaging of materials. The x-ray tube and the detector are set on the same side of the object. Thus, non-destructive evaluation of the wall of a tank, even when it is full, is possible without the requirement for the x-ray beam to cross the whole object. Several applications were tried in our laboratory. Besides 3D imaging, a method allowing thickness measurement of a wall was developed, which was especially suitable for multilayer compounds. The accuracy is mm. Compton scattering techniques also allow point-by-point density measurements in the near-surface zone of any component (even dense and bulky ones). An accuracy of 1% was achieved for light composite materials and also for dense components () provided by powder metallurgy. A new application allows us to perform 3D imaging using a linear accelerator (6 MeV) as the photon source. Thus, testing can be performed inside a tank, even through a thick and dense wall (8 mm of steel).

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.

In-line density measurement system using X-ray Compton scattering

Abstract A density measurement system based on X-ray Compton scattering has been constructed. Intended to be installed on an industrial production line, this computer-controlled system measures automatically the local density (within a few mm 3 ) of the middle products of powder metallurgy (green state). The measurement is nondestructive and is made from a single side of the sample. An industrial X-ray tube and a multihole collimator allow a measurement time of 40 s with an accuracy of better than 1%, for products obtained after compression of iron powder and whose density ranges from 6 to 7.3 g cm −3 .

Quantitative microtomography: measurement of density distribution in glass wool and local evolution during a one-dimensional compressive load

This paper proposes two applications of quantitative tomography. Assuming that the attenuation ratio of the x-ray beam throughout the investigated sample can be evaluated with high accuracy, this paper deals with quantitative characterization of glass wool structure. In a first part, we measure the 3D spatial distribution of density in a sample that has been extracted from a glass wool plate. The calibration is performed and an experimental validation of the technique is realized. The heterogeneity of the material is characterized and explained in relation to manufacturing methods. In a second step, we follow the evolution of this distribution during a mechanical test. For this purpose, an original device has been developed in the laboratory in order to perform a tomographic process during a mechanical load. The proposed method compares the density distribution between two different compression steps and determines the evolution of density inside the sample. The local strain is evaluated along the stress direction. These first results allow assumptions about the relationships between structure and properties to be expressed.

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.