Benoit Recur

Aeronautics composite material inspection with a terahertz time-domain spectroscopy system

Abstract. The usability of pulsed broadband terahertz radiation for the inspection of composite materials from the aeronautics industry is investigated, with the goal of developing a mobile time-domain spectroscopy system that operates in reflection geometry. A wide range of samples based on glass and carbon fiber reinforced plastics with various types of defects is examined using an imaging system; the results are evaluated both in time and frequency domain. The conductivity of carbon fibers prevents penetration of the respective samples but also allows analysis of coatings from the reflected THz pulses. Glass fiber composites are, in principle, transparent for THz radiation, but commonly with significant absorption for wavelengths >1  THz. Depending on depth, matrix material, and size, defects like foreign material inserts, delaminations, or moisture contamination can be visualized. If a defect is not too deep in the sample, its location can be correctly identified from the delay between partial reflections at the surface and the defect itself.

Propagation beam consideration for 3D THz computed tomography

In this paper, a model of the beam propagation is developed according to the physical properties of THz waves used in THz computed tomography (CT) scan imaging. This model is first included in an acquisition simulator to observe and estimate the impact of the Gaussian beam intensity profile on the projection sets. Second, the model is introduced in several inversion methods as a convolution filter to perform efficient tomographic reconstructions of simulated and real acquired objects. Results obtained with three reconstruction methods (BFP, SART and OSEM) are compared to the techniques proposed in this paper. We will demonstrate an increase of the overall quality and accuracy of the 3D reconstructions.

Mojette reconstruction from noisy projections

Apart from the usual methods based on the Radon theorem, the Mojette transform proposes a specific algorithm called Corner Based Inversion (CBI) to reconstruct an image from its projections. Contrary to other transforms, it offers two interesting properties. First, the acquisition follows discrete image geometry and resolves the well-known irregular sampling problem. Second, it updates projection values during the reconstruction such that the sinogram contains only data for not yet reconstructed pixels. Unfortunately, the CBI algorithm is noise sensitive and reconstruction from corrupted data fails. In this paper, we develop a new noise-robust CBI algorithm based on data redundancy and noise modelling in the projections. This algorithm is applied in discrete tomography from a Radon acquisition. Reconstructed image results are discussed and applications in usual tomography are detailed.

Rotations in the Mojette space

In this paper, we develop an exact, reversible and scale change rotation in the Mojette projection space. The whole process is performed using 1D fast operators and has the advantage to be consistent with standard tomographic geometry.

A Preliminary Study to Reduce the Missing Wedge Effect by Using a Noise Robust Mojette Reconstruction

Apart from the usual methods based on the Radon theorem, the Mojette transform proposes a specific algorithm called Corner Based Inversion (CBI) to reconstruct an image from its projections. Contrary to other transforms, it offers two interesting properties. First, the acquisition follows discrete image geometry and resolves the well-known irregular sampling problem. Second, it updates projection values during the reconstruction such that the sinogram contains only data for not yet reconstructed pixels. These properties could be a solution to reduce the missing wedge effect in tomography. Unfortunately, the CBI algorithm is noise sensitive and reconstruction from corrupted data fails. In this paper, we first develop and optimize a noise-robust CBI algorithm based on data redundancy and noise modelling in the projections. Afterwards, this algorithm is applied in discrete tomography from a specific Radon acquisition. Reconstructed image results are discussed and applications and perspectives to reduce the missing wedge effect are also developed.

Influence of the acquisition method on terahertz tomography

Terahertz tomography is a technique used in the field of contact-free and non destructive testing [1][2]. It can be achieved using a raster scanning system with three axes or two axes and a rotation stage. The setup uses a Gunn diode working at a frequency of 84 GHz as a source and a Schottky diode or pyroelectric element for the detection. A tripler can be added to the source to obtain a frequency of 252 GHz. The sample is hold on a rotating stage θ mounted on a two axis (X, Y) scanner. We propose to characterize the origin of the electronic noise induced by fluctuations in the detector and analyze the influence of the acquisition method on the reconstructed image in tomography.