A modified algebraic reconstruction technique taking refraction into account with an application in terahertz tomography

Abstract

Terahertz (THz) tomography is a rather novel technique for nondestructive testing that is particularly suited for the testing of plastics and ceramics. Previous publications showed a large variety of conventional algorithms adapted from computed tomography or ultrasound tomography which were directly applied to THz tomography. Conventional algorithms neglect the specific nature of THz radiation, i. e. refraction at interfaces, reflection losses and the beam profile (Gaussian beam), which results in poor reconstructions. The aim is the efficient reconstruction of the complex refractive index, since it indicates inhomogeneities in the material. A hybrid algorithm has been developed based on the algebraic reconstruction technique (ART). ART is adapted by including refraction (Snell's law) and reflection losses (Fresnel equations). Our method uses a priori information about the interface and layer geometry of the sample. This results in the "Modified ART for THz tomography", which reconstructs simultaneously the complex refractive index from transmission coefficient and travel time measurements.

Terahertz Differential Computed Tomography: a Relevant Nondestructive Inspection Application

“…As in [22] and in [13], we consider materials with a high refractive index. The reconstruction algorithm is stopped after 50 iterations since, during all the experiments we carried out, it seemed to be a good trade off between artefacts and convergence.…”

“…However, this method has a drawback [13], it can only be used to reconstruct a defect located in the central part of the cylinder. Moreover, as stated by Tepe et al [22], such a method is completely dedicated to reconstructing a cylinder and cannot be applied to objects with other shapes.…”

“…A first idea would be to model the THz beam propagation using a standard Snell & Descartes ray-tracing (as in X-ray CT). Since Tepe et al [22] say they neglect the Gaussian beam profile, we presume they used this approach. It has the advantage of easily modeling refraction, reflection and absorption.…”

“…The global procedure is illustrated in Fig.4: the beam is refracted at each interface and attenuated as it propagates through an object whose absorption index is known. As aknowledged by Tepe et al [22], this model cannot achieve a reliable projection operation since the intensity profile of the THz beam is ignored.…”

Hyperspectral Terahertz Tomography in Amplitude Contrast

“…CW narrowband systems will allow imaging in THz amplitude contrast at discrete wavelengths important for security and non-destructive testing. To achieve indirect imaging from the amplitude of the transmission beam hard-field tomography, it is also essential to understand and manage the soft-field scattered component in the transmitted THz signal; problems associated with THz computed tomography include Snell and Fresnel refraction [12][13][14][15], and Gaussian THz beam profile [16]. The novelty in this work is that using a pulsed THz tomography system wide spectral information is accessed, over one order of magnitude in wavelength, to study the quality of the hard-field reconstructed images as a function of wavelength.…”