A Priori Information and Off-Axis Measurements in Terahertz Tomography

Abstract

Terahertz tomography is a non-contact inspection technique to image objects from multiple angles and reconstruct their 3D volume from intensity and time-of-flight transmission data, without the need for radiation protection measures. Unlike X-rays, terahertz radiation is subject to strong diffraction and refraction when propagating through dielectric materials, which often deteriorate the image reconstruction quality. Our solution to this problem applies ray tracing, considering the beam shape and an a priori model of the sample under investigation to predict the beam paths of the terahertz radiation. We present two reconstruction methods based on the resulting beam path predictions yielding higher image quality. Method 1 filters out beams deviating strongly, thus removing induced artifacts and errors from the reconstruction image. Method 2 employs off-axis measurements that acquire data along the full detection plane and in this way detect even strongly deflected beams. Considering these beams and the information they carry in the reconstruction enhances the image quality. Applying these methods to terahertz tomography, even complicated structures can be imaged. We display the significant enhancements achieved with the two methods by comparing the reconstruction results of different polymeric samples.INDEX TERMS Terahertz radiation, computed tomography, non-destructive testing, a priori information, off-axis measurement, reconstruction algorithms, imaging, time-of-flight, conjugate gradient least square (CGLS).

Coherent Off-Axis Terahertz Tomography with a Multi-Channel Array and f-theta Optics

“…This allows a determination of a sample’s full refractive index from the measured amplitude reduction and time delay of the signal. The single-pixel setup is described in more detail in [ 23 , 24 ]. In [ 24 ], we presented an algorithm to include a priori information and off-axis measurements in the tomographic reconstruction process.…”

“…The single-pixel setup is described in more detail in [ 23 , 24 ]. In [ 24 ], we presented an algorithm to include a priori information and off-axis measurements in the tomographic reconstruction process. The latter requires time-costly scans of the whole detector plane, which have to be acquired by moving the single-pixel detector along the whole range of motion for every transmitter position and angle.…”

“…The image reconstruction employed in this work has been discussed at length in [ 23 ], and the incorporation procedure of a priori information is covered comprehensively in [ 24 ]. Here, we will therefore only give a brief introduction to the aforementioned concepts.…”

“…These optical effects, if not accounted for in the reconstruction scheme, result in image distortion and artifacts. To anticipate this, we designed a tomographic single-pixel setup featuring one transmitter and one independently moveable detector, allowing the detection of off-axis radiation deflected by the sample [ 24 ]. To be able to include the data from the off-axis measurements and additional a priori information about the sample in the reconstruction process, we implemented a flexible, iterative, and fast algorithm, the conjugate gradient least squares reconstruction algorithm [ 23 ].…”

“…To be able to include the data from the off-axis measurements and additional a priori information about the sample in the reconstruction process, we implemented a flexible, iterative, and fast algorithm, the conjugate gradient least squares reconstruction algorithm [ 23 ]. The imaging results can be observed in Figure 1 and [ 24 ].…”