3D TeraHertz Tomography

Transcription

1 3D TeraHertz Tomography B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. Desbarats, 3 1 LOMA, Université de Bordeaux / CNRS 2 ALPhANOV, Centre Technologique Optique et Lasers, Université de Bordeaux 3 LaBRI, Université de Bordeaux / CNRS 351 cours de la Libération talence Cedex, France Journées Problèmes Inverses - IMS - IMB - LaBRI - Avril /19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. Desbarats, 3 3D TeraHertz Tomography

2 Outline Introduction about TeraHertz Tomography Introduction about TeraHertz Tomography THz Imagery and properties of THz radiations, Reminder about tomographic principles, Two acquisition systems. TeraHertz Tomography : first results Reminder about usual tomographic reconstruction methods, Result comparisons (reconstruction method, the projection number, material characteristics), Acquisition limitations (CW and TDS). Inspection of opaque objects (composed of teflon, foam, metal,...), Archeology : imaging of potteries. Investigated acquisition optimizations and reconstruction models. 2/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

3 THz Imagery : Principles Properties THz region between microwave (100GHz) and infrared (10THz), Wavelength between 30µm and 3mm, Contrary to X-Ray THz radiation is non ionizing and low-energy (1 and 40meV ). Two Principles Passive Imagery : capture of THz natural emission object : security systems, Active Imagery : source focalized on the object + detector measuring variation : spectroscopy : to determine the composition of an object, 3D Laser scan : to acquire the external surface of the object, 3/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

4 THz Imagery : Principles Known limitations T-Ray propagation can not be correctly described by a geometrical ray line, Diffraction or scattering effects blur or deform the acquired signal, Complex objects signal analysis are complicated by multiple reflections and refractions of THz radiations. Signal advantages for THz Imagery The signal can go through particular matter : Transmission process, THz Imagery (applications in Archeology), Investigations in THz Tomography. 4/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

5 Reminder about tomographic principles Definition Imagery technique to reconstruct the volume of an object from a set of non invasive measures acquired from the exterior of the object. Modeled in continuous domain by the Radon Transform [Radon 1919] Acquisition proccess : to get the projections (direct transform), Reconstruction process : inverse Radon transform or Fourier space reconstruction. 5/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

6 Reminder about tomographic principles : Acquisition (a) Figure: (a) Projection line defined by an angle θ and a module ρ. Its value corresponds to the ray attenuation going through matter (data modeled by the function f (x, y)) along the blue line. (b) Data given by only one projection does not allow the reconstruction of f. (b) Direct Radon Transform R θ (ρ) = f (x, y)δ(ρ x cos θ y sin θ)dxdy (1) 6/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

7 Reminder about tomographic principles : Reconstruction (a) Figure: (a) Acquisition following several angles. (b) Intersection of data contained on the projection set allows a better recovering of the original domain f. (b) Retroprojection (Inverse Radon Transform) π f (x, y) = R θ (ρ)δ(ρ x cos θ y sin θ)dρdθ (2) 0 7/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

8 Two THz acquisition systems : Pulsed THz Acquisition Properties Pulse source used in spectrocopy, Non destructive and contactless imaging, Transparent to amorphous materials, plastic, tissues, paper, wood,... Strong interaction with polar molecules (H 2 O for instance), Acquisition system 8/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

9 Two THz acquisition systems : Pulsed THz Data analysis Acquisition data Amplitude, Temporal response (delay), Spectral response. Create sinograms from all available data. Ampl. Delay 9/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

10 Continuous Wave THz Tomography : Acquisition Acquisition System A continuous signal with a wavelength around 1mm is emitted by a diode Gunn. The signal is focalized on the objet (translation XZ - rotation Y), Signal attenuation is measured by a infrared thermal sensor. Differences Between pulsed and CW Acquisitions Pulsed Acquisition Continuous Wave Acq. Frequency 1 femtosecond (1 A continuous millimetre to 10THz (each wave (0.1 to 80MHz) 0.4THz) Type Transmission Transmission and Reflexion Scan Area mm mm 2 Resolution (diffraction limited) 10µm to 300µm 1 to 3mm Measures Temporal (Amplitude) Delay / Delay (Phase) / Spectral Acquisition between 2 and 3 days 3 hours (N θ = 18, ) Object Properties very low-density and low-thickness objects less sensitive to the density and thickness of acquired objects 0/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

11 TeraHertz Tomography : Reconstructions State of Art Investigated at the origin by Physician Labs (Fergusson [2004]), Coupled with Spectroscopy analysis and 3D laser scan (object surface), A method becames a standard : BFP, A main goal : reduce the acquisition time (i.e. projection number). Investigation about others methods Comparison analysis of the results obtained from BFP, SART and OSEM : Quality loss according to the number of projections (measured by SSIM), Precision comparison between the three methods (measured by PSF). 11/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

12 TeraHertz Tomography : Reminder about usual methods Backprojection of Filtered Projections with : θ(i θ ) = i θ π Nθ, W θ (ρ) = Nρ 2 ν= Nρ 2 N θ 1 I (i, j) = i θ =0 ν Nρ 2 ρs = Nρ 2 Nρ 2 ρ= Nρ 2 W θ(iθ ) (ρ) (ρ jcosθ(i θ ) isinθ(i θ )) (3) i 2πρs ν R θ (ρs )e ei 2πρν (BFP). Iterative methods (SART - OSEM) Iterative update of each pixel using all or a part of (sub-iterations) the projection set until convergence. N θ 1Nρ 1 pk(θ, ρ, i, j) R θ (ρ) I (i, j) k+1 = I (i, j) k i θ =0 iρ=0 R θ k (ρ) N θ 1 Nρ 1 pk(θ, ρ, i, j) i θ =0 iρ=0 (4) 12/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

13 TeraHertz Tomography : Precision (PSF) results (a) (b) Figure: Sinograms of two metallic bars (12 mm diameter) separated by 50 mm, with a projection number N θ = 72. (a) Ideal theoretical acquisition. (b) Acquisition using the millimeter wave tomographic scanner with the 110 GHz source. (a) (b) (c) (d) Figure: Cross sections of two metallic bars (12 mm diameter) separated by 50 mm.(a) Ideal theoretical image reconstruction from 3(a). (b) BFP reconstruction from 3(b). (c) SART reconstruction from 3(b). (d) OSEM reconstruction from 3(b). Figure: Intensity profiles along the horizontal line intercepting the center of both metallic bars. 3/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

14 TeraHertz Tomography : Quality (SSIM) results (a) (b) N θ = 12 N θ = 18 N θ = 24 N θ = 36 N θ = 72 (a) (b) Figure: (a) Photograph of original object : parallelepiped black foam (41 49) mm 2 with 2 holes, diameter 15 mm (1 hole with air and 1 hole containing a Teflon cylinder with a 6 mm cylindrical air hole inside). (b) Sinogram with N θ = 72 projections (lines) and Nρ = 128 samples per projection (columns). (c) Figure: Manufactured sample reconstructions using sinograms with 12, 18, 24, 36, 72 projections and the BFP (a), SART (b) and OSEM (c) methods. Figure: Manufactured sample SSIM parameter as a function of the projection number and the reconstruction method. 14/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

15 TeraHertz Tomography : Quality results according to material characteristics Phantoms reconstructed from CW acquisitions Acquisition of phantoms made of Foam or Teflon, with hole or metallic bars, 18 projections sized around Foam (n 0 = 1.05) Cyl./Hole Cube/Hole Cube/Metal Teflon (n 0 = 1.55) Cube/Hole Cube/Metal Cyl./Metal BFP SART EM Contours are well reconstructed (but hypersignal > refraction losses), Inner hole is visible, Metallic bar > hypersignal. Refractions are more important, Inner hole is visible in the cube but hypersignal, Matter coherence is lost. 15/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. Desbarats, 3 3D TeraHertz Tomography

16 Limitations of the Acquisition system Acquisition Limitations Acquisition limitations depend on physical and optical properties : Absorption (by polar molecules), Refraction Losses. TDS Refraction Losses CW Refraction Losses Hypersignal and Interfaces Hypersignal at interfaces due to the signal losses, Hypersignal at contours in reconstructed images, Non uniform signal loss near the interfaces. 16/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

17 Applications 3D Internal Inspection of industrial materials Archeology (volume inspection) 17/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

18 Applications : Video samples Matriochka Volume OrthoSlice Surface Figure: Matriochka Figure: Urne Volume Figure: Orthoslice Figure: Surface 18/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography

19 Conclusion and Perspective Conclusion Pulsed or Continuous THz allows 3D-Tomography of opaque objects but : Refraction losses makes it difficult in TDS to identify the amplitude and time delay, Non uniform losses around the interfaces. Perspectives Replace sensor/detector by 2D camera : to increase the acquisition time (but with the risk of ghosts), to get a subset of refracted signal and extract a priori object properties for the reconstruction. Improve the spatial resolution (using super-resolution), Develop specific reconstruction methods based on a non-linear / discontinuous attenuation to take into account the refraction losses. 19/19 B. Recur, 3 A. Younus, 1 S. Salort, 2 P. Mounaix, 1 B. Chassagne, 2 P. 3DDesbarats, TeraHertz 3 Tomography