CN105527306A - Method for three-dimensional imaging based on terahertz chromatography of planar array type detector - Google Patents

Method for three-dimensional imaging based on terahertz chromatography of planar array type detector Download PDF

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CN105527306A
CN105527306A CN201610029354.8A CN201610029354A CN105527306A CN 105527306 A CN105527306 A CN 105527306A CN 201610029354 A CN201610029354 A CN 201610029354A CN 105527306 A CN105527306 A CN 105527306A
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王大勇
李斌
戎路
王云新
万敏
黄昊翀
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Beijing University of Technology
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Abstract

The invention relates to a system and a method for three-dimensional imaging based on terahertz chromatography of a planar array type detector. According to the method disclosed by the invention, a first gold-plated paraboloid mirror and a second gold-plated paraboloid mirror are arranged in a mutually corresponding manner to form a beam expanded unit, wherein the beam expanded unit is arranged between a CO2 pumping terahertz laser and a sample to be detected and can be used for enlarging a terahertz spot output by the CO2 pumping terahertz laser by twice and enabling the propagation directions of output light to be parallel. The sample to be tested is placed between the second gold-plated paraboloid mirror and a pyroelectric image detector; a numerical value of projection transmitting the sample to be detected is collected by the pyroelectric image detector and is called as a projection drawing I (x, y). According to the system and the method disclosed by the invention, a diffraction effect of terahertz waves is introduced to the planar array type detector, processed data is propagated to the back surface of the sample by using an angular spectrum diffraction and propagation theory, and then filtered and reestablished with a filter back-projection algorithm in order to restrain the diffraction effect of terahertz waves, and finally three dimensional structure imaging inside the sample is obtained.

Description

A kind of method based on the three-dimensional imaging of planar array type detector terahertz tomographic
Technical field
The invention belongs to technical field of image processing originally, relate to a kind of method based on the three-dimensional imaging of planar array type detector chromatography, particularly a kind of method based on the three-dimensional imaging of planar array type detector terahertz tomographic.
Background technology
Refer generally to frequency and be called THz wave at the electromagnetic wave of 0.1THz-10THz (wavelength is 3mm-0.03mm).THz wave is between microwave and infrared waves, to nonmetal and apolar substance, there is penetrability, such as plastics, timber, clothes etc., and relatively X ray its there is low ionization, measured matter can not be destroyed because of photoionization, be more suitable for " In vivo detection ".Due to the inherent characteristics of THz wave, THz wave imaging technique is made to become one of study hotspot.Computer assisted tomography technology: utilize a branch of ray to carry out translation rotation sweep to a certain cross section of sample, the One Dimensional Projection data of material to attenuation coefficient value are obtained through the numerical value after object according to ray, obtained by tomographic reconstruction algorithm and rebuild image, rebuild the two-dimensional distribution that image is attenuation coefficient on xsect.The inherent characteristic of THz wave is combined with computer assisted tomography technology, the tomograph of sample interior can be obtained.Therefore, THz wave computer assisted tomography technology has huge application potential in fields such as medical imaging, safety inspection and industrial nondestructive testing.
At present, most terahertz tomographic imaging, still based on spot scan chromatography imaging method, needs longer acquisition time in image data process, and has very high requirement to the stability of Imaged samples.The present invention proposes a kind of continuous terahertz tomographic three-D imaging method based on planar array type detector, and compare Terahertz spot scan chromatography imaging method and save the horizontal and vertical projection acquisition time, a whole set of projection data acquisitions speed is fast.Wavelength due to Terahertz 7 orders of magnitude large compared to X ray, so perspective view quality receives the impact of the diffraction effect of Terahertz.In order to suppress the diffraction effect propagated in the outside THz wave of object, the data for projection utilizing angular spectrum diffraction propagation algorithm to be collected by detector passes back to be close to after object on tangent plane.Carry out image reconstruction with filter back-projection reconstruction algorithm fast to the perspective view after propagation, obtain the cross-sectional view of two-dimentional sample, the cross-sectional view eventually through the two-dimentional sample of different layers reconstructs sample three-dimensional internal structural drawing.The present invention not only can Quick Acquisition sample data for projection but also inhibit THz wave in the diffraction effect of object outside by angular spectrum diffraction propagation algorithm,
Summary of the invention
Feature of the present invention is planar array type detector, compares and a little sweeps the projection value that chromatographic imaging system can obtain big data quantity fast, but uses planar array type detector to have also been introduced the diffraction effect of THz wave.In order to suppress the diffraction effect of THz wave, utilizing angular spectrum diffraction communication theory that the data for projection after process is propagated into sample rear surface and carries out filter back-projection algorithm reconstruction, finally obtaining the three-dimensional structure imaging of sample interior.
For achieving the above object, the technical solution used in the present invention is a kind of method based on the three-dimensional imaging of planar array type detector terahertz tomographic, and the method realizes based on following system.
A kind of based on planar array type detector terahertz tomographic 3-D imaging system, the light path of this system comprises CO 2the gold-plated paraboloidal mirror 3 of pumping thz laser device the 1, first gold-plated paraboloidal mirror 2, second, testing sample 4, electric precise universal stage 5, pyroelectricity image detector 6.CO 2pumping thz laser device 1 is the continuous THz wave of 2.52THz (corresponding centre wavelength is 118.83 μm) for output center frequency, and real power is 70mW; First gold-plated paraboloidal mirror 2 and the mutual correspondence of the second gold-plated paraboloidal mirror 3 are arranged and are formed one and expand unit, and this expands unit and is arranged on CO 2between pumping thz laser device 1 and testing sample 4, expanding unit can by CO 2the Terahertz hot spot that pumping thz laser device 1 exports expands twice, and the direction of propagation exporting light is paralleled.Testing sample 4 is placed between the second gold-plated paraboloidal mirror 3 and pyroelectricity image detector 6, and need ensure that its tested area is less than beam diameter, and beam orthogonal is radiated on testing sample 4, and testing sample 4 is arranged on electric precise universal stage 5; Transmitted through the projection values of testing sample 4 gather by pyroelectricity image detector 6, be referred to as perspective view I (x, y).Remain unchanged in other situation, rotate electric precise universal stage 5, every 2 ° of records that once project, rotation angle range 0-180 °, altogether rotating and projection 90 times.
Based on a method for planar array type detector terahertz tomographic three-dimensional imaging, comprise angular spectrum diffraction communication process.Due to the diffraction effect of THz wave, after utilizing angular spectrum diffraction propagation algorithm to be passed back to by the data for projection in record plane to be close to object, tangent plane reaches the object suppressing diffraction effect.Set up coordinate system x-y and x o-y obe respectively record plane and object after tangent plane, z 1and z 2respectively represent record plane and object after tangent plane coordinate in the z-axis direction.Perspective view is I (x, y), and initial estimation phase place is δ (x, y), and record plane is d to the distance face of object back surface, namely records COMPLEX AMPLITUDE u (x, y, the z of plane 2).COMPLEX AMPLITUDE u (x, y, the z of following formula representative record plane 2) the rear tangent plane passing to testing sample 4 is propagated back to by angular spectrum diffraction, the COMPLEX AMPLITUDE u of the rear tangent plane of testing sample 4 can be obtained o(x o, y o, z 1).
with represent Fourier transform and inverse Fourier transform respectively, λ represents Terahertz wavelength, and k represents wave vector, N xand N y, Δ x and Δ y is respectively the number of pixels in the x and y direction of detector on record surface and pixel size.In order to obtain the drop shadow intensity value p (x propagating into object back surface o, y o), get u o(x o, y o, z 1) mould square, i.e. p (x o, y o)=| u o(x o, y o, z 1) | 2.
Based on a method for planar array type detector terahertz tomographic three-dimensional imaging, its chromatography three dimensional reconstructive CT method comprises three step by step rapid:
(1) COMPLEX AMPLITUDE on pyroelectricity image detector 6 is propagated back to pass to by angular spectrum diffraction to be close on the rear tangent plane of object, has 90 width drop shadow intensity figure p (x o, y o), be of a size of N 0× N 0.According to spot diameter size, effective drop shadow intensity figure is selected to be of a size of N × N.Choose out by drop shadow intensity's value of same a line of each angle, the two-dimensional array of a composition 90 × N, is referred to as sinogram p (t, θ).In sinogram p (t, θ) space, transverse axis represents projection angle, and Z-axis represents number of pixels.
(2) the two-dimensional cross sectional figure s (x', z) of filter back-projection reconstruction algorithm to testing sample 4 is adopted to rebuild.Filter back-projection reconstruction algorithm realizes based on iradon change, and wave filter uses Hamming wave filter, and wherein filter back-projection reconstruction algorithm belongs to common method.
(3) in order to reconstruct the interior three-dimensional structural drawing of testing sample 4, longitudinal direction is chosen the detector line number N meeting height of specimen, and the reconstruction of two-dimensional cross sectional figure is carried out to the sinogram p (t, θ) of every a line.The physical size of testing sample 4 is obtained by the product of pixel size on longitudinal direction and number of pixels.
Accompanying drawing explanation
Fig. 1: the process flow diagram of the inventive method;
Fig. 2: based on planar array type detector terahertz tomographic three-dimensional imaging index path;
In figure: 1, CO 2pumping thz laser device, the 2, first gold-plated paraboloidal mirror, the 3, second gold-plated paraboloidal mirror, 4, testing sample, 5, electric precise universal stage, 6, pyroelectricity image detector.
Fig. 3: angular spectrum diffraction propagation principle figure.
Fig. 4: tomography schematic diagram is the interaction figure between spatial domain schematic diagram and right figure frequency domain schematic diagram in figure;
Fig. 5: image rebuild by suction pipe, figure (a) is two-dimensional cross sectional figure, and figure (b) is three-dimensional internal structural drawing.
Embodiment
Describe exemplary embodiments of the present invention and feature thereof with reference to the accompanying drawings in detail.
A kind of method based on the three-dimensional imaging of planar array type detector terahertz tomographic of the present invention, is characterized in that its system comprises CO 2the gold-plated paraboloidal mirror 3 of pumping thz laser device the 1, first gold-plated paraboloidal mirror 2, second, testing sample 4, electric precise universal stage 5, pyroelectricity image detector 6, as shown in Figure 2.CO 2pumping thz laser device 1 is the continuous THz wave of 2.52THz (corresponding centre wavelength is 118.83 μm) for output center frequency, and real power is 70mW.The number of pixels of pyroelectricity image detector 6 is 320 × 320 pixels, and Pixel Dimensions is 80 μm × 80 μm, and sample frequency is 48Hz.
The rear tangent plane of sample is about 8.5cm to the distance d in detector panel face.The sample of imaging experiment is chosen for the suction pipe of polythene material, and suction pipe has isotropy, and the immovable characteristic of absorption coefficient, the theoretical absorption coefficient that diameter is about 6.02mm polythene material is about 0.1mm -1, be about 1.02 in the refractive index of THz wave band.Because spot diameter is greater than the diameter of sample, so hot spot shrouds whole sample all the time in the process of rotating acquisition projection.Pyroelectricity image detector detects the perspective view obtained and is of a size of 320 × 320 pixels.
First S1 with reference to summary of the invention, completes the collection of the perspective view at different projection angle, every 2 ° of records that once project, and rotation angle range 0-180 °, altogether rotating and projection 90 times.
S2, in order to suppress the diffraction effect propagated at the outside Terahertz of object, utilizes angular spectrum diffraction propagation algorithm to be passed back to by the data for projection that pyroelectric detector 6 collects to be close to after object on tangent plane, as shown in Figure 3.Set up coordinate system x-y and x o-y obe respectively record plane and object after tangent plane.Drop shadow intensity is I (x, y), and initial estimation phase place is δ (x, y), namely records COMPLEX AMPLITUDE u (x, y, the z of plane 2) be:
u ( x , y , z 2 ) = I ( x , y ) exp [ j δ ( x , y ) ]
Angular spectrum diffraction is utilized to propagate complex amplitude u (x, y, the z of record surface 2) propagate into tangent plane after the object of distance d, i.e. the COMPLEX AMPLITUDE u of tangent plane after object o(x o, y o, z 1).Angular spectrum diffraction communication theory is the Helmholtz equation strictly observing scalar diffraction, then:
U o ( f x , f y , z 1 ) = U ( f x , f y , z 2 ) exp ( j 2 π z 1 - f x 2 - f y 2 )
U (f in formula x, f y, z 2) and U o(f x, f y, z 1) represent u (x, y, z respectively 2) and u o(x o, y o, z 1) through the frequency spectrum of Fourier transform.Utilize inverse Fourier transform can obtain the COMPLEX AMPLITUDE of tangent plane after object.
In formula with represent Fourier transform and inverse Fourier transform respectively.G (f x, f y) representation space transport function, expression formula is:
G ( f x , f y ) = exp { j 2 π z ( 1 λ ) 2 - ( x N x Δ x ) 2 - ( y N y Δ y ) 2 }
In formula, k represents wave vector, N xand N y, Δ x and Δ y is respectively the number of pixels in the x and y direction of detector on record surface and pixel size.Angular spectrum is propagated and mainly be make use of a Fourier transform and an inverse Fourier transform, and the pixel dimension size therefore after angular spectrum propagation and CCD Pixel Dimensions are consistent.In order to obtain the drop shadow intensity value p (x propagating into object back surface o, y o), get u o(x o, y o, z 1) mould square, i.e. p (x o, y o)=| u o(x o, y o, z 1) | 2.Can the projection value of Quick Acquisition big data quantity although experiment imaging system employs planar array type detector, simultaneously data for projection receives the diffraction effect of THz wave, utilizes angular spectrum diffraction propagation algorithm to be passed back to by the data for projection on record surface and is close to the object that object back surface can reach the diffraction effect suppressed in the outside THz wave of object.
S3 sets up the angle that cartesian coordinate system x '-z ', rotating coordinate system t-s and polar coordinate system ω-θ, θ are coordinate axis z ' and coordinate axis t, as shown in Figure 4.Choose object individual layer xsect to analyze, the value that object s (x ', z ') cuts into slices in same angle in the two-dimensional Fourier transform that the one-dimensional Fourier transform of the parallel projection of angle θ equals object s (x ', z ').On angle θ direction, One Dimensional Projection data are represented by p (t, θ).
Object s (x ', z ') can represent with s (t, θ) in rotating coordinate system, make S (β, v) represent the two-dimensional frequency distribution of object, then:
Make t=x'sin θ+z'cos θ, substituted into above formula, can obtain:
In formula, * represents convolution algorithm, and P (ω, θ) represents the Fourier transform of data for projection p (t, θ).The two-dimensional cross sectional figure s (x ', z ') of sample at certain one deck can be obtained eventually through above formula.The wave filter used in this method is Hamming wave filter.Measurement obtain two-dimensional cross sectional figure s (x ', z ') in suction pipe wall two ends differ 80 pixels, and be multiplied by the Pixel Dimensions 80 μm of pyroelectric detector, the diameter calculating suction pipe is about 6.4mm, with the relative error of the straw diameter of actual measurement 6%, as shown in Figure 5.
S4, in order to reconstruct the interior three-dimensional structural drawing of testing sample 4, longitudinal direction is chosen detector 68 row meeting height of specimen, and the sinogram p (t, θ) of every a line is carried out to the reconstruction of two-dimensional cross sectional figure.Obtained the physical size of testing sample 4 by the product of pixel size on longitudinal direction and number of pixels, be about 5.44mm, as shown in Figure 5.

Claims (4)

1., based on a planar array type detector terahertz tomographic 3-D imaging system, it is characterized in that: the light path of this system comprises CO 2pumping thz laser device (1), the first gold-plated paraboloidal mirror (2), the second gold-plated paraboloidal mirror (3), testing sample (4), electric precise universal stage (5), pyroelectricity image detector (6); CO 2pumping thz laser device (1) is the continuous THz wave of 2.52THz for output center frequency, and real power is 70mW; First gold-plated paraboloidal mirror (2) and the mutual correspondence of the second gold-plated paraboloidal mirror (3) are arranged and are formed one and expand unit, and this expands unit and is arranged on CO 2between pumping thz laser device (1) and testing sample (4), expanding unit can by CO 2the Terahertz hot spot that pumping thz laser device (1) exports expands twice, and the direction of propagation exporting light is paralleled; Testing sample (4) is placed between the second gold-plated paraboloidal mirror (3) and pyroelectricity image detector (6), and need ensure that its tested area is less than beam diameter, beam orthogonal is radiated on testing sample (4), and testing sample (4) is arranged on electric precise universal stage (5); Transmitted through the projection values of testing sample (4) gather by pyroelectricity image detector (6), be referred to as perspective view I (x, y); Remain unchanged in other situation, rotate electric precise universal stage (5), every 2 ° of records that once project, rotation angle range 0-180 °, altogether rotating and projection 90 times.
2. the one of application rights requirement described in 1 is based on planar array type detector terahertz tomographic 3-D imaging system, and a kind of method based on the three-dimensional imaging of planar array type detector terahertz tomographic, is characterized in that: the method comprises angular spectrum diffraction communication process; Due to the diffraction effect of THz wave, after utilizing angular spectrum diffraction propagation algorithm to be passed back to by the data for projection in record plane to be close to object, tangent plane reaches the object suppressing diffraction effect; Set up coordinate system x-y and x o-y obe respectively record plane and object after tangent plane, z 1and z 2respectively represent record plane and object after tangent plane coordinate in the z-axis direction; Perspective view is I (x, y), and initial estimation phase place is δ (x, y), and record plane is d to the distance face of object back surface, namely records COMPLEX AMPLITUDE u (x, y, the z of plane 2); COMPLEX AMPLITUDE u (x, y, the z of following formula representative record plane 2) the rear tangent plane passing to testing sample (4) is propagated back to by angular spectrum diffraction, the COMPLEX AMPLITUDE u of the rear tangent plane of testing sample (4) can be obtained o(x o, y o, z 1);
with represent Fourier transform and inverse Fourier transform respectively, λ represents Terahertz wavelength, and k represents wave vector, N xand N y, Δ x and Δ y is respectively the number of pixels in the x and y direction of detector on record surface and pixel size; In order to obtain the drop shadow intensity value p (x propagating into object back surface o, y o), get u o(x o, y o, z 1) mould square, i.e. p (x o, y o)=| u o(x o, y o, z 1) | 2.
3. a kind of method based on the three-dimensional imaging of planar array type detector terahertz tomographic according to claim 2, is characterized in that: chromatography three dimensional reconstructive CT method comprises three step by step suddenly,
(1) COMPLEX AMPLITUDE on pyroelectricity image detector (6) is propagated back to pass to by angular spectrum diffraction to be close on the rear tangent plane of object, has 90 width drop shadow intensity figure p (x o, y o), be of a size of N 0× N 0; According to spot diameter size, effective drop shadow intensity figure is selected to be of a size of N × N; Choose out by drop shadow intensity's value of same a line of each angle, the two-dimensional array of a composition 90 × N, is referred to as sinogram p (t, θ); In sinogram p (t, θ) space, transverse axis represents projection angle, and Z-axis represents number of pixels;
(2) the two-dimensional cross sectional figure s (x', z) of filter back-projection reconstruction algorithm to testing sample (4) is adopted to rebuild; Filter back-projection reconstruction algorithm realizes based on iradon change, and wave filter uses Hamming wave filter, and wherein filter back-projection reconstruction algorithm belongs to common method;
(3) in order to reconstruct the interior three-dimensional structural drawing of testing sample (4), longitudinal direction is chosen the detector line number N meeting height of specimen, and the reconstruction of two-dimensional cross sectional figure is carried out to the sinogram p (t, θ) of every a line; The physical size of testing sample (4) is obtained by the product of pixel size on longitudinal direction and number of pixels.
4. a kind of method based on the three-dimensional imaging of planar array type detector terahertz tomographic according to claim 2, is characterized in that:
S1 is reference system structure first, completes the collection of the perspective view at different projection angle, every 2 ° of records that once project, and rotation angle range 0-180 °, altogether rotating and projection 90 times;
S2, in order to suppress the diffraction effect propagated at the outside Terahertz of object, to utilize angular spectrum diffraction propagation algorithm to be passed back to by the data for projection that pyroelectric detector (6) collects to be close to after object on tangent plane; Set up coordinate system x-y and x o-y obe respectively record plane and object after tangent plane; Drop shadow intensity is I (x, y), and initial estimation phase place is δ (x, y), namely records COMPLEX AMPLITUDE u (x, y, the z of plane 2) be:
u ( x , y , z 2 ) = I ( x , y ) exp [ j δ ( x , y ) ]
Angular spectrum diffraction is utilized to propagate complex amplitude u (x, y, the z of record surface 2) propagate into tangent plane after the object of distance d, i.e. the COMPLEX AMPLITUDE u of tangent plane after object o(x o, y o, z 1); Angular spectrum diffraction communication theory is the Helmholtz equation strictly observing scalar diffraction, then:
U o ( f x , f y , z 1 ) = U ( f x , f y , z 2 ) exp ( j 2 π z 1 - f x 2 - f y 2 )
U (f in formula x, f y, z 2) and U o(f x, f y, z 1) represent u (x, y, z respectively 2) and u o(x o, y o, z 1) through the frequency spectrum of Fourier transform; Utilize inverse Fourier transform can obtain the COMPLEX AMPLITUDE of tangent plane after object;
In formula with represent Fourier transform and inverse Fourier transform respectively; G (f x, f y) representation space transport function, expression formula is:
G ( f x , f y ) = exp { j 2 π z ( 1 λ ) 2 - ( x N x Δ x ) 2 - ( y N y Δ y ) 2 }
In formula, k represents wave vector, N xand N y, Δ x and Δ y is respectively the number of pixels in the x and y direction of detector on record surface and pixel size; Angular spectrum is propagated and mainly be make use of a Fourier transform and an inverse Fourier transform, and the pixel dimension size therefore after angular spectrum propagation and CCD Pixel Dimensions are consistent; In order to obtain the drop shadow intensity value p (x propagating into object back surface o, y o), get u o(x o, y o, z 1) mould square, i.e. p (x o, y o)=| u o(x o, y o, z 1) | 2; Although experiment imaging system employs the projection value of planar array type detector Quick Acquisition big data quantity, simultaneously data for projection receives the diffraction effect of THz wave, utilizes angular spectrum diffraction propagation algorithm to be passed back to by the data for projection on record surface and is close to the object that object back surface can reach the diffraction effect suppressed in the outside THz wave of object;
S3 sets up the angle that cartesian coordinate system x '-z ', rotating coordinate system t-s and polar coordinate system ω-θ, θ are coordinate axis z ' and coordinate axis t; Choose object individual layer xsect to analyze, the value that object s (x ', z ') cuts into slices in same angle in the two-dimensional Fourier transform that the one-dimensional Fourier transform of the parallel projection of angle θ equals object s (x ', z '); On angle θ direction, One Dimensional Projection data are represented by p (t, θ);
Object s (x ', z ') represent with s (t, θ) in rotating coordinate system, make S (β, v) represent the two-dimensional frequency distribution of object, then:
Make t=x'sin θ+z'cos θ, substituted into above formula, obtain:
In formula, * represents convolution algorithm, and P (ω, θ) represents the Fourier transform of data for projection p (t, θ); The two-dimensional cross sectional figure s (x ', z ') of sample at certain one deck can be obtained eventually through above formula; The wave filter used in this method is Hamming wave filter; Measurement obtains suction pipe wall two ends in two-dimensional cross sectional figure s (x ', z ') and differs 80 pixels, and is multiplied by the Pixel Dimensions 80 μm of pyroelectric detector, calculates the diameter of suction pipe;
S4, in order to reconstruct the interior three-dimensional structural drawing of testing sample (4), longitudinal direction is chosen detector 68 row meeting height of specimen, and the sinogram p (t, θ) of every a line is carried out to the reconstruction of two-dimensional cross sectional figure; The physical size of testing sample (4) is obtained by the product of pixel size on longitudinal direction and number of pixels.
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