JP2016057259A - Tomographic imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately acquire a tomographic image in a tomographic imaging apparatus using a terahertz wave.SOLUTION: A terahertz wave is polarized to a TM wave by a polarization controller 16 to irradiate a measurement sample 2. As a result, in order to have a property that reflectivity comes to zero when the TM wave is made incident at a predetermined angle (Brewster angle), accuracy of a tomographic image can be improved by reducing influence on reflection on the incident interface of the measurement sample 2.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a technique for taking a tomographic image using a terahertz wave.

  The terahertz wave can safely visualize the internal structure without being exposed to X-rays, and can measure the complex permittivity distribution using the fingerprint spectrum. A dielectric spectroscopic imaging system using continuous wave terahertz light generated using photoelectric technology has been developed and applied to 2D spectroscopic imaging of drugs (see Non-Patent Document 1).

  In addition, development of three-dimensional spectroscopic imaging has been studied, and a dielectric spectroscopic imaging system using a pulsed wave terahertz (Pulse-THz) based on optical technology has been developed and succeeded in 3D spectroscopic imaging (Non-Patent Document 2) , 3). Acquisition of a three-dimensional density distribution by 3D imaging can be fed back to the manufacturing process. For example, the medicine has various shapes, and various disturbances exist in the acquisition of tomographic images.

  Patent Document 1 discloses a homodyne detection electromagnetic wave spectroscopic measurement system using a continuously oscillating light source. The electromagnetic wave spectroscopic measurement system of Patent Document 1 photoelectrically converts an optical signal obtained by combining two continuous light waves having different frequencies to generate a terahertz wave, irradiates the object to be measured with the generated terahertz wave, and measures the target. The configuration is such that the terahertz wave transmitted through the object is received and the reference light combined by modulating the phase of one of the two continuous light waves is input to perform homodyne mixing.

  In the terahertz band, it is common to measure a complex dielectric constant of a measurement object by a free space method using a pseudo optical system using a lens. When a tomographic image is taken by CT (Computed Tomography), a measurement object is inserted between the lenses, and the insertion position is changed and rotated 360 degrees to obtain two-dimensional data of angles and positions. Then, by using the FBP (Filtered Back-Projection) method, which is a general technique in X-ray CT, on the acquired two-dimensional data, high-definition of the tomographic image by reconstruction of the tomographic image, image space filter, or the like. (See Non-Patent Document 4).

JP 2013-32933 A

Jae-Young Kim, et al., “Continuous-Wave THz Homodyne Spectroscopy and Imaging System With Electro-Optical Phase Modulation for High Dynamic Range”, IEEE transactions on terahertz science and technology, 2013, Vol. 3, No. 2, pp .158-164 B. Ferguson, et al., “Toward functional 3D T-ray imaging”, Physics in Medicine and Biology 47, Institute of Physics Publishing, 2002, pp.3735-3742 B. Recur, et al., “Invectigation on reconstruction methods applied to 3D terahertz computed tomography”, Optics Express, Optical Society of America, 14 March 2011, Vol. 19, No. 6, pp. 5105-5117 Masatoshi Taguchi, “Fundamentals of Image Reconstruction Method: Learning FBP Method”, 42nd Nuclear Medicine Subcommittee, Japan Society for Radiological Technology, April 1, 2001, pp.5-25

  However, when terahertz waves are applied to CT, there is a problem in that a CT image cannot be obtained accurately due to the influence of reflection and diffraction at the incident interface of the object to be measured, refraction in the medium, incident beam intensity distribution, and the like. .

  The present invention has been made in view of the above, and an object thereof is to obtain a tomographic image with higher accuracy in tomographic imaging using a terahertz wave.

  A tomography apparatus according to the present invention includes a terahertz wave generating means for photoelectrically converting an optical signal obtained by combining two continuous light waves having different frequencies to generate a terahertz wave, and polarizing the terahertz wave into a TM wave. Polarization control means for irradiating the measurement object, terahertz wave reception means for detecting the intensity of the terahertz wave transmitted through the measurement object, and changing the angle of the measurement object, the measurement object Signal processing means for reconstructing a tomographic image of the object to be measured from projection data of the intensity of the terahertz wave obtained by scanning in a predetermined direction.

  In the tomographic imaging apparatus, the signal processing unit detects an edge of the measurement target in the projection data, corrects the projection data according to an outer shape of the measurement target, and converts the tomographic image into an image. It is characterized by reconfiguring.

  In the tomographic imaging apparatus, the object to be measured is sealed in a container formed of a material matched to a dielectric constant of the object to be measured, and the terahertz wave is irradiated.

  In the tomographic imaging apparatus, the container includes an outer periphery having faces that are parallel to each other and always have an incident angle of 60 degrees or less.

  In the tomography apparatus, the terahertz wave receiving means inputs a reference beam obtained by modulating and combining one of the two continuous light waves, performs photoelectric conversion, and performs homodyne mixing with the terahertz wave. The phase is detected, and the signal processing unit reconstructs the tomographic image using the phase image detected by the terahertz wave receiving unit.

  In the tomographic imaging apparatus, the polarization control means also polarizes TE waves.

  According to the present invention, a tomographic image can be acquired with higher accuracy in tomographic imaging using a terahertz wave.

It is a functional block diagram which shows the structure of the tomography apparatus in this Embodiment. It is a figure which shows a mode that a terahertz wave injects into a measurement sample. It is a graph which shows the relationship between the incident angle of a TE wave and TM wave, and a reflectance when a terahertz wave enters a measurement sample from the atmosphere. It is a figure which shows a mode that a terahertz wave is radiate | emitted from a measurement sample to air | atmosphere. It is a graph which shows the relationship between the incident angle of TE wave and TM wave, and a reflectance when a terahertz wave radiate | emits from a measurement sample to air | atmosphere. It is a graph which shows the transmission intensity | strength when scanning to a Y-axis direction in a certain rotation angle (theta) when TE wave and TM wave inject into a measurement sample. It is a flowchart which shows the flow of the process which reconstructs a tomogram. It is a figure which shows the example of the measurement sample cassette which accommodates a measurement sample. It is a figure which shows another example of the measurement sample cassette which accommodates a measurement sample. It is a figure which shows another example of the measurement sample cassette which accommodates a measurement sample. It is a figure which shows the structure which polarizes a terahertz wave into a TE wave by a polarization controller.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a functional block diagram showing the configuration of the tomographic imaging apparatus in the present embodiment. The tomographic imaging apparatus shown in the figure includes light sources 11A and 11B, splitters 12A and 12B, couplers 13A and 13B, a phase modulator 14, a terahertz wave generator 15, a polarization controller 16, a terahertz wave receiver 17, and a phase detection amplifier. 18 and a signal processing unit 19.

  The light sources 11A and 11B output continuous light waves having different frequencies. Hereinafter, the continuous light wave output from the light source 11A is referred to as a first optical signal, and the continuous light wave output from the light source 11B is referred to as a second optical signal.

  The splitter 12A demultiplexes the first optical signal into two, and the splitter 12B demultiplexes the second optical signal into two.

  The coupler 13A combines the first optical signal demultiplexed by the splitter 12A and the second optical signal demultiplexed by the splitter 12B. The optical signal combined by the coupler 13A is input to the terahertz wave generator 15.

  The coupler 13B combines the first optical signal demultiplexed by the splitter 12A and the second optical signal phase-modulated by the phase modulator. The reference light combined by the coupler 13B is input to the terahertz wave receiver 17.

  The phase modulator 14 is disposed between the splitter 12B and the coupler 13B, and electrically modulates the phase of the second optical signal demultiplexed by the splitter 12B by an external control signal.

  The terahertz wave generator 15 photoelectrically converts the optical signal combined by the coupler 13A, and generates an electromagnetic wave (terahertz wave) having a frequency that matches the frequency difference between the first and second optical signals. The terahertz wave generator 15 is a single traveling carrier photodiode (UTC-PD).

  The polarization controller 16 polarizes the terahertz wave generated by the terahertz wave generator 15 into a TM wave and irradiates the measurement sample 2. A wire grid polarizer is used for the polarization controller 16. The terahertz wave irradiated to the measurement sample 2 has a measurement frequency of 1 THz and a beam diameter of 500 μm, for example.

  The terahertz wave receiver 17 receives the terahertz wave that has passed through the measurement sample 2, photoelectrically converts the reference light combined by the coupler 13 </ b> B, and detects the terahertz wave by homodyne mixing. The terahertz wave receiver 17 uses a photoconductive antenna (PCA).

  The phase detection amplifier 18 amplifies the terahertz wave detected by the terahertz wave receiver 17 and detects the phase.

  The signal processing unit 19 processes the amplitude and phase of the terahertz wave to obtain a tomographic image of the measurement sample 2. In this embodiment, two-dimensional projection data relating to amplitude is obtained by changing the angle around the z-axis and scanning in the y-axis direction, and the tomographic image is reconstructed by performing back-projection after filtering the obtained data. The FBP method is used.

  The measurement sample 2 is installed on a rotary stage that can move in the y-axis direction and can rotate about the z-axis. The measurement sample 2 is, for example, a tablet having a curved shape, a solution or an individual collected from a human or an animal.

  Next, regarding the TE wave and the TM wave, the incident angle and the reflectance when entering the measurement sample and emitting from the measurement sample will be described.

  First, an incident angle and a reflectance when a terahertz wave enters the measurement sample 2 will be described. FIG. 2 is a diagram illustrating a state in which the terahertz wave is incident on the measurement sample 2. FIG. 3 is a graph showing the relationship between the incident angle and the reflectance of the TE wave and the TM wave when entering the measurement sample 2 (refractive index n2 = 1.4) from the atmosphere (refractive index n1 = 1).

  As shown in FIG. 2, when entering the measurement sample 2 from the atmosphere, there is no Brewster angle when the electric field is a TE wave perpendicular to the incident surface, but when the electric field is a TM wave parallel to the incident surface, the Brewster angle is not present. There is a star angle. As shown in the graph of FIG. 3, the TM wave has a small reflectance with respect to the incident surface with respect to the incident angle, and the reflectance is 0.2 or less until the incident angle is about 80 degrees.

  Next, the incident angle and reflectance when the terahertz wave is emitted from the measurement sample 2 will be described. FIG. 4 is a diagram illustrating a state in which the terahertz wave is emitted from the measurement sample 2. FIG. 5 is a graph showing the relationship between the incident angle and the reflectance of the TE wave and the TM wave when emitted from the measurement sample 2 (refractive index n2 = 1.4) to the atmosphere (refractive index n1 = 1).

  As shown in FIG. 5, when emitted from the measurement sample 2 to the atmosphere, almost all electromagnetic waves are reflected at the boundary at an incident angle of about 40 degrees or more due to the total reflection condition, and are not emitted from the measurement sample 2. The electromagnetic wave reflected at the boundary is emitted by multiple reflection within the measurement sample, which causes a measurement error.

  Subsequently, the transmission intensity of the TE wave and the TM wave when the measurement sample having an elliptical cross section is scanned will be described.

  FIG. 6 is a graph showing the transmission intensity when scanning in the Y-axis direction at a certain rotation angle θ when TE waves and TM waves are incident on the measurement sample.

  A part of the terahertz wave is reflected on the incident surface of the measurement sample. Projection data obtained by the transmitted terahertz wave is convolved and integrated with a beam intensity distribution function. When the cross section of the measurement sample is elliptical as shown in FIG. 6, the incident angle is zero at the center of the measurement sample, and the incident angle increases as the distance from the center increases. As shown in FIG. 6, in the TE wave, a loss due to reflection on the surface of the measurement sample occurs as the incident angle increases from zero in the center of the measurement sample. On the other hand, a loss due to reflection similarly occurs in the TM wave, but the influence is small compared to the TE wave.

  Next, processing in which the signal processing unit reconstructs a tomographic image from scan data will be described.

  FIG. 7 is a flowchart showing a flow of processing for reconstructing a tomographic image.

  First, by changing the rotation angle θ around the z axis within a predetermined angle range and moving the measurement sample in the y axis direction, the incident position Y of the terahertz wave is changed, and the projection data I (Y, θ) Is acquired (step S11).

  Next, the edge of the projection data caused by reflection at the boundary surface of the measurement sample is detected for each line, and the line is shifted so that the edges of each line are aligned with the shape of the measurement sample with known external dimensions. Then, the projection data is corrected (step S12).

  Then, the variables Y and θ of the projection data are inversely Radon transformed into x and y, and the object function f (x, y) is acquired to reconstruct a tomographic image on the xy plane of the measurement sample (step S13).

  A high-loss region is formed at the edge of the tomographic image obtained by the tomographic imaging apparatus by reflection or diffraction at the boundary surface. When the measurement sample is circular, the value near the center is less influenced by reflection and diffraction and can be regarded as a true value.

  Next, a measurement sample cassette that stores a measurement sample when a tomographic image is taken will be described. As shown in the graph of FIG. 5, since the influence of interface reflection when emitting from the measurement sample to the atmosphere is large, the dielectric constant (refractive index) that can be internally sealed in accordance with the shape of the measurement sample A combined measurement sample cassette may be used.

  FIG. 8 is a diagram illustrating an example of a measurement sample cassette that stores a measurement sample. The measurement sample cassette 3 is a container that seals the measurement sample 2 with the dielectric constant adjusted to that of the measurement sample 2. For example, the material of the measurement sample cassette 3 is a polymer material such as a polyimide having a relative dielectric constant of 4 or less, quartz, or a liquid crystal polymer. The top and bottom surfaces of the measurement sample cassette 3 in FIG. 8A are sufficiently parallel to the wavelength λ of the terahertz wave, for example, λ / 10 or less. As shown in FIG. 8B, even when the measurement sample cassette 3 is rotated around the z-axis, total reflection does not occur at the exit interface, and a range up to 60 degrees (the Brewster angle of the TM wave) ( Good transmission characteristics are exhibited in the range of −60 degrees to 60 degrees.

  Further, in a range other than the above, another measurement sample cassette 3 for storing the measurement sample 2 as shown in FIG. 9 rotated 90 degrees may be prepared, and a range of 30 degrees to 150 degrees may be measured.

  FIG. 10 is a diagram showing still another measurement sample cassette 3. As shown in the figure, the outer periphery of the measurement sample cassette 3 may be a regular hexagon. The parallelism of the opposing surfaces of the measurement sample cassette 3 is, for example, λ / 10 or less. Since the measurement sample cassette 3 shown in FIG. 10 always has a surface with an incident angle of 60 degrees or less, measurement is possible over the entire circumference without replacing the measurement sample cassette 3. Note that the outer periphery of the measurement sample cassette 3 may not be a regular hexagon as long as the opposing surfaces are parallel and always have a surface with an incident angle of 60 degrees or less.

  Next, an embodiment using a phase image will be described.

  When a tomographic image of a measurement sample is photographed using only the amplitude of the terahertz wave, there is a problem that it is difficult to photograph a clear tomographic image when the terahertz wave absorption of the measurement sample is small and the amount of change in amplitude is small. When media having different dielectric constants are distributed in the measurement sample, it is possible to obtain a high-contrast tomographic image including a phase image by measuring the phase.

  In the present embodiment, the reference light obtained by combining the first optical signal and the second optical signal phase-modulated by the phase modulator 14 is used as a homodyne detection method to transmit the measurement sample 2. The terahertz wave phase can be measured.

  FIG. 11 is a diagram illustrating a configuration in which the terahertz wave is polarized into a TE wave by the polarization controller 16.

  Both the TE wave and the TM wave may be measured using the configuration of FIG. By taking the ratio of the transmitted signal intensity of the TE wave and the TM wave, the influence of fluctuations in the light source can be eliminated.

  As described above, according to the present embodiment, the polarization controller 16 polarizes the terahertz wave into a TM wave and irradiates the measurement sample 2 so that the TM wave is incident at a predetermined angle (Brewster angle). In this case, since the reflectance is zero, the influence of reflection at the incident interface of the measurement sample 2 can be reduced, and the accuracy of the tomographic image can be improved.

DESCRIPTION OF SYMBOLS 11A, 11B ... Light source 12A, 12B ... Splitter 13A, 13B ... Coupler 14 ... Phase modulator 15 ... Terahertz wave generator 16 ... Polarization controller 17 ... Terahertz wave receiver 18 ... Phase detection amplifier 19 ... Signal processing part 2 ... Measurement Sample 3 ... Measurement sample cassette

Claims (6)

Terahertz wave generating means for photoelectrically converting an optical signal obtained by combining two continuous light waves having different frequencies to generate a terahertz wave;
Polarization control means for polarizing the terahertz wave into a TM wave and irradiating the object to be measured;
Terahertz wave receiving means for detecting the intensity of the terahertz wave transmitted through the object to be measured;
Signal processing for reconstructing a tomographic image of the object to be measured from projection data of the intensity of the terahertz wave obtained by changing the angle of the object to be measured and scanning the object to be measured in a predetermined direction Means,
A tomographic imaging apparatus characterized by comprising:   The signal processing means detects an edge of the object to be measured in the projection data, corrects the projection data according to an outer shape of the object to be measured, and reconstructs the tomographic image. The tomography apparatus according to claim 1.   The tomographic imaging apparatus according to claim 1, wherein the target object is sealed in a container formed of a material matched to a dielectric constant of the target object and the terahertz wave is irradiated.   The tomography apparatus according to claim 3, wherein the container includes an outer periphery having faces that are parallel to each other and always have an incident angle of 60 degrees or less. The terahertz wave receiving means inputs and photoelectrically converts a reference light obtained by modulating and combining one phase of the two continuous light waves, detects the phase by homodyne mixing with the terahertz wave,
5. The tomographic imaging apparatus according to claim 1, wherein the signal processing unit reconstructs the tomographic image using a phase image detected by the terahertz wave receiving unit.   6. The tomographic imaging apparatus according to claim 1, wherein the polarization control means also polarizes TE waves.

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