WO2013153955A1 - Dispositif de correction d'image de projection de rayons x et procédé de correction d'image de projection de rayons x - Google Patents

Dispositif de correction d'image de projection de rayons x et procédé de correction d'image de projection de rayons x Download PDF

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WO2013153955A1
WO2013153955A1 PCT/JP2013/059175 JP2013059175W WO2013153955A1 WO 2013153955 A1 WO2013153955 A1 WO 2013153955A1 JP 2013059175 W JP2013059175 W JP 2013059175W WO 2013153955 A1 WO2013153955 A1 WO 2013153955A1
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projection
ray
correction function
correction
projection image
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PCT/JP2013/059175
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English (en)
Japanese (ja)
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豊 大竹
貴博 西畑
宏正 鈴木
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国立大学法人東京大学
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
    • G01N23/02—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material
    • G01N23/04—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
    • G01N23/02—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material
    • G01N23/04—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material
    • G01N23/046—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material using tomography, e.g. computed tomography [CT]
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2223/00—Investigating materials by wave or particle radiation
    • G01N2223/30—Accessories, mechanical or electrical features
    • G01N2223/345—Accessories, mechanical or electrical features mathematical transformations on beams or signals, e.g. Fourier

Definitions

  • the present invention relates to a technique for correcting an X-ray projection image for obtaining a CT cross-sectional image using X-ray CT.
  • the present invention relates to a technique for correcting an X-ray projection image so as to reduce or eliminate the influence of beam hardening.
  • X-ray CT Computed Tomography irradiates a subject (object) with X-rays from multiple directions, and based on the obtained X-ray intensity distribution (X-ray projection image), an image (cross-sectional image) inside the subject is obtained. It is a technology that can be reconfigured. Such X-ray CT has been widely used in medical applications, but in recent years, it has also been used for industrial purposes. When X-ray CT is used, a surface shape that cannot be seen from the outside can be specified, which is useful in, for example, dimensional inspection and reverse engineering of a product having a complicated shape.
  • ⁇ is a value called a linear attenuation coefficient, which is determined by the material at the position x.
  • the projection value P defined by logarithmic transformation is often used as follows.
  • the projection value is sometimes called the Ray-sum value.
  • ⁇ (x) may be zero in the air, and a constant value (here, ⁇ And so on)
  • FIG. 1A is a graph in which the projection values are plotted against the transmission length.
  • Beam hardening When the X-ray source is not a single color but a white X-ray source having a certain spectral distribution, the above linear relationship is not established. This is because the low energy component (soft X-ray) in white X-rays attenuates rapidly with respect to the transmission length, while the high energy component (hard X-ray) hardly attenuates. That is, in X-rays that have passed through a certain length, soft X-rays are lost and hard X-ray components remain. This is a phenomenon called beam hardening (beam hardening).
  • FIG. 1 (b) is a plot of the white X-ray projection values against the X-ray transmission length.
  • the projection value greatly increases (that is, the X-ray intensity greatly decreases) due to the attenuation of the soft X-rays.
  • the influence of hard X-rays having strong transmission power becomes relatively strong, so that the increase becomes moderate (that is, the X-ray intensity does not decrease so much).
  • Beam hardening artifact In X-ray CT, a cross-sectional image is reconstructed using this projection value. In this case, since the above linearity is assumed, in the case of a white X-ray source, an artifact occurs in the cross-sectional image. That is, the accuracy of the obtained cross-sectional image is deteriorated.
  • Patent Document 1 and Non-Patent Document 1 below propose a method of correcting and linearizing a projection value curve that is nonlinear by beam hardening.
  • Non-Patent Document 1 below proposes a correction method based on prior measurement using known materials. However, this is not only troublesome in preparation, but also has a problem that it cannot be applied to photographing conditions different from prior measurement.
  • preliminary reconstruction is performed using a sinogram (arrangement of X-ray projection images) before correction, the obtained reconstruction image is analyzed, and correction is performed by mapping of a non-linear curve. Proposes a technique to do.
  • this method has a problem that it requires time for correction because preliminary reconstruction and mapping are required.
  • Metal artifacts are often confused with beam hardening artifacts.
  • Metal artifacts are large noises that occur when there is a part of a material that absorbs X-rays very much, such as metal.
  • medical X-ray CT is generated by, for example, a metal prosthesis for dental caries treatment, for example, as described in Non-Patent Document 2 below, many developments have been made for a long time.
  • the cause of this metal artifact is that the intensity of the X-rays incident on the detector is significantly lowered due to the attenuation of the X-rays due to the portion where the X-ray absorption is very large, and falls below the detection limit of the detector. Therefore, the metal artifact is completely different from the beam hardening that is the subject of the present invention.
  • An object of the present invention is to provide a technique capable of correcting the influence of beam hardening in an X-ray projection image without requiring a prior learning procedure, preliminary reconstruction, or mapping.
  • the projection value calculation unit is configured to obtain, for each X-ray projection image, a total of projection values of each pixel of the X-ray projection image for a plurality of X-ray projection images obtained by projection from a plurality of directions.
  • the correction function assumption unit is configured to perform a process of assuming a temporary correction function for correcting the projection value,
  • the correction function determination unit determines an optimized correction function by correcting the temporary correction function so as to satisfy the condition that the sum of the corrected projection values is constant in each projection image.
  • the projection value correction unit is configured to correct the projection value using the optimized correction function.
  • the correction function determination unit obtains the optimized correction function so that a difference between a predetermined reference value that is not 0 and a sum of the projection values obtained by applying the provisional correction function is minimized.
  • the temporary correction function is an nth order polynomial
  • the X-ray projection image correction apparatus according to Item 1 or 2, wherein the correction function determination unit determines an unknown coefficient in the n-th order polynomial.
  • An X-ray CT apparatus comprising the X-ray projection image correction apparatus according to any one of items 1 to 4.
  • X-ray projection image correction method comprising the following steps: (1) A step of obtaining, for each X-ray projection image, a total of projection values of each pixel of the X-ray projection image for a plurality of X-ray projection images obtained by projection from a plurality of directions; (2) assuming a temporary correction function for correcting the projection value; (3) determining an optimized correction function by modifying the temporary correction function so as to satisfy the condition that the sum of the corrected projection values is constant in each projection image; (4) correcting the projection value using the optimized correction function;
  • (Item 7) A computer program for executing the X-ray projection image correction method according to item 6 by a computer.
  • FIG. 5 is a graph showing the relationship between transmission length L and projection value P for explaining the principle of beam hardening, in which FIG. (A) shows a case of monochromatic X-rays and FIG. (B) shows a case of white X-rays. Show. It is explanatory drawing which shows the outline of a X-ray CT apparatus.
  • 1 is a block diagram illustrating an overall configuration of an X-ray projection image correction apparatus according to an embodiment of the present invention. It is a flowchart which shows the outline
  • FIG. 4A is an example of a CT cross-sectional image obtained by the correction method of the present embodiment.
  • FIG. 5A shows a case where no correction is performed
  • FIG. 4A is an example of a CT cross-sectional image obtained by the correction method of the present embodiment
  • FIG. 5A shows a case where correction is not performed
  • FIG. 5B shows a case where correction is performed according to the present embodiment.
  • FIG. 9 is an explanatory diagram schematically illustrating FIG. 8. It is a graph which shows the change of the luminance value along the straight line A shown in FIG.
  • a horizontal axis shows length (arbitrary unit) and a vertical axis
  • shaft shows the normalized luminance value. It is a graph which shows the change of the luminance value along the straight line B shown in FIG. 9, Comprising: A horizontal axis shows length (arbitrary unit) and a vertical axis
  • shaft shows the normalized luminance value.
  • this X-ray CT apparatus includes an X-ray source 101 and a detector unit 102 as main components.
  • the X-ray source 101 is configured to emit white X-rays toward the detector unit 102.
  • the detector unit 102 is constituted by a set of a plurality (p pieces) of detectors 1021 to 102p.
  • the plurality of detectors correspond to pixels, so to speak, a projection value is calculated based on an X-ray luminance value (that is, energy value) detected by each detector, and an X-ray projection image can be constructed. That is, the projection image is composed of a set of pixel values for each detector.
  • a rotary table 103 is disposed between the X-ray source 101 and the detector unit 102, and an object 104 can be placed on the rotary table 103. Further, the turntable 103 can be rotated at every predetermined angle.
  • Each detector of the detector unit 102 can construct q X-ray projection images 1051 to 105q corresponding to each rotation angle by detecting X-rays transmitted through the object 104 (FIG. 2 ( b)).
  • a cross-sectional image can be obtained by performing reconstruction using a sinogram that is an array of X-ray projection images.
  • a sinogram that is an array of X-ray projection images.
  • the X-ray projection image correction apparatus of the present embodiment is used accompanying the above-described X-ray CT apparatus.
  • the X-ray projection image correction apparatus of this example includes a projection value calculation unit 1, a correction function assumption unit 2, a correction function determination unit 3, a projection value correction unit 4, and a reference value determination unit 5 ( (See FIG. 3).
  • the projection value calculation unit 1 receives a sinogram, which is a set of X-ray projection images obtained by projection from a plurality of directions, from the detector unit 102, and obtains the total of projection values (that is, pixel values) in each projection image. Has been.
  • the correction function assumption unit 2 is configured to perform a process of assuming a temporary correction function for correcting the projection value.
  • an n-order (for example, third-order) polynomial is used as a temporary correction function.
  • a process that assumes a temporary correction function means, for example, “a process that specifies a temporary correction function registered in advance in the system”, but “a temporary process that is dynamically The process of generating a correction function ”may be used.
  • the temporary correction function registered in advance on the system side may be one type or plural types.
  • the correction function determination unit 3 corrects the temporary correction function so as to satisfy the condition that “the total of the projection values after the correction process is constant in each projection image”, thereby optimizing the correction function. It is set as the structure which determines.
  • the correction function determination unit 3 of this example obtains an optimized correction function so that the difference between the predetermined reference value that is not 0 and the sum of the projection values obtained by applying the temporary correction function is minimized.
  • the processing is performed.
  • the correction function determination unit 3 determines an unknown coefficient in an n-th order polynomial that is a temporary correction function.
  • the projection value correction unit 4 is configured to correct the projection value using an optimized correction function.
  • the corrected projection value (or a projection image or sinogram formed thereby) is output to a reconstruction processing unit (not shown) for reconstruction processing.
  • the reference value determination unit 5 performs a process of determining a reference value used for optimization of the correction function.
  • the maximum value in the sum of the projection values for each projection image is used as the predetermined reference value.
  • CT sectional image generation procedure First, an overall procedure for generating a CT cross-sectional image will be described with reference to FIG.
  • Step SA-1 in FIG. 4 While rotating the object 104 by a predetermined angle, the object 104 is irradiated with white X-rays from the X-ray source 101, and a transmission image is detected by each detector of the detector unit 102.
  • One pixel of the projected image is constituted by the output of one detector.
  • each projection image corresponding to the angle between the object and the X-ray source can be acquired as in the conventional case. That is, in this example, one projection image is acquired corresponding to one angular relationship.
  • a two-dimensional detector using a cone beam is illustrated, but other imaging methods such as a one-dimensional detector using a fan beam can be used.
  • the X-ray projection image of this embodiment may be a one-dimensional image or a two-dimensional image.
  • the object is a single material, that is, the attenuation coefficient to X-rays is uniform.
  • Step SA-2 in FIG. 4 each projection image in the sinogram is corrected by the X-ray projection image correction apparatus of this example. Details of this procedure will be described later with reference to FIG.
  • Steps SA-3 and SA-4 in FIG. 4 Next, a CT cross-sectional image is reconstructed using a sinogram which is a set of corrected X-ray projection images.
  • the obtained CT cross-sectional image is output to a predetermined output destination (for example, a display, a printer, or a storage device). Since the reconstruction method itself may be the same as the conventional one, a detailed description of the reconstruction method is omitted.
  • Step SB-1 in FIG. 5 First, with respect to a plurality of projection images obtained by projection from a plurality of directions, the projection value calculation unit 1 calculates the total of projection values (that is, pixel values) in each projection image.
  • the total projection value can be expressed by the following formula, for example.
  • i represents the projection for each angle
  • j represents the detector number.
  • the detector number is a serial number assigned to each detector part, and is continuously assigned based on a predetermined rule even when the detector parts are two-dimensionally arranged. Therefore, P ij is a projection value (luminance value) in the j-th detector part in the projection at the i-th angle.
  • the total projection value for each projection image is constant for all i. This is shown in FIG. Since each X-ray passes through the same area no matter what direction is projected, if the projection value P ij and the transmission length x ij of the X-ray have a linear relationship, the projection value P in each projection The sum of ij should be equal.
  • the projected image of the object needs to be within the range of the entire detector section.
  • the conservation law is established when the linear relationship is satisfied.
  • white X-rays are used, actually, a projection value deviating from the conservation law (that is, non-linear) is obtained.
  • the reference value determining unit 5 determines a reference value for determining a correction function described later.
  • this reference value in this example, the largest one of the total projection values obtained in each projection i is used. Thereby, the scale of the projection value before and after correction can be roughly matched.
  • the reference value may be a constant other than 0 (usually a positive value).
  • Step SB-3 in FIG. 5 After or before and after Step SB-2, the correction function assumption unit 2 assumes a temporary correction function for correcting the projection value.
  • the following cubic polynomial is used as the provisional correction function F ().
  • a 1 , a 2 , and a 3 are coefficients, and are the elements of the vector a in the above example. These coefficients are unknown at this point.
  • n is a natural number
  • the provisional correction function F can be determined in view of the required accuracy and calculation speed. It is also possible to consider a correction function other than the nth order equation.
  • the correction function determination unit 3 determines the optimized correction function by correcting the temporary correction function so as to satisfy the condition that the sum of the corrected projection values in each projection image is constant. .
  • the following objective function J is considered.
  • the function F (specifically its coefficient) is determined so that the value of J is minimized.
  • Such a coefficient determination method is known as a least-square method, and the existing solution itself can be used, so that the description of the details of the solution is omitted.
  • an optimized correction function for removing the influence of beam hardening can be determined in this way.
  • the projection value correction unit 4 corrects the projection value using the optimized correction function. Thereby, a sinogram composed of a projection image in which the influence of beam hardening is removed or reduced can be obtained, and a high-accuracy CT cross-sectional image can be obtained by performing reconstruction using this.
  • the X-ray projection image correction method in the present embodiment can be executed by a computer by using an appropriate computer program.
  • the present embodiment it is possible to appropriately correct the X-ray projection image without performing prior learning.
  • this embodiment since it is not necessary to perform preliminary reconstruction, there is an advantage that the X-ray projection image correction process can be performed at high speed.
  • Example 1 A CT cross-sectional image obtained by the method of this example is shown in FIG. FIG. 7A shows the case without correction, FIG. 7B shows the case where the correction is made using the objective function J, and FIG. 7C shows the case where the correction is made using the objective function J ′. Since this object is a uniform material, the variation in luminance as shown in FIG. 7A indicates a deterioration in the accuracy of the cross-sectional image. In contrast, in FIG. 7B and FIG. 7C, the luminance variation is eliminated. Also from this result, the effect of improving the CT cross-sectional image accuracy by the method of this example can be understood.
  • Example 2 Evaluation results of CT cross-sectional images obtained by the method of this example will be described as Example 2 with reference to FIGS.
  • FIG. 8 is a photograph of an example of the obtained CT cross-sectional image
  • FIG. 9 is an explanatory diagram illustrating it.
  • “no correction” indicates that the correction according to this example was not performed
  • “correction” indicates that the correction was performed using the objective function J.
  • FIG. 10 shows a change in luminance value along the arrow direction of the straight line (dashed line) A shown in FIG.
  • the meanings of “without correction” and “with correction” in FIG. 10 are the same as those in FIG.
  • the CT value drops due to a cupping artifact as it goes to the center of the part.
  • the central part of the component near 200 on the scale on the horizontal axis
  • the central part of the component has a smaller value than the CT value of the surrounding air region (near 0 and 400 on the horizontal axis).
  • the cupping artifact is eliminated, and the value in the central portion can be corrected to substantially the same value as the CT value in the surrounding air region.
  • FIG. 11 shows a change in luminance value along the arrow direction of the straight line (dashed line) B shown in FIG.
  • the meanings of “without correction” and “with correction” in FIG. 11 are the same as described above.
  • the CT value fluctuates in an unstable manner, resulting in an unnatural valley.
  • this valley disappears and a substantially constant CT value is maintained in the part portion. Also from these results, the effect of improving the CT cross-sectional image accuracy by the method of this example can be understood.
  • each of the above-described components only needs to exist as a functional block, and does not have to exist as independent hardware.
  • a mounting method hardware or computer software may be used.
  • one functional element in the present invention may be realized by a set of a plurality of functional elements, and a plurality of functional elements in the present invention may be realized by one functional element.
  • each functional element constituting the present invention may exist in a discrete manner. If they exist in a discrete manner, necessary data can be transferred via a network, for example.
  • each function in each part can exist discretely.
  • each functional element in the present embodiment or a part thereof can be realized by using grid computing or cloud computing.

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Abstract

La présente invention porte sur un procédé permettant une correction pour l'impact de dureté de faisceau dans des images de projection de rayons X sans avoir besoin d'un processus d'apprentissage antérieur. Pour une pluralité d'images de projection de rayons X obtenues selon une projection depuis une pluralité de directions, la somme des valeurs de projection des pixels d'une image de projection de rayons X est trouvée pour chacune des images de projection de rayons X. Ensuite, une fonction de correction provisoire pour corriger des valeurs de projection est supposée, et une fonction de correction qui a été optimisée est déterminée. La détermination de cette fonction de correction est réalisée par modification de la fonction de correction provisoire de telle sorte que la condition suivante est satisfaite : la somme de valeurs de projection après correction est uniforme dans chacune des images de projection. Ensuite, à l'aide de la fonction de correction optimisée, la valeur de projection est corrigée.
PCT/JP2013/059175 2012-04-10 2013-03-28 Dispositif de correction d'image de projection de rayons x et procédé de correction d'image de projection de rayons x WO2013153955A1 (fr)

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Cited By (2)

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Publication number Priority date Publication date Assignee Title
CN105374014A (zh) * 2015-08-25 2016-03-02 上海联影医疗科技有限公司 图像校正方法及装置、医学图像生成方法及装置
JP2020094893A (ja) * 2018-12-12 2020-06-18 株式会社ニコン データ処理方法、データ処理装置、およびデータ処理プログラム

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JPH0327046B2 (fr) * 1984-08-29 1991-04-12 Tokyo Shibaura Electric Co
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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN105374014A (zh) * 2015-08-25 2016-03-02 上海联影医疗科技有限公司 图像校正方法及装置、医学图像生成方法及装置
JP2020094893A (ja) * 2018-12-12 2020-06-18 株式会社ニコン データ処理方法、データ処理装置、およびデータ処理プログラム
JP7200647B2 (ja) 2018-12-12 2023-01-10 株式会社ニコン データ処理方法、データ処理装置、およびデータ処理プログラム

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