WO2019012686A1 - Particle beam treatment device and drr image creation method - Google Patents

Particle beam treatment device and drr image creation method Download PDF

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WO2019012686A1
WO2019012686A1 PCT/JP2017/025693 JP2017025693W WO2019012686A1 WO 2019012686 A1 WO2019012686 A1 WO 2019012686A1 JP 2017025693 W JP2017025693 W JP 2017025693W WO 2019012686 A1 WO2019012686 A1 WO 2019012686A1
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ray
virtual
dimensional
drr image
image
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PCT/JP2017/025693
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French (fr)
Japanese (ja)
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啓太 吉田
卓紀 角尾
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株式会社日立製作所
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Priority to TW107106369A priority patent/TWI645836B/en
Publication of WO2019012686A1 publication Critical patent/WO2019012686A1/en

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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/03—Computed tomography [CT]
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N5/00—Radiation therapy
    • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy

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  • the present invention relates to a particle beam therapy apparatus for treating an affected area such as cancer by particle beam, and in particular to the creation of a DRR image used to position a patient.
  • a DRR Digital Reconstructed Radiography
  • the DRR image is compared with an X-ray image obtained by an X-ray imaging apparatus, and used for correcting or confirming positional deviation due to visual recognition by a doctor or an engineer or for automatically calculating positional deviation by a computer. .
  • the DRR image is a virtual x-ray directed from the virtual x-ray generator to the x-ray detector while the geometrical arrangement of the object and the x-ray generator and the x-ray detector are reproduced on a computer
  • a line segment is set, and a CT value on the line segment or a value obtained by converting the CT value into a line attenuation coefficient is line integrated to calculate a relative X-ray dose reaching each pixel.
  • a method has been proposed in which a DRR image is generated in consideration of a beam hardening phenomenon caused by the fact that the energy spectrum of the X-ray is not a monochromatic X-ray but an X-ray having a spread of the energy spectrum.
  • Patent Document 1 discloses that the effect of beam hardening when transmitting through the inside of the body and a region where CT values exist is taken into consideration. Further, in Patent Document 2, the feature amount of the tissue of the subject is extracted from the projection data, and the beam hardening correction memory divided in advance according to each feature amount is selected based on the extracted feature amount and corrected. It is disclosed that correction of beam hardening is performed separately for each part to remove CT artifacts.
  • CT artifact is removed by generating and correcting projection data for correction based on the amount of radiation hardening correction obtained from bone projection data at the time of reconstruction of a CT image. It is disclosed to improve.
  • Patent Document 4 discloses that the image quality is improved by removing the CT contrast agent artifact by extracting the feature amount of the contrast agent from the projection data and correcting the beam hardening caused by the contrast agent. It is done.
  • correction of beam hardening is performed with high accuracy by performing weighted addition of correction of beam hardening of a predetermined slice position including correction components of adjacent slices before and after from voxel data of CT. Is disclosed.
  • Patent Document 6 when a treatment is planned, CT information is stored along with CT imaging, and information associated with the beam hardening effect is stored, and in CT imaging in the treatment room, the position of the target is corrected when correction is performed to remove the beam hardening effect. It is disclosed that the relationship between the bed and the bed is recorded, and the relationship with the treatment planning bed position is calculated in the bed in the treatment room using the CT image, and the deviation is corrected in the bed.
  • JP, 2016-059612 A Japanese Patent Application Laid-Open No. 5-261022 Japanese Patent Application Laid-Open No. 10-075947 Japanese Patent Application Publication No. 2003-000580 JP, 2009-050413, A JP, 2011-010885, A
  • An object of the present invention is to provide a particle beam therapy system which provides a DRR image closer to an actual X-ray image, in order to solve the problems of the conventional DRR image as described above.
  • the present invention is directed to X-ray radiation emitted from an X-ray tube to a patient by perspective projection of virtual X-rays emitted from a virtual X-ray tube onto three-dimensional CT data inside the patient being irradiated.
  • a DRR image creation computing unit that creates a two-dimensional DRR image that simulates a two-dimensional X-ray image obtained by perspective projection of a line, and the DRR image creation computing unit generates virtual X-ray images for three-dimensional CT data
  • the DRR imaging processor emits X-rays emitted from the X-ray tube Based on the fact that the lines have different energy spectrum intensity distributions depending on the radiation direction, the energy spectrum intensity distribution of virtual X-rays is set at each position in the DRR image, It is intended to create a RR image.
  • the DRR image creation method according to the present invention is directed to a patient by perspectively projecting virtual X-rays emitted from a virtual X-ray tube onto three-dimensional CT data inside a patient who is an irradiation target for particle beam irradiation.
  • a DRR imaging method for creating a two-dimensional DRR image simulating a two-dimensional x-ray image obtained by irradiating an x-ray emitted from an x-ray tube comprising: energy of the x-ray emitted from the x-ray tube Measuring the spectral intensity distribution at a plurality of two-dimensional positions at a position from which a two-dimensional X-ray image is obtained; and virtual X-rays based on the energy spectral intensity distribution of X-rays measured at the plurality of two-dimensional positions; By setting the energy spectrum intensity distribution of the X-ray for each two-dimensional position of the virtual X-ray, the beam hardware based on the energy spectrum intensity distribution of the virtual X-ray is obtained for the three-dimensional CT data. And the step of creating the DRR image by making the virtual X-ray see through.
  • FIG. 5 is a block diagram showing an example of a hardware configuration of a DRR image creation computing unit and a patient positioning controller of the particle beam therapy system according to the first embodiment of the present invention. It is a figure explaining the energy spectrum of the X-ray emitted from a X-ray tube based on this invention. It is a figure explaining the operation
  • FIG. 1 is a block diagram conceptually showing an example of the configuration of a particle beam therapy system including the particle beam therapy system of the present invention.
  • the particle beam 2 emitted from the accelerator 1 that accelerates charged particles as a high energy charged particle beam passes through the vacuum duct 3 and is transported to the irradiation nozzle 4 provided downstream of the vacuum duct 3.
  • a deflection electromagnet for changing the traveling direction of the particle beam 2 is provided at a portion where the vacuum duct 3 is bent, but it is not shown in FIG.
  • the particle beam 2 is scanned in a two-dimensional direction perpendicular to the traveling direction of the particle beam 2 by a scanning electromagnet provided to the irradiation nozzle 4.
  • the scanned particle beam 2a is applied to the affected part 5 of a patient to be irradiated placed on a treatment table.
  • Various irradiation parameters at the time of irradiation are set by the treatment planning apparatus 10, and parameters of the respective devices of the accelerator 1 and the irradiation nozzle 4 for irradiation with the irradiation parameters are set by the system controller 20, and accelerator control is performed.
  • the command is transmitted to the apparatus 21 and the irradiation system control apparatus 22, and respective commands are output to the respective devices of the accelerator 1 and the irradiation nozzle 4.
  • an X-ray imaging apparatus 50 configured by flat panel detectors (FPDs) 52a and 52b (sometimes referred to as only the reference numeral 52 as a representative) and an X-ray imaging controller 53.
  • FPDs flat panel detectors
  • X-rays generated from the X-ray tube 51a and irradiated to the patient and perspective-projected X-rays are detected by the FPD 52a, and X-rays generated from the X-ray tube 51b and irradiated to the patient and perspective-projected are detected by the FPD 52b.
  • the X-ray imaging controller 53 controls the X-ray tubes 51a and 51b and the FPDs 52a and 52b, and acquires a two-dimensional X-ray image of the patient.
  • the irradiation dose to be applied to the affected area 5 is determined.
  • the irradiation dose is determined as a three-dimensional distribution in accordance with the shape of the affected area 5, that is, an irradiation dose distribution.
  • the treatment planning device 10 can determine irradiation parameters which are a set of various parameters of the accelerator 1 and the irradiation nozzle 4 for giving the irradiation dose distribution to the affected part 5.
  • the set of irradiation parameters can not be determined uniquely depending on the intensity of the particle beam, the diameter of the beam and the like. For this reason, an irradiation parameter that a user such as a doctor thinks is appropriate is determined.
  • three-dimensional CT data inside the patient including the affected area of the patient is generated based on, for example, an X-ray CT image or the like.
  • the three-dimensional CT data is used to determine the above-described irradiation dose distribution, and is also used to create a DRR image to be used in positioning of a patient when irradiating the affected part 5 with particle beams described later.
  • irradiation of particle beam to the affected area is performed once a day, divided into several tens of times.
  • a patient isocenter preset in the affected area in the two-dimensional X-ray image of the patient who is on the treatment table 62 acquired by the X-ray imaging controller 53 is determined by the irradiation nozzle 4
  • the position of the treatment table 62 is controlled so that the position of the patient resting on the treatment table 62 is positioned so as to match the isocenter of the device being carried out.
  • Positioning is created in advance by the DRR image creation computing unit 11 in the patient positioning controller 61 as an image simulating a two-dimensional X-ray image acquired by the X-ray image imaging controller 53 and an acquired two-dimensional X-ray image. This is performed by comparing the two-dimensional DRR image with one another and calculating the amount of positional deviation, and moving the treatment table 62 so that the amount of positional deviation becomes zero.
  • the treatment table 62 and the patient positioning controller 61 will be referred to as a patient positioning device 60 as devices used to position the patient.
  • the DRR image creation computing unit 11 and the patient positioning controller 61 may be realized by the same computer, and in this case, the DRR image creation computing unit 11 is physically included in the patient positioning device 60.
  • each device is controlled through the accelerator control device 21 and the irradiation system control device 22 according to the parameters of the accelerator 1 and the irradiation nozzle 4 determined in advance, and the affected part 5 is irradiated with the particle beam.
  • the radiation dose scheduled for the day is applied to the affected area, the radiation on that day is ended.
  • the patient positioning controller 61 is implemented as a computer including a processor 611, a memory 612, an input / output interface 613, and a display 614.
  • the DRR image creation computing unit 11 is also implemented as a computer including the processor 111, the memory 112, the input / output interface 113, and the display 114. That is, when the processor executes the program stored in the memory, the patient positioning controller 61 and the DRR image creation computing unit 11 are realized.
  • the patient positioning controller 61 and the DRR image creation computing unit 11 may be realized by the same computer.
  • the processor 611 and the processor 111 may be the same processor, and similarly, the display and the like may be the same display.
  • the DRR image is a virtual two-dimensional X-ray image obtained by perspective projection of a virtual X-ray emitted from a virtual X-ray source on three-dimensional CT data of a patient on a computer.
  • the DRR image is an image that simulates a two-dimensional X-ray image obtained by perspective projection of X-rays actually emitted from the X-ray tube to the patient.
  • virtual X-rays are transmitted to three-dimensional CT data by a method called ray casting method to create a two-dimensional X-ray image.
  • the DRR image creation computing unit 11 virtually reproduces the geometrical arrangement of the X-ray tube 51a, the patient 5 as the subject, and the FPD 52a as the X-ray detector in FIG. 1 to generate a DRR image.
  • the subject is given as three-dimensional CT data of the patient
  • the X-ray detector 52a is accumulated by accumulating the attenuation of the virtual X-ray by the CT value which is the value of the three-dimensional CT data for each position where the virtual X-ray passes.
  • a two-dimensional X-ray image, that is, a DRR image can be obtained by determining the X-ray intensity at the position of
  • X-rays generated from an X-ray tube are not single monochromatic X-rays in energy, but X-rays distributed in the energy spectrum. Therefore, conventionally, when creating a DRR image, using a virtual X-ray having an energy spectrum distribution as a virtual X-ray, the DRR image is created by calculating an X-ray intensity transmitted through three-dimensional CT data. . If there is a distribution in the energy spectrum of the X-ray, the absorption varies depending on the energy when transmitting three-dimensional CT data, so this calculation is included including this effect.
  • the lower the energy of the X-rays the larger the absorption and the larger the attenuation. Therefore, as the X-rays pass through the substance, the components with low energy decrease, and the energy spectrum distribution shifts to high energy. This effect is called beam hardening.
  • the X-ray tube 51 is configured to cause electrons generated from an electron source, which is the cathode 512, to collide with a target, which is the anode 511, to generate X-rays.
  • the energy of X-rays as photons emitted to the outside of the target differs depending on the position of the target at which the electrons collide. This is because X-rays are generated inside the target, and the generated X-rays pass through the inside of the target, and absorption of X-rays occurs in the process, and the intensity of the X-rays decreases. Differences occur in the intensity and energy spectral distribution. That is, due to the influence of beam hardening inside the target, the X-rays reaching the FPD 52 have different energy spectral distributions depending on the position even if there is no object in between. Furthermore, since the intensity is high on the cathode side and low on the anode side, unevenness occurs in the X-ray irradiated area. This phenomenon is called the heel effect.
  • FIG. 4 shows a schematic diagram in which the present invention is applied to ray casting.
  • Ray casting in the generation of DRR sets a ray connecting the X-ray tube and each pixel of the X-ray CT data in the geometrical arrangement of the X-ray imaging system reproduced on a computer, and a plurality of calculation points on this ray Is set, the CT value of the calculation point is corrected based on the accumulated CT value from the X-ray tube to the calculation point, and the CT value of the calculation point located on the ray is calculated using the CT value of each calculation point after correction.
  • This is a method of creating a DRR image based on the accumulated and accumulated CT values.
  • the attenuation of x-rays is determined by the attenuation coefficient and distance of each material.
  • the CT value is a relative value based on the linear attenuation coefficient of water at a predetermined energy, and the linear attenuation coefficient of each substance can be determined from the CT value.
  • the attenuation of X-rays is expressed by the following formula (1).
  • I is the X-ray intensity after transmission of the material
  • I 0 is the incident X-ray intensity
  • d i is the X-ray passing distance at each calculation point
  • ⁇ i is the linear attenuation coefficient of the material at each calculation point .
  • d i is the voxel size of the CT image at each calculation point, which is a fixed value according to the image. Since ⁇ i is a linear attenuation coefficient obtained from CT at each calculation point, ⁇ can also be regarded as an accumulated value of CT values. Also, diagnostic x-rays are not single energy, but have a width in the energy spectrum, and incident x-ray intensity is a function of energy. Also, the source weak coefficient is an energy function dependent on the energy of X-rays.
  • this X-ray intensity I when this X-ray intensity I is determined, a linear attenuation coefficient taking into consideration the energy spectral distribution of X-rays in the tissue at each calculation point is given, and the influence of so-called beam hardening gives FPD as a detector.
  • a DRR image is created by simulating the difference depending on the position reached by the X-ray.
  • the X-ray intensity can be accurately determined by giving linear attenuation coefficients in consideration of the energy spectral distribution of X-rays at all calculation points.
  • the X-ray intensity I (x, y) of each position on the detection surface of FPD is the energy of the X-ray reaching that position Since the value is obtained by integrating each X-ray intensity, it may be integrated by energy.
  • the energy spectral distribution of the virtual X-ray reaching the position (x, y) is represented as E (x, y).
  • I (x, y) can be calculated as in the following equation (2).
  • the X-ray intensity I at each position on the detection surface of the FPD can be obtained by operating the equation (2) as it is. Since a distribution of X-ray intensities by the detection plane of the FPD when a subject represented by three-dimensional CT data is viewed through virtual X-rays can be determined, a DRR image can be obtained from this X-ray intensity distribution.
  • X-rays are generated from the X-ray tube 51 used when actually irradiating particle beams, and X-ray spectra at a plurality of two-dimensional positions on the surface of the FPD 52 are detected.
  • the X-ray energy spectrum intensity distribution data as shown in FIG. 3 is obtained by using the measuring unit 522 for each two-dimensional position (step ST1).
  • X-rays are irradiated from two orthogonal directions to acquire two X-ray images, and DRR images corresponding to the respective X-ray images are required.
  • the X-ray energy spectrum intensity distribution of virtual X-rays reaching the detection plane of the FPD 52 from the virtual X-ray source It sets every (step ST2). Further, calculation points (i in equation (2)) of three-dimensional CT data are set for each virtual X-ray (step ST3).
  • the linear attenuation coefficient ⁇ j (E (x, y)) for each calculation point corresponding to the energy of each virtual X-ray is set using three-dimensional CT data, and each of the detection surfaces of the FPD is The X-ray intensity I (x, y) of the position is calculated and obtained (step ST4).
  • a DRR image is constructed based on the determined X-ray intensity I (x, y) (step ST5).
  • a DRR image including a difference in beam hardening effect in a subject (three-dimensional CT data) due to the difference in X-ray energy spectrum intensity distribution depending on the direction in which the X-ray tube emits X-rays. It is possible to obtain a DRR image closer to the X-ray image actually acquired.
  • the effective energy is calculated from the energy spectrum distribution of the incident X-ray, and the X-ray is regarded as a monochromatic X-ray for each position (x, y) of the FPD, and the X-ray intensity is a position at a unique energy.
  • the correction coefficient K (x, y) for the function or the reference X-ray intensity the integration of the energy spectral distribution can be eliminated and the amount of operation can be reduced.
  • the effective energy is an average of X-ray energy
  • beam hardening in the X-ray tube that is, when the heel effect occurs, the effective energy becomes large, and the heel effect can be expressed by the difference in effective energy. .
  • the correction coefficient K (x, y) due to the difference in the spectrum of the X-rays reaching the position of the FPD 52 is obtained by calculation.
  • X-ray energy spectrum intensity distribution data needs to be acquired in two dimensions, but it is not necessary to acquire it at precisely many positions.
  • FIG. 8 shows an example in which X-ray energy spectrum intensity distribution data are acquired at a total of nine places, and the correction coefficients at each position are found in a map.
  • the correction coefficient at the position where there is no X-ray energy spectrum intensity distribution data can be obtained by interpolation using the correction coefficient obtained by calculation at the position where there is X-ray energy spectrum intensity distribution data.
  • the correction coefficient map is stored as a map having a value for each two-dimensional small area of the FPD 52.
  • the correction factor is the effective energy (X-ray energy averaged) obtained from the energy spectrum distribution (ST11) measured at a predetermined measurement point on the FPD, and beam hardening, ie, the heel effect occurs in the X-ray tube.
  • the effective energy increases uniquely), and each correction coefficient is determined as a relative value based on the energy at a predetermined position. For example, when the portion where the heel effect is large is 1, the correction coefficient is determined by setting the small portion as 0.6. (ST12).
  • the correction coefficient K (x, y) at the two-dimensional position is determined by interpolation for the portion other than the measurement point (ST13).
  • DRR is generated (ST14 to ST18).
  • An incident X-ray obtained by multiplying the correction coefficient K (x, y) with respect to an incident X-ray with respect to an object at the time of DRR generation in consideration of the heel effect is determined (ST15).
  • the document value or the effective energy of the incident X-ray acquired in advance by simulation or measurement with respect to the original CT value set to the energy set at the time of calibration Converted to a CT value for (ST16).
  • ⁇ (d i ⁇ i ) in equation (3) is determined (ST17), and mapping is performed using two-dimensional coordinates to generate DRR (ST18).

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Abstract

This particle beam treatment device creates a digital reconstructed radiography (DRR) image, of 3D CT data of an internal part of a patient, by causing virtual x-rays to be transparent as a result of the inclusion of a beam hardening effect which is based on the energy spectrum strength distribution of the virtual x-rays, wherein a DRR image is created by setting the energy spectrum strength distribution of virtual x-rays for each of the positions in the DRR image, on the basis of the x-rays emitted from an x-ray tube having different x-ray spectrum strength distributions according to the emission direction.

Description

粒子線治療装置およびＤＲＲ画像作成方法Particle beam therapy apparatus and DRR image creation method
　本発明は、粒子線により癌などの患部を治療する粒子線治療装置、特に患者の位置決めを行うために用いるＤＲＲ画像の作成に関するものである。 The present invention relates to a particle beam therapy apparatus for treating an affected area such as cancer by particle beam, and in particular to the creation of a DRR image used to position a patient.
　粒子線治療において、Ｘ線ＣＴ装置によって収集した３次元の画像データから透視投影法を用いて生成するＤＲＲ（Digital Reconstructed Radiography）画像は、ビーム照射の際の患者位置決めに使用される。ＤＲＲ画像はＸ線撮像装置によって得られたＸ線画像と比較され、医師や技師等の視認による位置ずれの補正や確認に用いられることや、コンピュータによって自動で位置ずれを計算するために用いられる。ＤＲＲ画像は、被写体とＸ線発生装置およびＸ線検出器の幾何学的配置をコンピュータ上で再現した中で、仮想的なＸ線発生装置からＸ線検出器に向けて仮想的なＸ線として線分を設定し、線分上にあるＣＴ値またはＣＴ値を線減弱係数に変換した値を線積分して、各画素に到達する相対Ｘ線量を計算する。このとき、ＤＲＲ画像は、Ｘ線のエネルギースペクトルが単色Ｘ線ではなくエネルギースペクトルの拡がりを有するＸ線であることによるビームハードニング現象を考慮して生成される方法が提案されている。 In particle beam therapy, a DRR (Digital Reconstructed Radiography) image generated using perspective projection from three-dimensional image data collected by an X-ray CT apparatus is used for patient positioning during beam irradiation. The DRR image is compared with an X-ray image obtained by an X-ray imaging apparatus, and used for correcting or confirming positional deviation due to visual recognition by a doctor or an engineer or for automatically calculating positional deviation by a computer. . The DRR image is a virtual x-ray directed from the virtual x-ray generator to the x-ray detector while the geometrical arrangement of the object and the x-ray generator and the x-ray detector are reproduced on a computer A line segment is set, and a CT value on the line segment or a value obtained by converting the CT value into a line attenuation coefficient is line integrated to calculate a relative X-ray dose reaching each pixel. At this time, a method has been proposed in which a DRR image is generated in consideration of a beam hardening phenomenon caused by the fact that the energy spectrum of the X-ray is not a monochromatic X-ray but an X-ray having a spread of the energy spectrum.
　例えば、特許文献１では、体内およびＣＴ値の存在する領域を透過する際のビームハードニングによる影響を考慮することが開示されている。また、特許文献２では、投影データから被験者の組織の特徴量を抽出し、あらかじめ各特徴量に応じて区分けされたビームハードニング補正用メモリを、抽出した特徴量に基に選択して補正することにより、部位別にビームハードニングの補正を行い、ＣＴのアーチファクトを除去することが開示されている。 For example, Patent Document 1 discloses that the effect of beam hardening when transmitting through the inside of the body and a region where CT values exist is taken into consideration. Further, in Patent Document 2, the feature amount of the tissue of the subject is extracted from the projection data, and the beam hardening correction memory divided in advance according to each feature amount is selected based on the extracted feature amount and corrected. It is disclosed that correction of beam hardening is performed separately for each part to remove CT artifacts.
　さらに、特許文献３では、ＣＴ画像の再構成時に骨投影データから求めた線質硬化補正量を基に補正用投影データを生成して補正することにより、ＣＴのアーチファクトを除去して、画質の向上を図ることが開示されている。 Furthermore, in Patent Document 3, CT artifact is removed by generating and correcting projection data for correction based on the amount of radiation hardening correction obtained from bone projection data at the time of reconstruction of a CT image. It is disclosed to improve.
　特許文献４では、投影データから造影剤の特徴量を抽出し、造影剤に起因するビームハードニングを補正することにより、ＣＴの造影剤のアーチファクトを除去して、画質の向上を図ることが開示されている。 Patent Document 4 discloses that the image quality is improved by removing the CT contrast agent artifact by extracting the feature amount of the contrast agent from the projection data and correcting the beam hardening caused by the contrast agent. It is done.
　特許文献５では、ＣＴのボクセルデータから所定スライス位置のビームハードニングの補正を隣接する前後の複数スライスの補正成分も含めて加重加算して行うことにより、ビームハードニングの補正を高精度に行うことが開示されている。 In Patent Document 5, correction of beam hardening is performed with high accuracy by performing weighted addition of correction of beam hardening of a predetermined slice position including correction components of adjacent slices before and after from voxel data of CT. Is disclosed.
　特許文献６では、治療計画時にＣＴ撮影とともにビームハードニング効果に伴う情報を蓄積し、治療室でのＣＴ撮影で、そのビームハードニング効果を取り除く補正を入れてＣＴ再構成するとき、ターゲットの位置と寝台との関係を記録し、ＣＴ画像を使って治療室内寝台で治療計画寝台位置との関係を計算し、ずれを寝台で修正することが開示されている。 In Patent Document 6, when a treatment is planned, CT information is stored along with CT imaging, and information associated with the beam hardening effect is stored, and in CT imaging in the treatment room, the position of the target is corrected when correction is performed to remove the beam hardening effect. It is disclosed that the relationship between the bed and the bed is recorded, and the relationship with the treatment planning bed position is calculated in the bed in the treatment room using the CT image, and the deviation is corrected in the bed.
特開２０１６－０５９６１２号公報JP, 2016-059612, A 特開平５－２６１０９２号公報Japanese Patent Application Laid-Open No. 5-261022 特開平１０－０７５９４７号公報Japanese Patent Application Laid-Open No. 10-075947 特開２００３－０００５８０号公報Japanese Patent Application Publication No. 2003-000580 特開２００９－０５０４１３号公報JP, 2009-050413, A 特開２０１１－０１０８８５号公報JP, 2011-010885, A
　上記のように、従来、Ｘ線の照射対象である人体組織におけるビームハードニングの影響を考慮してＤＲＲ画像を作成する方法が種々提案されてきた。一方、Ｘ線発生装置であるＸ線管は、電子がターゲットに衝突する位置の違いによって、発生するＸ線のエネルギースペクトル分布が異なる。このため、Ｘ線が人体組織を通過する位置により、通過するＸ線のエネルギースペクトルが異なることになる。従来は、このＸ線エネルギースペクトル分布の空間的な差異による人体組織におけるビームハードニングの影響は考慮されていなかった。この影響により、ＤＲＲ画像とＸ線画像の差異が生じることとなっていた。 As described above, conventionally, various methods have been proposed for creating a DRR image in consideration of the effect of beam hardening in human tissue to be irradiated with X-rays. On the other hand, in the X-ray tube which is an X-ray generator, the energy spectrum distribution of the generated X-rays differs depending on the difference in the position where the electrons collide with the target. Therefore, the energy spectrum of the passing X-ray varies depending on the position where the X-ray passes through the human tissue. Heretofore, the effect of beam hardening in human tissue due to the spatial difference of the X-ray energy spectrum distribution has not been taken into consideration. This influence causes a difference between the DRR image and the X-ray image.
　この発明は、以上のような従来のＤＲＲ画像の問題点を解消すべく、実際のＸ線画像により近いＤＲＲ画像を提供する粒子線治療装置を得ることを目的とする。 An object of the present invention is to provide a particle beam therapy system which provides a DRR image closer to an actual X-ray image, in order to solve the problems of the conventional DRR image as described above.
　本発明は、粒子線を照射する照射対象である患者の内部の３次元ＣＴデータに仮想Ｘ線管から放射される仮想Ｘ線を透視投影することにより、患者にＸ線管から放射されるＸ線を透視投影して得られる２次元Ｘ線画像を模擬する２次元のＤＲＲ画像を作成するＤＲＲ画像作成演算器を備え、このＤＲＲ画像作成演算器は３次元ＣＴデータに対して、仮想Ｘ線のエネルギースペクトル強度分布に基づくビームハードニング効果を含んで、仮想Ｘ線を透視投影することによりＤＲＲ画像を作成する粒子線治療装置において、ＤＲＲ画像作成演算器は、Ｘ線管から放射されるＸ線が放射方向により異なるエネルギースペクトル強度分布を有することに基づいて、仮想Ｘ線のエネルギースペクトル強度分布を、ＤＲＲ画像内の位置毎に設定して前記ＤＲＲ画像を作成するものである。 The present invention is directed to X-ray radiation emitted from an X-ray tube to a patient by perspective projection of virtual X-rays emitted from a virtual X-ray tube onto three-dimensional CT data inside the patient being irradiated. A DRR image creation computing unit that creates a two-dimensional DRR image that simulates a two-dimensional X-ray image obtained by perspective projection of a line, and the DRR image creation computing unit generates virtual X-ray images for three-dimensional CT data In a particle beam therapy system that creates a DRR image by perspective projection of virtual X-rays, including a beam hardening effect based on the energy spectrum intensity distribution of the DRR imaging calculator, the DRR imaging processor emits X-rays emitted from the X-ray tube Based on the fact that the lines have different energy spectrum intensity distributions depending on the radiation direction, the energy spectrum intensity distribution of virtual X-rays is set at each position in the DRR image, It is intended to create a RR image.
　また、本発明に係るＤＲＲ画像作成方法は、粒子線を照射する照射対象である患者の内部の３次元ＣＴデータに仮想Ｘ線管から放射される仮想Ｘ線を透視投影することにより、患者にＸ線管から放射されるＸ線を照射して得られる２次元Ｘ線画像を模擬する２次元のＤＲＲ画像を作成するＤＲＲ画像作成方法であって、Ｘ線管から放射されるＸ線のエネルギースペクトル強度分布を、２次元Ｘ線画像を得る位置における複数の２次元位置で測定するステップと、仮想Ｘ線が、複数の２次元位置で測定されたX線のエネルギースペクトル強度分布に基づいて、仮想Ｘ線の２次元位置毎のＸ線のエネルギースペクトル強度分布を設定することにより、３次元ＣＴデータに対して、仮想Ｘ線のエネルギースペクトル強度分布に基づくビームハードニング効果を含んで、仮想Ｘ線を透視させることにより前記ＤＲＲ画像を作成するステップとを含んだものである。 In addition, the DRR image creation method according to the present invention is directed to a patient by perspectively projecting virtual X-rays emitted from a virtual X-ray tube onto three-dimensional CT data inside a patient who is an irradiation target for particle beam irradiation. A DRR imaging method for creating a two-dimensional DRR image simulating a two-dimensional x-ray image obtained by irradiating an x-ray emitted from an x-ray tube, comprising: energy of the x-ray emitted from the x-ray tube Measuring the spectral intensity distribution at a plurality of two-dimensional positions at a position from which a two-dimensional X-ray image is obtained; and virtual X-rays based on the energy spectral intensity distribution of X-rays measured at the plurality of two-dimensional positions; By setting the energy spectrum intensity distribution of the X-ray for each two-dimensional position of the virtual X-ray, the beam hardware based on the energy spectrum intensity distribution of the virtual X-ray is obtained for the three-dimensional CT data. And the step of creating the DRR image by making the virtual X-ray see through.
　この発明によれば、実際のＸ線画像により近いＤＲＲ画像を提供する粒子線治療装置を提供できる。 According to the present invention, it is possible to provide a particle beam therapy system which provides a DRR image closer to an actual X-ray image.
本発明の粒子線治療装置を含む線治療システムの一例としての粒子線治療システムの構成を示す概念図である。It is a conceptual diagram showing the composition of the particle therapy system as an example of the line therapy system containing the particle therapy system of the present invention. 本発明の実施の形態１による粒子線治療装置のＤＲＲ画像作成演算器および患者位置決め制御器のハード構成の一例を示すブロック図である。FIG. 5 is a block diagram showing an example of a hardware configuration of a DRR image creation computing unit and a patient positioning controller of the particle beam therapy system according to the first embodiment of the present invention. 本発明に基づく、Ｘ線管から放射されるＸ線のエネルギースペクトルを説明する図である。It is a figure explaining the energy spectrum of the X-ray emitted from a X-ray tube based on this invention. 本発明の実施の形態１による粒子線治療装置のＤＲＲ画像を得るための動作を説明する図である。It is a figure explaining the operation | movement for acquiring the DRR image of the particle beam therapy apparatus by Embodiment 1 of this invention. 本発明の実施の形態１による粒子線治療装置のＤＲＲ画像を得る工程を示すフロー図である。It is a flow figure showing the process of acquiring the DRR picture of the particle therapy system according to Embodiment 1 of the present invention. 本発明の実施の形態１による粒子線治療装置のＤＲＲ画像を得る一工程を説明する概念図である。It is a conceptual diagram explaining 1 process of obtaining a DRR image of particle beam therapy system by Embodiment 1 of the present invention. 本発明の実施の形態２による粒子線治療装置のＤＲＲ画像を得るための動作を説明する図である。It is a figure explaining the operation | movement for acquiring the DRR image of the particle beam therapy apparatus by Embodiment 2 of this invention. 本発明の実施の形態２による粒子線治療装置のＤＲＲ画像を得るためビームハードニング補正用データの一例を示す図である。It is a figure which shows an example of the data for beam hardening correction | amendment, in order to obtain the DRR image of the particle beam therapy system by Embodiment 2 of this invention. 本発明の実施の形態１による粒子線治療装置のＤＲＲ画像を得る工程を示すフロー図である。It is a flow figure showing the process of acquiring the DRR picture of the particle therapy system according to Embodiment 1 of the present invention.
実施の形態１．
　まず、本発明が適用される粒子線治療装置全体を説明する。図１が本発明の粒子線治療装置を含む粒子線治療システムの構成の一例を概念的に示すブロック図である。荷電粒子を加速する加速器１から高エネルギー荷電粒子ビームとして出射された粒子線２は、真空ダクト３内を通って、真空ダクト３の下流に設けられた照射ノズル４に輸送される。ここで、真空ダクト３が曲がっている部分には、粒子線２の進行方向を変化させるための偏向電磁石が設けられるが、図１では省略して図示している。粒子線２は、照射ノズル４に備えられた走査電磁石によって、粒子線２の進行方向に垂直な２次元方向に走査される。走査された粒子線２ａは治療台に載せられた照射対象である患者の患部５に照射される。照射する際の、種々の照射パラメータは治療計画装置１０で設定され、その照射パラメータで照射するための加速器１および照射ノズル４の各機器のパラメータが、システム制御装置２０で設定されて、加速器制御装置２１や照射系制御装置２２へ送信され、加速器１や照射ノズル４の各機器に対してそれぞれの指令が出力される。 Embodiment 1
First, an entire particle beam therapy system to which the present invention is applied will be described. FIG. 1 is a block diagram conceptually showing an example of the configuration of a particle beam therapy system including the particle beam therapy system of the present invention. The particle beam 2 emitted from the accelerator 1 that accelerates charged particles as a high energy charged particle beam passes through the vacuum duct 3 and is transported to the irradiation nozzle 4 provided downstream of the vacuum duct 3. Here, a deflection electromagnet for changing the traveling direction of the particle beam 2 is provided at a portion where the vacuum duct 3 is bent, but it is not shown in FIG. The particle beam 2 is scanned in a two-dimensional direction perpendicular to the traveling direction of the particle beam 2 by a scanning electromagnet provided to the irradiation nozzle 4. The scanned particle beam 2a is applied to the affected part 5 of a patient to be irradiated placed on a treatment table. Various irradiation parameters at the time of irradiation are set by the treatment planning apparatus 10, and parameters of the respective devices of the accelerator 1 and the irradiation nozzle 4 for irradiation with the irradiation parameters are set by the system controller 20, and accelerator control is performed. The command is transmitted to the apparatus 21 and the irradiation system control apparatus 22, and respective commands are output to the respective devices of the accelerator 1 and the irradiation nozzle 4.
　一方、例えば、患者のＸ線画像を取得して照射対象である患部５の位置などを確認し、患者の位置決めを行うために、Ｘ線管５１ａ、５１ｂ（代表として符号５１のみとして参照することもある）、フラットパネル検出器（ＦＰＤ）５２ａ、５２ｂ（代表として符号５２のみとして参照することもある）、Ｘ線画像撮像制御器５３によって構成されるＸ線撮像装置５０が設置されている。Ｘ線管５１ａから発生、患者に照射され透視投影されたＸ線はＦＰＤ５２ａで検出され、Ｘ線管５１ｂから発生、患者に照射され透視投影されたＸ線はＦＰＤ５２ｂで検出される。Ｘ線画像撮像制御器５３が、Ｘ線管５１ａ、５１ｂ、ＦＰＤ５２ａ、５２ｂを制御するとともに、患者の２次元Ｘ線画像を取得する。 On the other hand, for example, to obtain an X-ray image of a patient, confirm the position of the affected area 5 to be irradiated, etc., and position the patient, refer to the X-ray tubes 51a and 51b (only reference numeral 51 as a representative There is also provided an X-ray imaging apparatus 50 configured by flat panel detectors (FPDs) 52a and 52b (sometimes referred to as only the reference numeral 52 as a representative) and an X-ray imaging controller 53. X-rays generated from the X-ray tube 51a and irradiated to the patient and perspective-projected X-rays are detected by the FPD 52a, and X-rays generated from the X-ray tube 51b and irradiated to the patient and perspective-projected are detected by the FPD 52b. The X-ray imaging controller 53 controls the X-ray tubes 51a and 51b and the FPDs 52a and 52b, and acquires a two-dimensional X-ray image of the patient.
　以上の粒子線治療システムにより患者の患部５に治療用の放射線である粒子線を照射して腫瘍などの患部を治療する方法を簡単に説明する。まず、治療計画装置１０において、患部５に照射する照射線量を決定する。照射線量は、患部５の形状に合わせた３次元の分布、すなわち照射線量分布として決定される。照射線量分布が決定されると、治療計画装置１０において、患部５に照射線量分布を与えるための加速器１や照射ノズル４の種々のパラメータのセットである照射パラメータを決定することができる。ただし、粒子線の強度やビームの径などによって照射パラメータのセットは一意には決定できない。このため、医師などのユーザーが適切と考える照射パラメータを決定する。治療計画においては、患者の患部を含む患者内部の３次元ＣＴデータを、例えばＸ線ＣＴ画像などに基づいて生成する。この３次元ＣＴデータは、上記の照射線量分布を決定するときに用いられるとともに、後述する、粒子線を患部５に照射するときの患者の位置決めにおいて使用するＤＲＲ画像の作成に用いられる。 A method of treating an affected area such as a tumor by irradiating the particle beam which is a therapeutic radiation to the affected area 5 of a patient by the above particle beam therapy system will be briefly described. First, in the treatment planning apparatus 10, the irradiation dose to be applied to the affected area 5 is determined. The irradiation dose is determined as a three-dimensional distribution in accordance with the shape of the affected area 5, that is, an irradiation dose distribution. When the irradiation dose distribution is determined, the treatment planning device 10 can determine irradiation parameters which are a set of various parameters of the accelerator 1 and the irradiation nozzle 4 for giving the irradiation dose distribution to the affected part 5. However, the set of irradiation parameters can not be determined uniquely depending on the intensity of the particle beam, the diameter of the beam and the like. For this reason, an irradiation parameter that a user such as a doctor thinks is appropriate is determined. In the treatment planning, three-dimensional CT data inside the patient including the affected area of the patient is generated based on, for example, an X-ray CT image or the like. The three-dimensional CT data is used to determine the above-described irradiation dose distribution, and is also used to create a DRR image to be used in positioning of a patient when irradiating the affected part 5 with particle beams described later.
　粒子線治療の場合、患部への粒子線の照射は１日１回、数十回に分けて行われる。照射当日は、例えば、Ｘ線画像撮像制御器５３において取得された、治療台６２に載っている患者の２次元Ｘ線画像中の、患部に予め設定された患者アイソセンタが、照射ノズル４により決定されている機器のアイソセンタに合うように、治療台６２の位置を制御して、治療台６２に載っている患者の位置が、位置決めされる。位置決めは、患者位置決め制御器６１において、Ｘ線画像撮像制御器５３が取得した２次元Ｘ線画像と、取得される２次元Ｘ線画像を模擬する画像として予めＤＲＲ画像作成演算器１１において作成された２次元のＤＲＲ画像とを比較して、位置ずれ量を演算により求め、位置ずれ量が０となるよう治療台６２を移動させることにより行われる。ここでは、患者の位置決めを行うために用いられる装置として、治療台６２と患者位置決め制御器６１を患者位置決め装置６０と称することにする。また、ＤＲＲ画像作成演算器１１と患者位置決め制御器６１が同一の計算機で実現されてもよく、この場合は、ＤＲＲ画像作成演算器１１は物理的に患者位置決め装置６０に含まれることになる。 In the case of particle beam therapy, irradiation of particle beam to the affected area is performed once a day, divided into several tens of times. On the day of irradiation, for example, a patient isocenter preset in the affected area in the two-dimensional X-ray image of the patient who is on the treatment table 62 acquired by the X-ray imaging controller 53 is determined by the irradiation nozzle 4 The position of the treatment table 62 is controlled so that the position of the patient resting on the treatment table 62 is positioned so as to match the isocenter of the device being carried out. Positioning is created in advance by the DRR image creation computing unit 11 in the patient positioning controller 61 as an image simulating a two-dimensional X-ray image acquired by the X-ray image imaging controller 53 and an acquired two-dimensional X-ray image. This is performed by comparing the two-dimensional DRR image with one another and calculating the amount of positional deviation, and moving the treatment table 62 so that the amount of positional deviation becomes zero. Here, the treatment table 62 and the patient positioning controller 61 will be referred to as a patient positioning device 60 as devices used to position the patient. Also, the DRR image creation computing unit 11 and the patient positioning controller 61 may be realized by the same computer, and in this case, the DRR image creation computing unit 11 is physically included in the patient positioning device 60.
　位置決めが終わると、予め決定された加速器１や照射ノズル４のパラメータにより、加速器制御装置２１や照射系制御装置２２を介して各機器が制御され、患部５に粒子線が照射される。その日予定されている照射線量が患部に照射されるとその日の照射は終了する。 When the positioning is completed, each device is controlled through the accelerator control device 21 and the irradiation system control device 22 according to the parameters of the accelerator 1 and the irradiation nozzle 4 determined in advance, and the affected part 5 is irradiated with the particle beam. When the radiation dose scheduled for the day is applied to the affected area, the radiation on that day is ended.
　図２に示すように、患者位置決め制御器６１は、プロセッサー６１１、メモリ６１２、入出力インターフェース６１３、およびディスプレイ６１４を備えた計算機として実現される。同じく、ＤＲＲ画像作成演算器１１も、プロセッサー１１１、メモリ１１２、入出力インターフェース１１３、およびディスプレイ１１４を備えた計算機として実現される。すなわち、プロセッサーがメモリに記憶されたプログラムを実行することにより、患者位置決め制御器６１およびＤＲＲ画像作成演算器１１が実現される。上記のように、患者位置決め制御器６１とＤＲＲ画像作成演算器１１が同一の計算機で実現されても良い。この場合は、プロセッサー６１１とプロセッサー１１１とは同一のプロセッサであっても良く、同様に、ディスプレイなども同一のディスプレイであっても良い。 As shown in FIG. 2, the patient positioning controller 61 is implemented as a computer including a processor 611, a memory 612, an input / output interface 613, and a display 614. Similarly, the DRR image creation computing unit 11 is also implemented as a computer including the processor 111, the memory 112, the input / output interface 113, and the display 114. That is, when the processor executes the program stored in the memory, the patient positioning controller 61 and the DRR image creation computing unit 11 are realized. As described above, the patient positioning controller 61 and the DRR image creation computing unit 11 may be realized by the same computer. In this case, the processor 611 and the processor 111 may be the same processor, and similarly, the display and the like may be the same display.
　次に、ＤＲＲ画像の作成方法の詳細を説明する。ＤＲＲ画像は、計算機上で、患者の３次元ＣＴデータに仮想Ｘ線源から放射される仮想Ｘ線を透視投影して得られる仮想的な２次元のＸ線画像である。このように、ＤＲＲ画像は、実際に患者にＸ線管から放射されるＸ線を透視投影して得られる２次元Ｘ線画像を模擬する画像である。通常、レイキャスティング法と呼ばれる方法により仮想Ｘ線を３次元ＣＴデータに透過させて２次元Ｘ線画像を作成する。例えば、図１における、Ｘ線管５１ａ、被写体である患者５、およびＸ線検出器であるＦＰＤ５２ａの幾何学的配置を仮想的にＤＲＲ画像作成演算器１１により再現してＤＲＲ画像を生成する。このとき、被写体は患者の３次元ＣＴデータとして与えられ、仮想Ｘ線が通過する位置毎の３次元ＣＴデータの値であるＣＴ値による仮想Ｘ線の減衰を累積することによりＸ線検出器５２ａの位置でのＸ線強度を求めることで２次元Ｘ線画像、すなわちＤＲＲ画像が得られる Next, details of the method of creating a DRR image will be described. The DRR image is a virtual two-dimensional X-ray image obtained by perspective projection of a virtual X-ray emitted from a virtual X-ray source on three-dimensional CT data of a patient on a computer. Thus, the DRR image is an image that simulates a two-dimensional X-ray image obtained by perspective projection of X-rays actually emitted from the X-ray tube to the patient. Usually, virtual X-rays are transmitted to three-dimensional CT data by a method called ray casting method to create a two-dimensional X-ray image. For example, the DRR image creation computing unit 11 virtually reproduces the geometrical arrangement of the X-ray tube 51a, the patient 5 as the subject, and the FPD 52a as the X-ray detector in FIG. 1 to generate a DRR image. At this time, the subject is given as three-dimensional CT data of the patient, and the X-ray detector 52a is accumulated by accumulating the attenuation of the virtual X-ray by the CT value which is the value of the three-dimensional CT data for each position where the virtual X-ray passes. A two-dimensional X-ray image, that is, a DRR image can be obtained by determining the X-ray intensity at the position of
　一般に、Ｘ線管から発生されるＸ線は、エネルギーが単一の単色Ｘ線ではなく、エネルギースペクトルに分布があるＸ線である。このため、従来、ＤＲＲ画像を作成するとき、仮想Ｘ線として、エネルギースペクトル分布を有する仮想Ｘ線を用いて、３次元ＣＴデータを透過したＸ線強度を演算してＤＲＲ画像を作成していた。Ｘ線のエネルギースペクトルに分布があると、３次元ＣＴデータを透過させた場合、エネルギーにより吸収が異なるため、この効果を含んで演算する。エネルギースペクトルに分布があるＸ線が物質を透過するとき、Ｘ線のエネルギーが低いほどより吸収が大きく、減衰が大きい。したがってＸ線は物質を通過するに従いエネルギーが低い成分が少なくなり、エネルギースペクトル分布として高いエネルギーにシフトする。この効果がビームハードニングと呼ばれている。 In general, X-rays generated from an X-ray tube are not single monochromatic X-rays in energy, but X-rays distributed in the energy spectrum. Therefore, conventionally, when creating a DRR image, using a virtual X-ray having an energy spectrum distribution as a virtual X-ray, the DRR image is created by calculating an X-ray intensity transmitted through three-dimensional CT data. . If there is a distribution in the energy spectrum of the X-ray, the absorption varies depending on the energy when transmitting three-dimensional CT data, so this calculation is included including this effect. When X-rays having a distribution in the energy spectrum pass through a substance, the lower the energy of the X-rays, the larger the absorption and the larger the attenuation. Therefore, as the X-rays pass through the substance, the components with low energy decrease, and the energy spectrum distribution shifts to high energy. This effect is called beam hardening.
　従来は、Ｘ線のエネルギースペクトル分布はＸ線が透過する場所によらず同じとして、どの場所でも同じエネルギースペクトル分布のＸ線が透過するとしてビームハードニングの効果を演算していた。本発明者らは、場所によらず同じエネルギースペクトル分布のＸ線が入射して透過するとしてビームハードニングの効果を演算したのでは、Ｘ線画像を高精度に模擬するＤＲＲ画像が得られないことに気付いた。Ｘ線管５１は、図３に示すように、陰極５１２である電子源から発生される電子を陽極５１１であるターゲットに衝突させてＸ線を発生するように構成されている。ここで、電子が衝突するターゲットの位置によりターゲット外部に放射される光子としてのＸ線のエネルギーが異なる。これは、Ｘ線がターゲット内部で発生するためであり、発生したＸ線はターゲット内部を通過し、その過程でＸ線の吸収が生じ、Ｘ線の強度が低下するため、ターゲットの位置により、強度およびエネルギースペクトル分布に差が生じる。すなわち、ターゲット内部でのビームハードニングの影響により、ＦＰＤ５２に到達するＸ線は、間に被写体が無い場合でも位置によりエネルギースペクトル分布が異なる。さらに、陰極側で強度が高く、陽極側で強度が低くなるため、Ｘ線照射領域でむらが生じる。この現象はヒール効果と呼ばれている。 Conventionally, assuming that the energy spectral distribution of X-rays is the same regardless of where X-rays are transmitted, the effect of beam hardening has been calculated as X-rays of the same energy spectral distribution are transmitted anywhere. If we calculate the effect of beam hardening on the assumption that X-rays of the same energy spectral distribution are incident and transmitted regardless of location, we can not obtain DRR images that simulate X-ray images with high accuracy. I realized that. As shown in FIG. 3, the X-ray tube 51 is configured to cause electrons generated from an electron source, which is the cathode 512, to collide with a target, which is the anode 511, to generate X-rays. Here, the energy of X-rays as photons emitted to the outside of the target differs depending on the position of the target at which the electrons collide. This is because X-rays are generated inside the target, and the generated X-rays pass through the inside of the target, and absorption of X-rays occurs in the process, and the intensity of the X-rays decreases. Differences occur in the intensity and energy spectral distribution. That is, due to the influence of beam hardening inside the target, the X-rays reaching the FPD 52 have different energy spectral distributions depending on the position even if there is no object in between. Furthermore, since the intensity is high on the cathode side and low on the anode side, unevenness occurs in the X-ray irradiated area. This phenomenon is called the heel effect.
　そこで、エネルギースペクトル分布および強度、すなわちエネルギースペクトル強度分布が場所により異なる仮想Ｘ線を、３次元ＣＴデータに透過させてＤＲＲ画像を作成することを提案するのである。図４に本発明をレイキャスティングに適用した模式図を示す。ＤＲＲの生成におけるレイキャスティングは、コンピュータ上に再現したＸ線撮影系の幾何学的配置におけるＸ線管とＸ線ＣＴデータの各画素とを結ぶ光線を設定し、この光線上に複数の計算点を設定し、計算点のＣＴ値をＸ線管から計算点までの累積ＣＴ値に基づいて補正し、補正後の各計算点のＣＴ値用いて前記光線上に位置する計算点のＣＴ値を累積し、累積されたＣＴ値に基づいてＤＲＲ画像を作成する手法である。 Therefore, it is proposed to transmit a virtual X-ray having different energy spectrum distribution and intensity, that is, energy spectrum intensity distribution depending on the location, to three-dimensional CT data to create a DRR image. FIG. 4 shows a schematic diagram in which the present invention is applied to ray casting. Ray casting in the generation of DRR sets a ray connecting the X-ray tube and each pixel of the X-ray CT data in the geometrical arrangement of the X-ray imaging system reproduced on a computer, and a plurality of calculation points on this ray Is set, the CT value of the calculation point is corrected based on the accumulated CT value from the X-ray tube to the calculation point, and the CT value of the calculation point located on the ray is calculated using the CT value of each calculation point after correction. This is a method of creating a DRR image based on the accumulated and accumulated CT values.
　X線の減弱は各物質の減弱係数と距離によって決まる。ＣＴ値は所定エネルギーにおける水の線減弱係数を基準とした相対値となっており、ＣＴ値から各物質の線減弱係数を求めることができる。X線の減弱は下記式（１）で表される。
Figure JPOXMLDOC01-appb-M000001
 ここで、Ｉは物質の透過後のX線強度、Ｉ０は入射X線強度、ｄｉは各計算点においてX線の通過する距離、μｉは各計算点における物質の線減弱係数である。 The attenuation of x-rays is determined by the attenuation coefficient and distance of each material. The CT value is a relative value based on the linear attenuation coefficient of water at a predetermined energy, and the linear attenuation coefficient of each substance can be determined from the CT value. The attenuation of X-rays is expressed by the following formula (1).
Figure JPOXMLDOC01-appb-M000001
 Here, I is the X-ray intensity after transmission of the material, I 0 is the incident X-ray intensity, d i is the X-ray passing distance at each calculation point, and μ i is the linear attenuation coefficient of the material at each calculation point .
　ＤＲＲを生成する際、ｄｉは各計算点におけるＣＴ画像のボクセルサイズであり、画像に応じた固定値となる。μｉは各計算点におけるＣＴから求めた線減弱係数となるため、上記ΣはＣＴ値の累積値とも見なすことができる。また、診断Ｘ線は単一エネルギーではなく、エネルギースペクトルに幅を持っており、入射Ｘ線強度はエネルギーの関数となる。また、線源弱係数はＸ線のエネルギーに依存するエネルギー関数である。 When generating DRR, d i is the voxel size of the CT image at each calculation point, which is a fixed value according to the image. Since μ i is a linear attenuation coefficient obtained from CT at each calculation point, Σ can also be regarded as an accumulated value of CT values. Also, diagnostic x-rays are not single energy, but have a width in the energy spectrum, and incident x-ray intensity is a function of energy. Also, the source weak coefficient is an energy function dependent on the energy of X-rays.
　本発明では、このＸ線強度Ｉを求めるときに、各計算点での組織におけるＸ線のエネルギースペクトル分布を考慮した線減弱係数を与え、いわゆるビームハードニングの影響が、検出器であるＦＰＤにＸ線が到達する位置により異なることを模擬してＤＲＲ画像を作成する。全ての計算点で、それぞれＸ線のエネルギースペクトル分布を考慮した線減弱係数を与えて演算することで精度良くＸ線強度が求まる。すなわち、ＦＰＤの検出面の位置での２次元座標を（ｘ,ｙ）とすると、ＦＰＤの検出面における各位置のＸ線強度Ｉ（ｘ,ｙ）は、その位置に到達するＸ線のエネルギー毎のＸ線強度を積算した値となるため、エネルギーで積分すれば良い。位置（ｘ,ｙ）に到達する仮想Ｘ線のエネルギースペクトル分布をＥ（ｘ,ｙ）と表す。そのとき、各計算点におけるＸ線強度も線減弱係数もエネルギーに対応した値となるので、Ｉ（ｘ,ｙ）は次の式（２）のようにして演算できる。
Figure JPOXMLDOC01-appb-M000002
 In the present invention, when this X-ray intensity I is determined, a linear attenuation coefficient taking into consideration the energy spectral distribution of X-rays in the tissue at each calculation point is given, and the influence of so-called beam hardening gives FPD as a detector. A DRR image is created by simulating the difference depending on the position reached by the X-ray. The X-ray intensity can be accurately determined by giving linear attenuation coefficients in consideration of the energy spectral distribution of X-rays at all calculation points. That is, assuming that the two-dimensional coordinate at the position of the detection surface of FPD is (x, y), the X-ray intensity I (x, y) of each position on the detection surface of FPD is the energy of the X-ray reaching that position Since the value is obtained by integrating each X-ray intensity, it may be integrated by energy. The energy spectral distribution of the virtual X-ray reaching the position (x, y) is represented as E (x, y). At that time, since both the X-ray intensity and the linear attenuation coefficient at each calculation point have values corresponding to the energy, I (x, y) can be calculated as in the following equation (2).
Figure JPOXMLDOC01-appb-M000002
　例えば、式（２）をそのまま演算することにより、ＦＰＤの検出面における各位置でのＸ線強度Ｉが求まる。仮想Ｘ線により３次元ＣＴデータで表される被写体を透視したときのＦＰＤの検出面におけるによるＸ線強度の分布が求まるので、このＸ線強度分布によりＤＲＲ画像を得ることができる。 For example, the X-ray intensity I at each position on the detection surface of the FPD can be obtained by operating the equation (2) as it is. Since a distribution of X-ray intensities by the detection plane of the FPD when a subject represented by three-dimensional CT data is viewed through virtual X-rays can be determined, a DRR image can be obtained from this X-ray intensity distribution.
　以上の説明をまとめると、図５に示すフロー図のようになる。まず、図６に示すように、実際に粒子線照射するときに使用するＸ線管５１からＸ線を発生させて、ＦＰＤ５２の面における、複数の２次元位置でのＸ線スペクトルを、スペクトル検出器５２２により測定し、図３で示したような、Ｘ線エネルギースペクトル強度分布データを各２次元位置について取得する（ステップＳＴ１）。通常、位置決めには、図１で示すように、直交する２方向からＸ線を照射して２つのＸ線画像を取得し、それぞれのＸ線画像に対応したＤＲＲ画像が必要となる。このため、Ｘ線管５１ａとＦＰＤ５２ａにより取得するＸ線画像、Ｘ線管５１ｂとＦＰＤ５２ｂにより取得するＸ線画像、および、それぞれに対応したＤＲＲ画像を作成する必要がある。したがって、Ｘ線管５１ａから発生させたＸ線によるＦＰＤ５２ａの位置における２次元のＸ線エネルギースペクトル強度分布データ、およびＸ線管５１ｂから発生させたＸ線によるＦＰＤ５２ｂの位置における２次元のＸ線エネルギースペクトル強度分布データの、２つの２次元のＸ線エネルギースペクトル強度分布データを取得する。 It will become like the flowchart shown in FIG. 5 if the above description is put together. First, as shown in FIG. 6, X-rays are generated from the X-ray tube 51 used when actually irradiating particle beams, and X-ray spectra at a plurality of two-dimensional positions on the surface of the FPD 52 are detected. The X-ray energy spectrum intensity distribution data as shown in FIG. 3 is obtained by using the measuring unit 522 for each two-dimensional position (step ST1). Usually, as shown in FIG. 1, for positioning, X-rays are irradiated from two orthogonal directions to acquire two X-ray images, and DRR images corresponding to the respective X-ray images are required. Therefore, it is necessary to create an X-ray image acquired by the X-ray tube 51a and the FPD 52a, an X-ray image acquired by the X-ray tube 51b and the FPD 52b, and a DRR image corresponding to each. Therefore, two-dimensional X-ray energy spectrum intensity distribution data at the position of FPD 52a by X-rays generated from X-ray tube 51a, and two-dimensional X-ray energy at the position of FPD 52b by X-rays generated from X-ray tube 51b Two two-dimensional X-ray energy spectral intensity distribution data of spectral intensity distribution data are acquired.
　次に、測定した各位置におけるＸ線エネルギースペクトル強度分布に基づいて、仮想Ｘ線源からＦＰＤ５２の検出面に到達する仮想Ｘ線のＸ線エネルギースペクトル強度分布を、ＦＰＤ５２の検出面の２次元位置毎に設定する（ステップＳＴ２）。さらに、それぞれの仮想Ｘ線毎に、３次元ＣＴデータの計算点（式（２）のｉ）を設定する（ステップＳＴ３）。３次元ＣＴデータを用いて、各仮想Ｘ線のエネルギーに対応した、計算点毎の線減弱係数μｊ（Ｅ（ｘ,ｙ））を設定して式（２）によりＦＰＤの検出面における各位置のＸ線強度Ｉ（ｘ,ｙ）を演算して求める（ステップＳＴ４）。求めたＸ線強度Ｉ（ｘ,ｙ）に基づいてＤＲＲ画像を構成する（ステップＳＴ５）。 Next, based on the measured X-ray energy spectrum intensity distribution at each position, the X-ray energy spectrum intensity distribution of virtual X-rays reaching the detection plane of the FPD 52 from the virtual X-ray source It sets every (step ST2). Further, calculation points (i in equation (2)) of three-dimensional CT data are set for each virtual X-ray (step ST3). The linear attenuation coefficient μ j (E (x, y)) for each calculation point corresponding to the energy of each virtual X-ray is set using three-dimensional CT data, and each of the detection surfaces of the FPD is The X-ray intensity I (x, y) of the position is calculated and obtained (step ST4). A DRR image is constructed based on the determined X-ray intensity I (x, y) (step ST5).
　以上により、Ｘ線発生管がＸ線を放射する方向によりＸ線エネルギースペクトル強度分布が異なることによる、被写体（３次元ＣＴデータ）におけるビームハードニング効果の違いを含んだＤＲＲ画像を構成することができ、実際に取得するＸ線画像により近いＤＲＲ画像を得ることができる。 According to the above, it is possible to construct a DRR image including a difference in beam hardening effect in a subject (three-dimensional CT data) due to the difference in X-ray energy spectrum intensity distribution depending on the direction in which the X-ray tube emits X-rays. It is possible to obtain a DRR image closer to the X-ray image actually acquired.
実施の形態２．
　実施の形態１で説明した方法ではＦＰＤの検出面に対応した各２次元位置ごとの全エネルギースペクトルの積分計算を行う必要があり、演算量が多くなる。そこで、入射のＸ線のエネルギースペクトル分布から実効エネルギーを算出し、ＦＰＤの各位置（ｘ,ｙ）毎にＸ線が単色Ｘ線であるとみなし、Ｘ線の強度を一意のエネルギーにおける位置の関数、または基準のＸ線強度、に対する補正係数Ｋ（ｘ,ｙ）とすることで、エネルギースペクトル分布の積分をなくし演算量を少なくできる。ここで、実効エネルギーをX線のエネルギーを平均化したものとすると、Ｘ線管内においてビームハードニング、すなわちヒール効果が生じると実効エネルギーは大きくなり、実効エネルギーの違いでヒール効果を表すことができる。 Second Embodiment
In the method described in the first embodiment, it is necessary to perform integral calculation of the entire energy spectrum for each two-dimensional position corresponding to the detection surface of the FPD, and the amount of calculation increases. Therefore, the effective energy is calculated from the energy spectrum distribution of the incident X-ray, and the X-ray is regarded as a monochromatic X-ray for each position (x, y) of the FPD, and the X-ray intensity is a position at a unique energy. By using the correction coefficient K (x, y) for the function or the reference X-ray intensity, the integration of the energy spectral distribution can be eliminated and the amount of operation can be reduced. Here, assuming that the effective energy is an average of X-ray energy, beam hardening in the X-ray tube, that is, when the heel effect occurs, the effective energy becomes large, and the heel effect can be expressed by the difference in effective energy. .
　式（２）を上記で説明した補正係数Ｋ（ｘ,ｙ）を用いた式に変換すると式（３）となる。
Figure JPOXMLDOC01-appb-M000003
 When the equation (2) is converted into the equation using the correction coefficient K (x, y) described above, the equation (3) is obtained.
Figure JPOXMLDOC01-appb-M000003
　測定したＸ線エネルギースペクトル強度分布データを用いて、ＦＰＤ５２の位置に到達するＸ線のスペクトルの違いによる補正係数Ｋ（ｘ,ｙ）を演算により求める。Ｘ線エネルギースペクトル強度分布データは、２次元で取得する必要があるが、緻密に多くの位置で取得する必要はない。例えば、図８に、合計９か所でＸ線エネルギースペクトル強度分布データを取得し、それぞれの位置での補正係数を求めた結果をマップに記入した例を示す。Ｘ線エネルギースペクトル強度分布データが無い位置での補正係数は、Ｘ線エネルギースペクトル強度分布データが有る位置において演算で求めた補正係数を用いて補間により求めることができる。補間により求めた補正係数を含めて、補正係数マップとして記憶しておく。補正係数マップは、ＦＰＤ５２の２次元の小領域毎に値があるマップとして記憶する。レイキャスティングにより求めた各位置におけるＸ線強度Ｉに、該当する位置が含まれる領域の補正係数を乗ずることで、位置決め時に取得する実際のＸ線画像に、より近いＤＲＲ画像を得ることができる。 Using the measured X-ray energy spectrum intensity distribution data, the correction coefficient K (x, y) due to the difference in the spectrum of the X-rays reaching the position of the FPD 52 is obtained by calculation. X-ray energy spectrum intensity distribution data needs to be acquired in two dimensions, but it is not necessary to acquire it at precisely many positions. For example, FIG. 8 shows an example in which X-ray energy spectrum intensity distribution data are acquired at a total of nine places, and the correction coefficients at each position are found in a map. The correction coefficient at the position where there is no X-ray energy spectrum intensity distribution data can be obtained by interpolation using the correction coefficient obtained by calculation at the position where there is X-ray energy spectrum intensity distribution data. It is stored as a correction coefficient map including the correction coefficient obtained by interpolation. The correction coefficient map is stored as a map having a value for each two-dimensional small area of the FPD 52. By multiplying the X-ray intensity I at each position obtained by ray casting by the correction coefficient of the region including the corresponding position, it is possible to obtain a DRR image closer to the actual X-ray image acquired at the time of positioning.
　ここで、補正係数の求め方を含めて、ＤＲＲ画像を得るまでの工程の具体例を、図９のフロー図に示す。補正係数は、ＦＰＤ上の所定の測定点で測定したエネルギースペクトル分布(ＳＴ１１)から求めた実効エネルギー（Ｘ線のネルギーを平均化したもので、Ｘ線管内でビームハードニング、すなわちヒール効果が生じると実効エネルギーは大きくなる）として一意に求め、所定位置におけるエネルギーを基準とした相対値として、各補正係数を定める。例えばヒール効果が大きい箇所を１とした場合に、小さい場所を０．６とするなどして補正係数を決定する。(ＳＴ１２)。測定箇所以外の部分は補間により、２次元位置での補正係数Ｋ(ｘ,ｙ)を求める(ＳＴ１３)。求めた補正係数を基にＤＲＲの生成を行う(ＳＴ１４～ＳＴ１８)。ＤＲＲ生成時の被写体に対する入射Ｘ線に対して、ヒール効果を考慮し、補正係数Ｋ(ｘ，ｙ)を乗じた入射Ｘ線とする(ＳＴ１５)。被写体内を通る各計算点においては、キャリブレーション時に設定されたエネルギーに対して設定された元のＣＴ値に対して、あらかじめ文献値または、シミュレーションや実測により取得していた入射Ｘ線の実効エネルギーに対するＣＴ値に変換する(ＳＴ１６)。光軸上の各計算点で変換したＣＴ値を累積することにより式（３）のΣ（ｄｉμｉ）を求め（ＳＴ１７）、２次元座標でマッピングし、ＤＲＲを生成する(ＳＴ１８)。 Here, a specific example of the process up to obtaining a DRR image including how to determine the correction coefficient is shown in the flowchart of FIG. The correction factor is the effective energy (X-ray energy averaged) obtained from the energy spectrum distribution (ST11) measured at a predetermined measurement point on the FPD, and beam hardening, ie, the heel effect occurs in the X-ray tube. The effective energy increases uniquely), and each correction coefficient is determined as a relative value based on the energy at a predetermined position. For example, when the portion where the heel effect is large is 1, the correction coefficient is determined by setting the small portion as 0.6. (ST12). The correction coefficient K (x, y) at the two-dimensional position is determined by interpolation for the portion other than the measurement point (ST13). Based on the determined correction coefficient, DRR is generated (ST14 to ST18). An incident X-ray obtained by multiplying the correction coefficient K (x, y) with respect to an incident X-ray with respect to an object at the time of DRR generation in consideration of the heel effect is determined (ST15). At each calculation point passing through the inside of the subject, the document value or the effective energy of the incident X-ray acquired in advance by simulation or measurement with respect to the original CT value set to the energy set at the time of calibration Converted to a CT value for (ST16). By accumulating the CT values converted at each calculation point on the optical axis, Σ (d i μ i ) in equation (3) is determined (ST17), and mapping is performed using two-dimensional coordinates to generate DRR (ST18).
　なお、本発明は、その発明の範囲内において、各実施の形態を組み合わせたり、実施の形態を適宜、変形、省略したりすることが可能である。 In the present invention, within the scope of the invention, it is possible to combine each embodiment or to appropriately modify or omit the embodiments.
１０　治療計画装置、１１　ＤＲＲ画像作成演算器、５０　Ｘ線撮像装置、５００　３次元ＣＴデータ、５１ａ、５１ｂ　Ｘ線管、５１０　仮想Ｘ線管、６０　患者位置決め装置 Reference Signs List 10 treatment planning device, 11 DRR image formation computing unit, 50 X-ray imaging device, 500 three-dimensional CT data, 51a, 51b X-ray tube, 510 virtual X-ray tube, 60 patient positioning device

Claims (5)

  1. 　粒子線を照射する照射対象である患者の内部の３次元ＣＴデータに仮想Ｘ線管から放射される仮想Ｘ線を透視投影することにより、前記患者にＸ線管から放射されるＸ線を透視投影して得られる２次元Ｘ線画像を模擬する２次元のＤＲＲ画像を作成するＤＲＲ画像作成演算器を備え、このＤＲＲ画像作成演算器は前記３次元ＣＴデータに対して、前記仮想Ｘ線のエネルギースペクトル強度分布に基づくビームハードニング効果を含んで、前記仮想Ｘ線を透視投影することにより前記ＤＲＲ画像を作成する粒子線治療装置において、
    前記ＤＲＲ画像作成演算器は、前記Ｘ線管から放射されるＸ線が放射方向により異なるエネルギースペクトル強度分布を有することに基づいて、前記仮想Ｘ線のエネルギースペクトル強度分布を、前記ＤＲＲ画像内の位置毎に設定して前記ＤＲＲ画像を作成することを特徴とする粒子線治療装置。     By perspectively projecting virtual X-rays emitted from a virtual X-ray tube onto three-dimensional CT data inside a patient to be irradiated that emits particle beams, X-rays emitted from the X-ray tube are transmitted to the patient The DRR image creation computing unit is configured to create a two-dimensional DRR image simulating a two-dimensional X-ray image obtained by projection, and the DRR image creation calculator is configured to generate the virtual X-ray with respect to the three-dimensional CT data. A particle beam therapy system for producing the DRR image by perspective projection of the virtual X-ray, including a beam hardening effect based on energy spectrum intensity distribution,
    The DRR image creation computing unit determines an energy spectral intensity distribution of the virtual X-ray within the DRR image based on the fact that the X-ray emitted from the X-ray tube has an energy spectral intensity distribution different depending on the radiation direction. A particle beam therapy system characterized in that the DRR image is created by setting for each position.
  2. 　前記ＤＲＲ画像作成演算器は、前記ＤＲＲ画像内の位置毎に設定した前記仮想Ｘ線のエネルギースペクトル強度分布に対応した補正係数を、前記ＤＲＲ画像内の位置毎に設定して、前記補正係数を用いて前記仮想Ｘ線が透視投影されたＸ線強度を演算して前記ＤＲＲ画像を作成することを特徴とする請求項１に記載の粒子線治療装置。 The DRR image creation computing unit sets a correction coefficient corresponding to the energy spectrum intensity distribution of the virtual X-ray set for each position in the DRR image for each position in the DRR image, and sets the correction coefficient. The particle beam therapy system according to claim 1, wherein the DRR image is created by calculating an X-ray intensity obtained by perspective projection of the virtual X-ray using the virtual X-ray.
  3. 　前記患者の位置と前記粒子線の位置とを合わせるための患者位置決め装置を備え、前記患者位置決め装置は、前記Ｘ線管により前記患者にＸ線を照射して得られる前記２次元Ｘ線画像と、前記ＤＲＲ画像作成演算器により作成された前記ＤＲＲ画像とを比較して、患者を位置決めすることを特徴とする請求項１または２に記載の粒子線治療装置。 A patient positioning device for aligning the position of the patient with the position of the particle beam, the patient positioning device comprising the two-dimensional x-ray image obtained by irradiating the patient with the x-ray by the x-ray tube; The particle beam therapy system according to claim 1 or 2, wherein a patient is positioned by comparing the DRR image created by the DRR image creation computing unit.
  4. 　粒子線を照射する照射対象である患者の内部の３次元ＣＴデータに仮想Ｘ線管から放射される仮想Ｘ線を透視投影することにより、前記患者にＸ線管から放射されるＸ線を照射して得られる２次元Ｘ線画像を模擬する２次元のＤＲＲ画像を作成するＤＲＲ画像作成方法であって、
    前記Ｘ線管から放射されるＸ線のエネルギースペクトル強度分布を、前記２次元Ｘ線画像を得る位置における複数の２次元位置で測定するステップと、
    前記仮想Ｘ線が、前記複数の２次元位置で測定されたX線のエネルギースペクトル強度分布に基づいて、前記仮想Ｘ線の前記２次元位置毎のＸ線のエネルギースペクトル強度分布を設定することにより、前記３次元ＣＴデータに対して、前記仮想Ｘ線のエネルギースペクトル強度分布に基づくビームハードニング効果を含んで、前記仮想Ｘ線を透視させることにより前記ＤＲＲ画像を作成するステップと、を含んだことを特徴とするＤＲＲ画像作成方法。     Irradiating the patient with X-rays emitted from the X-ray tube by perspectively projecting virtual X-rays emitted from the virtual X-ray tube onto the three-dimensional CT data inside the patient to be irradiated with particle beams Method for creating a two-dimensional DRR image simulating a two-dimensional X-ray image obtained by
    Measuring an energy spectral intensity distribution of X-rays emitted from the X-ray tube at a plurality of two-dimensional positions at positions where the two-dimensional X-ray image is obtained;
    By setting the energy spectrum intensity distribution of the X-rays for each of the two-dimensional positions of the virtual X-rays, based on the energy spectrum intensity distribution of the X-rays measured at the plurality of two-dimensional positions. Creating a DRR image by causing the virtual X-ray to see through the three-dimensional CT data, including a beam hardening effect based on the energy spectrum intensity distribution of the virtual X-ray DRR image creation method characterized in that.
  5. 前記仮想Ｘ線を透視させることにより前記ＤＲＲ画像を作成するステップにおいて、前記複数の２次元位置で測定されたエネルギースペクトル強度分布に基づいて、前記２次元位置毎に予め設定した補正係数を用いて前記仮想Ｘ線が透視投影されたＸ線強度を演算することにより前記ＤＲＲ画像を作成することを特徴とする請求項４に記載のＤＲＲ画像作成方法。 In the step of creating the DRR image by making the virtual X-ray see through, using a correction coefficient preset for each of the two-dimensional positions, based on the energy spectrum intensity distribution measured at the plurality of two-dimensional positions. The DRR image creation method according to claim 4, wherein the DRR image is created by calculating an X-ray intensity obtained by perspective projection of the virtual X-ray.
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