JP2008119095A - X-ray ct system and scattering correction method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable the correction of a high-precision scattering according to the size of a specimen or the like. <P>SOLUTION: An X-ray tube and an X-ray detector are arranged facing each other sandwiching the specimen to reconstruct a CT image from a projection data obtained by scanning the subject. The X-ray CT system is provided with a means for determining the transmission length of X rays transmitted from the projection data, a means which determines the angle of scattering of scattered rays entering respective X-ray detection surfaces from the transmission length previously determined and the positional relationship with the X-ray detector, a means which determines differentiated sectional areas of the respective scattered rays from information by which the relationship is previously regulated between the angle of scattering determined and the differentiated sectional areas of scattering according to the angle of scattering, a means which determines a scattering coefficient indicating the ratio of the intensity of the scattered rays with respect to the intensity of the X rays transmitted from the differentiated sectional areas determined and a means which carries out the correction of scattering in the projection data from the intensities of X rays detected on the respective X-ray detection surfaces and the scattering coefficients determined corresponding to the respective X rays transmitted. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an X-ray CT apparatus and a scatter correction method. More specifically, the present invention includes an X-ray tube and an X-ray detector facing each other with a subject interposed therebetween, and a CT tomogram is reproduced based on projection data obtained by scanning the subject. The present invention relates to an X-ray CT apparatus and an X-ray scattering correction method thereof.

  X-rays are scattered in all directions when passing through the subject, and the X-ray detector also observes X-rays from directions other than the transmission lines (direct lines) due to the scattered lines. Since the line deteriorates the CT image, it is necessary to remove the influence of the scattered radiation.

Conventionally, a predetermined reference phantom is scanned by adjusting a collimator so that X-rays (direct rays) are sequentially incident on each row of a multi-row detector, and scattered ray correction data is obtained based on the obtained projection data. There is known an X-ray CT system that creates and corrects scattered radiation of projection data obtained by scanning a subject using the correction data (Patent Document 1).
JP 2005-46199 A

  However, in practice, since the size (physique, site) and internal tissue of the subject vary, the type, amount, and direction of scattered radiation also vary from subject to subject. Therefore, with the conventional method using the scattered radiation correction coefficient obtained with the reference phantom as it is, more accurate scattered radiation correction cannot be performed.

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide an X-ray CT apparatus and a scatter correction method capable of efficiently performing scatter correction with higher accuracy according to the subject size and the like. There is to do.

  An X-ray CT apparatus according to a first aspect of the present invention includes an X-ray tube and an X-ray detector facing each other with a subject interposed therebetween, and reconstructs a CT tomogram based on projection data obtained by scanning the subject. In the line CT apparatus, the transmission length calculation means for determining the transmission length of transmitted X-rays based on the projection data, and the scattered radiation incident on each X-ray detection surface based on the positional relationship between the determined transmission length and the X-ray detector A scattering angle calculating means for obtaining the scattering angle of the differential angle, and a differential cross-sectional area calculating means for obtaining a differential cross-sectional area of each scattered ray based on information that preliminarily defines a relationship between the scattering angle and the differential cross-sectional area of the scattering by the obtained scattering angle Scattering coefficient calculation means for obtaining a scattering coefficient representing the ratio of scattered radiation intensity to transmitted X-ray intensity based on the obtained differential cross-sectional area, X-ray intensity detected on each X-ray detection surface, and corresponding to each transmitted X-ray Based on the scattering coefficient In which and a correcting means for performing scatter correction of the shadow data.

  In the present invention, the transmission length reflecting the subject size and the internal tissue can be obtained by the configuration for obtaining the transmission length of the transmitted X-ray based on the projection data of the subject. In addition, the scattering angle (scattering ray) according to the actual projection is obtained by obtaining the scattering angle of the scattered radiation incident on each X-ray detection surface based on the positional relationship between the transmission length reflecting the subject and the X-ray detector. Can be extracted. In addition, with the configuration for obtaining the differential cross-sectional area (scattering coefficient) of scattering at this scattering angle, more accurate scattered radiation can be extracted according to the subject size (physique, site) and internal tissue, and fine scattering correction can be performed.

An X-ray CT apparatus according to a second aspect of the present invention includes an X-ray tube and an X-ray detector facing each other with a subject interposed therebetween, and reconstructs a CT tomogram based on projection data obtained by scanning the subject. In the line CT apparatus, the transmission length calculation means for determining the transmission length of transmitted X-rays based on the projection data, and the scattered radiation incident on each X-ray detection surface based on the positional relationship between the determined transmission length and the X-ray detector The scattering angle calculation means for obtaining the scattering angle of the light source, and the differential for obtaining the differential cross-sectional area for each type of scattered radiation based on the information defining the relationship between the scattering angle and the differential cross-sectional area of the scattering for each scattering type in advance by the obtained scattering angle A cross-sectional area calculating means, a photon energy calculating means for obtaining a photon energy of transmitted X-rays based on information that preliminarily defines a relationship between an X-ray transmission length and an X-ray photon energy transmitted through the section by the determined transmission length; Above Scattering ratio calculation means for obtaining the ratio of the scattering cross section for each type generated in the transmission based on the information preliminarily defining the relationship between the photon energy and the scattering cross section for each type of scattering based on the calculated photon energy; Scattering coefficient calculation means for obtaining a scattering coefficient representing the ratio of the scattered radiation intensity to the transmitted X-ray intensity based on the differential sectional area synthesized by the ratio of the scattering sectional area for each type obtained above, and detected by each X-ray detection surface Correction means for correcting the scattering of the projection data based on the X-ray intensity and the scattering coefficient obtained corresponding to each transmitted X-ray.

  In the present invention, it is possible to extract the scattering properties such as coherent (Rayleigh) scattering and incoherent (Compton) scattering, which are suitable for obtaining a differential sectional area for each type of scattering based on the obtained scattering angle. In addition, the actual scattering situation (property) is more faithfully constructed by combining the differential cross sections by type with the ratio of the cross sections by type determined based on the photon energy of transmitted X-rays in line with the actual scan. The reflected scattering coefficient can be obtained.

  In the third aspect of the present invention, the correction means is obtained corresponding to the X-ray intensity detected in each X-ray detection row arranged in the body axis direction of the subject and the transmitted X-rays arranged in the body axis direction. Based on each scattering coefficient, scattering correction of projection data in the X-ray detection row direction is performed. The present invention is suitable for application to scattering correction in the direction of the subject body axis.

  In the fourth aspect of the present invention, the correcting means includes the X-ray intensity detected on each X-ray detection surface arranged in the channel direction, and each scattering coefficient obtained corresponding to each transmitted X-ray arranged in the channel direction. Based on the above, scatter correction of projection data in the channel direction is performed. The present invention is suitable for application to scatter correction in the channel direction of an X-ray detector.

  A scattering correction method according to a fifth aspect of the present invention includes an X-ray tube and an X-ray detector facing each other with a subject interposed therebetween, and reconstructs a CT tomographic image based on projection data obtained by scanning the subject. A method of correcting a scattering of a CT apparatus, wherein a transmission length of transmitted X-rays is obtained based on the projection data, and is incident on each X-ray detection surface based on a positional relationship between the obtained transmission length and the X-ray detector. A step of obtaining a scattering angle of the scattered radiation, a step of obtaining a differential sectional area of each scattered radiation based on information in which a relationship between the scattering angle and the differential sectional area of the scattering is previously defined by the obtained scattering angle, and the obtained differential breaking A step of obtaining a scattering coefficient representing a ratio of the scattered radiation intensity to the transmitted X-ray intensity based on the area; an X-ray intensity detected on each X-ray detection surface; and a scattering coefficient determined corresponding to each transmitted X-ray Projection data scattering based on In which and a correcting step of performing positive.

A scatter correction method according to a sixth aspect of the present invention includes an X-ray tube and an X-ray detector facing each other with a subject interposed therebetween, and reconstructs a CT tomogram based on projection data obtained by scanning the subject. A method of correcting a scattering of a CT apparatus, wherein a transmission length of transmitted X-rays is obtained based on the projection data, and is incident on each X-ray detection surface based on a positional relationship between the obtained transmission length and the X-ray detector. A step of obtaining a scattering angle of the scattered radiation, and a step of obtaining a differential sectional area for each type of scattered radiation based on information in which the relationship between the scattering angle and the differential sectional area of the scattering is defined in advance for each scattering type based on the obtained scattering angle; Determining the photon energy of transmitted X-rays based on information that preliminarily defines the relationship between the X-ray transmission length and the photon energy of X-rays transmitted through the section based on the determined transmission length; A step of obtaining a ratio of a scattering cross section by type generated by the transmission based on information preliminarily defining a relationship between a photon energy and a scattering cross section for each type of scattering, and the obtained differential cross section by type according to the obtained type. A step of obtaining a scattering coefficient representing the ratio of the scattered radiation intensity to the transmitted X-ray intensity based on the differential sectional area synthesized by the ratio of the scattering sectional area, the X-ray intensity detected on each X-ray detection surface, and each transmitted X-ray And a correction step for correcting the scatter of the projection data based on the scattering coefficient obtained corresponding to the above.

  In the seventh aspect of the present invention, the correction step is obtained corresponding to the X-ray intensity detected in each X-ray detection row arranged in the body axis direction of the subject and each transmitted X-ray arranged in the body axis direction. Based on each scattering coefficient, scattering correction of projection data in the X-ray detection row direction is performed.

  In the eighth aspect of the present invention, the correction step includes the X-ray intensity detected on each X-ray detection surface arranged in the channel direction, and each scattering coefficient obtained corresponding to each transmitted X-ray arranged in the channel direction. Based on the above, scatter correction of projection data in the channel direction is performed.

  As described above, according to the present invention, it is possible to perform more accurate correction of scattered radiation according to the subject, which greatly contributes to improving the performance and reliability of X-ray CT imaging.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings. Note that the same reference numerals denote the same or corresponding parts throughout the drawings. FIG. 1 is a configuration diagram of an X-ray CT apparatus 200 according to an embodiment. This X-ray CT apparatus has an imaging table 10 on which an object is placed and moved in the body (z) axis direction, and an object using an X-ray cone beam. A scanning gantry 20 that collects data by the axial / helical scan of the above, and an operation console 1 that performs remote control of the imaging table 10 and the scanning gantry 20 and that allows the operator to perform various setting operations.

  An operation console 1 collects projection data acquired by an input device 2 that receives an input from an operator, a central processing unit (CPU) 3 that performs scattering correction processing and image reconstruction processing according to the present invention, and a scanning gantry 20. A data acquisition buffer 5, a monitor 6 that displays a CT tomogram reconstructed based on projection data, and a storage device 7 that stores various programs, data, and X-ray CT images for realizing the functions of the apparatus. . In addition, the imaging table 10 includes a drive mechanism unit and a top plate (cradle) 12 on which a subject is placed and put into and out of a bore (cavity) of the scanning gantry 20.

  Further, the scanning gantry 20 controls the X-ray tube 21, the X-ray controller 22 that controls the tube voltage and tube current of the X-ray tube 21, and the thickness (slice thickness) of the X-ray beam in the z-axis direction. A collimator 23, a multi-row detector 24 that can simultaneously acquire X-ray projection data for a plurality of rows, a data acquisition device DAS (Data Acquisition System) 25 that collects projection data for each row, an X-ray tube 21 and a multi-row Control signals are exchanged between the operation console 1 and the imaging table 10, the rotation unit 15 that supports the row detector 24 and the like so as to be rotatable around the subject body axis, the rotation unit controller 26 that controls the rotation unit 15. And a controller 29.

  FIG. 2 is an external perspective view illustrating an X-ray imaging system according to the embodiment. The X-ray tube 21 and the multi-row detector 24 are provided opposite to each other, and these are supported so as to be rotatable around the subject body axis (z axis). The multi-row detector 24 includes about 1000 X-ray detection elements in the channel (x-axis) direction, and these are arranged in a plurality of rows (eight rows in the illustrated example) in the row R (z-axis) direction.

Collection of projection data by such a configuration is performed as follows. With the subject positioned in the cavity of the scanning gantry 20, the subject is irradiated with the X-ray beam from the X-ray tube 21. In this state, the X-ray beam from the X-ray tube 21 passes through the subject and enters the detector rows of the multi-row detector 24 all at once. The data collection unit 25 generates projection data corresponding to each detector row of the multi-row detector 24 and stores these in the data collection buffer 5. Further, the same X-ray projection is performed at each view angle i where the gantry 15 is slightly rotated, and thus projection data for one rotation of the gantry is collected and accumulated. At the same time, the imaging table 10 is moved intermittently / continuously in the direction of the body axis according to the axial / helical scan method, thus collecting and accumulating all projection data for the required imaging area of the subject. Then, the CPU 3 reconstructs a CT tomogram of the subject by performing scatter correction described later based on projection data obtained in parallel with the scan after the completion of all the scans, and displays this on the monitor 6. .

  FIGS. 3 and 4 are diagrams (1) and (2) for explaining various tables according to the embodiment, and show physical relationships between various tables used in X-ray scattering correction described later. FIG. 3A shows the relationship between the photon energy of X-rays transmitted through the subject (eg, representative of water) and the scattering cross-section (area display of the probability that the photons are scattered) σ. Medical X-rays are polychromatic X-rays having an energy distribution of about 0.1 to several MeV. If the energy distribution is divided, the probability (cross-sectional area) of each type of scattering that occurs correspondingly is calculated. Is done. The main scattering observed in this energy region includes photoelectric effect (photoelectric absorption), Rayleigh scattering (coherent scattering), and Compton scattering (incoherent scattering). In general, the cross-sectional area of photoelectric absorption is dominant for low energy photons of 0.1 MeV or less, and this is deeply related to attenuation of transmitted X-rays and hardening of the radiation. In addition, coherent scattering is dominant for the low energy photons, and the coherent scattering rapidly decreases as the energy increases. In the subsequent energy region (0.1 <hν <1 MeV), incoherent Compton scattering becomes dominant, and the scattering gradually decreases as the energy increases.

  In this embodiment, a large number of scattering experiments are performed in advance, or a large amount of well-known experimental data is used, and these are statistically processed, so that the relationship between the photon energy and each type of scattering cross section is determined in advance. It is possible to determine a scattering cross section σ for each type by creating a table of prescribed photon energy-scattering cross sections and referring to this table by photon energy.

  FIG. 3B shows the relationship between the transmission length of transmitted X-rays transmitted through the subject and the photon energy. When the specimen size is small and the transmission length is short, the attenuation of photon energy is relatively gradual. However, when the specimen size is large and the transmission length is long, the photon energy generated in the section decreases and photoelectric absorption occurs. The effect also increases and the photon energy decays rapidly. In the present embodiment, in the same manner as described above, a transmission length-photon energy table is created in advance, and by referring to this table by the transmission length of the X-rays transmitted through the subject, the type of scattering in the section can be determined. It is possible to determine the photon energy that characterizes the proportion.

  FIG. 4 shows the relationship between the scattering angle of X-rays transmitted through the subject and the differential cross section (the probability of scattering per scattering angle) for each type of scattering. In addition, the scattering angle in this Embodiment represents angle (theta) of the scattered radiation with respect to the advancing direction of a transmission line (direct line). In general, the differential cross section of coherent scattering such as Rayleigh is dominant in areas where the scattering angle is small, and the differential cross section of incoherent scattering such as Compton is dominant in areas where the scattering angle is large. Since the figure shows the maximum differential cross section normalized to 1 for each type of scattering, in actual scattering correction, the proportion of type-specific scattering is obtained from the characteristics of FIG. 3A by photon energy. Thus, the differential cross sections are synthesized at this ratio. In the present embodiment, in the same manner as described above, a table of scattering angle-differential cross-sectional area is created in advance, and a differential cross-sectional area for each type can be obtained by referring to this table with the X-ray scattering angle. is there.

FIG. 5 is a flowchart of X-ray CT imaging processing according to the embodiment, and shows a case where image reconstruction is performed by performing scatter correction described later on projection data obtained by scanning a subject. Preferably, this process is performed after a scout scan of the subject is performed in advance. In step S11, various scan parameters for the axial / helical scan of the subject are set. The scan width of the collimator 23 is set so that transmission lines (direct lines) are incident on all X-ray detector rows R1 to R8. In step S12, input of setting confirmation (CONFIRM) is awaited. Eventually, when input, the subject is scanned in step S13. In step S14, projection data is collected and stored in a memory. In step S15, it is determined whether or not all scans for the required imaging area have been completed. If not, the process returns to step S13.

  Thus, when all the scans are completed, the preprocessing for the projection data is performed in step S16. This preprocessing includes reference correction, channel sensitivity correction, and the like. In step S17, an X-ray transmission length estimation process, which will be described later, is performed based on the projection data after the preprocessing, and the transmission length of each transmission X-ray according to the size (physique) of the subject is obtained. In step S18, an X-ray scattering coefficient calculation process described later is performed based on the X-ray transmission length data, and an X-ray scattering coefficient corresponding to the subject size is obtained. In step S19, the scattering component included in each projection data is corrected using the obtained scattering coefficient.

For example, the intensity scatter _R1 (j) of the scattered radiation incident on the channel j of the detector array R1 is the X-ray intensity p (R2, j) -p (R8, j incident on the other detector arrays R2 to R8. ) By the sum of values weighted by the scattering coefficient α that specifies the relationship between the detection rows. That is, the scattered doses scat _R1 (j) to scat _R8 (j) incident on the channel j of each detector row R1 to _R8 are obtained by the following equations.

Here, p (R1, j) is the X-ray intensity (projection data) incident on the channel j of the first detection row R1, and the scattering coefficient α _R12 (R1, j) is the channel j of the first row detector. It represents the rate at which scattered radiation caused by incident transmitted lines (direct lines) is incident on channel j of the second row detector. Others are the same. Then, for example, the projection data p ′ (scattered ray compensated by subtracting the scattered ray intensity scatR1 (j) from the projection data p (R1, j) of the detector array R1.
R1, j) is obtained. The same applies to the other detector rows R2 to R8.

  In step S20, back projection processing is performed using the projection data after scattering correction, and a CT tomographic image is reconstructed. In step S21, the obtained CT tomographic image is displayed on the screen.

FIG. 6 is a flowchart of the X-ray transmission length estimation processing according to the embodiment, and shows a case where the subject is scanned and the transmission length of the transmission X-ray is obtained directly from the projection data. FIG. 7 shows an image of X-ray transmission length estimation processing. This figure shows projection data in the channel direction j of a certain detection row with respect to view angles i ₀ (θ = 0 °), i ₄₅ (θ = 45 °), and i ₉₀ (θ = 90 °). In step S31, the projection data of the subject is read corresponding to a certain view angle. In step S32, projection data I _j (j = 1 to _m ) of a portion exceeding a predetermined threshold TH is extracted. Here, j represents the relative channel number assigned to the projection data of the portion exceeding the predetermined threshold value TH. Preferably, in step S33, projection data for a plurality of channels are averaged to reduce the influence of noise components. At step S34 obtains the transmission length t _j corresponding to the transmission X-ray of each channel on the basis of the projection data said averaged and stored in the memory.

That is, in general, the j-th projection data I _j of the subject is given by

here,
I ₀ : X-ray emission intensity (reference data)
μ _b : X-ray attenuation count representing the subject t _j : Expressed by X-ray transmission length. Solving this equation for the transmission length t _j of X-ray,

Is obtained. Note that the X-ray attenuation count μ _b representing the subject uses the same default value as that of acrylic or the like, but is configured to be set in advance by the operator according to the outer size of the subject and the imaging region. You may do it.

  FIG. 7 shows a profile of X-ray transmission length at each view angle. When the view angle is 0 °, X-rays pass through the subject through a relatively short distance from the front surface to the back surface, so that the transmission length is relatively short. On the other hand, when the view angle is 90 °, X-rays pass through the subject through a relatively long distance from the left side surface to the right side surface, so that the transmission length is relatively long. When the view angle is 45 °, it is between these.

  In step S35, it is determined whether or not the transmission length for all projection data of the subject has been obtained. If NO, the process returns to step S31, and if YES, the process is exited. Thus, the total transmission length reflecting the size (physique) of the subject is obtained.

  FIG. 8 is a flowchart of the X-ray scattering coefficient calculation process according to the embodiment. Based on the positional relationship between the X-ray transmission length of the subject and the X-ray detector, more accurate X-ray scattering faithful to the actual scattering state. The case where the coefficient is obtained is shown. FIG. 9 shows an image of the X-ray scattering coefficient calculation process. In step S41, the transmission length data of the subject is read corresponding to a certain view angle. In step S42, the transmission length-photon energy table is referred to by the transmission length data, and the photon energy that characterizes the scattering in the transmission length is obtained. When the X-ray transmission length is short, the photon energy is relatively large, and when the X-ray transmission length is long, the photon energy is relatively small.

  In step S43, the photon energy-scattering cross section table is referred to with the obtained photon energy, and the ratio of coherent scattering and incoherent scattering generated with the photon energy is obtained. In FIG. 3A, when the X-ray transmission length is short and the photon energy (average photon energy) transmitted through this section is large, incoherent (Compton) scattering is dominant. When the length is long and the photon energy (average photon energy) transmitted through this section is small, the ratio of coherent (Rayleigh) scattering is large.

In step S44, the scattering angle data of the scattered radiation is obtained corresponding to each of the X-ray detection rows R1 to R8 based on the information on the mounting position (top height, etc.) and size (physique) of the subject. FIG. 9A shows a side cross-sectional view of the view angle i ₀ when X-rays pass through the subject from the front surface to the back surface. The center (starting point) of scattering can be set on the subject body axis CLb, for example. The body axis CLb of the subject can be automatically set by parameter setting before the start of scanning or based on the transmission length of each X-ray obtained after the scanning. Now, direct radiation to the detector row R1 of the X-ray is assumed a state in which the incident direction from the starting point of the body axis CLb the detection surface of the detection column R2~R8, representative of all the scattering in the transmission interval t _R1 Such scattered rays are generated. Since these geometric dimensions are known in the CPU 3 from the transport position of the subject, the position of the X-ray detector, etc., the respective scattering angle data θ _{R12 to} θ _R18 are obtained by calculation. When the subject size is large, the body axis CLb moves to the CLb ′ field position, and appropriate scattering angle data can be obtained according to this. The same applies to the view angles i ₄₅ and i ₉₀ .

In step S45, the scattering angle-differential cross-sectional area table is referred to with the obtained scattering angle data, and the differential cross-sectional area of scattering corresponding to each of the scattering angles θ _{R12 to} θ _R18 is determined for each scattering tail type. In step S46, the obtained differential cross-sectional areas are synthesized at the ratio of the type obtained in step S43, and the scattering coefficients α _{R12 to} α _R18 are obtained. In step S47, it is determined whether or not the total scattering coefficient has been obtained. If NO, the process returns to step S41. If YES, the process is exited. These scattering coefficients are used in the scattering correction process in step S19.

  In the above embodiment, the scattered radiation correction processing in the direction of the subject body axis (z axis) has been described, but the present invention is not limited to this. It is clear that the present invention can be applied to correction of scattered radiation in the channel direction of the X-ray detector.

In the above embodiment, the transmission length (subject size) of transmitted X-rays is obtained based on projection data obtained by scanning the subject. However, the present invention is not limited to this. For example, even if the subject is scanned in advance from at least two orthogonal directions (for example, view angles i ₀ , i ₉₀ ) and the approximate size (X-ray transmission length) of the subject is obtained based on the obtained projection data, The present invention can be realized.

  Moreover, in the said embodiment, although this invention was demonstrated with the specific numerical value, this invention is not limited to these numerical values.

  Moreover, although the preferred embodiment of the present invention has been described, it goes without saying that various changes in the configuration, control, processing, and combination of each part can be made without departing from the spirit of the present invention.

1 is a configuration diagram of an X-ray CT apparatus according to an embodiment. It is a figure explaining the X-ray imaging system by embodiment. It is FIG. (1) explaining the various tables by embodiment. It is FIG. (2) explaining the various tables by embodiment. It is a flowchart of the X-ray CT imaging process by embodiment. It is a flowchart of the X-ray transmission length estimation process by embodiment. It is an image figure of the X-ray transmission length estimation process by embodiment. It is a flowchart of the X-ray scattering coefficient calculation process by embodiment. It is an image figure of the X-ray-scattering-coefficient calculation process by embodiment.

Explanation of symbols

1 Operation console 2 Input device 3 Central processing unit (CPU)
5 Data collection buffer 6 Monitor 7 Storage device 10 Shooting table 12 Top plate (cradle)
DESCRIPTION OF SYMBOLS 15 Rotation part 20 Scanning gantry 21 X-ray tube 23 Collimator 24 Multi-row detector 25 Data acquisition device

Claims (8)

In an X-ray CT apparatus comprising an X-ray tube and an X-ray detector facing each other with a subject interposed therebetween, and reconstructing a CT tomogram based on projection data obtained by scanning the subject,
A transmission length calculating means for determining a transmission length of transmitted X-rays based on the projection data;
A scattering angle calculating means for obtaining a scattering angle of scattered radiation incident on each X-ray detection surface based on the positional relationship between the obtained transmission length and the X-ray detector;
Differential cross-sectional area calculating means for obtaining a differential cross-sectional area of each scattered ray based on information that preliminarily defines the relationship between the scattering angle and the differential cross-sectional area of scattering by the obtained scattering angle;
A scattering coefficient computing means for obtaining a scattering coefficient representing a ratio of scattered radiation intensity to transmitted X-ray intensity based on the obtained differential cross-sectional area;
X-ray CT comprising correction means for correcting scattering of projection data based on the X-ray intensity detected on each X-ray detection surface and the scattering coefficient obtained corresponding to each transmitted X-ray apparatus. In an X-ray CT apparatus comprising an X-ray tube and an X-ray detector facing each other with a subject interposed therebetween, and reconstructing a CT tomogram based on projection data obtained by scanning the subject,
Transmission length calculation means for determining the transmission length of transmitted X-rays based on the projection data;
A scattering angle calculating means for determining a scattering angle of scattered radiation incident on each X-ray detection surface based on the positional relationship between the obtained transmission length and the X-ray detector;
Differential cross-sectional area calculating means for obtaining a differential cross-sectional area for each type of scattered radiation based on information that preliminarily defines the relationship between the scattering angle and the differential cross-sectional area of the scattering for each type of scattering by the obtained scattering angle;
A photon energy calculating means for obtaining a photon energy of a transmitted X-ray based on information that preliminarily defines a relationship between an X-ray transmission length and a photon energy of the X-ray transmitted through the section by the determined transmission length;
Scattering ratio calculation means for obtaining a ratio of the scattering cross section by type that occurs in the transmission based on information that prescribes the relationship between the photon energy and the scattering cross section by the type of scattering in advance by the obtained photon energy,
A scattering coefficient calculating means for obtaining a scattering coefficient representing a ratio of scattered radiation intensity to transmitted X-ray intensity based on a differential sectional area obtained by combining the determined differential sectional area by type at a ratio of the determined scattering sectional area by type;
X-ray CT comprising correction means for correcting scattering of projection data based on the X-ray intensity detected on each X-ray detection surface and the scattering coefficient obtained corresponding to each transmitted X-ray apparatus. The correcting means detects X-rays based on the X-ray intensity detected in each X-ray detection row arranged in the body axis direction of the subject and each scattering coefficient obtained corresponding to each transmitted X-ray arranged in the body axis direction. The X-ray CT apparatus according to claim 1, wherein scattering correction is performed on projection data in a column direction. The correcting means scatters projection data in the channel direction based on the X-ray intensity detected on each X-ray detection surface arranged in the channel direction and each scattering coefficient obtained corresponding to each transmitted X-ray arranged in the channel direction. The X-ray CT apparatus according to claim 1, wherein correction is performed. A scatter correction method for an X-ray CT apparatus comprising an X-ray tube and an X-ray detector facing each other across a subject, and reconstructing a CT tomogram based on projection data obtained by scanning the subject,
Obtaining a transmission length of transmitted X-rays based on the projection data;
Obtaining a scattering angle of scattered radiation incident on each X-ray detection surface based on the positional relationship between the obtained transmission length and the X-ray detector;
Obtaining a differential cross-sectional area of each scattered ray based on information that preliminarily defines the relationship between the scattering angle and the differential cross-sectional area of scattering by the obtained scattering angle;
Obtaining a scattering coefficient representing a ratio of scattered radiation intensity to transmitted X-ray intensity based on the obtained differential cross-sectional area;
A scatter correction method comprising: a correction step of performing scatter correction of projection data based on the X-ray intensity detected on each X-ray detection surface and a scattering coefficient obtained corresponding to each transmitted X-ray . A scatter correction method for an X-ray CT apparatus comprising an X-ray tube and an X-ray detector facing each other across a subject, and reconstructing a CT tomogram based on projection data obtained by scanning the subject,
Obtaining a transmission length of transmitted X-rays based on the projection data;
Obtaining a scattering angle of scattered radiation incident on each X-ray detection surface based on the positional relationship between the obtained transmission length and the X-ray detector;
Obtaining a differential cross-sectional area for each type of scattered radiation based on information that prescribes the relationship between the scattering angle and the differential cross-sectional area of scattering according to the scattering angle determined in advance according to the scattering angle;
Obtaining the photon energy of transmitted X-rays based on information defining the relationship between the X-ray transmission length and the photon energy of X-rays transmitted through the section in advance by the determined transmission length;
Determining the ratio of the type-specific scattering cross-section generated by the transmission based on the information that prescribes the relationship between the photon energy and the scattering cross-section in advance for each type of scattering by the obtained photon energy;
Obtaining a scattering coefficient representing a ratio of scattered radiation intensity to transmitted X-ray intensity based on a differential sectional area obtained by synthesizing the obtained differential sectional area by type at a ratio of the obtained scattering sectional area by type;
A scatter correction method comprising: a correction step of performing scatter correction of projection data based on the X-ray intensity detected on each X-ray detection surface and a scattering coefficient obtained corresponding to each transmitted X-ray . In the correction step, X-ray detection is performed based on the X-ray intensity detected in each X-ray detection row arranged in the body axis direction of the subject and each scattering coefficient obtained corresponding to each transmitted X-ray arranged in the body axis direction. 7. The scatter correction method according to claim 5, wherein scatter correction is performed on projection data in the column direction. The correction step scatters projection data in the channel direction based on the X-ray intensity detected on each X-ray detection surface arranged in the channel direction and each scattering coefficient obtained corresponding to each transmitted X-ray arranged in the channel direction. The scattering correction method according to claim 5 or 6, wherein correction is performed.

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