WO2018062308A1 - X-ray ct device, method for measuring electron density and effective atomic number, ct scanning method, and inspection method - Google Patents

Abstract

An X-ray CT device is provided with a processing unit for processing data obtained by detecting X-rays radiated from an X-ray light source of a light source unit and transmitted through a measurement object, the processing unit finding an absorption coefficient µ(k) for each energy region k of each calibration tool from data obtained by measuring two or more types of calibration tools in a plurality of energy regions, calculates calibration coefficients F(k), G(k) for the energy regions k on the basis of equation (3) from the absorption coefficient found for each calibration tool, finds an absorption coefficient µ(k) for each energy region k of a measurement object from data obtained by measuring a measurement object in each of a plurality of energy regions, and calculates the electron density ρ_e and the effective atomic number Z of the measurement object on the basis of equation (4) from the calibration coefficients F(k), G(k) for the energy regions k and the absorption coefficients µ(k) for the energy regions k of the measurement object.

Description

X-ray CT apparatus, electron density and effective atomic number measurement method, CT inspection method, inspection method

The present invention relates to an X-ray CT apparatus and an electron density and effective atomic number measurement method using the X-ray CT apparatus. The present invention also relates to a CT inspection method for a subject using an X-ray CT apparatus, and an inspection method for inspecting the presence or absence of explosives inside an object using an X-ray CT apparatus.

CT diagnosis is indispensable for cancer diagnosis.
CT information provides information indispensable not only for diagnostic imaging but also for treatment planning that affects the quality of treatment in radiation therapy of cancer.
A CT image is basically composed of CT values obtained from the magnitude of subject attenuation.
In the X-ray treatment plan, a CT value is converted into an electron density using a CT value-electron density conversion table, and an X-ray or heavy particle beam treatment plan is made based on the information on the electron density.

In heavy ion radiotherapy, it has been proposed to obtain an electron density distribution with an accuracy of ± 1% or less for water-equivalent sites and ± 2% or less for organs such as lungs (IPEM81).
In X-ray irradiation therapy (stereoscopic radiation therapy (SRT), intensity-modulated radiation therapy (IMRT)), it has been proposed to obtain the electron density with an accuracy of ± 3% (Japan Society for Radiological Technology, (Kyoto), 2003).
However, according to a research survey (see Non-Patent Document 1), in more than 200 facilities in Japan, the numerical value of the CT value-electron density conversion table varies widely, with an average of ± 6%, a deviation from the average value. Is reported to be 25% at the maximum, and the quantitativeness is not always sufficient with the current technology.

As a solution to the variation in the numerical value of the CT value-electron density conversion table, two-color X-ray CT (Dual Source CT) using X-rays of two types of energy or CT images using X-ray energy information Photon Counting CT is proposed.
Patent Documents 1 to 11 describe techniques relating to the device configuration of Photon Counting CT and a data reconstruction program.

Also, a method for obtaining the electron density / effective atomic number from the CT value by using the apparatus and program described in Patent Document 13 using the apparatus described in Patent Document 12 is described. Further, examples are published in academic literature (see Non-Patent Literature 2) by the same inventor.

JP 2016-32635 A Japanese Patent Laid-Open No. 2016-16130 JP 2015-223350 A Japanese Patent Laying-Open No. 2015-180859 JP2015-160135A JP 2015-144809 A JP 2015-144808 A Japanese Patent Laying-Open No. 2015-62657 JP2015-131028A JP 2014-140707 A JP 2014-128456 A JP 2007-271468 A JP 2009-53090 A

However, the technique relating to the device configuration and data reconstruction program of Photon Counting CT disclosed in Patent Document 1 to Patent Document 11 does not solve the method of acquiring the electron density from the CT value with high accuracy.

Further, in the methods disclosed in Patent Document 13 and Non-Patent Document 2, the measurement error of the electron density of the embodiment described in Non-Patent Document 2 is about 10%, and the measurement accuracy of the electron density is the conventional X There is no inventive step compared to line CT. This is because errors due to multiple factors such as background and multiple scattering in actual measurement are included.

In addition, the electron density can be acquired with high accuracy in applications other than patient treatment purposes, such as for identifying the material to be measured, such as luggage inspection, and for inspecting cracks, defects, etc. of the object to be measured. May be required.

In response to the above-described problems, the present invention provides an X-ray CT apparatus and an X-ray CT that can estimate the electron density and effective atomic number inside a measurement target (subject or other object) with high accuracy. The present invention provides a method for measuring electron density and effective atomic number using an apparatus. The present invention also provides a CT inspection method and inspection method to which the electron density and effective atomic number measurement method of the present invention is applied.

The X-ray CT apparatus of the present invention has an X-ray light source and is capable of irradiating an object to be measured with X-rays in a plurality of energy regions, and is measured by being irradiated from the X-ray light source of the light source part. A detection unit that detects X-rays transmitted through an object and a processing unit that processes data of X-rays detected by the detection unit are provided. The processing unit uses two or more types of calibration jigs composed of a single element or compound that are homogeneous and capable of calculating effective atomic numbers and electron densities, and each calibration jig is provided with a plurality of calibration jigs. The absorption coefficient μ (k) in each energy region k of each calibration jig is obtained from the data measured and detected in each energy region, and the absorption coefficient μ (k) in each energy region k of each calibration jig. ), The calibration coefficients F (k) and G (k) in the respective energy regions k are calculated based on the following mathematical formula (3).

[In Equation (3), ρ _e represents the electron density of the calibration jig, and Z represents the effective atomic number of the calibration jig. ]

Furthermore, the processing unit obtains an absorption coefficient μ (k) in each energy region k of the measurement target from data detected by measuring the measurement target in each of the plurality of energy regions, and obtains an absorption coefficient μ (k) in each energy region k of the measurement target. From the absorption coefficient μ (k) and the calibration coefficients F (k) and G (k) in each energy region k, the electron density ρ _e and the effective atomic number Z of the measurement target are calculated based on the following formula (4). calculate.

[In Formula (4), ρ _e represents the electron density of the measurement object, and Z represents the effective atomic number of the measurement object. ]

The method for measuring the electron density and effective atomic number of the present invention is a method for measuring the electron density and effective atomic number of an object to be measured using an X-ray CT apparatus, which is homogeneous and calculates the effective atomic number and electron density. Using two or more types of calibration jigs composed of a single element or compound, each calibration jig is measured in a plurality of energy regions with an X-ray CT system, and each calibration is performed. An absorption coefficient μ (k) in each energy region k of the jig is determined. Then, calibration coefficients F (k) and G (k) in each energy region k are calculated from the absorption coefficient μ (k) in each energy region k of each calibration jig based on the following formula (3). To do.

[In Equation (3), ρ _e represents the electron density of the calibration jig, and Z represents the effective atomic number of the calibration jig. ]

Further, the measurement object is measured in each of a plurality of energy regions by an X-ray CT apparatus to obtain an absorption coefficient μ (k) in each energy region k of the measurement object, and the absorption coefficient in each energy region k of the measurement object. Based on μ (k) and calibration coefficients F (k) and G (k) in each energy region k, the electron density ρ _e and the effective atomic number Z of the measurement target are calculated based on the following formula (4). .

[In Formula (4), ρ _e represents the electron density of the measurement object, and Z represents the effective atomic number of the measurement object. ]

The CT examination method of the present invention is a CT examination method for performing a CT examination on a human subject, and in the method for measuring electron density and effective atomic number of the present invention, the subject to be measured is the subject. The electron density and effective atomic number of the subject are measured, and an electron density map inside the subject is obtained based on the measured electron density and effective atomic number of the subject.

The inspection method of the present invention is an inspection method for inspecting an object and detecting the presence or absence of explosives in the object, in the method for measuring electron density and effective atomic number of the present invention, with the measurement object as an object, The presence or absence of explosives is detected by measuring the electron density and effective atomic number of the object and specifying the substance inside the object based on the measured electron density and effective atomic number of the object.

According to the present invention described above, the calibration coefficient obtained by measuring the calibration jig is used to calibrate the actual measurement value to be measured. The error factor can be eliminated at the same time, and the electron density and effective atomic number of the measurement object can be calculated with high accuracy.

According to the CT examination method of the present invention, since the electron density and effective atomic number of the subject to be measured can be calculated with high accuracy, a highly accurate electron density map of the human body can be obtained, and the treatment plan can be obtained. Sometimes it is possible to estimate a more accurate range calculation.
Therefore, it is possible to improve the irradiation accuracy during X-ray therapy or heavy particle beam therapy.

According to the inspection method of the present invention, since the effective atomic number and electron density of the object to be measured can be calculated with high accuracy, it is possible to detect the substance existing inside the measurement target with high positional accuracy. Therefore, the presence or absence of explosives can be accurately detected.
Therefore, if the inspection method of the present invention is applied to baggage and cargo inspection, it becomes possible to perform inspection with high accuracy.

In addition, when the X-ray CT apparatus of the present invention and the method for measuring electron density and effective atomic number of the present invention are applied to inspection of an object other than a subject, the effective atomic number and electron density of the object to be measured Therefore, it is possible to detect a substance (for example, dangerous substance, corrosive substance), a defect, or the like existing inside the object with high positional accuracy.
Therefore, if the X-ray CT apparatus of the present invention and the method for measuring electron density and effective atomic number of the present invention are applied to the inspection of the presence or absence of product defects, it becomes possible to perform inspection with high accuracy.

In addition, according to the present invention, since the electron density of the measurement target is calculated using the calibration coefficient calculated in advance by the measurement of the calibration jig, it is compared with the conventional method such as when successive approximation is performed. Thus, it is possible to greatly reduce the amount of calculation when calculating the electron density. As a result, the electron density can be calculated in a short time, and there is no need to use a large computer, and the calculation can be performed by a personal computer or the like.

1 is a schematic configuration diagram of an embodiment of an X-ray CT apparatus of the present invention. It is a flowchart explaining the process of calculating a calibration coefficient, an electron density, and an effective atomic number in the X-ray CT apparatus of FIG. It is a schematic block diagram of other embodiment of the X-ray CT apparatus of this invention. It is a schematic block diagram of the X-ray CT apparatus used for the measurement of an Example. A to D are diagrams showing results of calculating calibration coefficients from measurement results of Examples. A to C are diagrams showing the results of calculating electron density and effective atomic number in Examples.

First, prior to the description of specific embodiments of the present invention, the outline and principle of the present invention will be described.

An X-ray CT (Computed Tomography) apparatus of the present invention (hereinafter also abbreviated as “the apparatus of the present invention”) has an X-ray light source, and can irradiate a measurement object with X-rays in a plurality of energy regions. A light source unit, a detection unit that detects X-rays emitted from the X-ray light source of the light source unit and transmitted through the object to be measured, and a processing unit that processes data of X-rays detected by the detection unit. The processing unit uses two or more types of calibration jigs composed of a single element or compound that are homogeneous and capable of calculating effective atomic numbers and electron densities, and each calibration jig is provided with a plurality of calibration jigs. From the data measured and detected in each energy region, the absorption coefficient in each energy region of each calibration jig is obtained, and the calibration coefficient in each energy region is obtained from the absorption coefficient in each energy region of each calibration jig. The absorption coefficient in each energy region of the measurement target is obtained from the data detected by measuring the measurement target in each of the plurality of energy regions, and the absorption coefficient in each energy region of the measurement target and each energy region The electron density and effective atomic number of the measurement object are calculated from the calibration coefficient in FIG.

The electron density and effective atomic number measurement method of the present invention (hereinafter also abbreviated as “the measurement method of the present invention”) is a method of measuring the electron density and effective atomic number of a measurement object using an X-ray CT apparatus. . And, using two or more kinds of calibration jigs composed of a single element or compound that is homogeneous and whose effective atomic number and electron density can be calculated, each calibration jig is obtained by an X-ray CT apparatus. Measure each of the energy regions to obtain the absorption coefficient in each energy region of each calibration jig, and calculate the calibration coefficient for each energy region from the absorption coefficient in each energy region of each calibration jig. Calculate and measure the measurement object in each of a plurality of energy regions using an X-ray CT apparatus to obtain an absorption coefficient in each energy region of the measurement object. The absorption coefficient in each energy region of the measurement object and each energy From the calibration coefficient in the region, the electron density and effective atomic number of the measurement object are calculated.

The CT examination method of the present invention is a CT examination method for performing a CT examination on a human subject. In the measurement method of the present invention, the measurement target is the subject, and the electron density and effective atomic number of the subject are determined. Based on the measured electron density and effective atomic number of the subject, an electron density map inside the subject is obtained.

The inspection method of the present invention is an inspection method for inspecting an object and detecting the presence or absence of explosives in the object. In the measurement method of the present invention, the electron density and effective atomic number of the object are set as the object to be measured. , And the presence or absence of explosives is detected by identifying the substance inside the object based on the measured electron density and effective atomic number.

The apparatus of the present invention basically has a processing unit in the apparatus. The processing unit processes data detected by the detection unit during X-ray CT measurement by a computer program.
For example, the measurement unit including the light source unit and the detection unit may have a configuration in which a processing unit is built in the measurement unit, or a configuration in which a processing unit is attached outside the measurement unit.

The measurement method of the present invention uses an X-ray CT (Computed Tomography) apparatus.
As the X-ray CT apparatus used by the measurement method of the present invention, not only the above-described apparatus of the present invention, that is, an X-ray CT apparatus provided with a processing unit that calculates the electron density and effective atomic number of the measurement target, It is also possible to use an X-ray CT apparatus having another configuration (for example, a conventionally known X-ray CT apparatus).
The measurement method of the present invention includes a case where data processing such as calculation of the electron density and effective atomic number of the measurement target is performed outside the X-ray CT apparatus. For example, the data measured by the X-ray CT apparatus is stored in a storage medium, and the data stored in the storage medium is processed externally, or the data is transmitted to the external processing apparatus by wired or wireless processing. It is also possible to process data by a program on the network.

In the apparatus of the present invention and the measurement method of the present invention, the X-ray CT apparatus irradiates the object to be measured with X-rays in a plurality (two or more) of energy regions and transmits the X-rays transmitted through the object to be measured in the energy regions. Detect every time. As long as this condition is satisfied, each configuration of the light source unit that emits X-rays from the X-ray light source and the detection unit that detects X-rays transmitted through the measurement object in the X-ray CT apparatus uses various configurations. It is possible.

Examples of the combination of the light source unit and the detection unit of the X-ray CT apparatus in the apparatus of the present invention and the measurement method of the present invention include the following configurations (A) to (C).

(A) The light source unit includes at least a first X-ray light source that emits X-rays of the first energy and a second X-ray light source that emits X-rays of a second energy different from the first energy. It is the composition which has.
The detection unit has a detector for each X-ray light source of the light source unit.
As the configuration of the light source unit, for example, as disclosed in FIG. 3 of Japanese Patent Laid-Open No. 10-104175, a high energy X-ray bundle is irradiated from the first X-ray light source, and the second X-ray light source is used. A configuration for irradiating a low-energy X-ray beam can be employed.
The light source unit may have three or more X-ray light sources having different energy.
The X-ray light source can be configured to irradiate monochromatic X-rays or to irradiate continuous X-rays having a certain energy distribution. When the X-ray light source is configured to emit a certain amount of continuous X-rays, the “first energy” and the “second energy” described above are referred to as “first energy region” and “second energy”. Read as "Area".
In the configuration of (A), the accuracy of calculation such as the electron density can be increased when the X-ray to be irradiated is narrower in energy distribution, such as monochromatic X-ray.

(B) The light source unit can change the energy of X-rays emitted from one X-ray light source by changing the acceleration voltage supplied to the X-ray light source.
The detection unit has a detector for one X-ray light source of the light source unit.
By changing the acceleration voltage or the like supplied to one X-ray light source (X-ray tube), the energy of X-rays irradiated from the X-ray light source is changed. Then, for example, X-ray irradiation of the first energy, X-ray irradiation of the second energy, X-ray irradiation of the third energy, etc. X-ray measurement of each energy is performed.
The X-ray light source can be configured to irradiate monochromatic X-rays or to irradiate continuous X-rays having a certain energy distribution. When the X-ray light source is configured to emit a certain amount of continuous X-rays, the above-mentioned “X-ray energy” is read as “X-ray energy region”.
In the configuration of (B), the accuracy of calculation such as the electron density can be increased when the X-ray to be irradiated has a narrow energy distribution such as a monochromatic X-ray.

(C) In the X-ray energy distribution irradiated from the X-ray light source of the light source unit, the detection unit is configured to discriminate and detect two or more energy regions for each energy region (see, for example, Patent Document 13 above). reference).
A so-called photon counting CT apparatus employs this configuration.
The X-ray light source of the light source unit is preferably configured to irradiate continuous X-rays and has a spectral distribution over a relatively wide energy range.
In this configuration, two or more energy regions can be measured simultaneously by one X-ray irradiation, so that the exposure dose can be reduced. Further, since continuous X-rays are used instead of monochromatic X-rays, no large-scale equipment is required.
Therefore, when this configuration is adopted, it can be easily installed in a hospital or the like.
When the light source unit and the detection unit have the configuration (C), the X-ray light source of the light source unit and the detector of the detection unit can adopt the same configuration as a conventionally known photon counting CT apparatus.

Note that each of the above-described configurations (A) to (C) is a basic configuration. As a modification, for example, in each basic configuration, a plurality of X-ray light sources that irradiate the same energy may be provided.

When the light source unit is composed of a plurality of X-ray light sources, for example, the X-ray irradiation directions of the plurality of X-ray light sources are made different from each other in the same manner as the configuration described in Patent Document 13 (FIGS. 2, 4 and the like). It becomes possible.
For example, in the configuration of (A) described above, the X-ray irradiation directions by a plurality of X-ray light sources can be made different from each other.
When the X-ray irradiation directions of the plurality of X-ray light sources are different from each other, it is possible to perform measurement by simultaneously irradiating X-rays from the plurality of X-ray light sources.

In the apparatus of the present invention and the measurement method of the present invention, a calibration jig as a standard substance is used.
As the calibration jig, a substance composed of a single element or a compound that is homogeneous and can calculate the effective atomic number Z _eff and the electron density ρ _e is used.
As a substance used as a calibration jig, a substance composed of any element or compound can be used as long as the effective atomic number Z _eff and the electron density ρ _e can be calculated.
However, since an error in the calculated electron density increases as the atomic number increases, it is preferable to use a substance composed of elements having an atomic number in the range of 1 to 30.

For some elements, the theoretical values of the known effective atomic number Z _eff and electron density ρ _e for that element can be used.
For other elements and compounds, the effective atomic number Z _eff and the electron density ρ _e can be calculated using calculation formulas described in the literature.
The formula for calculating the effective atomic number Z _eff is, for example, Khan's The Physics of Radiation Therapy, Faiz M. Khan, John P. Gibbons (Jr.), Lippincott Wiliams & Wikins, 2014 (p.78 formula (6.4)). Are listed.
The calculation formula of the electron density ρ _e is described in, for example, J Med Phys. 2009 Jul-Sep; 34 (3): 176-179. Doi: 10.4103 / 0971-6203.54853 (the last formula in the introduction).
For example, a substance selected from carbon, magnesium, and aluminum can be used for the calibration jig. These carbon, magnesium, and aluminum are materials that have not been used as calibration jigs in the past.
In addition, although a liquid or the like is possible as the calibration jig, it is desirable to use a substance that can maintain a solid state as stably as possible at room temperature.

It is desirable that the calibration jig has a shape that can be easily handled and measured. For example, a quadrangular prism shape, a cylindrical shape, and the like can be given.
As for the size of the calibration jig, an appropriate size is selected in accordance with the type of object to be measured and the required accuracy of electron density. For example, the thickness of the portion through which X-rays are transmitted is set to be about several mm to several cm.

Subsequently, the principle of the present invention will be described.

When X-ray absorption coefficient of an object to be measured is obtained by irradiating two or more kinds of X-rays of energy k, the absorption coefficient μ (k) is obtained by the following equation (1) (for example, M. Torikoshi et al. al., Phys. Med. Boil, 48, 673, (2003)).

Conventionally, for example, from the above formula (1), the absorption coefficient μ is measured with monochromatic X-rays of two energies, and the electron density ρ _e is measured using the measured value of the absorption coefficient μ and the theoretical value of the atomic number Z. I was looking for. Further, for example, an effective value for the atomic number Z _eff and the electron density ρ _e are obtained by sequentially approximating the atomic number Z by entering an appropriate value.
Since the measured value of the absorption coefficient μ includes an error and background, it is difficult to accurately obtain the electron density ρ _e from the measured value of the absorption coefficient μ and the theoretical value of the atomic number Z.
Further, if approximation is performed sequentially, the amount of calculation becomes enormous, so that the calculation takes time, and calculation processing becomes difficult with a personal computer or the like.

On the other hand, in the present invention, the effective atomic number Z _eff and the electron density ρ _e to be measured are obtained by a method different from the conventional method such as sequential approximation.
Hereinafter, the method of the present invention will be described.

In the above formula (1), it is assumed that the coefficients F and G have little atomic number dependency, and therefore have no atomic number dependency.
Assuming that there is no atomic number dependency, the formula (1) is expressed as the following formula (2).

Further, when the formula (2) is modified, the following formula (3) is obtained.

Here, two or more kinds of substances composed of a single element or a compound, which are homogeneous and can calculate the effective atomic number Z _eff and the electron density ρ _e, are used for each substance. The absorption coefficient μ at the energy of is measured. Then, using the fact that the expression (3) is linear with respect to Z ⁴ , the measured value of the absorption coefficient μ of each substance, the value of the atomic number Z and the electron density ρ _e (known value) at a certain energy k. F (k) and G (k) at the energy k are obtained from the theoretical value or the calculated value.

Furthermore, the equation (2) can be transformed as the following equation (4).

If the absorption coefficient μ (k) of the object to be measured is measured with a plurality (two or more) of energy k and F (k) and G (k) corresponding to the same energy k are obtained in advance, formula (4) Is linear with respect to F (k) / G (k), the electron density ρ _e to be measured is obtained.

In the present invention, based on the principle described above, as described above, a calibration jig is used for a material composed of a single element or a compound that is homogeneous and can calculate the effective atomic number Z _eff and the electron density ρ _e. Two or more kinds of calibration jigs having different elements are prepared.
Then, a calibration jig is measured in advance to obtain a calibration coefficient.
Thereafter, the measurement target (subject or other object) is measured, and the measured value of the measurement target is calibrated using the obtained calibration coefficient, and the effective atomic number Z _eff and the electron of the measurement target are measured. Find the density.
Hereinafter, these procedures will be described in more detail.

First, with respect to each calibration jig, the absorption coefficient μ (k) is measured at each energy k of a plurality (two or more) of energy k, and F (k) and G corresponding to each energy k are measured. (K) is obtained, and the obtained F (k) and G (k) are used as calibration coefficients for the respective energy k.
Specifically, since Equation (3) is linear with respect to Z ⁴ , at each energy k, two or more types of calibration are performed with Z ⁴ as the horizontal axis and μ / ρ _e as the vertical axis. Plot the jig value. Calibration coefficients F (k) and G (k) at the energy k can be obtained from the slope of the straight line obtained by plotting and the intercept of the straight line.
At this time, in order to obtain a straight line, it is necessary to plot two or more points, so two or more kinds of calibration jigs are used.

Next, the absorption coefficient μ (k) of the measurement target (subject or other object) is measured at each of the plurality (two or more) of energy k.
Then, using the measured value of the absorption coefficient μ (k) of the measurement object at each energy k and the calibration coefficients F (k) and G (k) obtained in advance as described above, the electron density of the measurement object ρ _e is calculated.
Specifically, since Equation (4) is linear with respect to F (k) / G (k), μ (k) / G (k) with F (k) / G (k) as the horizontal axis. ) Is plotted on the vertical axis, and the value of each energy k of two or more energies k is plotted. Z ⁴ ρ _e is determined from the slope of the straight line obtained by plotting, and ρ _e is determined from the intercept of the straight line. And from these values, the electron density ρ _e and the effective atomic number Z _eff of the measurement object can be obtained.
At this time, in order to obtain a straight line, it is necessary to plot two or more points, so measurement and calculation of a calibration coefficient are performed with a plurality (two or more) of energy k.

According to the present invention, by adopting the above-described method, the effective atomic number Z _eff and the electron density ρ _e of the measurement target can be calculated with high accuracy.

According to the CT examination method of the present invention, since the electron density and effective atomic number of the subject to be measured can be calculated with high accuracy, a highly accurate electron density map of the human body can be obtained, and the treatment plan can be obtained. Sometimes it is possible to estimate a more accurate range calculation.
Therefore, it is possible to improve the irradiation accuracy during X-ray therapy or heavy particle beam therapy.

According to the inspection method of the present invention, since the effective atomic number and electron density of the object to be measured can be calculated with high accuracy, it is possible to detect the substance existing inside the measurement target with high positional accuracy. Therefore, the presence or absence of explosives can be accurately detected.
Therefore, if the inspection method of the present invention is applied to baggage and cargo inspection, it becomes possible to perform inspection with high accuracy.

In addition, when the apparatus of the present invention and the measurement method of the present invention are applied to the inspection of an object other than the subject, it becomes possible to calculate the effective atomic number and electron density of the object to be measured with high accuracy. It is possible to detect a substance (for example, a dangerous substance, a corrosive substance), a defect, and the like existing inside an object with high positional accuracy without being destroyed.
Therefore, if the apparatus of the present invention and the measurement method of the present invention are applied to the inspection of the presence or absence of product defects, it becomes possible to perform a highly accurate inspection.

When the absorption coefficient μ of the measurement target is measured, the actual measurement value includes factors such as background and multiple scattering.
If the electron density is obtained directly from the measurement value and atomic number value as in the conventional method, factors such as background and multiple scattering cannot be sufficiently eliminated, and the accuracy of the electron density deteriorates.
On the other hand, by adopting the method of the present invention and performing calibration using the calibration coefficient obtained by measuring the calibration jig, a plurality of error factors such as background and multiple scattering included in the actual measurement value are obtained. At the same time, and the electron density and effective atomic number can be calculated with high accuracy.

Further, according to the present invention, the electron density of the measurement object is calculated using the calibration coefficient calculated in advance from the measurement data of the calibration jig, so that it is compared with the conventional method such as when successive approximation is performed. Thus, it is possible to greatly reduce the amount of calculation when calculating the electron density. As a result, the electron density can be calculated in a short time, and there is no need to use a large computer, and the calculation can be performed by a personal computer or the like.

The frequency of the measurement of the calibration jig does not have to be performed once for each measurement target, and depends on the required accuracy, but it takes into account deterioration of the X-ray tube and fluctuations in the spectrum state. It is desirable to increase the frequency (for example, several times a day to about once every several days).
For example, it can be about once a day. In this case, for example, it is conceivable to measure the calibration jig at the start of daily work.

In the measurement of the calibration jig, it is desirable to perform measurement at the same position as the position where the measurement object is measured. By measuring at the same position, the influence on the measured value of the absorption coefficient due to the distance from the light source can be eliminated.
The accuracy of the measurement position of the calibration jig depends on the accuracy of the required electron density. When the subject is a measurement object, for example, it may be within ± 1% (about 2 mm) with respect to the affected part of the subject of about 20 cm.

In the apparatus of the present invention and the measurement method of the present invention, a configuration in which the calibration jig is installed at the measurement position and removed from the measurement position by an operator's hand or a machine outside the apparatus is also conceivable. Then, the measurement of the calibration jig becomes complicated.
Therefore, in order to easily measure the calibration jig, the X-ray CT apparatus has a switching mechanism for switching between the measurement of the calibration jig and the measurement of the measurement object such as the subject. Is desirable. For example, a calibration jig is put in and out by a switching mechanism such as an arm or a slider so that the measurement of the calibration jig and the measurement of the measurement target can be switched.
As the switching mechanism, a configuration in which switching is performed by manually moving a slider or the like, or a configuration in which switching is performed by pressing a button can be considered, but more preferably, a configuration in which switching is performed automatically by control of a computer program or the like. And
For example, when measuring the calibration jig at the start of work described above, after the power is turned on, the calibration jig is automatically installed at the measurement position, the measurement of the calibration jig is measured, and the calibration jig is used. What is necessary is just to set it as the structure controlled automatically so that accommodation of a jig | tool is performed sequentially.
The configuration having a switching mechanism for switching between the calibration jig and the measurement target also includes the processing unit in the X-ray CT apparatus of the present invention including the processing unit that calculates the electron density and effective atomic number of the measurement target. The present invention can also be applied to other X-ray CT apparatuses (for example, conventionally known X-ray CT apparatuses) that are not.
Further, the X-ray CT apparatus can be configured such that the calibration jig can be taken out.

Subsequently, specific embodiments of the present invention will be described.

FIG. 1 shows a schematic configuration diagram of an embodiment of the X-ray CT apparatus of the present invention.
This embodiment is a case where the present invention is applied to an X-ray CT apparatus having a measurement object as a subject.

The X-ray CT apparatus 1 shown in FIG. 1 includes two parts, an X-ray transmission intensity measurement unit 10 and an electron density and effective atomic number calibration system unit 20. The X-ray transmission intensity measurement unit 10 includes a light source unit and a detection unit of the apparatus of the present invention. The electron density and effective atomic number calibration system unit 20 includes a processing unit of the apparatus of the present invention.

The X-ray transmission intensity measurement unit 10 includes an X-ray light source 11, a slit or collimator 12, a bed apparatus 14, a slit or collimator 15, an energy discrimination X-ray intensity detector 16, and a data collection unit 17.
A slit or collimator 12 is disposed under the X-ray light source 11, and X-rays are irradiated downward from the X-ray light source 11.
An energy discriminating X-ray intensity detector 16 is disposed below a subject (patient P) 30 to be measured via a slit or collimator 15.
The data collection unit 17 is connected to the energy discrimination X-ray intensity detector 16 and collects data detected by the energy discrimination X-ray intensity detector 16.
As shown in the lower part of FIG. 1, the couch device 14 has an XYZθ precision automatic stage, and is further provided with a switching mechanism (not shown). With the switching mechanism, as shown by the arrows in the figure, the calibration jig 13 and the subject (patient P) 30 can be switched to perform each measurement.
The calibration jig 13 is a substance composed of a single element or compound serving as a standard substance that is homogeneous and capable of calculating the effective atomic number and electron density, and two or more different types are prepared. Each calibration jig 13 is switched by a switching mechanism, and measurement of the calibration jig 13 is sequentially performed.

The electron density and effective atomic number calibration system unit 20 includes a control unit 21, a calibration unit 22, and an analysis unit 23. The electron density and effective atomic number calibration system unit 20 is configured by a personal computer or the like and operates by a computer program. The calibration unit 22 and the analysis unit 23 are included in the processing unit of the apparatus of the present invention.
The control unit 21 controls the X-ray light source 11, the slit or collimator 12, the switching mechanism between the calibration jig 13 and the subject 30, and the data collection unit 17 of the X-ray transmission intensity measurement unit 10.
The calibration unit 22 calculates a calibration coefficient from the data collected by the data collection unit 17 of the X-ray transmission intensity measurement unit 10.
The analysis unit 23 analyzes the electron density and the effective atomic number by applying the calibration coefficient calculated by the calibration unit 22.

Next, the operation of the X-ray CT apparatus 1 in FIG. 1 will be described.
FIG. 2 shows a flowchart of the process of calculating the calibration coefficient, electron density, and effective atomic number in the X-ray CT apparatus 1 of FIG.

As shown in the flowchart of FIG. 2, in step S1, calibration measurement using the calibration unit 22 is started.
Next, in step S <b> 2, the calibration jig 13 is installed at a corresponding portion (measurement position) of the subject 30 of the bed apparatus 14. That is, the calibration jig 13 is placed at the measurement position by the operation of the switching mechanism.

Next, CT imaging of the calibration jig 13 is performed in step S3. That is, X-rays are emitted from the X-ray light source 11, X-rays transmitted through the calibration jig 13 are detected by the energy discrimination X-ray detector 16, and the detected data is collected by the data collection unit 17. When the CT imaging of the calibration jig 13 is completed, the calibration jig 13 is removed from the measurement position by the switching mechanism and stored in a predetermined place.

Since there are two or more types of calibration jigs 13, each calibration jig 13 is installed at the measurement position of the calibration jig 13, CT imaging of the calibration jig 13, and the calibration jig taken. The removal from 13 measurement positions is repeated.

Next, in step S4, an absorption coefficient is obtained. That is, the absorption coefficient μ (k) is acquired in the calibration unit 22 from the data of the calibration jig 13 collected by the data collection unit 17.

Next, in step S5, a calibration coefficient is calculated. That is, in the calibration unit 22, the calibration coefficient F (k) is calculated from the absorption coefficient μ (k) and the values (theoretical values or calculated values) of the effective atomic number Z _eff and the electron density ρ _e of each calibration jig 13. , G (k).

Next, in step S6, measurement of the subject 30 using the analysis unit 23 is started.
In step S <b> 7, the subject (patient P) 30 is placed on the bed apparatus 14.

Next, CT imaging of the subject 30 is performed in step S8. That is, X-rays are emitted from the X-ray light source 11, X-rays transmitted through the subject 30 are detected by the energy discrimination X-ray detector 16, and the detected data is collected by the data collection unit 17.

Next, in step S9, an absorption coefficient is obtained. That is, the analysis unit 23 acquires the absorption coefficient μ (k) of the subject 30 from the data of the subject 30 collected by the data collecting unit 17.

Next, in step S10, calibration coefficients are applied. That is, the calibration coefficients F (k) and G (k) previously calculated in step S5 are applied to the absorption coefficient μ (k) of the subject 30.

Next, in step S11, the electron density and effective atomic number are analyzed. That is, the analysis unit 22 analyzes the electron density ρ _e and the effective atomic number Z _eff of the subject 30 from the absorption coefficient μ (k) of the subject 30 and the calibration coefficients F (k) and G (k).

Thereafter, in step S12, it is determined whether to measure another subject 30.
When measuring another subject 30, the process returns to step S6 and the other subject 30 is measured.
If the other subject 30 is not measured, the measurement is terminated.

In this way, by calibrating the measured value of the subject 30 with the calibration coefficients F (k) and G (k) calculated using the calibration jig 13, the electron density and effective atomic number of the subject 30 are corrected. Can be calculated with high accuracy.

According to the above-described embodiment, the calibration coefficient is calculated from the absorption coefficient acquired by the measurement of the calibration jig 13, and the calibration coefficient is applied to the absorption coefficient acquired by the measurement of the subject 30. The effective atomic number and electron density of the subject 30 are calculated. Therefore, a plurality of error factors such as background and multiple scattering included in the actual measurement value can be eliminated simultaneously, and the effective atomic number Z _eff and the electron density ρ _e of the subject 30 can be calculated with high accuracy. Become.
As a result, a highly accurate electron density map of the human body can be obtained, so that it is possible to estimate a more accurate range calculation at the time of treatment planning.
Therefore, it is possible to improve the irradiation accuracy during X-ray therapy or heavy particle beam therapy.

In addition, since the electron density of the subject 30 is calculated using the calibration coefficient calculated from the measurement data of the calibration jig 13 in advance, the electron density is compared with a conventional method such as when successive approximation is performed. The amount of calculation when calculating the density can be greatly reduced. As a result, the electron density can be calculated in a short time, and there is no need to use a large computer, and the calculation can be performed by a personal computer or the like.

FIG. 3 shows a schematic configuration diagram of another embodiment of the X-ray CT apparatus of the present invention.
This embodiment is a case where the present invention is applied to an X-ray CT apparatus in which an object to be measured is an object other than a subject.

The X-ray CT apparatus 40 shown in FIG. 3 includes two parts, an X-ray transmission intensity measurement unit 50 and an electron density and effective atomic number calibration system unit 60. The X-ray transmission intensity measurement unit 50 includes a light source unit and a detection unit of the apparatus of the present invention. The electron density and effective atomic number calibration system unit 60 includes a processing unit of the apparatus of the present invention.

The X-ray transmission intensity measuring unit 50 includes an X-ray light source 51, an XYZ precision automatic stage 52, a slit or collimator 53, an XYZθ precision automatic stage 56, a slit or collimator 57, an energy discrimination X-ray intensity detector 58, and an XYZ precision automatic stage 59. Have
A slit or collimator 53 is disposed in front of the X-ray light source 51, and X-rays are irradiated forward from the X-ray light source 51. The position of the X-ray light source 51 can be changed by an XYZ precision automatic stage 52.
The XYZθ precision automatic stage 56 is provided with a switching mechanism (not shown). With the switching mechanism, as shown by the arrows in the figure, the calibration jig 54 and the measurement target (object) 55 can be switched to perform each measurement. Further, the XYZθ precision automatic stage 56 can change the position and orientation of the calibration jig 54 or the measuring object 55 installed thereon.
An energy discriminating X-ray intensity detector 58 is disposed in front of the calibration jig 54 or the measurement object 55 via a slit or collimator 57. The position of the energy discrimination X-ray detector 58 can be changed by an XYZ precision automatic stage 59.
The calibration jig 54 is a substance composed of a single element or compound serving as a standard substance that is homogeneous and capable of calculating an effective atomic number and an electron density, and two or more different kinds are prepared. Each calibration jig 54 is switched by a switching mechanism, and the measurement of the calibration jig 54 is sequentially performed.

The electron density and effective atomic number calibration system unit 60 includes a control unit 61, a calibration unit 62, and an analysis unit 63. The electron density and effective atomic number calibration system unit 60 is configured by a personal computer or the like and operates by a computer program. The calibration unit 62 and the analysis unit 63 are included in the processing unit of the apparatus of the present invention.
Functions and operations of the control unit 61, the calibration unit 62, and the analysis unit 63 are almost the same as those of the control unit 21, the calibration unit 22, and the analysis unit 23 of the electron density and effective atomic number calibration system unit 20 of the X-ray CT apparatus 1 of FIG. It is the same.
The control unit 61 includes the X-ray light source 51, the XYZ precision automatic stage 52, the XYZθ precision automatic stage 56, the energy discrimination X-ray intensity detector 58, and the XYZ precision automatic stage 59 of the X-ray transmission intensity measurement unit 50, and calibration. Control is performed on a switching mechanism that switches between the jig 54 and the measurement target 55.
The calibration unit 62 calculates a calibration coefficient from the data collected by the X-ray transmission intensity measurement unit 50.
The analysis unit 63 analyzes the electron density and effective atomic number by applying the calibration coefficient calculated by the calibration unit 62.

In the X-ray CT apparatus 40, the calibration jig 54 is measured and the calibration coefficients F (k), G are obtained by the same procedure as the flowchart shown in FIG. 2 of the X-ray CT apparatus 1 of the previous embodiment. The processes of (k) calculation, measurement of the measurement target 55, and analysis of the electron density ρ _e and effective atomic number Z _eff of the measurement target 55 are performed.
The electron density and effective atomic number of the measurement target 55 are calculated with high accuracy by calibrating the measurement value of the measurement target 55 using the calibration coefficients F (k) and G (k) calculated using the calibration jig 54. can do.

According to the above-described embodiment, the calibration coefficient is calculated from the absorption coefficient acquired by the measurement of the calibration jig 54, and the calibration coefficient is applied to the absorption coefficient acquired by the measurement of the measurement target 55. The effective local number and electron density of the measurement object are calculated. Therefore, a plurality of error factors such as background and multiple scattering included in the actual measurement value can be eliminated simultaneously, and the effective atomic number Z _eff and the electron density ρ _e of the measurement target 55 can be calculated with high accuracy. Become.
Thereby, it becomes possible to detect a substance (for example, a dangerous substance, a corrosive substance), a defect, and the like existing in the measurement object 55 with non-destructiveness with high positional accuracy.

In addition, since the electron density of the measurement object 55 is calculated using the calibration coefficient calculated in advance from the measurement data of the calibration jig 54, the electron density is compared with the conventional method such as when successive approximation is performed. The amount of calculation when calculating the density can be greatly reduced. As a result, the electron density can be calculated in a short time, and there is no need to use a large computer, and the calculation can be performed by a personal computer or the like.

The X-ray CT apparatuses 1 and 40 of the above-described embodiments are each configured to have an X-ray light source that irradiates continuous X-rays and an energy discrimination detector, as in the conventional photon counting CT apparatus. That is, the combination of the light source unit and the detection unit has the configuration (C) described above.
The apparatus and method of the present invention are not limited to the configuration (C), and can also be applied to the configurations (A) and (B) described above.

When the present invention is applied to the configuration of (A), for example, when measuring a calibration jig and measuring a measurement target, X-ray irradiation with different energy (or energy region) from each X-ray light source is performed. Are performed sequentially or simultaneously. Thereby, data necessary for calculating the calibration coefficients F (k) and G (k) and the electron density of the measurement object are obtained.

When the present invention is applied to the configuration of (B), for example, when measuring a calibration jig and measuring a measurement target, X-ray irradiation and change of energy (or energy region) of the irradiated X-rays are performed. , X-ray irradiation is repeated to measure a plurality of energies (or energy regions) with X-rays. Thereby, data necessary for calculating the calibration coefficients F (k) and G (k) and the electron density of the measurement object are obtained.

Actually, the calibration coefficient was calculated using a calibration jig, and the measurement target was analyzed by applying the calibration coefficient.

As calibration jigs, three types of carbon C, magnesium Mg, and aluminum Al were prepared.
Specifically, a cylindrical calibration jig having a diameter of 5 mm made of each element was prepared.
In this example, three types of calibration jigs themselves were measured.

FIG. 4 shows a schematic configuration diagram of the X-ray CT apparatus used for the measurement of the example.
An X-ray CT apparatus 70 shown in FIG. 4 has an X-ray light source 72, an XYZ precision automatic stage 73, a slit 74, an XYZθ precision automatic stage 76, an energy discrimination X-ray intensity detector 77, and an XYZ precision automatic stage on a surface plate 71. 78. Further, a CT imaging control PC (personal computer) 79 is provided outside the surface plate 71.

A slit 74 is disposed in front of the X-ray light source 72, and X-rays are irradiated forward from the X-ray light source 72. The X-ray light source 72 is disposed on an XYZ precision automatic stage 73, and the position can be changed by the XYZ precision automatic stage 73.
On the XYZθ precision automatic stage 76, a sample 75 serving as a calibration jig and a measurement object is arranged. The XYZθ precision automatic stage 76 can change the position and orientation of the sample 75 disposed thereon.
On the XYZ precision automatic stage 78 in front of the sample 55, an energy discrimination X-ray intensity detector 77 is arranged. The position of the energy discrimination X-ray detector 77 can be changed by an XYZ precision automatic stage 78.
As the X-ray light source 72, a W anode X-ray light source was used. A photon counting line detector was used as the energy discrimination X-ray intensity detector 77.
The surface plate 71 is 1 m long by 2 m wide.

Although not shown in detail in FIG. 4, the CT imaging control PC 79 includes an X-ray light source 72 and an XYZ precision automatic stage 73, an XYZθ precision automatic stage 76 for the sample 75, an energy discrimination X-ray intensity detector 77 and Each is connected to an XYZ precision automatic stage 78.

First, CT imaging was performed using each calibration jig of carbon C, magnesium Mg, and aluminum Al as a sample 75.
The X-ray light source 72 generated continuous X-rays in a wide energy range and irradiated the sample 75.
Then, X-rays transmitted through the sample 75 were detected and discriminated into energy regions of 50 to 60 KeV, 60 to 70 keV, 70 to 80 keV, and 80 to 90 keV by the energy discrimination X-ray detector 77. Then, the absorption coefficient μ (k) in each energy region k was calculated from the detected values in each energy region.

From the calculated absorption coefficient (k), the theoretical value of the atomic number Z and the electron density ρ _e of each calibration jig (C, Mg, Al), μ / ρ _e and Z ⁴ are calculated, and each energy region , The values of μ / ρ _e and Z ⁴ of three calibration jigs were plotted, and three approximate straight lines were obtained.
The results for each energy region are shown in FIGS. 5A-5D. FIG. 5A shows the results in the energy region 50-60 keV, FIG. 5B shows the results in the energy region 60-70 keV, FIG. 5C shows the results in the energy region 70-80 keV, and FIG. 5D shows the results in the energy region 80-90 keV. Show.

5A to 5D, in any energy region, the three points are arranged in a substantially straight line, and the approximate straight line is obtained with high accuracy. The approximate line is μ / ρ _e = F (k) × Z ⁴ + G (k), and the calibration coefficients F (k) and G (k) of each energy region k are obtained from the slope and intercept of the approximate line.
In the energy region 50 to 60 keV shown in FIG. 5A, F (k) = 2.0976 × 10 ⁻²⁹ and G (k) = 6.0186 × 10 ⁻²⁵ .
In the energy region 60 to 70 keV shown in FIG. 5B, F (k) = 1.244 × 10 ⁻²⁹ and G (k) = 5.943 × 10 ⁻²⁵ .
In the energy region 70 to 80 keV shown in FIG. 5C, F (k) = 9.585 × 10 ⁻³⁰ and G (k) = 5.587 × 10 ⁻²⁵ .
In the energy region 80 to 90 keV shown in FIG. 5D, F (k) = 6.5554 × 10 ⁻³⁰ and G (k) = 5.4434 × 10 ⁻²⁵ .

Next, CT imaging was again performed using each calibration jig (C, Mg, Al) as a sample 75 as a measurement object.
The X-ray light source 72 generated continuous X-rays in a wide energy range and irradiated the sample 75.
Then, X-rays transmitted through the sample 75 were detected and discriminated into energy regions of 50 to 60 KeV, 60 to 70 keV, 70 to 80 keV, and 80 to 90 keV by the energy discrimination X-ray detector 77. Then, the absorption coefficient μ (k) in each energy region k was calculated from the detected values in each energy region.

From the calculated absorption coefficient μ (k) and the previously determined calibration coefficients F (k) and G (k) of each energy region k, μ (k) / G (k) and F (k) / G ( k) is calculated, and in each of the three samples 75, μ (k) / G (k) and F (k) / G (k) of four energy regions are plotted, and four points are approximated. A straight line was obtained.
The results for each sample are shown in FIGS. 6A-6C. The result of carbon C is shown in FIG. 6A, the result of magnesium Mg is shown in FIG. 6B, and the result of aluminum Al is shown in FIG. 6C.

6A to 6D, in any sample, four points are arranged in a substantially straight line, and an approximate straight line is obtained with high accuracy. An approximate straight line is defined as μ (k) / G (k) = ρ _e Z ⁴ {F (k) / G (k)} + ρ _e , and ρ _e Z ⁴ and ρ of each sample are obtained from the slope and intercept of the approximate straight line. _e was determined.
In the carbon C sample shown in FIG. 6A, ρ _e Z ⁴ = 5.7328 × 10 ²⁶ and ρ _e = 5.6629 × 10 ²³ were obtained.
In the magnesium Mg sample shown in FIG. 6B, ρ _e Z ⁴ = 1.1265 × 10 ²⁸ and ρ _e = 5.2196 × 10 ²³ were obtained.
In the aluminum Al sample shown in FIG. 6C, ρ _e Z ⁴ = 2.1417 × 10 ²⁸ and ρ _e = 7.6542 × 10 ²³ were obtained.

Then, the effective atomic number Z _eff was calculated from the obtained ρ _e Z ⁴ and ρ _e . Moreover, the ratio of error was calculated _| required from the theoretical value of electron density (rho) _e and effective atomic number _Zeff of each element, and the analysis result of the calculated electron density and effective atomic number. The results of each sample are summarized in Table 1.
In Table 1, since the theoretical value is 3 digits after the decimal point, the value of the analysis result shown above is rounded off to the fourth digit after the decimal point. The error ratio is two significant figures.

From Table 1, it can be seen that the relative errors are electron density ρ _e <0.7% and effective atomic number Z _eff <6.0%, which can be calculated with high accuracy.

In the above-described embodiment, a calibration jig made of a single element is used as a measurement object as it is, but a compound or alloy having two or more of these elements or an object containing any of these elements is used. Similarly, the electron density and effective atomic number can be calculated with high accuracy as the measurement target.
In the above-described embodiment, a calibration jig made of a single element is used. However, a substance composed of a compound that is homogeneous and capable of calculating an effective atomic number and electron density is used for the calibration jig. Similarly, the electron density and effective atomic number of the measurement target can be calculated with high accuracy.

The present invention is not limited to the configurations of the above-described embodiments and examples, and various other configurations can be adopted within the scope of the present invention.

1,40,70 X-ray CT apparatus, 10,50 X-ray transmission intensity measurement unit, 11,51,72 X-ray light source, 12,15,53,57 slit or collimator, 13,54 calibration jig, 14 bed Equipment, 16, 58, 77 Energy discriminating X-ray intensity detector, 17 Data collection unit, 20, 60 Electron density and effective atomic number calibration system unit, 21, 61 control unit, 22, 62 calibration unit, 23, 63 analysis unit , 30 subject, 52, 59, 73, 78 XYZ precision automatic stage, 55 measurement object, 56, 76 XYZθ precision automatic stage, 71 surface plate, 74 slits, 75 samples, 79 CT control PC for CT imaging

Claims (10)

1. A light source unit having an X-ray light source and capable of irradiating an object to be measured with X-rays in a plurality of energy regions;
   A detection unit for detecting X-rays irradiated from the X-ray light source of the light source unit and transmitted through the object to be measured;
   A processing unit for processing X-ray data detected by the detection unit;
   The processing unit uses two or more kinds of calibration jigs made of a single element or compound that are homogeneous and capable of calculating effective atomic number and electron density, and each of the calibration jigs is a plurality of the calibration jigs. An absorption coefficient μ (k) in each energy region k of each calibration jig is obtained from data detected and detected in each energy region, and an absorption coefficient in each energy region k of each calibration jig. From μ (k), calibration coefficients F (k) and G (k) in each energy region k are calculated based on the following formula (3).
   

   

   [In Formula (3), ρ _e represents the electron density of the calibration jig, and Z represents the effective atomic number of the calibration jig. ]
   Further, the processing unit obtains an absorption coefficient μ (k) in each energy region k of the measurement object from data obtained by measuring and detecting the measurement object in each of the plurality of energy regions. From the absorption coefficient μ (k) in the energy region k and the calibration coefficients F (k) and G (k) in each energy region k, the electron density ρ _{e of the} measurement object is calculated based on the following equation (4). And calculate the effective atomic number Z
   

   

   [In Formula (4), ρ _e represents the electron density of the measurement object, and Z represents the effective atomic number of the measurement object. ]
   X-ray CT system.
2. The X-ray CT apparatus according to claim 1, further comprising a switching mechanism that switches between the calibration jig and the measurement target.
3. The X-ray CT apparatus according to claim 2, wherein the switching mechanism is controlled to automatically switch the calibration jig and the measurement object.
4. The X-ray light source of the light source unit is configured to irradiate continuous X-rays having a spectrum over the plurality of energy regions, and the detection unit is configured to discriminate and detect X-rays for each specific energy region. The X-ray CT apparatus according to any one of claims 1 to 3.
5. A method for measuring an electron density and an effective atomic number of a measurement object using an X-ray CT apparatus,
   Using two or more kinds of calibration jigs composed of a single element or compound that is homogeneous and whose effective atomic number and electron density can be calculated, each of the calibration jigs is obtained by the X-ray CT apparatus. , Respectively measuring in a plurality of energy regions, to determine the absorption coefficient μ (k) in each energy region k of each calibration jig,
   Based on the following equation (3), calibration coefficients F (k) and G (k) in each energy region k are calculated from the absorption coefficient μ (k) in each energy region k of each calibration jig. ,
   

   

   [In Formula (3), ρ _e represents the electron density of the calibration jig, and Z represents the effective atomic number of the calibration jig. ]
   With the X-ray CT apparatus, the measurement object is measured in each of the plurality of energy regions, and the absorption coefficient μ (k) in each energy region k of the measurement object is obtained.
   From the absorption coefficient μ (k) in each energy region k of the measurement object and the calibration coefficients F (k) and G (k) in each energy region k, the measurement is performed based on the following equation (4). Calculate the target electron density ρ _e and effective atomic number Z
   

   

   [In Formula (4), ρ _e represents the electron density of the measurement object, and Z represents the effective atomic number of the measurement object. ]
   Measuring method of electron density and effective atomic number.
6. The X-ray CT apparatus irradiates an object to be measured with continuous X-rays having a spectrum extending over the plurality of energy regions from an X-ray light source, and discriminates and detects X-rays transmitted through the object to be measured for each specific energy region. The method for measuring electron density and effective atomic number according to claim 5, wherein
7. The method for measuring an electron density and an effective atomic number according to claim 5 or 6, wherein the X-ray CT apparatus is configured to be able to take out the calibration jig.
8. The method for measuring electron density and effective atomic number according to any one of claims 5 to 7, wherein a substance selected from carbon, magnesium, and aluminum is used for the calibration jig.
9. A CT examination method for conducting a CT examination on a human subject,
   9. The method of measuring electron density and effective atomic number according to claim 5, wherein the measurement object is the object, and the electron density and effective atomic number of the object are measured. ,
   A CT examination method for obtaining an electron density map inside the subject based on the measured electron density and effective atomic number of the subject.
10. An inspection method for inspecting an object and detecting the presence or absence of explosives in the object,
    The method of measuring electron density and effective atomic number according to any one of claims 5 to 8, wherein the measurement object is the object, the electron density and effective atomic number of the object are measured,
    An inspection method for detecting the presence or absence of the explosive by identifying a substance inside the object based on the measured electron density and effective atomic number of the object.

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