JP2008224609A - Threshold determination method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide threshold determination method that can more efficiently determine energy threshold which can separate scattering components. <P>SOLUTION: In this method, radiation detector 3 uses photon counting method to detect radiation R from radiation source 2 through collimator 6, defines the sensing area of rectilinear propagation component of the radiation as the first area A and also that of scattering component as the second areas B1, B2 among the sensing area of the detector, based on the detected data, computing the mean-value ratio of these counted values in the first and second areas (first count ratio). Next, the detector detects radiation which passed through radiated object 5, while raising the energy threshold, to compute the mean-value ratio of those counted values in the first and second areas (second count ratio), based on detected data of each energy threshold. Then, the second count ratio is selected from a plurality of energy thresholds under specific requirements, based on the first count ratio to define the energy threshold for the second count ratio as the threshold for discriminating the rectilinear propagation component. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for determining an energy threshold for energy discrimination in an energy discrimination type photon counting radiation detector.

Conventionally, as a radiation detector used as a technology in this field, for example, there is one disclosed in Patent Document 1. Patent Document 1 discloses an imaging device used for nuclear medicine diagnosis and the like, and this imaging device detects an RI (radiation isotope) administered to a subject and obtains an image. In the imaging apparatus described in Patent Document 1, the radiation detection unit outputs an energy signal corresponding to the energy of the incident radiation. Then, the energy signals are discriminated in a predetermined energy window determined by the two reference signals (energy threshold values), and the detected energy signals are counted to obtain detection data. In the technique described in Patent Document 1, the reference signal (energy threshold) for dividing the energy window is set so that the energy window has a predetermined width based on the RI peak energy value, and the measurement is performed. Sometimes the energy window is shifted to reduce the effects of scattered radiation.
JP-A-8-166457

  However, in the technique described in Patent Document 1, since the shift amount of the energy window is simply calculated based on the peak energy value of RI and the peak energy value of Compton scattering, for example, white X For radiation having a continuous energy distribution such as a line, it has been difficult to efficiently remove scattered components.

  Therefore, an object of the present invention is to provide a threshold determination method capable of determining an energy threshold that can more efficiently separate scattered components.

  A threshold value determination method according to the present invention is a threshold value determination method for determining an energy threshold value for energy discrimination in an energy discrimination type radiation detector having a radiation detection unit in which a plurality of radiation detection elements are arranged. (1) a first radiation detection step in which radiation output from the radiation source is detected by a radiation detector; and (2) output from the radiation source in the radiation detection unit based on detection data in the first radiation detection step. Radiation detection in which a region including a plurality of radiation detection elements that detect a straight component in the emitted radiation is set as the first region, and a scattered component is detected in the radiation output from the radiation source in the radiation detection unit A region setting step for setting a region including a plurality of elements as the second region; and (3) the first region based on the detection data in the first radiation detection step. First count ratio calculation for calculating, as a first count ratio, a ratio of an average value of count values associated with a plurality of radiation detection elements in the second region to an average value of count values associated with a plurality of radiation detection elements And (4) an object to be irradiated is disposed between the radiation source and the radiation detector, and the radiation output from the radiation source and passed through the object to be irradiated is first to nth for energy discrimination by the radiation detector. A second radiation detection step for detecting each energy threshold value, and (5) a plurality of radiations in the first region based on detection data for each of the first to n-th energy threshold values in the second radiation detection step. The second count ratio is calculated as a ratio of the average value of the count values associated with the detection elements to the average value of the count values associated with the plurality of radiation detection elements in the second region. A count ratio calculating step; and (6) a second count ratio selected based on a predetermined condition determined based on the first count ratio among n second count ratios among the first to nth energy thresholds. And a threshold value determining step for determining the associated energy threshold value as a straight component discrimination threshold value.

  In the above method, in the first radiation detection step, when detecting the radiation output from the radiation source by the radiation detector, the geometrical arrangement of the radiation source and the radiation detector and the focus of the radiation source in the radiation source are determined. Radiation is mainly irradiated to a plurality of local radiation elements determined by size, shape, etc., but radiation is also detected by surrounding radiation detection elements due to scattering in the air. Accordingly, the plurality of radiation detection elements included in the radiation detection unit include a radiation detection element that detects a straight traveling component of radiation output from the radiation source and a radiation detection element that detects a scattering component. It will be. In the first radiation detection step, the straight component is dominant in the radiation detected by the radiation detector. For this reason, the count value corresponding to the detection result of the radiation detection element detecting the straight component increases, and the count value corresponding to the detection result of the radiation detection element detecting the scattering component decreases. Therefore, based on the detection data, it is possible to select a region including the radiation detection element that detects the straight component and a region including the radiation detection element that detects the scattering component in the radiation detection unit.

  Therefore, in the above method, the region setting step sets, as the first region, a region including a plurality of radiation detection elements that detect a straight component of radiation incident on the radiation detector in the radiation detection unit based on the detection data. In the radiation detection unit, a region including a plurality of radiation detection elements that detect the scattered component is set as the second region. Then, the ratio of the average value of the count values in the second area to the average value of the count values in the first area is calculated as the first count ratio by the first count ratio calculation step.

  Moreover, in the said method, the radiation which detected the radiation which passed the to-be-irradiated object arrange | positioned between a radiation source and a radiation detector is detected by a radiation detector by implementing a 2nd radiation detection process. At this time, radiation is detected using the first to nth energy thresholds. In the second count ratio calculation step, the second count ratio is calculated for each of the first to nth energy thresholds. As a result, n second count ratios are acquired.

  In the threshold value determining step, the energy threshold value corresponding to the second count ratio selected from the first to n-th energy threshold values according to a predetermined condition determined by the first count ratio among n second count ratios. Is determined as a straight component discrimination threshold.

  In the detection data having a larger energy threshold value among the first to nth energy threshold values, a large amount of scattered components are removed, so that the second count ratio becomes small and approaches the first count ratio. Therefore, by selecting the second count ratio according to a predetermined condition determined by the first count ratio and determining the corresponding energy threshold value as the straight component discrimination threshold value, the scattered components can be more reliably separated and go straight. It is possible to determine an energy threshold that can more accurately discriminate components.

  In the threshold setting method, the first region and the second region are set with reference to the actual detection data, and the straight component discrimination threshold is determined using the count values in the first region and the second region. Therefore, by using the straight component discrimination threshold as an energy threshold, it is possible to acquire detection data in which the straight component in the radiation detected by the radiation detector is dominant.

  Therefore, by detecting the radiation that has passed through the irradiated object by irradiating the irradiated object with the straight component discrimination threshold set, detection data with more scattered components removed and the straight traveling component being dominant Based on this, it is possible to form a transmission image. As a result, the thickness, material, and the like of the irradiated object can be acquired more accurately, and a transmission image with a clearer outline of the irradiated object can be obtained.

  The threshold determination method according to the present invention further includes a reference count ratio calculation step of calculating a reference count ratio serving as a reference for determining the straight component discrimination threshold based on the first count ratio. In the determination step, it is effective to select the second count ratio from the n second count ratios with a predetermined condition being closest to the reference count ratio.

  In this case, the reference count ratio is preferably calculated by multiplying the first count ratio by a preset scattering component allowable value.

  The count ratio in the scattering region used when calculating the first count ratio is due to the scattering component caused by the radiation that does not pass through the irradiated object, and includes a predetermined radiation source and a radiation detector. It is unique to each system. Therefore, when the object to be irradiated is arranged, the amount of scattering component allowable for the first count ratio is determined in advance as the scattering component allowable value, and the reference count ratio is calculated by multiplying the first count ratio. Thus, it is possible to obtain detection data from which a desired amount of the scattered component has been removed.

  Further, in the threshold value determining step of the threshold value determining method according to the present invention, the energy that becomes a substantially minimum count value among the count values acquired by the radiation detector while changing the energy threshold value in a state where no radiation is output from the radiation source. It is preferable to further include an initial threshold value determining step for determining the threshold value as an initial threshold value.

  The detection data obtained by energy discrimination using the initial threshold value determined as described above includes detection data corresponding to the scattering component in addition to the straight traveling component. Therefore, by subtracting the detection data acquired using the straight component discrimination threshold from the detection data acquired using only the initial threshold, the initial threshold is set as the lower limit threshold and the straight component discrimination threshold is set as the upper threshold. The detection data can be acquired in the energy region determined as follows and the energy region in which the scattering component is dominant.

  Furthermore, in the threshold value determining step of the threshold value determining method according to the present invention, it is preferable that the second count ratio is selected from the n second count ratios with a predetermined condition being closest to the first count ratio.

  Among the first to nth energy thresholds, the energy threshold corresponding to the second count ratio selected on condition that it is closest to the first count ratio among the n second count ratios is a straight component discrimination threshold. Since it is possible to separate more scattered components by determining as, it is possible to more accurately discriminate the straight component.

  In the region setting step of the threshold value determination method according to the present invention, the first region is set as a region including a plurality of radiation detection elements associated with a count value equal to or greater than the first count value in the radiation detection unit, and The two regions are preferably set as regions including a plurality of radiation detection elements associated with a count value equal to or smaller than the second count value smaller than the first count value in the radiation detection unit.

  According to the threshold value determination method of the present invention, it is possible to determine an energy threshold value that can more efficiently separate scattered components.

  Hereinafter, embodiments of a threshold value determination method according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.

  The present invention relates to a method for determining an energy threshold value for energy discrimination in a radiation detector that detects radiation R by a photon counting method. The radiation detector to which the present invention is applied is suitably used for a nondestructive inspection system for carrying out baggage inspection at airports, inspection of contaminants in foods, and the like.

  FIG. 1 is a block diagram of a nondestructive inspection system including a radiation detector to which an embodiment of the threshold value determination method of the present invention is applied. FIG. 1 also shows an object to be inspected (irradiated object) 5 as a scatterer to be inspected by the nondestructive inspection system 1 and a collimator 6 to be used when determining an energy threshold for energy discrimination described later. ing.

  As shown in FIG. 1, a nondestructive inspection system 1 includes a radiation source 2 that outputs radiation R, a radiation detector 3 that is an energy discrimination type and detects radiation R by a photon counting method, and a radiation detector 3. The input / output device 4 is electrically connected. The input / output device 4 includes an input unit 41 such as a keyboard and a display unit 42 such as a display, and inputs instructions from the operator, numerical data, and the like to the radiation detector 3 through the input unit 41. At the same time, the display unit 42 displays data (detection data, numerical data, etc.) from the radiation detector 3. The input / output device 4 may be a part of the radiation detector 3 and may be provided integrally with the radiation detector 3.

  The radiation detector 3 includes a radiation detection unit 31, an amplification unit 32, a voltage comparison unit 33, a threshold setting unit 34, a counting processing unit 35, a storage unit 36, and a control unit 37. Yes.

The radiation detection unit 31 is a line sensor (see FIG. 3) in which a plurality of radiation detection elements 31 _{1 to} 31 _m (m is an integer of 2 or more) are arranged in a line, and each of the radiation detection elements 31 _{1 to} 31. _m converts the incident radiation R into a pulse signal having a peak value corresponding to the energy and outputs the pulse signal. Examples of the radiation detection elements 31 _{1 to} 31 _m include those using cadmium telluride (CdTe).

The amplifying unit 32 is provided at the subsequent stage of the radiation detecting unit 31, and amplifies the pulse signals output from the radiation detecting elements 31 _{1 to} 31 _m and shapes the waveforms, and then inputs them to the voltage comparing unit 33.

The voltage comparison unit 33 discriminates the pulse signals input in association with the radiation detection elements 31 _{1 to} 31 _m in accordance with the energy threshold set by the threshold setting unit 34 and outputs the pulse signal to the counting processing unit 35. More specifically, the voltage comparison unit 33 corresponds to the energy threshold value, compares the voltage value input by the threshold setting unit 34 with the peak value of the pulse signal input from the amplification unit 32, and sets the threshold value. A pulse signal having a peak value equal to or higher than the comparison voltage value set by the unit 34 is discriminated and output to the counting processing unit 35.

The counting processing unit 35 counts the pulse signal output from each of the radiation detection elements 31 _{1 to} 31 _m and subjected to energy discrimination by the voltage comparison unit 33, and outputs a count value that is the count result to each of the radiation detection elements 31 _{1 to} 31 _m. To the storage unit 36. The storage unit 36 records the count value from the count processing unit 35 in association with each of the radiation detection elements 31 _{1 to} 31 _m , and records various other information required for radiation detection.

  The control unit 37 includes a calculation unit 37A that performs a predetermined calculation and a comparison unit 37B that compares various data in order to determine an energy threshold set by the threshold setting unit 34. The control unit 37 has a function of controlling the radiation detector 3 based on information recorded in the storage unit 36 and a command input through the input unit 41.

In the radiation detector 3, the pulse signals output from the radiation detection elements 31 _{1 to} 31 _{m included} in the radiation detection unit 31 are amplified by the amplification unit 32 and waveform-shaped, and then input to the voltage comparison unit 33. .

  The voltage comparison unit 33 compares the voltage value as the energy threshold set by the threshold setting unit 34 with the peak value of the pulse signal, and discriminates a pulse signal having a peak value equal to or higher than the voltage value serving as a reference for comparison. Input to the counting processing unit 35.

The count processing unit 35 counts the pulse signal input from the voltage comparison unit 33 and inputs the count value as the count result to the storage unit 36. The storage unit 36 records the count value input from the count processing unit 35 as detection data in association with each of the radiation detection elements 31 _{1 to} 31 _m .

Then, the control unit 37 inputs the detection data stored in the storage unit 36 to the input / output device 4. When detection data is input from the radiation detector 3, the input / output device 4 maps the corresponding count values to the positions of the radiation detection elements 31 _{1 to} 31 _m and displays them on the display unit 42 as a count distribution diagram. To do. In this count distribution diagram, an inspection object 5 that is a scatterer is disposed between the radiation source 2 and the radiation detector 3, and the radiation R that has passed through the inspection object 5 is detected by the radiation detector 3. In this case, it corresponds to a transmission image of the inspection object 5. Therefore, the radiation detection unit 31 functions as an image sensor, and in this case, the radiation detection elements 31 _{1 to} 31 _m function as pixels.

  Here, a method of determining an energy threshold for energy discrimination of the pulse signal by the voltage comparison unit 33 will be described, and the function of the control unit 37 which is one feature in the radiation detector 3 will be described in more detail.

  As shown in FIG. 2, the threshold determination method includes an initial threshold determination step S1, a first radiation detection step S2, a region setting step S3, a first count ratio calculation step S4, and a reference count ratio calculation step S5. The second radiation detection step S6, the second count ratio calculation step S7, and the threshold value determination step S8.

  In the initial threshold value determination step S1, the threshold value is set so that the count value output from the count processing unit 35 in a state in which the radiation R is not output from the radiation source 2 is substantially minimum, that is, preferably 0 or substantially 0. . In this step, while increasing the energy threshold value by the threshold value setting unit 34, the energy threshold value when each count value is preferably 0 or almost 0 is set as the initial threshold value. This initial threshold is for removing the so-called dark count. Here, the initial threshold value is the energy threshold value when the count value is preferably 0 or almost 0, but the count value output from the count processing unit 35 is minimized within an allowable numerical range. The energy threshold may be used.

  In the next first radiation detection step S2, the radiation detector 3 detects the radiation R in the arrangement relationship shown in FIG. In FIG. 3, the components other than the radiation detector 31 of the radiation detector 3 and the input / output device 4 are not shown. This first radiation detection step S2 will be described in more detail.

  As shown in FIG. 3, the collimator 6 is disposed in front of the radiation source 2, and the radiation R output from the radiation source 2 is narrowed by the collimator 6 and then directly detected by the radiation detector 3. In this detection, the energy threshold value of the radiation detector 3 is set as the initial threshold value acquired in the initial threshold value determining step S1.

The detection data of the radiation R in the first radiation detection step S2 is recorded in the storage unit 36 and displayed on the display unit 42 as a count distribution chart as shown in FIG. FIG. 4 is a schematic diagram of the count distribution based on the detection data in the first radiation detection step. The horizontal axis shown in FIG. 4 represents the positions of the radiation detection elements (pixels) 31 _{1 to} 31 _m , and the vertical axis represents the count value.

  In the first radiation detection step S2, since the inspection object 5 is not arranged between the collimator 6 and the radiation detector 3, ideally the geometry of the radiation source 2, the collimator 6 and the radiation detection unit 31 is used. The radiation R is detected only in the irradiation region determined by the target arrangement. However, in reality, the radiation R is detected around the irradiation region due to scattering when passing through the collimator 6, scattering in the air after passing through the collimator 6, and the like. Therefore, the count distribution displayed on the display unit 42 is, as shown in FIG. 4, a region where a straight component that is output from the radiation source 2 and is not scattered is detected and has a larger count value, and positions on both sides thereof. Thus, a substantially rectangular count distribution consisting of a region where a scattered component is detected and a smaller count value is obtained.

In the region setting step S3, based on the count distribution displayed on the display unit 42, as shown in FIG. 1 area) A is set as A, and areas located on both sides of the direct incident area A and having a smaller count value in response to detection of the scattering component of the radiation R are designated as scattering areas (second areas) B1 and B2. Set. In this setting, the operator inputs information for designating a radiation detection element to be a boundary pixel between the direct incident area A and the scattering areas B1 and B2 among the radiation detection elements 31 _{1 to} 31 _m while viewing the count distribution. It inputs by inputting to the radiation detector 3 through.

More specifically, the operator sets the direct incident area A by designating the radiation detection elements 31 _a1 and 31 _a2 which are the boundary pixels on both sides of the area to be the direct incident area A through the input unit 41. . In addition, the operator sets the scattering regions B1 and B2 by designating the radiation detection elements 31 _b1 and 31 _b2 which are the boundary pixels on the direct incident region A side of the regions to be the scattering regions B1 and B2, respectively. In the setting of the scattering regions B1 and B2, when the radiation detection elements 31 _b1 and 31 _b2 are designated, the control unit 37 of the radiation detector 3 is positioned at both ends of the line sensor from each of the radiation detection elements 31 _b1 and 31 _b2. The regions up to the radiation detecting elements 31 ₁ and 31 _m to be set may be set as the scattering regions B1 and B2, respectively. The designation information of the direct incident area A and the scattering areas B1 and B2 input through the input unit 41 is recorded in the storage unit 36.

The radiation detection elements 31 _a1 and 31 _b1 may be different as shown in FIG. The same applies to the radiation detection elements 31 _a2 and 31 _b2 .

In the first count ratio calculation step S4, the arithmetic unit 37A of the radiation detector 3, the average value M1 _D of the count value of the direct incidence region A stored in the storage unit 36, the count of the scattering region B1, the B2 An average value of the values M1 _S is calculated, and M1 _S / M1 _D is calculated as the first count ratio C1.

In the reference count ratio calculation step S5, and calculates a reference count ratio C _B by multiplying the scatter component tolerance S the operator to set the first count ratio C1. The reference count ratio C _B separates the scattered component from the radiation R to be detected, in other words, the count ratio to be a reference for determining the energy threshold to discriminate the rectilinear wave. The reference count ratio C _S and the scattering component allowable value S will be further described later.

  In the threshold value determination method of this embodiment, as shown in FIG. 2, the second radiation detection step S6 is performed after the reference count ratio calculation step S5. In this step, the object 5 to be inspected, which is a scatterer, is arranged between the collimator 6 and the radiation detector 3 in the arrangement relationship in the first radiation detection step S2 shown in FIG. . After the radiation R is output from the radiation source 2 and passed through the collimator 6, the radiation R that has passed through the inspection object 5 is detected by the radiation detector 3. In the second radiation detection step S6, the radiation R is detected at each of the first to nth energy thresholds while sequentially increasing the energy threshold for energy discrimination by a predetermined step value. Note that the first energy threshold value may be the same as or different from the initial threshold value. Detection data for each of the first to nth energy threshold values is recorded in the storage unit 36, respectively.

The second count ratio calculation step S7 is performed while detecting the radiation R in the second radiation detection step S6. That is, every time the threshold setting unit 34 increases the voltage value corresponding to the energy threshold by a predetermined step value, in other words, the calculation unit 37A is directly incident on each voltage value corresponding to the first to nth energy thresholds. A second count ratio C2 is calculated as a ratio (M2 _S / M2 _D ) of the average value M2 _S of the count values in the scattering regions B1 and B2 to the average value M2 _D of the count values in the region A. Since the second count ratio C2 is calculated for each of the first to nth energy thresholds, n second count ratios C2 _{1 to} C2 _n are calculated as a result. The n second count ratios C2 _{1 to} C2 _n are recorded in the storage unit 36.

Here, the second count ratio calculation step S7 is performed while detecting the radiation R in the second radiation detection step S6, but the following may be performed. That is, in the second radiation detection step S <b> 6, the radiation R is detected using the first to nth energy thresholds, and the detection data of each energy threshold is recorded in the storage unit 36. Then, after the completion of the second radiation detection step S6, the second count ratio calculation step S7 is performed, and n second count ratios C2 _{1 to} C2 _n are calculated using each detection data in the storage unit 36. May be.

In the threshold value determining step S8, comparing unit 37B by the control unit 37 has is compared with n second count ratio _C2 1 -C2 _n, and a reference count ratio _{C S} calculated in reference count ratio calculation step S5. The straight comparison section 37B is, among the energy threshold of the first to n, the energy threshold corresponding to the reference count ratio _{C S} nearest second count ratio C2 from the second count ratio _C2 1 -C2 _n Determined as a component discrimination threshold. The straight component discrimination threshold is a scattering component removal threshold because it is used to remove the scattered component from the radiation R incident on the radiation detector 31 and discriminate the straight component.

  Conversely, it can be said that the straight component discrimination threshold is a threshold set when the scattered component is selectively acquired together with the initial threshold. When the radiation R is detected using the initial threshold as the energy threshold, the detection data includes a scattering component in addition to the straight traveling component. Therefore, the scattering component has the initial threshold as the lower limit threshold, and the straight component discrimination threshold. Is included in the energy region with the upper limit threshold. Therefore, for example, the scattered data is obtained by subtracting the detection data when the radiation R is detected using the straight component discrimination threshold as the energy threshold from the detection data when the radiation R is detected using only the initial threshold as the energy threshold. can do. Hereinafter, an energy region in which the initial threshold is a lower threshold and the straight component discrimination threshold is an upper threshold is also referred to as a scattering component acquisition region. The straight component discrimination threshold and the initial threshold are recorded in the storage unit 36. When these threshold values are used as energy threshold values, the threshold value setting unit 34 reads out from the storage unit 36 and inputs voltage values corresponding to them to the voltage comparison unit 33.

Here, the reference count ratio _CS will be described.

In the second radiation detection step S6, when the energy threshold value is increased, detection data from which scattered components are removed as the energy threshold value increases is acquired. Therefore, theoretically, the second count ratio C2 _k (k is an integer from 1 to n) and the first count ratio C1 at a certain energy threshold value coincide. However, depending on the measurement conditions (for example, the characteristics of the inspection object 5 and the radiation source 2), the lower limit value of the second count ratio C2 is normally reached without reaching the first count ratio C1.

Accordingly, for example, the inspection object 5 is allowed to be allowed to what extent the situation in the case where the inspection object 5 is not disposed with respect to the scattering components in the scattering regions B1 and B2 when the inspection object 5 is disposed. The operator determines in advance based on the degree of radiation scattering that can be determined and predicted based on the material, thickness, etc., and sets the value as the scattering component allowable value S. The scattered component tolerance S count ratio obtained by multiplying the first count ratio C1, in the first to n-th reference count ratio C _S should be a reference for determining the rectilinear wave discrimination threshold among the energy threshold of is there. Thus, in order to calculate the reference count ratio _{CS based} on the first count ratio C1 and the scattering component allowable value S, the scattering component is removed from the radiation R incident on the radiation detection unit 31 by an amount desired by the operator. Is possible.

  The scattering component allowable value S may be input in advance by the operator through the input unit 41 to the storage unit 36 before the initial threshold value determining step S1, or after the first count ratio calculating step S4, the first count ratio C1. You may input according to the magnitude | size of.

In the above description, the scattering component permissible value S type is operator, but calculation unit 37A is to calculate a reference count ratio C _S, after the first count ratio calculation step S4, the operator, first count after calculating the reference count ratio C _S is multiplied by the scattered component tolerance S to the ratio C1, it may be input to the radiation detector 3 through the input unit 41 and the reference count ratio C _S.

  Next, a method for determining the straight component discrimination threshold will be described more specifically using experimental examples and numerical examples shown in FIGS.

FIG. 5 is a diagram showing a count distribution based on detection data in the first radiation detection step. An X-ray tube is used as the radiation source 2 when acquiring the detection data of FIG. 5, and a line sensor in which ₆₄ radiation detection elements 31 _{1 to} 31 ₆₄ are arranged in a line is used as the radiation detection unit 31. is doing. The collimator 6 uses a slit. Furthermore, the energy threshold value for detecting the radiation R is 20 keV, and this energy threshold value is determined by performing the initial threshold value determining step S1.

  As shown in FIG. 5, a larger count value is acquired at the central portion of the radiation detection unit 31, and a smaller count value is acquired in an area located outside the center. Based on this count distribution, the direct incident area A and the scattering areas B1 and B2 are set as shown in FIG. 5, and the first count ratio C1 is calculated. In the example of FIG. 5, the first count ratio C1 is 0.05.

  FIG. 6 is a diagram showing a count distribution based on detection data in the second radiation detection step. In FIG. 6, the horizontal axis indicates the position of the radiation detection element, and the vertical axis indicates the relative count value. The relative count value is a count value normalized by a count value associated with one radiation detection element in the direct incident area A.

上記第１〜第１０のエネルギー閾値毎の検出データに基づくカウント分布では、図６に示すように、エネルギー閾値が増加するにつれて散乱領域Ｂ１，Ｂ２における相対カウント値が徐々に小さくなる。これは、エネルギー閾値が増加するにつれて散乱成分がより除去されるためである。 When the detection data shown in FIG. 6 is acquired, a 10 mm-thick SUS plate is arranged as the inspection object 5 between the collimator 6 and the radiation detection unit 31. In addition, in the second radiation detection step S6 when acquiring the detection data shown in FIG. 6, the first energy threshold is set to 20 keV, the step value is increased from the first energy threshold to 10 keV, and increased to 110 keV. To the tenth energy threshold. In this case, Table 1 shows numerical values of the first to tenth energy thresholds.

In the count distribution based on the detection data for each of the first to tenth energy threshold values, as shown in FIG. 6, the relative count values in the scattering regions B1 and B2 gradually decrease as the energy threshold value increases. This is because the scattered components are more removed as the energy threshold increases.

Of the second count ratios C2 _{1 to} C2 ₁₀ calculated based on the detection data for each energy threshold shown in FIG. 6, the second count ratios B ₁ , B ₅ , B ₆ , B ₇ , and B ₈ are each 0. .25, 0.22, 0.2, 0.18, and 0.14.

As described above, since the first count ratio C1 is 0.05, for example by setting the scattering component tolerances as 3, reference count ratio C _S becomes 0.15.

In this case, among the second count ratio _C2 1 _{-C2 10,} the second count ratio C2 closest to the reference count ratio _{C S} is 0.14 of a second count ratio C2 _8. Therefore, to determine the 90keV which is a value of the energy threshold of the 8 corresponding to the second count ratio C2 ₈ as rectilinear wave discrimination threshold.

According to the threshold value determining method, the direct incident area A and the scattering areas B1 and B2 are set based on the actual detection data in the first radiation detection step S2, and the direct incident area A and the scattering areas B1 and B2 are set. The first count ratio C1 is calculated using the average values M1 _D and M1 _S of the count values. Then, based on the first count ratio C1, it calculates a reference count ratio C _S as a reference for determining the rectilinear wave discrimination threshold (upper limit threshold value of the scattered component acquisition region).

And based on the average values M2 _D and M2 _S of the count values in the direct incident area A and the scattering areas B1 and B2 in the actual detection data for each of the first to n-th energy threshold values in the second radiation detection step S6. second count ratio C2 ₁ -C2 _n were calculated respectively Te, determines the rectilinear wave discrimination threshold relative to the reference count ratio C _S.

  In this case, referring to the count distribution based on the actual detection data, the direct incident area A and the scattering areas B1 and B2 are set, and all the count values in the direct incident area A and the scattering areas B1 and B2 are used. However, the straight component discrimination threshold is determined. As a result, by using the straight component discrimination threshold as the energy threshold for energy discrimination, the scattered component can be more efficiently separated from the radiation R detected by the radiation detector 3, and as a result, the straight component is more accurately detected. It is possible to discriminate and acquire them.

  Therefore, when the inspection object 5 is inspected in the nondestructive inspection system 1, the radiation R that has passed the inspection object 5 by irradiating the inspection object 5 with the radiation R is set to the radiation component threshold value as the energy threshold value. By detecting with the detector 3, transmission data (or absorption data) in which more scattered components are removed and noise is reduced can be acquired. Therefore, for example, the thickness, material, shape, etc. of the inspection object 5 can be acquired more accurately.

  Further, the detection data obtained by setting the straight component discrimination threshold (upper threshold of the scattered component acquisition region) as the energy threshold is subtracted from the detection data acquired by setting only the initial threshold (lower threshold of the scattered component acquisition region) as the energy threshold. Thus, data in which the scattering component is dominant can be acquired. Then, by using data in which the scattering component is dominant, it is possible to acquire information or the like that captures a change in shape with little dependence on the thickness of the inspection object 5.

  Furthermore, the detection data when the threshold value for straight component discrimination is used and the detection data using the scattering component acquisition region as the energy region for acquiring the scattering component are compared, or predetermined calculation is performed on them. Thus, it is possible to obtain the thickness, material, shape, and the like of the inspection object 5 with higher accuracy than the analysis using the radiation transmission image without energy discrimination.

  When performing nondestructive inspection of the inspection object 5 using the nondestructive inspection system 1, a collimator 6 may be installed between the radiation source 2 and the inspection object 5. The collimator 6 may be installed without reducing the focal point of the radiation generating source in the radiation source 2 or by setting the focal point of the radiation generating source to a desired shape.

As mentioned above, although embodiment of the threshold value determination method of this invention was described, this invention is not limited to the said embodiment. For example, in the radiation detection unit 31, a plurality of radiation detection elements 31 _{1 to} 31 _m are arranged in a line shape, but may be arranged in a two-dimensional shape. Also in this case, the direct incident region and the scattering are designated by designating the radiation detection element that becomes the boundary between the direct incident region (first region) and the scattering region (second region) based on the detection data acquired in the first radiation detection step S2. What is necessary is just to set an area | region. As described above, by using the radiation detection unit 3 in which the radiation detection elements are two-dimensionally arranged, and setting the straight component discrimination threshold as the energy threshold and detecting the radiation R, a transmission image with a clearer contour can be obtained. Can be acquired.

  In the flowchart shown in FIG. 2, the initial threshold value determining step is provided. However, for example, when only a straight component is selectively acquired, the initial threshold value determining step may not be performed.

Further, when the direct incident area A and the scattering areas B1 and B2 are set with the count distribution shown in FIG. 4, the operator actually sees the shape of the count distribution and the boundary between the direct incident area A and the scattering areas B1 and B2. The radiation detection elements 31 _a1 , 31 _a2 , 31 _b1 and 31 _b2 are described as being input, but the present invention is not limited to this. For example, a first count value that serves as a reference for determining the direct incident area A and a second count value that serves as a reference for determining the scattering areas B1 and B2 and is smaller than the first count value are set in advance. The incident area A and the scattering areas B1 and B2 may be set by using them.

Specifically, a region including a plurality of radiation detection elements in the radiation detection unit 31 associated with a count value equal to or greater than the first count value in the detection data in the first radiation detection step S2 is defined as a direct incident region A. A region including a plurality of radiation detection elements in the radiation detection unit 31 associated with a count value equal to or smaller than the second count value smaller than the first count value may be set as the scattering regions B1 and B2. In this case, by inputting the first count value and the second count value in the storage unit 36 in advance, the control unit 37 directly sets the incident area A and the scattering areas B1 and B2 based on the detection data. Is also possible. The first count value is equal to or more than the theoretical count value determined according to, for example, the theoretical energy of radiation output from the radiation source 2 and the detection sensitivity of the radiation detection elements 31 _{1 to} 31 _m. Just keep it a little smaller.

Furthermore, in the threshold value determination method according to the flowchart shown in FIG. 2, the reference count ratio calculation step S5 is provided, but the reference count ratio calculation step S5 may not be provided. For example, n second count ratios C2 _{1 to} C2 _n are displayed on the display unit 42, and an operator selects a desired second count based on the first count ratio C1 from the second count ratios C2 _{1 to} C2 _n. The ratio C2 may be selected and specified through the input unit 41. Furthermore, the reference count ratio may not be used. For example, the second count ratio C2 indicating the value closest to the first count ratio C1 among the _n second count ratios C2 _{1 to} C2 n calculated from the detection data for each of the first to nth energy thresholds. The associated energy threshold may be determined as the straight component discrimination threshold.

  Furthermore, in the above description, the object to be arranged to determine the straight component discrimination threshold is the inspection object 5 itself that is an actual inspection object, but is not limited to this, and the straight component discrimination threshold or the like is not limited thereto. It is also possible to use an object to be disposed between the radiation source 2 and the radiation detector 3 when determining the threshold value, different from the object to be irradiated as the actual inspection object 5. For example, in the case of acquiring ideal data that can be originally shown by the inspected object 5, the object to be irradiated when determining a threshold such as a straight component discrimination threshold is a scatterer separately prepared for data acquisition or a problem in advance. An equivalent product of the object to be inspected 5 that has been found to be absent is preferable.

  In the description so far, the initial threshold value and the straight component acquisition threshold value are determined by performing predetermined detection in a state where the collimator 6 is disposed between the radiation source 2 and the radiation detector 3. As described above, for example, when the focal point of the radiation source in the radiation source 2 is small, or when the focal shape of the radiation source itself is a desired irradiation shape, the collimator 6 may not be disposed. Moreover, the radiation source 2 is not specifically limited, The X-ray tube as mentioned above, a radioisotope, etc. are illustrated.

It is a block diagram of an example of a nondestructive inspection system. It is a flowchart of a threshold value determination method. It is a figure which shows the detection method of the radiation in a 1st radiation detection process. It is a schematic diagram of count distribution which is the detection data in a 1st radiation detection process. It is a figure which shows count distribution which is the detection data in the 1st radiation detection process in an experiment example. It is a figure which shows count distribution which is the detection data in the 2nd radiation detection process in an experiment example.

Explanation of symbols

1 ... Nondestructive inspection system, 2 ... radiation source, 3 ... radiation detector, 5 ... object to be inspected (an object to be irradiated), 6 ... collimator, 31 ... radiation detection unit, ₃₁ 1 to 31 m _... radiation detection element, A ... direct incident area (first area), B1, B2 ... scattering area (second area), R ... radiation.

Claims (6)

A threshold determination method for determining an energy threshold for energy discrimination in an energy discrimination type radiation detector having a radiation detection unit in which a plurality of radiation detection elements are arranged and detecting radiation by photon counting ,
A first radiation detection step of detecting radiation output from a radiation source by the radiation detector;
Based on the detection data in the first radiation detection step, the radiation detection unit first includes a region including a plurality of radiation detection elements that detect a straight component of the radiation output from the radiation source. A region setting step of setting as a second region a region including a plurality of the radiation detection elements that detect a scattering component of the radiation output from the radiation source in the radiation detection unit,
Based on the detection data in the first radiation detection step, the plurality of radiation detection elements in the second region with respect to an average value of count values associated with the plurality of radiation detection elements in the first region. A first count ratio calculating step of calculating a ratio of the average values of the associated count values as a first count ratio;
An object to be irradiated is disposed between the radiation source and the radiation detector, and radiation output from the radiation source and passed through the object to be irradiated is first to nth for energy discrimination by the radiation detector. A second radiation detection step for detecting each energy threshold value;
Based on detection data for each of the first to n-th energy threshold values in the second radiation detection step, the first value relative to an average value of count values associated with the plurality of radiation detection elements in the first region. A second count ratio calculating step of calculating a second count ratio as a ratio with an average value of the count values associated with the plurality of radiation detection elements in two regions,
Among the first to n-th energy thresholds, an energy threshold associated with the second count ratio selected according to a predetermined condition determined based on the first count ratio among n second count ratios A threshold value determining step for determining a straight component discrimination threshold value;
A threshold value determining method comprising: A reference count ratio calculating step of calculating a reference count ratio as a reference for determining a straight component discrimination threshold for discriminating the straight component from the radiation incident on the radiation detector based on the first count ratio; And
2. The threshold value determining step, wherein the second count ratio is selected from n second count ratios with the predetermined condition being closest to the reference count ratio. Threshold determination method.   The threshold value determination method according to claim 2, wherein the reference count ratio is calculated by multiplying the first count ratio by a preset scattering component allowable value.   The method further includes an initial threshold value determining step of determining, as an initial threshold value, an energy threshold value that is a substantially minimum count value among count values acquired by the radiation detector while changing an energy threshold value in a state where no radiation is output from the radiation source. The threshold value determination method according to claim 1, wherein the threshold value determination method is a threshold value determination method.   2. The threshold value determining step, wherein the second count ratio is selected from n second count ratios with the predetermined condition being closest to the first count ratio. Threshold determination method.   In the region setting step, the first region is set as a region including a plurality of the radiation detection elements associated with a count value greater than or equal to a first count value in the radiation detection unit, and the second region is the radiation. 2. The threshold value determination method according to claim 1, wherein a region including a plurality of the radiation detection elements associated with a count value equal to or smaller than a second count value smaller than the first count value in the detection unit is set.

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