JP2005149762A - X-ray inspection device, and tube voltage/tube current control method of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tube voltage/tube current control method of an X-ray inspection device finding a physically optimum X-ray condition even for an unknown substance to be inspected depending on a transmission data thereof. <P>SOLUTION: Transmissivity of an intended observation part of the substance is measured from the transmission data of the substance obtained at an X-ray detector 3, and the tube voltage of an X-ray control part 6 is controlled so that the measured transmissivity comes to a prescribed value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an X-ray inspection apparatus among non-destructive inspection apparatuses, and in particular, an X-ray inspection apparatus or an X-ray fluoroscopic apparatus that creates a transmission image of a subject from transmission data of the subject, or transmission data of the subject. The present invention relates to an X-ray inspection apparatus or X-ray tomography apparatus for creating a cross-sectional image or a three-dimensional image of a specimen, and a tube voltage / tube current adjusting method thereof.

  The inside of industrial products such as electronic parts and aluminum castings is inspected by an X-ray fluoroscopic inspection apparatus. In such an X-ray fluoroscopic examination apparatus, the tube voltage V and the tube current I of the X-ray tube can be changed manually, and an optimum image is obtained by performing settings according to the subject while observing the transmission image. Yes.

  When the tube voltage is changed, the energy E of each X-ray photon increases and the number N of photons increases. When the tube current is changed, E does not change and N increases.

  The detector detects X-rays with two-dimensional resolution, and the output of each detection element is proportional to the total amount of X-ray energy (E × N) received by each element. A transmission image is created by assigning light and dark according to the output of each detection element. On the other hand, the transmission ability of X-rays increases as E increases.

  When the tube voltage and the tube current are changed manually, for example, a tube voltage (that is, E) that can pass through the subject is selected first. However, if E is too high, the contrast of the image is lowered and the best image is not obtained. Therefore, the tube voltage is lowered and the tube current is increased to supplement the output. If the tube voltage is lowered too much, the contrast will be too high, resulting in a whiteout or blackout image.

  In this way, it is troublesome to manually set the tube voltage and tube current each time the subject is replaced or the observation field of view is changed, and there is a problem that it depends on the skill of the operator. In Document 1 and Patent Document 2, a transmission image is taken in, image-processed and fed back, and a tube voltage and a tube current are automatically set.

  On the other hand, in the case of a computed tomography apparatus (CT), the tube voltage that does not cause insufficient transmission is selected empirically, and the optimum condition setting has not been performed.

In Patent Document 1 and Patent Document 2, optimum X-ray conditions (tube voltage and tube current) are automatically set even for an unknown subject based on the transmission image. The optimum conditions are automatically set to these conditions, assuming that the tube voltage and tube current in which the brightness range in which the transmission image is to be observed is within the brightness range suitable for visual observation are the optimum X-ray conditions.
JP 2002-14059 A JP 2003-173895 A

  The optimum X-ray condition as the gradation adjustment described above is different from the physical optimum X-ray condition. The physical optimum X-ray condition is a condition that gives the maximum S / N ratio (signal / noise) to the structure of the observation portion, and is irrelevant to the visibility of the transmitted image (tone adjustment).

  The present invention has been made in view of the above, and an object of the present invention is to provide an X-ray capable of obtaining a physically optimum X-ray condition for an unknown subject based on the transmission data. An object of the present invention is to provide an inspection device and a method for adjusting the tube voltage and tube current.

In order to solve the above-described problems, an invention described in claim 1 includes an X-ray controller that controls a tube voltage and a tube current applied to an X-ray tube, an X-ray detector that detects an X-ray transmitted through a subject, In the X-ray inspection apparatus having means for creating a transmission image of the subject from the transmission data of the subject obtained by the X-ray detector,
Means for measuring the transmittance of a desired observation portion of the subject from the transmission data;
And means for adjusting the tube voltage in the X-ray controller so that the measured transmittance becomes a predetermined value.

In order to solve the above-described problems, an invention according to claim 2 is directed to an X-ray controller that controls a tube voltage and a tube current applied to an X-ray tube, an X-ray detector that detects an X-ray transmitted through a subject, and In the tube voltage / tube current adjustment method of the X-ray inspection apparatus, comprising means for creating a transmission image of the subject from the transmission data of the subject obtained by the X-ray detector,
Measuring the transmittance of a desired observation portion of the subject from the transmission data;
Adjusting the tube voltage in the X-ray control unit so that the measured transmittance becomes a predetermined value.

In the configuration according to claims 1 and 2, when the tube voltage and the tube current are changed, the attenuation index τ (giving transmission of e ^−τ ) and the (X-ray quantum) noise to the observation portion of the subject change. However, the condition that the ratio between the attenuation index τ and the noise is maximum can be proved that the transmittance is about 20% (+ 10% −5%) when the product of the tube voltage and the tube current is constant ( Later).

  On the other hand, in the X-ray controller, the maximum tube current is generally limited so that the product of the tube current and the tube voltage is constant.

  Therefore, the tube voltage is adjusted so that the transmittance of the observation portion becomes a predetermined value (about 20% (+ 10% -5%)) even for an unknown subject, and the tube current is set to the maximum value with respect to the tube voltage. By doing so, it is possible to set the tube voltage and tube current at which the ratio between the attenuation index τ of the observed portion and its noise is maximized.

  The transmission image under these conditions is an image that can physically distinguish the structure of the observation part to the maximum extent, and the optimum X-ray condition can be obtained.

In order to solve the above-described problem, an invention described in claim 3 includes an X-ray controller that controls a tube voltage and a tube current applied to an X-ray tube, an X-ray detector that detects an X-ray transmitted through a subject, and X means having scanning means for scanning the transmission direction in a plurality of directions, and means for creating a cross-sectional image or a three-dimensional image of the subject from transmission data of the subject in a plurality of directions obtained by the X-ray detector. In line inspection equipment,
Means for setting a maximum attenuation path, which is an X-ray path having substantially the smallest transmission of the transmission data;
Means for measuring the transmittance of this maximum attenuation path;
And means for adjusting the tube voltage in the X-ray controller so that the measured transmittance becomes a predetermined value.

In order to solve the above-described problem, an invention described in claim 4 is an X-ray control unit that controls a tube voltage and a tube current applied to an X-ray tube, an X-ray detector that detects X-rays transmitted through a subject, and X means having scanning means for scanning the transmission direction in a plurality of directions, and means for creating a cross-sectional image or a three-dimensional image of the subject from transmission data of the subject in a plurality of directions obtained by the X-ray detector. In the tube voltage / tube current adjustment method for wire inspection equipment,
Setting a maximum attenuation path, which is an X-ray path having the smallest transmission of the transmission data;
Measuring the transmission of this maximum attenuation path;
Adjusting the tube voltage in the X-ray control unit so that the measured transmittance becomes a predetermined value.

  In the X-ray inspection apparatus for creating a cross-sectional image or a three-dimensional image, the ratio between the attenuation index τ in the maximum attenuation path and its noise is the maximum among the transmission data in a plurality of directions. It can be shown that the X-ray condition is optimal (described later).

  Therefore, looking at the shape of the subject, estimating the maximum attenuation path and setting the transmission direction, adjusting the transmission direction while viewing the transmission data under the provisional conditions, and setting the maximum attenuation path (giving transmission direction) To do.

When the tube voltage and the tube current are changed, the attenuation index τ (which gives transmission of e ^−τ ) and its (X-ray quantum) noise with respect to the maximum attenuation path change, and the ratio of the attenuation index τ and the noise changes. It can be proved that the transmittance is about 20% (+ 10% -5%) when the product of the tube voltage and the tube current is constant (described later).

  On the other hand, in the X-ray controller, the maximum tube current is generally limited so that the product of the tube current and the tube voltage is constant.

  Therefore, even for an unknown subject, the tube voltage is adjusted so that the transmittance of the maximum attenuation path becomes a predetermined value (about 20% (+ 10% -5%)), and the tube current is the maximum value for the tube voltage. As a result, the tube voltage and tube current at which the ratio between the attenuation index τ of the maximum attenuation path and its noise can be maximized can be set. As described above, the best cross-sectional image or three-dimensional image can be created from the transmission data under the X-ray conditions.

In order to solve the above problems, an invention according to claim 5 is an X-ray control unit that controls a tube voltage and a tube current applied to an X-ray tube, an X-ray detector that detects X-rays transmitted through a subject, and X means having scanning means for scanning the transmission direction in a plurality of directions and means for creating a cross-sectional image or a three-dimensional image of the subject from transmission data of the subject in a plurality of directions obtained by the X-ray detector. In line inspection equipment,
Means for setting a maximum attenuation path, which is an X-ray path having substantially the smallest transmission of the transmission data;
Means for collecting transmission data in this maximum attenuation path;
Means for calculating the tube voltage and the tube current from which the ratio of the attenuation index τ in the maximum attenuation path and the noise spreading to the attenuation index τ is the largest from the transmission data in the maximum attenuation path;
And means for setting a tube voltage and a tube current in the X-ray control unit with reference to the calculated values.

In order to solve the above problems, an invention described in claim 7 includes an X-ray controller that controls a tube voltage and a tube current applied to an X-ray tube, an X-ray detector that detects an X-ray transmitted through a subject, X means having scanning means for scanning the transmission direction in a plurality of directions, and means for creating a cross-sectional image or a three-dimensional image of the subject from transmission data of the subject in a plurality of directions obtained by the X-ray detector. In the tube voltage / tube current adjustment method for wire inspection equipment,
Setting a maximum attenuation path, which is an X-ray path having the smallest transmission of the transmission data;
Collecting transmission data along this maximum attenuation path;
Calculating the tube voltage and tube current at which the ratio of the attenuation index τ and the noise spreading to τ in the maximum attenuation path is the largest from the transmission data in the maximum attenuation path;
A step of setting a tube voltage and a tube current in the X-ray control unit with reference to the calculated value.

  In the X-ray inspection apparatus for creating a cross-sectional image or a three-dimensional image, the ratio between the attenuation index τ in the maximum attenuation path and its noise is the maximum among the transmission data in a plurality of transmission directions. It can be shown that the X-ray condition is as follows (described later).

  Therefore, looking at the shape of the subject, estimating the maximum attenuation path and setting the transmission direction, adjusting the transmission direction while viewing the transmission data under the provisional conditions, and setting the maximum attenuation path (giving transmission direction) To do.

When the tube voltage and tube current are changed, the attenuation index τ (which gives attenuation of e ^−τ ) and its noise change with respect to the maximum attenuation path, respectively. The tube voltage and tube current at which the ratio of the attenuation index τ in the maximum attenuation path to the noise becomes maximum can be calculated (described later). As described above, the best cross-sectional image or three-dimensional image can be created from the transmission data under the X-ray conditions.

  In order to solve the above problems, the inventions of claims 6 and 8 are the inventions according to claims 5 and 7, wherein the calculation is performed for a plurality of combinations of one tube voltage and the maximum tube current set for the tube voltage. The gist is to calculate the ratio and calculate the tube voltage and the tube current at which the ratio is the largest.

  In the configurations of the sixth and eighth aspects, the optimum condition can be calculated with high accuracy regardless of the function of the set maximum tube current (with respect to the tube voltage).

  According to the present invention, an X-ray inspection apparatus capable of obtaining a physically optimum X-ray condition based on a transmission image of an unknown subject and a tube voltage / tube current adjustment method thereof are provided. it can.

(First embodiment)
FIG. 1 is a schematic configuration diagram according to the first embodiment. This configuration is the same as a general X-ray fluoroscopic inspection apparatus.

  The X-ray generator 1 includes an X-ray tube and a high voltage generator, and generates an X-ray beam 2 from an X-ray focal point F. The X-ray detector 3 detects X-rays transmitted through the subject 4 with a two-dimensional resolution and outputs transmission data as a video signal. The computer 5 digitally converts the video signal with an internal capture board and takes it in and displays it as a transparent image.

  The X-ray controller 6 controls the tube voltage and tube current of the X-ray generator so as to become set values. The operator can arbitrarily set the tube voltage and the tube current on the panel of the X-ray control unit 6 (or via the computer 5), but the X-ray control unit 6 is set when the allowable maximum tube current is exceeded. The maximum allowable tube current is automatically controlled. Normally, the maximum allowable tube current is controlled in proportion to the reciprocal of the tube voltage, and the product of the tube voltage and the tube current is constant, which is called isowatt control.

  Next, the operation of this embodiment will be described. First, when the operator sets the subject 4 and turns on the X-ray, the computer 5 displays a transmission image of the subject 4. Next, the tube voltage is adjusted so that the transmittance of the portion of the transmission image to be observed is about 20% (+ 10% -5%) (the transmittance does not depend on the tube current). The tube current is the maximum tube current for this tube voltage. Thereby, the portion to be observed becomes the best image.

  To adjust the transmittance, a straight line ROI is set on the transmission image so as to cross the observation part, and the profile of this part is displayed (by the computer 5 program), and the brightness of the observation part is compared with the brightness of the air part. And do it. If there is no air portion in the transmission image, the subject is taken in and out of the X-ray beam 2 for comparison.

  Next, the best image obtained by setting the tube voltage and tube current is subjected to gradation conversion so as to be an easy-to-view image (performed by a program of the computer 5) and observed.

  The reason why the best image is obtained when the transmittance is about 20% (+ 10% -5%) will be described in order.

First, the principle of optimum V and I is “optimal V and I are V and I that maximize τ / στ, where τ is the attenuation index and στ is the noise of the attenuation index”. Here, the attenuation index τ corresponds to the line integral of the absorption coefficient of the subject along the X-ray path, and is τ when the attenuation is e ^−τ .

  When τ / στ is maximum, the transmission image is an image that can physically distinguish the structure of the observation portion to the maximum extent. For example, if this image B (linear scale) is converted into an image having an attenuation index τ (≡ln (B0 / B)), this corresponds to a shaded image of a negative photograph, and the shade width (contrast) / shading noise is The largest and best image.

  τ / στ is maximized on the upper limit line of the V and I movable ranges (this is because τ is the same for the same V, and στ decreases as I increases). Therefore, the optimum condition is on the isowatt line (V · I = constant) of the maximum output (Watt).

  Next, in the case of isowatt control, it is shown that τ / στ is maximized when the transmittance is about 20% (+ 10% −5%). Start with general theory, then move to Isowat.

In general, how to change the tube current I when the tube voltage V is changed: I kV ^k (1)
And Next, the number of X-ray photons received per pixel is N with the subject and N0 without. N0 is roughly proportional to I · V ^k1 , so substituting equation (1) here,
N0∝I ・ V ^k1 ∝V ^{(k + k1)} (2)
It becomes. According to the X-ray data, k1 is a numerical value of about 1.2. The damping index τ is
Since τ≡ln (B0 / B) ≒ ln (N0 ・ V / (N ・ V)),
τ = ln (N0 / N) (3)
The noise στ can be calculated by
στ = √ ((∂τ / ∂N) ² ) · σ _N = 1 / N · √ (N) = 1 / √ (N) (4)
It can be calculated with Here, this noise is X-ray quantum noise. Using equations (3) and (4), στ / τ is
στ / τ = 1 / √ (N0 · e ^−τ ) · 1 / τ (5)
And represented only by N0 and τ. Next, if the differential d (στ / τ) / dV is expanded using the partial differential of τ and N0,
d (στ / τ) / dV
= ∂ (στ / τ) / ∂τ · dτ / dV + ∂ (στ / τ) / ∂N0 · dN0 / dV (6)
It becomes. Substituting equation (5) on the right side and performing partial differentiation,
d (στ / τ) / dV = 1 / √ (N0)
· E ^{τ / 2} / τ · {(1 / 2-1 / τ) · dτ / dV-1 / (2 · N0) · dN0 / dV} (7)
It becomes. στ / τ is minimized when the right side of Equation (7) is 0, and the equation that gives the optimum V is
(1 / 2-1 / τ) · dτ / dV−1 / (2 · N0) · dN0 / dV = 0 (8)
It becomes. Here, dN0 / dV is obtained from the equation (2).
dN0 / dV = (k + k1) · N0 / V (9)
It can be expressed as Further, since dτ / dV is fixed at the transmission length, it is proportional to the differential dμ / dV of the absorption coefficient μ. V dependence of μ μ∝V ^k2 (10)
Then,
dτ / dV = k2 · τ / V (11)
It can be expressed as Substituting equations (9) and (11) into equation (8) and solving for τ,
(1 / 2-1 / τ) · k2 · τ-1 / (2 · N0) · (k + k1) · N0 = 0
τ = 2 + (k + k1) / k2 (12)
It becomes. V giving this τ is the optimum V.

  Next, specific numerical values will be considered in Expression (12). In the case of isowatt control, k is −1, and k1 is a constant of about 1.2. k2 is a gentle function of V. FIG. 2 shows a typical lnμ-ln (V) curve and its slope k2. In general, k2 converges to about −3 when V decreases, regardless of the element, and converges to about −0.5 when V increases. Although the transition region varies depending on the element, it is about −3 at 60 kV or less for normal light elements and about −0.5 at 100 kV or more.

FIG. 3 shows τ (optimal τ) that gives the optimum V and transmittance (= e ^−τ ) corresponding to this τ. The optimum τ is approximately 1.9 to 1.6 (substantially depending on V), and the corresponding transmittance (optimum transmittance) is 15 to 20%. When the range is expanded in consideration of the error ± 0.2 of k1, the optimum τ is 2 to 1.2, and the optimum transmittance is 14 to 30%. Therefore, if the numerical values are rounded and the tube voltage V is adjusted so that the transmittance is about 20% (+ 10% -5%) (the tube current is maximum), τ / στ of the portion to be observed becomes approximately maximum, and the physical The best image.

  The optimal transmittance of 20% is premised on isowatt control. In the case of a microfocus X-ray generator having an X-ray focal point of several μm, the X-ray control unit 6 performs isowatt control of the maximum tube current over the entire usable tube voltage, but the X-ray focal point is large (X In the case of X-ray generators, isowatt control is often impossible with a small tube voltage. However, in practice, the influence on the optimum τ is small. For example, at a low voltage, the maximum tube current often reaches a constant value. In this case, using k = 0 and k2 = −3 (since V is small), the optimum transmittance is 20%, and the isowatt The result is no different. Therefore, a tube voltage adjustment method for making the transmittance about 20% (+ 10% -5%) can be used.

  Although the V and I adjustments are finished, the best transmission image is physically obtained, but it is usually not visually easy to see (that is, it is usually too dark). Next, the operator performs gradation conversion (by manually changing the window width window level of the display program or the offset and gain of the capture board), and adjusts to a screen that is visually easy to see and inspects the subject.

  According to this embodiment mentioned above, there exist the following effects.

  When the tube voltage V and the tube current I are changed, the attenuation index τ and its noise change for a certain observation portion of the subject. According to the first embodiment, by adjusting V and I by isowatt control and adjusting the transmittance of the observation portion to about 20% (+ 10% -5%), the attenuation index τ and its noise are observed with respect to the observation portion. Therefore, the transmission image under this condition is an image in which the structure of the observation portion can be physically distinguished to the maximum extent. For example, if this image B is converted into an image having an attenuation index τ (≡ln (B0 / B)), this corresponds to a gray image of a negative photograph, and the maximum gray level width (contrast) / dark noise is the best. It is an image. As another example, when the subject is a homogeneous material and the thickness t is constant, the thickness difference Δt can be identified most finely. This indicates that the discrimination using a linear transmission meter (linear penetrometer) according to the JIS standard is best.

  As a result, the best physically transmitted image can be easily obtained for an unknown subject without depending on the skill of the operator.

(Modification of the first embodiment)
In the first embodiment, the maximum tube current I is selected (for reasons of X-ray focal spot size, image saturation, subject exposure, etc.). Good. In this case, the set V and I are automatically the optimum V and I (in the case of isowatt control) with a small output (Watt).

  In the first embodiment, when the transmittance is adjusted to about 20% (+ 10% -5%), V may be adjusted by changing the transmittance depending on V as shown in FIG. Further, for the non-isowatt region of the allowable tube current, τ may be calculated by equation (12) according to k of the current limit curve, and the transmittance calculated from τ (depending on V) may be used. .

(Second Embodiment)
FIG. 4 is a schematic configuration diagram in the second embodiment. The same components as those in FIG. This configuration is the same as that of a general computed tomography apparatus (CT) having a two-dimensional X-ray detector. A rotation table 7 for placing and rotating the subject 4 and a mechanism control unit 8 for controlling the rotation table 7 are added to the configuration of FIG. In addition, the computer 5 has a CT scan function for collecting a large number of transmission images whose directions are changed by rotating the subject 4, and a reconstruction function for creating a cross-sectional image or a three-dimensional image from the large number of transmission images. Has been.

  Next, the operation of this embodiment will be described. First, the operator places the subject 4 on the rotary table 7. At this time, the specimen is placed so that the longitudinal direction (maximum attenuation direction) is the X-ray direction when viewing the shape of the subject. Next, when the X-ray is turned on, the computer 5 displays a transmission image of the subject 4. The operator adjusts the transmission direction (rotation) while viewing the transmission image under the provisional X-ray condition, and sets the maximum attenuation path (providing the transmission direction). To be precise, the rotation position is adjusted so that the minimum transmittance in the data area of the cross-sectional image (used for creation) on the transmission image is minimized. The X-ray path that gives the minimum transmittance when combined is defined as the maximum attenuation path.

  Next, the tube voltage is adjusted so that the transmittance of the maximum attenuation path of the transmission image is about 20% (+ 10% -5%) (the transmittance does not depend on the tube current). The tube current is the maximum tube current for this tube voltage. To adjust the transmittance, a straight line ROI is set on the transmission image so as to cross the maximum attenuation path, and the profile of this part is displayed (performed by the program of the computer 5). The brightness of the maximum attenuation path and the brightness of the air part are displayed. To compare.

  As a result, as in the case of the first embodiment, τ / στ of the maximum attenuation path is approximately maximized, and V and I are set such that the maximum attenuation path is physically the best transmission image.

  Next, when the operator issues a CT scan command, the computer 5 rotates the subject 4 to collect a large number of transmission images whose directions are changed, and re-creates a cross-sectional image or a three-dimensional image from the large number of transmission images. Constitute. The reconstruction of the cross-sectional image is performed by first logarithmically transforming the transmission image to create an image having an attenuation index τ, applying a filtering process to the image of τ, and then back-projecting the image on the cross-sectional image matrix. A three-dimensional image is created based on a plurality of cross-sectional images.

  Next, the best cross-sectional image is when V / I at which τ / στ in the maximum attenuation path in the cross-sectional image (used for creation) data area on the transmission image is maximized. Show. First, the number of X-ray photons received per pixel is N (i) with the subject and N0 without. Here, i indicates a data position. (N0 does not depend on i.) Then, it can be inferred as follows.

  1. From the equation (3), τ is maximum at a position where N is minimum on the data.

  2. From equation (4), on the data, στ is the maximum at the position where N is the minimum.

  3. Since τ is backprojected to obtain a cross-sectional image, the cross-sectional image value is approximately proportional to the maximum τ. That is, when V is changed, the maximum τ changes, and the cross-sectional image value changes proportionally.

4). Since τ is backprojected to obtain a cross-sectional image, the noise σP at one point P in the cross-sectional image is σP∝√ {(average of all backprojection directions of στ of the path passing through the P point) ² } (13)
It becomes. Therefore (because of the root mean square), the maximum στ is dominant in the total backprojection direction.

  From this, it can be said that the maximum noise in the cross-sectional image is substantially proportional to the maximum στ. That is, when V and I are changed, the maximum στ changes, and the maximum noise changes approximately proportionally.

  From 5, 3 and 4, the SN ratio of the cross-sectional image {the cross-sectional image value / maximum noise in the cross-sectional image} (when V and I are changed) changes approximately in proportion to {maximum τ / maximum στ}. .

  From 6.5, 1, 2, the SN ratio of the cross-sectional image {cross-sectional image value / maximum noise in the cross-sectional image} is approximately proportional to τ / στ in the maximum attenuation path.

  7). Therefore, the cross-sectional image is the best when V and I have the maximum τ / στ in the maximum attenuation path in the transmission data used when the cross-sectional image is reconstructed.

  As described above, the best cross-sectional image can be obtained by setting the transmittance in the maximum attenuation path to about 20% (+ 10% −5%). Since the three-dimensional image is obtained based on this cross-sectional image, the best three-dimensional image can be obtained similarly.

  According to this embodiment mentioned above, there exist the following effects. That is, according to the second embodiment, by changing V and I by isowatt control, the transmittance in the maximum attenuation path of the subject is about 20% (+ 10% -5%), so that the maximum attenuation path The ratio between the attenuation index τ and its noise can be maximized. Since the SN ratio {cross-sectional image value / maximum noise in the cross-sectional image} of the cross-sectional image obtained by back projecting the attenuation index τ is substantially proportional to τ / στ in the maximum attenuation path, A cross-sectional image is obtained. Further, the best three-dimensional image can be obtained based on this cross-sectional image.

(Modification of the second embodiment)
In the second embodiment, the X-ray detector 3 has a two-dimensional resolution, but the present invention can be similarly applied to a case of a one-dimensional resolution along the rotation plane.

  In addition, in the second embodiment, similarly to the modification of the first embodiment, the tube current I may not be the maximum but may be a small value. In this case, the set V and I are automatically the optimum V and I with a small output (Watt). Similarly to the modification of the first embodiment, V may be adjusted by changing the transmittance depending on V.

  In the second embodiment, it is not always necessary to perform the CT scan with the optimum V and I obtained, and V and I may be set with reference to it (by rounding the numerical values). In CT, it is often better not to finely adjust the X-ray conditions V and I for each subject. This is because correction data must be re-acquired each time the conditions change (contrast balance changes when V changes), a group of samples are often photographed under the same conditions, and image evaluation By avoiding complications. For this reason, imaging conditions V and I (V and the maximum I with respect thereto) are determined in advance (for example, 10 kV steps), and a condition exceeding the obtained optimum V is selected to perform CT scanning. Here, it is safer to select V exceeding the optimum V. This is because τ / στ decreases rapidly when the tube voltage V is lowered from the optimum V, whereas it gradually decreases on the increasing side. FIG. 5 is a calculation example of τ / στ. This is the case where the subject is aluminum of a certain thickness, and I is changed by isowatt control over the entire V region.

(Third embodiment)
The configuration of the third embodiment is the same as that of the second embodiment of FIG. In the third embodiment, V and I automatic adjustment functions are added as functions of the computer 5 to the second embodiment.

  In FIG. 4, FDD is a detection distance (Focus to Detector Distance). Although not shown, the rotary table 7 and the X-ray detector 3 can be moved in the FDD direction, and the imaging magnification can be changed.

  The computer 5 includes a CPU, a memory, a disk, a display, a keyboard, and the like. The computer 5 reads and displays the device status such as the detection distance FDD, moves the FDD by an operator command input, and converts the transmission image from the X-ray detector 3 into digital data by an internal capture board. And capture.

  The computer 5 calculates the optimum tube voltage V and tube current I from the captured image by calculating the optimum X-ray condition and sends it to the X-ray controller 6, which calculates the V and I of the X-ray generator 1. Adjust to the value. This is one loop of feedback. Since there is actually a calculation error, it can be adjusted to the optimum value at high speed by repeating this. The calculation of the optimum V and I is performed by a software program of the computer 5.

  Note that the tube voltage and tube current have a range limited by the X-ray control unit 10. This is achieved by limiting the heat of the target and the capacity of the high pressure generating part. In addition, a limit range for optimum control can be set separately within this range (for example, in the case of a microfocus X-ray tube, if the current is taken too much, the focus may be increased, so the tube current is limited to a small value. .)

  Next, the operation of this embodiment will be described. The point of the operation of the present invention is a V, I automatic adjustment function (hereinafter referred to as auto V, I) in the computer 5. This will be explained in the following order.

  The operator places the subject 4 on the rotary table 7. At this time, the specimen is placed so that the longitudinal direction (maximum attenuation direction) is the X-ray direction when viewing the shape of the subject. Next, when the X-ray is turned on, the computer 5 displays a transmission image of the subject 4. The operator adjusts the transmission direction (rotation) while viewing the transmission image under the provisional X-ray condition, and sets the maximum attenuation path (providing the transmission direction). To be precise, the rotation position is adjusted so that the minimum transmittance in the data area of the cross-sectional image (used for creation) on the transmission image is minimized. The X-ray path that gives the minimum transmittance when combined is defined as the maximum attenuation path.

  Next, the operator commands auto V and I (input to the computer). Auto V, I software automatically sets V and I that maximize τ / στ for the maximum attenuation path.

Here, the formulation and principle of the auto V and I will be described before explaining the operation of the auto V and I. First, the brightness B (brightness) of the transmission image (linear scale) is expressed by a pixel value of 0 to 255, for example, but B is approximately proportional to the total amount of X-ray energy / pixel / sample (ignoring the hardening of the line quality). Then, the basic formula,
B = B0 (n) · m · n ^ake · (FDD0 / FDD) ²
Exp {-uv (v0.n) .t} + Bof (14)
n = V / v0 (15)
m = I / ai0 (16)
It is represented by Here, using v0 and ai0 as the minimum setting units for V and I, natural numbers n and m are used in place of V and I. ake is a constant of about 2.2, uv (v) is the absorption coefficient of the subject, t is the subject transmission length, and Bof is the brightness when the X-ray is OFF. uv (v) uses a similar function because the exact value of the subject itself is unknown. Since ^{nake in the} basic formula (14) has an error in a wide area of n, the error is absorbed by B0 (n). B0 (n) is a value obtained in advance by calibration (described later). Next, BN (number of photons / pixel / sample) is proportional to B / V (ignoring hardening of the line),
BN = a0 · (B−Bof) / n (17)
It is represented by Here, since a relative BN may be obtained, a0 may be an arbitrary constant.

Next, the principle of Auto V, I is
“Optimum V and I are V and I that maximize τ / στ, where τ is the attenuation index and στ is the noise of the attenuation index.”
It is. Here, the attenuation index τ corresponds to the line integral of the absorption coefficient of the subject along the X-ray path, and is τ when the attenuation is e ^−τ . If the photon number BN ₀ is BN (brightness B ₀ is B) after being attenuated, the attenuation index τ is
Since τ≡ln ((B ₀ −Bof) / (B−Bof)) ≈ln (BN0 · V / (BN · V)),
τ = ln (BN ₀ / BN) (18)
Calculated by The noise σ _BN and σ _BN0 (root mean square error) of BN and BN ₀ are almost photon noise.
σ _BN = √ (BN) (19)
σ _BN0 = √ (BN ₀ / k) (20)
Calculated by Here, k is an integral multiple for obtaining air data. The noise of τ, that is, the noise στ that spreads to τ is
στ = √ {((∂τ / ∂BN) · σ _BN ) ² + ((∂τ / ∂BN ₀ ) · σ _BN0 ) ² }
= √ {((1 / BN) · √ (BN)) ² + ((1 / BN ₀ ) · √ (BN ₀ / k)) ² }
= √ {(1 / √ (BN)) ² + (1 / √ (k · BN ₀ )) ² } (21)
Where BN << k · BN _0, so στ is
στ = 1 / √ (BN) (22)
Calculated by

  Next, FIG. 6 shows the optimum V and I search principle. n and m are movable within the X-ray device limit (ABCDEF). This is a region between the tube current upper limit mmax (n) and the tube current lower limit mmin (n). In addition, when the condition that the brightest part (AIR part) is not saturated is entered, it is necessary that mir (n) is a line immediately before AIR part = saturation, so that n <mair (n). The hatched area (ABCGHF) in the figure.

The maximum value of τ / στ is on the upper limit line (ABCGH) of the V and I movable ranges. This can be seen from the fact that for the same n, τ is the same, and as m increases, στ decreases. From this, the search principle is
“V and I are varied along the upper limit line of the V and I range of motion, and τ / στ is calculated to find the maximum point.”
(Here, the entire V and I range of motion may be calculated, but only the calculation is wasted and the result does not change.)

<Auto V, I>
FIG. 7 is a flowchart of Auto V and I. The operation of Auto V and I will be described with reference to FIG.

  S1: Read FDD value.

  Read the detection distance FDD.

S2: the initial position n _A, m _A calculation.

The initial positions n _A and m _A are the intersections of the AIR part = the line mail (n) immediately before saturation and the diagonal line FD restricted by the X-ray apparatus. This ensures that the first image is not always saturated. mail (n) is obtained from the equation (14).
mair (n) = int {(Bh-Bof) / B0 (n) ^.n -ake. (FDD0 / FDD) ^-2 } (23)
It turns out that it becomes. Here, Bh is the brightness immediately before saturation, which is a constant. Change n and calculate this formula to find the point that crosses the straight line. When the intersection is outside the point D, the point D is set as the initial position.

  When determining the initial position, for example, a horizontal line passing through the point D may be used instead of the diagonal line FD.

  S3: i is repeated from 1 to ilmt.

  ilmt is the feedback limit number when it does not converge, and is set to 8, for example.

S4: X-ray irradiation starts at n _A and m _A.

S5: The brightness Bmeas of the maximum attenuation path at the current position (n _A , m _A ) is obtained.

  A transmission image is taken in and the brightness Bmeas of the maximum attenuation path is obtained.

  The maximum attenuation path is specified by the operator in advance on the screen, or the computer automatically obtains the maximum attenuation path. That is, the darkest portion on the slice plane of the cross-sectional image on the screen is set as the maximum attenuation path. When simultaneously photographing a large number of cross sections at once, the darkest portion in the data area of the cross-sectional image (used for creation) on the screen is set as the maximum attenuation path.

  S6: Transmission length tmeas calculation.

Here, the transmission length tmeas in the maximum attenuation path of the subject is obtained using Bmeas of S5. First, the AIR portion brightness Bair is an expression derived from the expression (14),
Bair = B0 (n _A ) · m _A · n _A ^ake · (FDD0 / FDD) ² + Bof (24)
Calculated by Using this, tmeas is an equation derived from equation (14),
tmeas = ln {(Bair−Bof) / (Bmeas−Bof)} / uv (v0 · n _A ) (25)
Calculated by

S7: Optimal positions n _B and m _{B are} calculated.

  The following calculation is performed with n = nmin to nmax.

AIR part = line just before saturation, mir (n) can be calculated from equation (23), so the upper limit line m (n) is the smaller of m (n) = mmax (n) and mir (n) (26) )
It can be calculated with Next, since tmeas is known, the attenuation index τ is expressed by the following equation:
τ = uv (v0 · n) · tmeas (27)
Where B and BN are equations using equations (14) and (17),
B = B0 (n) · m (n) · n ake · (FDD0 / FDD) 2 · exp (-τ) + Bof ... (28)
BN = a0 · (B−Bof) / n (29)
It can be calculated with Furthermore, the noise στ of τ is expressed by the equation (22).
στ = 1 / √ (BN) (22)
And τ / στ is obtained.

Τ / στ is calculated for all n, m (n) (excluding points where m (n) <mmin (n)), and the point giving the maximum τ / στ is defined as n _B , m _B.

  S8: Convergence determination.

When the change from n _A , m _A to n _B , m _B is smaller than the specified value, it is determined that the convergence has occurred, and the process proceeds to S11 through the i loop. When it is larger, the process proceeds to S9.

S9: Substitution calculation of n _A = n _B and m _A = m _B.

  Set the optimal position to the current position. S4 to S9 are one feedback for V and I control.

  S10: S4 to S9 are repeated for i.

  Usually, since the absorption coefficient uv (v) is different from that of the subject, there is an error in one feedback. Therefore, the feedback is repeatedly converged. If it does not converge even ilmt times, it is aborted and the process proceeds to S11.

  S11: Optimal V and I settings.

The optimum V (= v0 · n _B ) and I (= ai0 · m _B ) are set.

  <> End.

  After the auto V and I are finished, the optimum V and I in the maximum attenuation path are set.

  Next, when the operator issues a CT scan command, as in the second embodiment, the computer 5 collects a large number of transmission images whose directions are changed by rotating the subject 4, and from these transmission images, A cross-sectional image or a three-dimensional image is created.

  As a result, the best cross-sectional image or three-dimensional image can be obtained as in the second embodiment.

<Calibration>
Next, the operation at the time of calibration of B0 (n) will be described.

First, the subject 4 is not present, and the FDD is fixed to a constant value FDD0. Next, the tube voltage vcal (i) is set, and the tube current Ical (i) is set so that the transmitted image has an intermediate brightness (Bcal (i)). Vcal (i) is changed and repeated, the measured value,
imax, vcal (i), Ical (i), Bcal (i), FDD0
To the computer. The computer calibration program is a formula,
ncal (i) = vcal (i) / v0 (30)
mcal (i) = Ical (i) / ai0 (31)
Ncal (i) and mcal (i) are calculated by the following equation, and B0 (n) at the calibration point is calculated from the equation (14):
B0 (ncal (i)) = (Bcal (i) -Bof) / mcal (i) · ncal (i) ^-ake ...... (32)
Calculate with Since B0 (ncal (i)) is skipped, B0 (n) is obtained and stored at n = nmin to nmax by interpolation calculation (interpolation or extrapolation). Further, FDD0 is stored.

  <> End.

  According to this embodiment mentioned above, there exist the following effects. That is, when the tube voltage V and the tube current I are changed, the attenuation index τ and the noise thereof change for a certain observation portion of the subject. According to the third embodiment, it is possible to automatically calculate and set V and I that maximize the ratio of the attenuation index τ in the maximum attenuation path and the noise based on the transmission data. Since the SN ratio {cross-sectional image value / maximum noise in the cross-sectional image} of the cross-sectional image obtained by back projection is substantially proportional to τ / στ in the maximum attenuation path, the best cross-sectional image can be obtained. Further, the best three-dimensional image can be obtained based on this cross-sectional image.

  According to the third embodiment, since n and m are changed along the upper limit line of the V and I movable ranges, τ / στ is calculated, and the maximum point is searched for. Optimum conditions can be calculated easily and accurately with any function of the minimum tube current (vs. tube voltage). That is, even if there are curves and corners as shown in FIG. 6, any X-ray control unit can be handled by simply changing the stored arrays mmax (n) and mmin (n). Strictly speaking, since there is a minimum setting unit, the V and I movable ranges are stepped. However, according to the present embodiment, the optimum condition can be calculated in consideration of the unevenness of the steps, and the calculation is accurate. . In particular, this effect is significant in the case of an X-ray control unit having a large minimum setting unit. In addition, the calculation is simple because only the upper limit line is required instead of the entire V and I movable ranges.

  According to the third embodiment, the optimum X-ray condition is automatically set by feeding back the transmission data of the subject, and the correction of V and I in one loop of the feedback is adjusted to almost the optimum value at one time. Since it is a correction, convergence can be accelerated.

  In the CT having the configuration as shown in FIG. 4, the communication time between the computer 5 that processes the transmission image and the X-ray control unit 6 operating by its own CPU, and the response of the X-ray tube control from the X-ray control unit. Since the time is long, the time for one loop of feedback cannot be shortened. Even in such a case, according to the present invention, it is possible to converge at a high speed because one correction is a correction that almost matches the optimum value at one time.

  As a result, an optimum cross-sectional image or an optimum three-dimensional image can be obtained by simply obtaining an optimum transmission image for the maximum attenuation path for an unknown subject without depending on the skill of the operator. it can.

(Modification of the third embodiment)
(1) Modification 1: In the third embodiment, the auto V and I send the optimum V and I values to the X-ray control unit for feedback, but only the obtained optimum V and I values are displayed. You can also. In this case, the operator reads the display and inputs it to the X-ray control unit. If it does in this way, it can use effectively in the case of a X-ray fluoroscopic inspection apparatus in which an X-ray control part and a personal computer are not connected by communication.

  (2) Modification 2: Although the maximum attenuation path is set manually in the third embodiment, it can be automatically set.

  In this case, when an auto V or I command is issued, the computer 5 temporarily sets V (usually selecting the maximum V) and I and performs a temporary CT scan. A large number of transmission images whose directions are changed by rotating the subject 4 are collected, but the number of collection directions is smaller than that of a full-scale CT scan. From this large number of transmission images, a transmission image that minimizes the minimum transmittance in the data area of the cross-sectional image (used for creation) is selected, set to the rotational position corresponding to this transmission image, and the minimum transmission position on the image Is set as the maximum attenuation path. Next, auto V and I are performed. As a result, the optimum V and I can be set fully automatically.

  (3) Modification 3: In the third embodiment, one type of absorption coefficient uv (v) is stored. However, various types of absorption coefficients uv (v) can be stored and switched. In this case, the operator inputs and selects, for example, for an aluminum casting, for a substrate, for a capacitor, for a battery, or for iron according to the subject. Thereby, convergence can be made quickly and accurately. Moreover, it is also possible to automatically select the optimum uv (v) from the history of measured Bmeas in one subject in Auto V and I.

  (4) Modification 4: In the third embodiment, the X-ray detector 3 has a two-dimensional resolution. However, the present invention can be similarly applied to a one-dimensional resolution along the rotation plane.

(5) Modification 5: In the third embodiment, only the photon noise is considered as the noise στ spreading to τ. However, the present invention is not limited to this, and other noises can also be considered. As an example, if there is a constant noise σ _B (on the scale of brightness B) in an X-ray detector, for example, στ is
στ = √ {1 / BN + (σ _B / (B−Bof)) ² } (33)
It becomes. The first term in √ is a photon noise component, and the second term is an X-ray detector noise component. In this equation, σ _B may be not the noise of the X-ray detector but the digitization noise of the capture board. When considering other noises, it is necessary to obtain BN not as a relative value but as an absolute value. Therefore, the coefficient a0 in the equation (16) is calibrated in advance and correctly obtained.

  (6) Modification 6: In the third embodiment, as described in the modification of the second embodiment, it is not always necessary to perform the CT scan with the obtained optimum V and I. V, I may be set.

  (7) Modification 7: In the third embodiment, the tube current upper limit mmax (n) may be set in consideration of the X-ray focal spot size. That is, in the case of a microfocus X-ray tube, if the current is taken too much, the focal point may become large. In this case, it is preferable to limit the tube current to a small value.

  (8) Modification 8: The second and third embodiments are examples in which a third-generation CT that rotates a subject is used as an apparatus for creating a cross-sectional image (or a three-dimensional image). Regardless of the method, the V and I adjusting method according to the present invention can be used, and the same effect can be obtained. That is, the X-ray generator and the X-ray detector may be rotated while the subject is fixed, and the second, fourth, and fifth generation systems may be used. Further, even a tomographic apparatus such as a laminograph (or tomosynthesis) can be used as long as it uses X-rays.

  Note that the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention at the stage of implementation. In addition, the embodiments may be appropriately combined as much as possible, and in that case, combined effects can be obtained. Furthermore, the above embodiments include inventions at various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. For example, when an invention is extracted by omitting some constituent elements from all the constituent elements shown in the embodiment, when the extracted invention is implemented, the omitted part is appropriately supplemented by a well-known common technique. It is what is said.

1 is a schematic configuration diagram of an X-ray inspection apparatus in which a first embodiment of a tube voltage / tube current adjusting method according to the present invention is implemented. The figure which shows the typical ln ((micro | micron | mu))-ln (V) curve in the same embodiment, and its inclination k2. The figure which shows (tau) which provides the optimal V in the same embodiment, and the transmittance | permeability corresponding to this (tau). The schematic block diagram of the X-ray inspection apparatus with which 2nd Embodiment of the tube voltage and tube current adjustment method which concerns on this invention is implemented. The figure which shows the example of calculation of (tau) / (sigma) tau in the embodiment. The figure which shows the search principle of the optimal V and I in the embodiment. Flow chart of auto V, I in the same embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... X-ray generator, 2 ... X-ray beam, 3 ... X-ray detector, 4 ... Subject, 5 ... Computer, 6 ... X-ray control part, 7 ... Rotary table, 8 ... Mechanism control part.

Claims (8)

From the X-ray controller for controlling the tube voltage and tube current applied to the X-ray tube, the X-ray detector for detecting the X-ray transmitted through the subject, and the transmission data of the subject obtained by the X-ray detector In an X-ray inspection apparatus having means for creating a transmission image of the subject,
Means for measuring the transmittance of a desired observation portion of the subject from the transmission data;
An X-ray inspection apparatus comprising: means for adjusting a tube voltage in the X-ray control unit so that the measured transmittance becomes a predetermined value. From the X-ray controller for controlling the tube voltage and tube current applied to the X-ray tube, the X-ray detector for detecting the X-ray transmitted through the subject, and the transmission data of the subject obtained by the X-ray detector In the tube voltage / tube current adjustment method of an X-ray inspection apparatus having means for creating a transmission image of the subject,
Measuring the transmittance of a desired observation portion of the subject from the transmission data;
Adjusting the tube voltage in the X-ray control unit so that the measured transmittance becomes a predetermined value. A method for adjusting the tube voltage and tube current of the X-ray inspection apparatus. An X-ray controller for controlling tube voltage and tube current applied to the X-ray tube, an X-ray detector for detecting X-rays transmitted through the subject, scanning means for scanning the transmission direction in a plurality of directions, and the X-rays In an X-ray examination apparatus having means for creating a cross-sectional image or a three-dimensional image of the subject from transmission data in a plurality of directions of the subject obtained by a detector,
Means for setting a maximum attenuation path, which is an X-ray path having substantially the smallest transmission of the transmission data;
Means for measuring the transmittance of this maximum attenuation path;
An X-ray inspection apparatus comprising: means for adjusting a tube voltage in the X-ray control unit so that the measured transmittance becomes a predetermined value. An X-ray controller for controlling tube voltage and tube current applied to the X-ray tube, an X-ray detector for detecting X-rays transmitted through the subject, scanning means for scanning the transmission direction in a plurality of directions, and the X-rays In a tube voltage / tube current adjustment method for an X-ray inspection apparatus, comprising means for creating a cross-sectional image or a three-dimensional image of the subject from transmission data in a plurality of directions of the subject obtained by a detector,
Setting a maximum attenuation path, which is an X-ray path having the smallest transmission of the transmission data;
Measuring the transmission of this maximum attenuation path;
Adjusting the tube voltage in the X-ray control unit so that the measured transmittance becomes a predetermined value. A method for adjusting the tube voltage and tube current of the X-ray inspection apparatus. An X-ray controller for controlling tube voltage and tube current applied to the X-ray tube, an X-ray detector for detecting X-rays transmitted through the subject, scanning means for scanning the transmission direction in a plurality of directions, and the X-rays In an X-ray examination apparatus having means for creating a cross-sectional image or a three-dimensional image of the subject from transmission data in a plurality of directions of the subject obtained by a detector,
Means for setting a maximum attenuation path, which is an X-ray path having substantially the smallest transmission of the transmission data;
Means for collecting transmission data in this maximum attenuation path;
Means for calculating a tube voltage and a tube current at which the ratio of the attenuation index τ in the maximum attenuation path and the noise spreading to the attenuation index τ is the largest from the transmission data in the maximum attenuation path;
An X-ray inspection apparatus comprising: means for setting a tube voltage and a tube current in the X-ray control unit with reference to the calculated value.   The calculating means calculates the ratio for a plurality of combinations of one tube voltage and the maximum tube current set for the tube voltage, and calculates the tube voltage and the tube current at which this ratio is the largest. The X-ray inspection apparatus according to claim 5, wherein: An X-ray controller for controlling tube voltage and tube current applied to the X-ray tube, an X-ray detector for detecting X-rays transmitted through the subject, scanning means for scanning the transmission direction in a plurality of directions, and the X-rays In a tube voltage / tube current adjustment method for an X-ray inspection apparatus, comprising means for creating a cross-sectional image or a three-dimensional image of the subject from transmission data in a plurality of directions of the subject obtained by a detector,
Setting a maximum attenuation path, which is an X-ray path having the smallest transmission of the transmission data;
Collecting transmission data along this maximum attenuation path;
Calculating the tube voltage and the tube current at which the ratio of the attenuation index τ in the maximum attenuation path and the noise spreading to the attenuation index τ is the largest from the transmission data in the maximum attenuation path;
A tube voltage / tube current adjusting method for an X-ray inspection apparatus, comprising: setting a tube voltage and a tube current in the X-ray control unit with reference to the calculated values.   In the calculating step, the ratio is calculated for a plurality of combinations of one tube voltage and the maximum tube current set for the tube voltage, and the tube voltage and the tube current at which the ratio is maximized are calculated. The tube voltage / tube current adjusting method for an X-ray inspection apparatus according to claim 7, wherein:

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