JPH06179A - Method and instrument for measuring bone-salt quantity - Google Patents

Abstract

PURPOSE:To eliminate a counting dropout error by irradiating the examinee with attenuated X-rays via an attenuation filter, determining an attenuation coefft. without contg. the counting dropout error and ascertaining the content of the counting dropout error from the ratio of the estimated air count rate estimated therefrom and the actually measured air count rate. CONSTITUTION:The X-rays from an X-ray generator 10 is converged by a collimator 14 and is detected 12 after the transmission through the attenuation filter 16. The attenuation filter 16 determines the attenuated air count rate CABSi by attenuating the X-rays to the counting dropout threshold intensity of the detector 12a or below at a known rate. The addition character (i) indicates the number of a detector 12a. The filter 16 is then removed and the actually measured air count rate CAIRi of the intensity of the X-rays transmitted through the air layer. The estimated air count rate CTRUEi which does not contain the error is calculated by multiplying the CAIRi by a constant alpha. The optimum correction coefft. is formed from the ratio of the CTRUEi and the CAIRi for each of the respective detectors 12a. The actually measured sount rate Ci determined by irradiating the examinee is corrected by the optimum correction coefft. As a result, the error by the counting dropout and the variation of the detectors is prevented..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bone mineral content measuring method and apparatus for measuring the bone mineral content in a subject (mainly a human body) by irradiation with X-rays.

[0002]

2. Description of the Related Art A bone mineral content measuring device for measuring the amount of mineral components such as calcium (bone mineral) in bone is known and is used for diagnosing osteoporosis.

A conventional general bone mineral content measuring device is an X-ray generator for generating X-rays, an X-ray detector for detecting X-rays transmitted through a living body, and the detected data is analyzed to analyze bone mineral content. And an arithmetic unit for obtaining the quantity.

Here, the living body is roughly divided into "bone" and "soft tissue" having different X-ray absorption rates,
The measured data includes data on bone and data on soft tissue. Therefore, as is well known, in the conventional bone mineral content measuring device, the bone mineral content is analyzed from two types of data obtained by sequentially or simultaneously irradiating two X-rays having different energies. (For example, JP-A-2-278974 or USP 5,020,0
85). It should be noted that the X-ray irradiation site of a living body is wrapped with a bag of water having an X-ray absorption rate equivalent to that of soft tissue with a certain thickness, and two irradiations of high energy irradiation and weak energy irradiation are not performed, and one energy is used. A method for measuring bone mineral content by performing X-ray irradiation is also known.

In the conventional bone mineral content measuring device, the X-ray detecting device is composed of a single X-ray detector. However, in order to remarkably shorten the inspection time, it is necessary to use an X-ray detection device composed of a plurality (for example, 90) of X-ray detectors (for example, semiconductor detectors) arranged in one dimension or two dimensions. Occurs. However, in order to realize this, it must be taken into consideration that the sensitivity (counting rate) of each X-ray detector fluctuates due to various factors such as manufacturing problems, changes over time, and changes in temperature and humidity. This has been a major factor preventing the introduction of an X-ray detection device composed of a plurality of X-ray detectors into a bone mineral content measuring device.

[0006]

By the way, in general,
The pulse detection type X-ray detector has a certain restriction on the X-ray intensity that can be effectively detected, and when the X-ray intensity exceeds a certain value, pulse separation cannot be performed, and a count counting error occurs. . In other words, when the X-ray intensity becomes larger than a certain fixed value ("counting threshold intensity"), the actual count rate gradually deviates from the true count rate, and the count rate significantly decreases near the count limit. Have characteristics.

Incidentally, it is possible to lower the irradiation X-ray intensity below the "counted-down threshold intensity", but in that case, the measurement accuracy will be lowered. In short, it is necessary to increase the intensity of X-rays as much as possible, as long as it does not affect the living body more than necessary.

On the other hand, each X-ray detector does not have uniform detection sensitivity in a strict sense. Therefore, the error (counting error) when detecting the above-mentioned high-intensity X-rays is not uniform and varies among the X-ray detectors. Therefore, in performing highly accurate bone mineral content measurement, it is necessary to individually correct the detection error for each X-ray detector.

The present invention has been made in view of the above problems of the prior art, and its object is to solve the problem of counting error in the X-ray detector, and moreover, due to the difference in characteristics between the X-ray detectors. An object is to provide a bone mineral content measuring device that can solve the problem.

[0010]

In order to achieve the above object, the method for measuring bone mineral content according to claim 1 comprises a preparatory step of preliminarily obtaining a correction coefficient for a count rate in an X-ray detection apparatus, and a test object. A measurement irradiation step of irradiating X-rays to obtain a measured count rate, a correction step of correcting the measured count rate using the correction coefficient when a count rate correction is necessary, and a bone mineral based on the measured count rate A bone mineral amount measuring method comprising: a bone mineral amount calculating step of calculating the amount of X-rays, wherein the preparing step comprises: An X-ray irradiation by inserting an attenuation filter that attenuates X-rays, and an X-ray irradiation without any intervention between the X-ray generation device and the X-ray detection device, and an attenuation irradiation process for obtaining an attenuation air count rate. Air irradiation step to obtain the measured air count rate by performing Based on the estimated air count rate calculated on the basis of, and the air optimum correction coefficient which is the ratio of the measured air count rate, a correction coefficient calculation step of calculating an optimum correction coefficient for each count rate is included. Characterize.

Further, the bone mineral content measuring device according to claim 4 is
An X-ray generator that generates X-rays, an X-ray detector that detects X-rays, an arithmetic unit that calculates a bone mineral content from an actual count rate obtained by the X-ray detector, and the X-ray generator. An X-ray filter inserted between the X-ray detection device and an attenuation filter for reducing X-rays from the X-ray generation device to a count-down threshold intensity of the X-ray detection device or less. A filter moving device that inserts the X-ray attenuation filter in the X-ray beam path between the X-ray generation device and the X-ray detection device at the time of attenuation irradiation performed before actual irradiation of X-rays on a sample. And a correction coefficient calculation calculation device for calculating a correction coefficient based on the attenuation count rate obtained by the attenuation irradiation.

[0012]

According to the above construction, the attenuation air count rate is obtained in the attenuation irradiation step with the attenuation filter interposed.
This attenuated air count rate theoretically does not include counting error. That is, the attenuation filter attenuates the X-rays from the X-ray generator to below the count-down threshold intensity in the X-ray detector, and the attenuation air count rate is
It shows the true count rate at the X-ray intensity at that time.

Then, in the air irradiation step, the actually measured air count rate is obtained. This air count rate is presumed to contain a counting error.

Therefore, the estimated air count rate calculated on the basis of the attenuated air count rate becomes close to the true count rate in the air irradiation without attenuation, and the estimated air count rate and the actually measured air count rate are combined. The air optimum correction coefficient, which is the ratio, indicates the magnitude of the counting error. Therefore, since the degree of error is known from the air optimum correction coefficient, the actual optimum correction coefficient is calculated from it.

If the optimum correction coefficient is obtained for each X-ray detector, the variation between the detectors can be eliminated. At that time, if a counting-off standard correction curve is prepared in advance, the correction curve is corrected based on the air optimum correction coefficient of each detector, and the optimum correction coefficient at each count rate for each detector is simplified. You can ask.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings.

First, the bone mineral content measuring method according to the present invention will be described with reference to FIG.

In FIG. 1, (A) shows an attenuation irradiation step. The X-rays generated by the X-ray generator 10 are
After the beam is focused by the collimator 14, it passes through the attenuation filter 16 and becomes an X-ray beam 18, which is detected by the X-ray detection device 12.

Here, the attenuation filter 16 has a known attenuation rate, and attenuates the X-rays below the count-down threshold intensity in the X-ray detector 12. The attenuation filter 16 is composed of, for example, an aluminum plate having a thickness of 1 cm. In this example, the X-ray detection device 12 is, for example, 90 X-ray detectors 12a arranged one-dimensionally or two-dimensionally. A semiconductor detector, for example, is used as the X-ray detector 12a.

As shown in the drawing, in this embodiment, the collimator 14 obtains a fan-beam-shaped X-ray beam 18.

As described above, by performing the attenuation irradiation with the attenuation filter 16 interposed, it is possible to obtain the "attenuation air count rate" C _ABSi which does not include the counting error. Here, the subscript i indicates the i-th number of the X-ray detector 12a.

Next, in the air irradiation step shown in FIG.
The attenuation filter 16 inserted in the attenuation irradiation step shown in (A) is pulled out, and X-ray irradiation is performed without any intervention. That is, the intensity of the X-rays transmitted through the air layer is counted as the “actual measurement air counting rate” C _AIRi .

On the other hand, as will be described in detail later on the basis of the above-mentioned attenuated air count rate C _ABSi , the "estimated air count rate" C _TRUEi
Is calculated.

After the above, the actually measured irradiation step shown in FIG. 1C is executed. That is, the subject 20 is placed between the X-ray generator 10 and the X-ray detector 12, and the actual measurement count rate C _i is obtained by the irradiation of X-rays. This actual count rate C
_{After i} is multiplied by the optimum correction coefficient γ _i to correct and eliminate the counting error, the bone mineral content of the subject 20 is actually calculated. In the measurement irradiation step (C), in principle, high-energy X-ray irradiation and low-energy X-ray irradiation are performed, and from the two types of count rates obtained by these two irradiations, soft tissue in the living body is obtained. The amount of bone mineral is calculated while separating the data regarding. Of course, there are cases where high-energy X-rays and low-energy X-rays are simultaneously irradiated.

The principle of the bone mineral content measuring method according to the present invention outlined above will be described in more detail.

FIG. 2 shows the count rate C for each detector i.
Is shown as a graph.

The attenuation air weak count rate C _ABSi actually obtained in the attenuation irradiation step shown in FIG. _1A is expected to cause a counting error due to the action of the attenuation filter 16, as shown in the figure. It is set to the threshold value C _K or less. Here, since the detectors 12a have different sensitivities, the respective attenuation count rates _CABSi are not uniform.

The measured air count rate C _AIRi shown in FIG.
It was actually obtained in the air irradiation step shown in FIG. 1B, and it is expected that this includes a counting error.

On the other hand, the estimated air count rate C shown in FIG.
_TRUEi is obtained by _multiplying the attenuation count rate C _ABSi by a constant α for a predetermined estimation calculation. Specifically, the following first
The estimated air count rate C _TRUEi is calculated by calculating the _equation .

C _TRUEi = C _ABSi × α (1) Therefore, the estimated air count rate C _TRUEi is estimated to be close to the true count rate calculated based on the count rate C _ABSi without counting error. It

The difference 100 between the estimated air count rate C _TRUEi and the measured air count rate C _AIRi is considered to indicate the magnitude of the count rate corresponding to the counting error.

Therefore, it is conceptually understood that the "air optimum correction coefficient" is obtained by the ratio γ _Bi between the estimated air count rate C _TRUEi and the actually measured air count rate C _AIRi . As understood from FIG. 2, each X-ray detector 1
The optimum air correction coefficient γ _Bi is calculated for each 2a.

The above-mentioned estimated calculation constant α is based on the X-ray intensity on the X-ray detection device 12 calculated from the intensity of the X-rays generated by the X-ray generation device 10 and the attenuation rate of the attenuation filter 16. , Which is obtained in advance by experiments or the like, and how to obtain α will be described in detail later.

FIG. 3 shows a standard correction curve F (C) showing the value of the correction coefficient γ for each count rate. In the present embodiment, only one standard correction curve F (C) has been obtained in advance by experiments or the like, and the individual correction curve F ′ _i (C) for each detector is the standard correction curve F (c). It is required by modifying. This will be specifically described.

First, the air standard correction coefficient γ _Ai at the measured air count rate C _AIRi is calculated by the following second equation.

Γ _Ai = F (C _AIRi ) (2) That is, the measured air count rate C _{AIRi is added to the} standard correction curve F (C).
By substituting, the air standard correction coefficient γ _Ai can be obtained. However, the sensitivity of the X-ray detector changes due to long-term use or temperature and humidity fluctuations, and the true correction curve for eliminating counting errors does not always match the standard correction curve. Therefore, the individual correction curve F ′ _i (C) is obtained as follows.

That is, the air optimum correction coefficient γ _Bi at the actually measured air count rate C _AIRi is specifically calculated by the following third formula.

Γ _Bi = C _TRUEi / C _AIRi (3) As described above, the standard air standard correction coefficient γ _Ai and the optimum air correction coefficient γ _Bi which are assumed in advance at the measured air count rate C _AIRi , And the curve optimization coefficient δ _i is defined as follows.

Δ _i = (γ _Bi −1) / (γ _Ai −1) (4) The ratio indicated by the proportional coefficient δ _i is set above the standard correction curve F (C) for the threshold value _CK or more. If moved downward, the individual correction curve F ′ _i (C) can be obtained.

[0040] As described above, individual correction curve _F'i (C)
If found, the actual measurement count rate C _i obtained by performing the actual measurement irradiation
By making the curve correspond to that curve, the optimum correction coefficient γ _i can be instantly read.

Of course, it is possible to prepare an individual correction curve for each X-ray detector in advance, but there is a demerit that a storage area for storing all the correction curves is required and the calculation becomes complicated.

Next, the bone mineral content measuring apparatus according to the present invention will be described. FIG. 4 is a block diagram showing the overall configuration of the bone mineral content measuring device according to the present invention. Each arithmetic unit basically corresponds to each arithmetic operation described above, and will be described in detail below.

In the first computing unit 22, the attenuated air count rate C _ABSi obtained in the attenuated irradiation step is input, the calculation of the above-mentioned first equation is executed, and the estimated air count rate C _TRUEi is calculated.
The estimated air count rate C _TRUEi is _equal to the first
After being temporarily stored in the memory 24, it is read out and sent to the second computing unit 26 and the fourth computing unit 28. The second
The memory 23 stores a constant α for estimation calculation.

The actually measured air count rate C _AIRi obtained in the air irradiation step is input to the second calculator 26, and the curve optimization coefficient _δi for correcting the standard correction curve F (C) described above is calculated. ing. Specifically, the following fifth formula has been calculated.

Δ _i = (C _TRUEi / C _AIRi −1) / F (C _AIRi ) −1 (5) That is, according to the fifth formula, the air optimum correction coefficient γ _Bi is used as a basis and the air standard correction coefficient. From the ratio with γ _Ai ,
The curve optimization coefficient δ _i has been calculated. Then, after the curve optimization coefficient is once stored in the fourth memory 30,
It is sent to the third computing unit 32. The third memory 3
In FIG. 4, only one standard correction curve F (C) is stored and read by the second calculator 26 and the third calculator 32.

The third computing unit 32 judges the necessity of the correction, and when the correction is judged to be necessary, the corrected individual correction curve F ′ _i (C) is obtained, and at the same time, the actually measured count rate is obtained. C
_i is multiplied by the optimum correction coefficient γ _i specified from the individual correction curve.

That is, in the third arithmetic unit, when the measured count rate C _i is smaller than the threshold value C _K , that is, when it is determined that the counting error is not included, a new measured count rate C ′ _{i is} added. As, the input C _i is output as it is.

On the other hand, when the measured count rate C _i is equal to or greater than the threshold value C _K , the following sixth equation is calculated.

C ′ _i = {(F (C _i ) −1) × δ _i +1} × C _i (6) Here, the sixth formula will be described with reference to FIG.

Standard correction curve F at the actual count rate C _i
(C) When the correction coefficient γ _(P1) at point P1 above is read,
This is F _(Ci) .

On the other hand, when the correction coefficient γ _(P2) at _P2 on the individual correction curve F ′ _{i (C)} is obtained, γ = on the graph of FIG.
Above 1.0, the ratio of γ _(P1) and γ _(P2) is approximately equal to the ratio of γ _Ai and γ _Bi .

Therefore, the following equation is derived.

(Γ _(P2) −1) / (γ _(P1) −1) = (γ _(P2) −1) / (F _(Ci) −1) = δ1 When this is transformed, γ _(P2) −1 = Δ _i (F _(Ci) -1) When further modified, γ _(P2) = δ _i (F _(Ci) -1) +1, which is called the optimum correction coefficient γ _i . Then the multiplied to the measured count rate C _i and the 6 expression for calculating the actual count rate _C'i after correction.

C ′ _i = γ _i × C _i = {(F _(Ci) −1) γ _i +1} × C _{i As} described above, in the third computing unit 32, the new coefficient multiplied by the correction coefficient is added. The measured count rate C ′ _i is obtained and sent to the fourth computing unit 28.

The controller 34 controls the arithmetic units 22, 2
6 and 32 are controlled, and write / read control of each memory is performed. Further, the controller 34 includes the attenuation filter 1 between the X-ray generator 10 and the X-ray detector 12.
The filter moving device 36 for inserting or withdrawing 6 is controlled.

[0056] The fourth calculator 28, based on the measured count rate _C'i which correction has been made, by conventional same calculation method, and performs calculation of bone mineral density.

Next, a series of steps from the attenuated irradiation to the actual display of the bone mineral content according to the bone mineral content measuring method of the present embodiment will be described in more detail.

In FIG. 5, in S101, the filter moving device 36 inserts the attenuation filter 16 between the X-ray generator 10 and the X-ray detector 12. S102
Then, the attenuation irradiation is executed, and the attenuation count rate C _ABSi is obtained. In S103, the inserted attenuation filter is pulled out. Then, in S104, the true counting rate C _TRUE is estimated from the attenuation counting rate C _ABSi . After that, in S105, air irradiation is executed and the actually measured air count rate C _AIRi is obtained.

In S106, the standard air correction coefficient γ _Ai is calculated based on the standard correction curve F (C), and in S107, the air optimum correction coefficient γ _Bi is calculated.

Then, in S108, the curve optimization coefficient δ
_i is calculated, and the optimum correction coefficient γ _i is calculated in S109 based on this result.

The operations of S106 to S108 are the second operation.
This corresponds to the calculation of the calculator 26. The calculation of S109 and the calculation of S112 described later correspond to the calculation of the third calculator 32. Incidentally, each process of S101 to S109 is defined as a preparation process.

In S110, actual measurement irradiation is performed with the subject interposed.

Then, in S111, it is judged whether or not the actually measured count rate C _i is larger than the threshold value C _K , that is, whether or not a counting error may occur. Correction of the count rate C _i is executed.

Of course, in each of the steps from S101 to S112, X-ray irradiation of two types of energy, high and low, the counting rate and the calculation are executed as necessary. This is for discriminating between soft tissue and bone and is the same as in the past.

[0065] In S113, based on the newly determined actual count rate _C'i, the conventional same calculation method, the amount of bone mineral is analyzed. Then, in S114, the result is displayed.

Next, how to obtain the constant α will be described.

FIG. 6 shows each step for obtaining α. In practice, the series of steps shown in FIG. 6 is executed before the apparatus is shipped from the factory, and α is set in the apparatus. Remembered. In S201, as shown in FIG. 7, the stepwise phantom 38 having the X-ray attenuation rate corresponding to the soft tissue of the human body is irradiated with the X-rays, and in S202, the counting rate as the irradiation result is shown. Not stored in memory. S
At 203, it is determined whether or not X-ray irradiation has been executed for each step of the phantom 38, and if not completed, S201 is executed again, and if completed, S204 is executed. In S204, a graph as shown in FIG. 8 is created. That is, a graph in which the horizontal axis represents the X-ray intensity and the vertical axis represents the count rate is created. Since the thickness of each step of the phantom 38 and the X-ray attenuation rate thereof are known, the X-ray intensity in the X-ray detector can be easily calculated from the X-ray intensity set by the X-ray generator. It
As shown in FIG. 8, when a certain threshold strength is exceeded,
The counting rate tends to decrease due to the occurrence of the counting error. Therefore, the threshold strength is determined by the inflection point of the characteristic curve 200. Along with this, Phantom 3
The count rate at the so-called air strength when 8 is not interposed can be read by extrapolating the characteristic curve 200 (shown by a broken line in the drawing). That is S2
05 and S206.

Thereafter, in S207, the phantom 38 is pulled out, the attenuation filter 16 is inserted in its place, and the attenuation irradiation is executed. Count rate at this time (attenuation count rate) C
The ratio of the count rate C _TRUEi obtained by extrapolation to 2 is a constant α, which is calculated in S208. The standard correction curve described above is also obtained in the same manner as above based on the theory shown in FIG.

The present invention can be applied to the case where there is one X-ray detector or the case where there are a plurality of X-ray detectors. When there are a plurality of X-ray detectors, it is possible to eliminate variations among the detectors by applying the correction process to all the detectors in a unified manner. Further, the present invention can be applied not only to the pulse detection type measurement but also to the integration detection type measurement if a similar problem occurs.

[0070]

As described above, according to the method and apparatus for measuring bone mineral content according to the present invention, the attenuation count rate that does not include the counting error is obtained by performing the attenuation irradiation through the attenuation filter and estimating from it. Based on the ratio between the estimated air count rate and the actually measured air count rate, the content ratio of the counting error can be recognized and the correct correction coefficient can be calculated. Therefore, according to the present invention, the counting error can be eliminated and the variation between the X-ray detectors can be eliminated, and the bone mineral content can be accurately measured.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing an attenuation irradiation step, an air irradiation step, and an actually measured irradiation step in a bone mineral content measuring method according to the present invention.

FIG. 2 is a graph showing a mutual relationship among an attenuation count rate, an actually measured air count rate, and an estimated air count rate.

FIG. 3 is a characteristic diagram showing a counting-down standard correction curve.

FIG. 4 is a block diagram showing an overall configuration of a bone mineral content measuring device according to the present invention.

FIG. 5 is a flowchart showing specific steps of a bone mineral content measuring and measuring method according to the present invention.

FIG. 6 is a flowchart showing steps for obtaining a constant α.

FIG. 7 is a conceptual diagram showing irradiation of a phantom 38.

FIG. 8 is a characteristic diagram showing a graph for obtaining a constant α.

[Explanation of symbols]

 10 X-ray generator 12 X-ray detector 16 Attenuation filter 18 X-ray beam 20 Subject F (C) Counting standard correction curve

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Koichi Kawamura 6-22-1 Mure, Mitaka City, Tokyo Aloka Co., Ltd.

Claims (4)

[Claims] 1. A preparatory step of previously obtaining a correction coefficient for a count rate in an X-ray detection apparatus, an actual measurement irradiation step of irradiating an object with X-rays to obtain an actual count rate, and a case where the count rate correction is necessary. A bone mineral content measuring method comprising: a correction step of correcting the actual measurement count rate using the correction coefficient; and a bone mineral content calculation step of calculating a bone mineral content based on the actual measurement count rate, The preparatory step is to insert an attenuation filter that attenuates X-rays below the count-down threshold intensity between the X-ray generation device and the X-ray detection device to remove X-rays.
Attenuation irradiation step of irradiating X-rays to obtain an attenuated air count rate, and air to obtain an actually measured air count rate by irradiating X-rays without any intervention between the X-ray generator and the X-ray detector. Optimum to calculate the optimum correction coefficient for each count rate based on the irradiation step, the air optimum correction coefficient that is the ratio of the estimated air count rate calculated based on the attenuated air count rate, and the measured air count rate. A method for measuring bone mineral content, comprising: a correction coefficient calculating step. 2. The bone mineral content measuring method according to claim 1, wherein the X-ray detection device is composed of a plurality of X-ray detectors.
A method for measuring bone mineral content, characterized in that the air optimum correction coefficient is obtained for each line detector, and the optimum correction coefficient for each count rate is calculated. 3. The bone mineral content measuring method according to claim 2, wherein a counting-down standard correction curve is prepared in advance, and the counting-down standard correction curve is corrected based on the air optimum correction coefficient,
A method for measuring bone mineral content, characterized in that a curve optimization coefficient is calculated for each X-ray detector. 4. An X-ray generator for generating X-rays, an X-ray detector for detecting X-rays, an arithmetic unit for calculating a bone mineral content from an actual count rate obtained by the X-ray detector, An X-ray filter inserted between an X-ray generation device and the X-ray detection device, wherein the X-rays from the X-ray generation device are attenuated below the count-down threshold intensity of the X-ray detection device. An attenuation filter and the X-ray attenuation filter in the X-ray beam path between the X-ray generation device and the X-ray detection device during the attenuation irradiation performed before the X-ray actual measurement irradiation to the subject. A bone mineral density measuring device comprising: a filter moving device to be inserted; and a correction coefficient calculating device for calculating a correction coefficient based on the attenuation count rate obtained by the attenuation irradiation.