JP2008206560A - Bone salt quantity measuring apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bone salt quantity measuring apparatus enabling simple bone salt determination which can be widely used for medical examinations with less burdens even for an elderly person or the like and realizing measurement accuracy which can be utilized for early detection of osteoporosis and follow-up for treatment. <P>SOLUTION: The bone salt quantity measuring apparatus for photographing the X-ray image of an object H and measuring the bone salt quantity of the object H on the basis of obtained image data is provided with an X-ray source 8 for irradiating the object H with X-rays, an X-ray detector 11 for recording the X-ray image corresponding to the irradiated X-ray dose and a water tank 30 for holding a liquid object 31 for including the object H during photographing. The X-ray source 8 is a molybdenum anode X-ray tube, and the X-ray detector 11 is arranged such that a dynamic range is 3 digits or more and a distance from the object H is 15 cm or more. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a bone mineral content measuring device, and more particularly to a bone mineral content measuring device that measures bone mineral content using single energy X-rays.

  Conventionally, in order to diagnose osteoporosis at a medical site, bone mineral content measurement (bone mineral content determination) has been performed. Examples of the bone mineral quantification method used for measuring the bone mineral content include a dual energy X-ray absorption method (Dual X-ray Absorptiometry: DXA method) and a quantitative ultrasonic bone density measurement method (Quantitative Ultrasound: QUS method). ), Quantitative X-ray CT examination (Quantitative Computed Tomography: QCT), single energy X-ray absorption (Single X-ray Absorptiometry: SXA), etc. The X-ray absorption method has become a standard method for measuring bone mineral content.

  Specifically, for example, in the case of using a dual energy X-ray absorption method, an image signal of a difference signal is acquired by performing energy subtraction between a plurality of images obtained by irradiating radiation having different energies. Then, subtraction is performed between the image signal for correction averaged in the main scanning direction and / or sub-scanning direction and the difference signal to obtain a correction difference signal, and bone mineral quantitative analysis is performed using the correction difference signal. A method is known (see, for example, Patent Document 1).

  As an apparatus using a quantitative X-ray CT examination method, there is an apparatus for calculating a bone density from a CT value for each pixel in a specific bone tissue obtained by discriminating and extracting a CT tomographic image. It has been proposed (see, for example, Patent Document 2).

  As a means for measuring bone mineral content using the single energy X-ray absorption method, the X-ray absorption value of the subject is measured by irradiating the subject with single energy X-rays. Using a single type of filter made of only a single metal material to make the line monochromatic, and optimizing the filter thickness, collimator diameter, tube voltage and current, X-rays can be produced with a simple configuration. A bone mineral content measuring device that is monochromatic has been proposed (see, for example, Patent Document 3).

Further, when measuring bone density, the bone density adapter is moved so that the monochromator is arranged below the diaphragm device, and when performing general X-ray imaging, the bone density adapter is moved so that the monochromator is retracted from below the diaphragm device. Thus, there has been proposed an X-ray imaging apparatus with a bone density measuring function that enables bone density measurement and general X-ray imaging with one apparatus (see, for example, Patent Document 4).
JP-A-5-111480 JP 2005-192657 A JP 2005-319236 A Japanese Patent Laid-Open No. 10-314154

By the way, osteoporosis increases with aging, and fractures of elderly people, especially femoral neck fractures, are likely to be bedridden. In particular, it is said that in women over 40 years old, the amount of bone mineral is drastically reduced, and when it becomes 0.70 g / cm ² or less, fractures are likely to occur. Increasing the number of patients with osteoporosis is not only a problem for individual patients, but also a major problem in Japan, where the elderly population is increasing, which has spurred an increase in medical expenses.

  It is said that osteoporosis can be considerably prevented by appropriate exercise and taking enough calcium from the diet. Therefore, it is important for individuals to accurately grasp their own bone mineral amount at an early stage and strive to prevent osteoporosis. Therefore, today, bone mineral quantification is being screened.

However, the change in the amount of bone mineral is very small, and the measurement error of bone mineral content measurement for prevention and early detection of osteoporosis is required to be around 1%. Moreover, even if calcium is sufficiently ingested and proper exercise is continued, the increase in bone mineral content is considered to be about 0.5% per year, and osteoporosis prevention, early detection and treatment are appropriately performed. For this purpose, it is preferable to periodically conduct a bone mineral content examination for a long period of at least several years and observe the progress. Therefore, it is required to be able to easily measure the amount of bone mineral for prevention of osteoporosis, early detection, and the like.
In addition, since osteoporosis is a disease that affects a large number of elderly people as described above, it is preferable to measure the amount of bone mineral in such a way as to minimize the burden on those who undergo examination.

However, the dual energy X-ray absorption method has a drawback that it takes several minutes or more to measure although the measurement accuracy is excellent. For this reason, it is not preferable as a screening method regularly performed by the elderly. In addition, in the case of the dual energy X-ray absorption method, information cannot be obtained as a high-resolution image simultaneously with the measurement of the bone mineral content.
The quantitative X-ray CT examination method can obtain information three-dimensionally, but has a drawback that the exposure dose is large, and is not suitable for examination (especially examination regularly performed over a long period of time).
Furthermore, the quantitative ultrasonic bone densitometry using an ultrasonic device has problems such as measurement accuracy, and the evaluation of the effectiveness of medical examination has just begun, and is not yet suitable for practical use.

  On the other hand, the monoenergetic X-ray absorption method can measure the amount of bone mineral by photographing a subject using an X-ray film or the like, and is the most simple method for bone mineral quantification. is there. However, because the measurement accuracy is low, it currently only plays an auxiliary role in other methods, and this method alone cannot be used for medical applications such as early detection of osteoporosis and confirmation of progress. There is.

  Therefore, the present invention has been made to solve the above-described problems, and it is possible to perform simple bone mineral quantification that can be used for a wide range of medical examinations with less burden even for elderly people and the like. An object of the present invention is to provide a bone mineral content measuring apparatus capable of realizing measurement accuracy usable for follow-up observation for early detection and treatment of osteoporosis.

In order to solve the above problem, the bone mineral content measuring device according to claim 1,
A bone mineral content measuring apparatus for taking an X-ray image of a subject and measuring the bone mineral content of the subject based on the obtained image data,
An X-ray source for irradiating the subject with X-rays;
An X-ray detector for recording an X-ray image corresponding to the incident X-ray dose irradiated from the X-ray source;
A liquid material holding member for holding a liquid material containing the subject at the time of imaging,
The X-ray source is a molybdenum anode X-ray tube;
The X-ray detector is characterized in that the dynamic range is 3 digits or more and the distance from the subject is 15 cm or more.

The invention according to claim 2 is the bone mineral content measuring device according to claim 1,
The X-ray source is characterized in that a tube voltage is set to 20 kVp or more and 39 kVp or less.

The invention according to claim 3 is the bone mineral content measuring device according to claim 1 or 2,
The dynamic range of the X-ray detector is 7 digits or less.

The invention according to claim 4 is the bone mineral content measuring device according to any one of claims 1 to 3,
The X-ray detector is arranged so that a distance from the subject is 1 m or less.

The invention according to claim 5 is the bone mineral content measuring device according to any one of claims 1 to 4,
It is a human finger or foot.

The invention according to claim 6 is the bone mineral content measuring device according to any one of claims 1 to 5,
It can also serve as a breast image photographing device for photographing a breast image.

When measuring the amount of bone mineral by the single energy X-ray absorption method, it is necessary to irradiate the subject with monochromatic X-rays. According to the invention described in claim 1, strong characteristic X-rays are emitted. Since a molybdenum anode X-ray tube that radiates and has excellent monochromaticity as compared with an X-ray tube of a tungsten anode is used as an X-ray source, an accurate measurement of bone mineral content can be performed.
Further, when X-rays pass through an object (subject), scattered X-rays having various energies are generated as secondary X-rays, and these scattered X-rays become noise in the X-ray image. In this regard, by separating the X-ray detector by 15 cm or more from the subject, scattered X-rays can be reduced without reducing the primary X-ray dose due to the so-called Gradel effect, and the monochromaticity is more excellent with the X-ray detector. The X-ray transmission X-ray dose can be captured.
In addition, when the dynamic range of the X-ray detector is about two digits, sufficient image contrast cannot be obtained in a bone portion having a large X-ray absorption particularly when irradiated with low-energy X-rays. Although accurate measurement of the amount is difficult, in the present invention, since the dynamic range of the X-ray detector is as wide as 3 digits or more, X-rays that greatly decrease in the bone can be measured sufficiently, and the bone mineral content can be measured. There is an effect that the accuracy can be increased.

If the tube voltage of the X-ray source is too low, even if the subject is irradiated with X-rays, the X-rays are almost absorbed by the bone part and the transmitted X-ray dose necessary for measuring the bone mineral content cannot be obtained.
In this respect, according to the invention described in claim 2, since the tube voltage is 20 kVp or more, there is an effect that the transmitted X-ray dose necessary for measuring the bone mineral content can be obtained.
On the other hand, when the tube voltage exceeds 39 kVp, the bremsstrahlung X-rays other than the 18 keV characteristic X-rays increase more than necessary and the measurement accuracy deteriorates. However, according to the present invention, the tube voltage is set to 39 kVp or less. Therefore, the measurement accuracy is not degraded.

  According to the third aspect of the present invention, since the dynamic range of the X-ray detector is 7 digits or less, the time spent for image data processing is short, and the apparatus cost can be reduced. Play.

  According to the fourth aspect of the present invention, since the distance between the X-ray detector and the subject is 1 m or less, the X-ray dose to be detected by the X-ray detector is not greatly reduced, and the bone Increases the accuracy of salt measurement. Further, since the distance between the X-ray detector and the subject is short, there is an effect that the apparatus can be downsized.

  According to the fifth aspect of the present invention, since the subject is a human finger or foot, it is not necessary to take an image in a recumbent position, for example, a simple test in which an examination is performed by holding a hand over a subject table. The bone mineral content can be measured by the technique. For this reason, there is little burden on the subject, and there is an effect that the bone mineral content can be easily measured even when the subject is an elderly person, for example.

According to the sixth aspect of the present invention, it is possible to use both a breast imaging apparatus used for breast cancer screening and the like and a bone mineral quantitative examination apparatus as a single unit. In this regard, breast cancer screening is recommended for women over the age of 40 who are particularly prone to develop osteoporosis, and breast cancer screening is performed every year or at least every two years. For this reason, for example, in a subject who performs breast cancer screening, bone mineral content quantitative examination can be performed at the time of breast cancer screening, and the bone mineral content quantitative examination can be easily performed.
Particularly, in recent years, a technique for configuring a phase contrast imaging is known for a mammography apparatus (see, for example, Japanese Patent Laid-Open No. 2001-238871), and a molybdenum anode X-ray tube is used for this imaging, and phase contrast is further improved. In order to obtain the image, the subject and the X-ray detector can be photographed apart from each other, so that a Gradel effect that reduces scattered X-rays can be obtained. Further, in such an apparatus, since the X-ray intensity is greatly reduced in order to perform magnified imaging, a CR cassette or the like having a dynamic range of 4 digits is used. Therefore, by measuring the bone mineral content using such a mammography apparatus, it is possible to perform a bone mineral quantitative examination simply and accurately without increasing the number of apparatuses.

  Below, one Embodiment of the bone mineral content measuring apparatus 1 which concerns on this invention is described, referring FIGS. 1-10. However, the scope of the invention is not limited to the illustrated examples.

  FIG.1 and FIG.2 shows the structural example of the bone mineral content measuring apparatus 1 in this embodiment. The bone mineral content measuring device 1 is connected to a communication network (not shown; hereinafter simply referred to as “network”) such as a LAN (Local Area Network) via a switching hub (not shown), for example. The measurement result of the bone mineral content measured by the bone mineral content measuring device 1 is displayed on a monitor or the like of a terminal device (not shown) connected to the network, or the measurement result is output to an output device (not shown) connected to the network. ) May be output from a film.

  In addition, the structure of the bone mineral content measuring apparatus 1 is not limited to what was illustrated here, For example, the bone mineral content measurement apparatus 1 may also output the output (display or film output) of the bone mineral content measurement result. You may comprise.

  As shown in FIG. 1, in the bone mineral content measuring apparatus 1 according to the present embodiment, a support base 3 is provided on an imaging main body 4 serving as a base so as to be movable up and down with respect to the support base 2. A substantially rectangular parallelepiped imaging main body 4 is supported on the support base 3 via a support shaft 5 so as to be rotatable in a clockwise direction and a counterclockwise direction when viewed from the direction of arrow A.

The support base 3 is provided with a drive device 6 that drives its elevation and rotation of the support shaft 5. The drive device 6 includes a known drive motor or the like (not shown), and the support base 3 and the imaging main body 4 are moved up and down according to the position of the subject H.
In the present embodiment, the bone mineral content measuring apparatus 1 is configured to be able to perform imaging for measuring bone mineral content (bone mineral content determination) and breast imaging with a single device. The support base 3 and the imaging main body 4 are movable according to the position of the subject H suitable for each imaging.
That is, when imaging a finger for bone mineral content measurement (bone mineral content determination), the position of the subject H suitable for imaging is a position near the subject's chest or lower than the chest, The position can be adjusted so that the subject can take a posture that is less likely to get tired when his / her finger is immersed in a water tank 30 (described later) placed on a subject table 14 (described later).
When a breast image is captured, the position of the subject H suitable for imaging is a position near the chest of the standing subject, and the support base 3 and the imaging main body 4 are the subject H. The height and position of the breast can be adjusted according to the position of the breast.

  As shown in FIG. 2, the photographing main body 4 is provided with a holding member 7 along the vertical direction. An X-ray source 8 that emits X-rays to the subject H is attached to the upper portion of the holding member 7. A power supply unit 9 for applying a tube voltage and a tube current is connected to the X-ray source 8 via a support shaft 5, a support base 3, and an imaging main body unit 4. An aperture 10 for adjusting the X-ray irradiation field is provided at the X-ray emission port of the X-ray source 8 so as to be freely opened and closed. The photographing main body 4 is provided with a filter (not shown). For example, molybdenum, rhodium, aluminum, or the like can be used as the filter.

  In the present embodiment, a molybdenum anode X-ray tube is applied as the X-ray source 8. By using a molybdenum anode X-ray tube as the X-ray source 8, a strong characteristic X-ray having an energy of 18 keV can be emitted from the X-ray source 8. Further, the X-ray tube of molybdenum anode is characterized by being superior in monochromaticity as compared with the X-ray tube of tungsten anode.

  The X-ray source 8 is preferably a rotary anode X-ray tube. In this rotary anode X-ray tube, X-rays are generated when an electron beam emitted from the cathode collides with the anode. This is incoherent (incoherent) like natural light, and is not divergent X-rays but divergent light. If the electron beam continues to hit the place where the anode is fixed, the anode is damaged by the generation of heat. Therefore, in an X-ray tube that is usually used, the anode is rotated to prevent the anode from being shortened. The rotating anode X-ray tube causes an electron beam to collide with a surface of a certain size of the anode, and the generated X-ray is emitted toward the subject H from the plane of the anode of the certain size. The size of the plane viewed from the irradiation direction (subject direction) is called a focus.

The focus size D (μm) indicates the length of one side when the focus is a square, the length of the short side when the focus is a rectangle or a polygon, and the diameter when the focus is a circle. The larger the focal spot size D, the larger the X-ray dose irradiated.
The focal spot size of the X-ray tube of the X-ray source 8 used in this embodiment is preferably 1 μm to 500 μm. If it is smaller than 1 μm, a sufficient amount of X-rays that pass through the subject H cannot be obtained within a few seconds. On the other hand, if it is larger than 500 μm, the blur of the image becomes large and the measurement accuracy is lowered.

  In addition, the bone mineral content measuring apparatus 1 can adjust the set value of the tube voltage of the power supply unit 9 so that the tube voltage is adjusted and applied to the X-ray source 8. X-rays can be irradiated with an energy amount.

In the present embodiment, the bone mineral content measuring apparatus 1 can measure the bone mineral content and take a breast image as described above. When measuring the bone mineral content, the X-ray source is used. The tube voltage applied to 8 is preferably set to 20 kVp or more and 39 kVp or less, and more preferably set tube voltage is 25 kVp or more and 35 kVp or less. If the tube voltage is too low, the irradiated X-rays are all absorbed by the bone of the subject H, and the transmitted X-ray dose necessary for measurement cannot be obtained. On the other hand, when the tube voltage exceeds 39 kVp, the bremsstrahlung X-rays other than the 18 keV characteristic X-rays increase more than necessary, and the measurement accuracy deteriorates.
By setting the tube voltage in this way, the bone mineral content measuring apparatus 1 can perform image capturing in a low energy band where the tube voltage is low when measuring the bone mineral content.

Conventional bone imaging has been performed using a conventional silver halide photographic light-sensitive material having a dynamic range of about two digits. Therefore, imaging is performed by setting the tube voltage applied to the X-ray source 8 to 50 kVp or more. It was. By setting the tube voltage so high, an image contrast that can withstand diagnosis and the like can be obtained even when the dynamic range is about two digits. On the other hand, when the setting of the tube voltage applied to the X-ray source 8 is lower than 50 kVp, an image contrast that can withstand diagnosis and the like can be obtained for soft tissue such as skin, but a bone part having a large X-ray absorption. However, sufficient contrast cannot be obtained. However, in the present embodiment, as will be described later, since a digital X-ray detector having a wide dynamic range is used as the X-ray detector 11, it can sufficiently withstand diagnosis and the like even for X-rays that greatly decrease in the bone. Contrast can be obtained and bone mineral content can be measured.
In addition, the tube voltage at the time of measuring the amount of bone mineral is not limited to the range illustrated here.

  One end of a detector holding unit 12 that holds an X-ray detector 11 that detects X-rays transmitted through the subject H is attached to the lower part of the holding member 7. As the X-ray detector 11, for example, a digital X-ray detector such as a CR (Computed Radiography) cassette or an FPD (Flat panel X-ray detector) containing a stimulable phosphor sheet can be applied. The X-ray detector 11 may be a digital X-ray detector, and is not limited to CR or FPD. Conventionally, silver halide photographic light-sensitive materials have been widely used for X-ray imaging. However, when silver halide photographic light-sensitive materials are used, in order to measure the amount of X-ray transmitted through the specimen, first, development processing is performed. It takes time and is low in accuracy, such as calculating after measuring the image density of the subsequent film. In the case of a silver halide photographic light-sensitive material, the dynamic range is about two digits, and accurate measurement is impossible. On the other hand, if a digital X-ray detector is used, image data can be obtained directly as a digital image signal, which can be processed accurately and quickly.

In the present embodiment, the X-ray detector 11 has a dynamic range of 3 digits or more.
Here, in this embodiment, the density range of the image detected by the bone mineral content measuring apparatus 1 corresponds to an X-ray absorption coefficient of 3 digits or more. That is, the X-rays irradiated to the subject H are attenuated (decreased) to several tenths by passing through the bone part. The X-ray intensity distribution on the surface of the two-dimensional X-ray detector 11 usually has a difference (variation) of about twice as much as the maximum value and the minimum value of the X-ray intensity. For this reason, it is necessary to perform X-ray intensity correction (shading correction described later) on the surface of the X-ray detector 11 in order to accurately measure the bone mineral content. Considering such X-ray attenuation, necessity of correction, and the like, the dynamic range of the X-ray detector 11 needs to be at least three digits or more in order to accurately measure the amount of bone mineral. Since the dynamic range is 3 digits or more, it is possible to achieve higher sensitivity and higher resolution than when shooting with X-ray film or the like, and in a low energy band where the tube voltage applied to the X-ray source 8 is low. Even when the image is taken, it is possible to detect an image with good contrast.
The dynamic range is not particularly limited as long as it is 3 digits or more, and the wider the dynamic range, the more desirable it is in consideration of measurement errors and the like. However, if the dynamic range exceeds 7 digits, it takes time to process the image data and the apparatus becomes expensive. Therefore, the dynamic range is preferably 7 digits or less.

The relative position of the X-ray source 8 and the detector holding unit 12 is fixed, and the distance is R. In FIG. 2, the distance from the X-ray source 8 to the subject H is r1, and the distance between the subject H and the X-ray detector is r2.
In addition, without making R constant, both the distance from the X-ray source 8 to the subject and the distance from the subject to the X-ray detector 11 may be appropriately changed.

Further, the distance from the subject H to the X-ray detector 11 is 15 cm or more.
When X-rays pass through the subject H, scattered X-rays having various energies are generated from the subject H as secondary X-rays. Thus, the distance from the subject H to the X-ray detector 11 is 15 cm. By separating from the above, scattered X-rays can be reduced without reducing the primary X-ray dose by the so-called Gradel effect. In this way, by separating the distance between the subject H and the X-ray detector 11, scattered X-rays can be removed, and the energy width of the X-rays detected by the X-ray detector 11 can be prevented from widening. X-rays with higher monochromaticity can be obtained, which is preferable.
However, when the distance from the subject H to the X-ray detector 11 exceeds 1 m, the apparatus becomes very large, and the X-ray dose to be detected by the X-ray detector 11 is greatly reduced by the distance square law. If the distance is too far, the image will not be sharp. For this reason, the distance from the subject H to the X-ray detector 11 is preferably 1 m or less.
As a method for removing scattered X-rays, it is generally known to use an X-ray grid. Although it is possible to use an X-ray grid in this embodiment as well, it is preferable not to use the X-ray grid because primary X-rays are reduced when the X-ray grid is used.

  An X-ray dose detector 13 for detecting the irradiated X-ray dose is provided below the holding member 7 and on the lower surface of the detector holder 12.

  Between the X-ray source 8 and the detector holding unit 12, a flat object table 14 is provided so that one end thereof is attached to the holding member 7. The subject table 14 is connected to a position adjustment device 15 including a motor or the like that changes the position with respect to the holding member 7 in order to enable imaging magnification adjustment (position adjustment in the height direction) during phase contrast imaging in breast image imaging. Has been.

  A water tank 30 for placing the finger of the subject who is the subject H at the time of radiographing is detachably placed on the subject table 14 in the X-ray irradiation region irradiated from the X-ray source 8. Yes. In the present embodiment, the water tank 30 is a liquid material holding member that holds a liquid material 31 that encloses the subject H at the time of imaging, and the subject H is placed inside the water tank 30 when the subject H is placed in the water tank 30. An amount of the liquid material 31 that is fully immersed is held. In the present embodiment, X-ray imaging is performed in a state where the subject H is submerged in the liquid material 31 having a certain depth and the subject H is sufficiently immersed.

  Although the depth (amount) of the liquid material 31 held in the water tank 30 varies depending on the subject H, it is preferably about 3 cm to 10 cm for a finger and about 5 cm to 15 cm for a foot, for example.

The liquid 31 is preferably water because it is the simplest, cheap and safe. You may use what added the fragrance | flavor, disinfectant, the pigment | dye, etc. to water, and gave the device which increases a patient's safety. Moreover, it is a preferable aspect to use the liquid material 31 that is closer to human flesh and body fluid, rather than water. For example, a hyaluronic acid solution, a gelatin solution, a glycerin solution, a mannose solution, a rice juice, a starch powder, or the like can be used alone or in a solution with water.
Further, the liquid material 31 may be held in the water tank 30 after being put in a plastic bag or the like so that the subject H does not directly touch the liquid material 31.
Further, the liquid 31 or the liquid 31 in a bag or the like may be heated to about body temperature and used.

Further, an attachment portion (not shown) for attaching the compression plate 21 is provided above the position of the holding member 7 where the subject table 14 is attached in FIGS. 1 and 2.
In the present embodiment, the bone mineral content measuring apparatus 1 can capture a breast image in addition to measuring the bone mineral content (bone mineral content determination) as described above. For example, as shown in FIG. 3, the water tank 30 is removed from the subject table 14, and one end of the compression plate 21 is attached to the attachment portion of the holding member 7 so that the compression plate 21 and the subject table 14 are substantially parallel. Attach to. Then, the breast is placed as the subject H on the subject table 14, and the subject H is pressed and fixed by the compression plate 21 from above to perform imaging.
The compression plate 21 is movable along the holding member 7 while maintaining a state parallel to the subject table 14. The movement of the compression plate 21 can be applied either automatically or manually.

In the present embodiment, a protector 25 is provided on the lower surface of the subject table 14 so as to extend in a substantially vertical direction so that the subject can reach the photographing position without hitting his / her leg. As a result, the subject can be positioned at the imaging position without hitting the detector holding unit 12 with his / her leg.
Note that the compression plate 21 and the protector 25 are not essential components, and the compression plate 21 and the protector 25 may not be used.

  As shown in FIG. 4, the imaging main unit 4 includes a control device configured to include a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like (all not shown). 22 is provided. An X-ray dose detection unit 13, a power supply unit 9, a driving device 6, and a position adjustment device 15 are connected to the control device 22 via a bus 23. The control device 22 includes an input device 24a including a keyboard and a touch panel (not shown) for inputting photographing conditions and the like, a position adjustment switch for adjusting the position of the subject table 14, and a CRT display or a liquid crystal display. An operation device 24 having a display device 24b is connected.

  The ROM of the control device 22 stores a control program and various processing programs for controlling each part of the bone mineral content measuring device 1, and the CPU cooperates with the control program and the various processing programs to store the bone mineral content. The operation of each part of the measuring apparatus 1 is comprehensively controlled, X-ray imaging is performed, and image data of the X-ray image is generated.

  For example, the CPU controls the driving device 6 on the basis of the imaging conditions of the subject, the type of imaging such as measurement of bone mineral content or breast imaging, and the imaging body 4 is set to the height of the subject. While raising and lowering to the combined height, the support shaft 5 is rotated to adjust the X-ray irradiation angle. Then, the position of the subject table 14 is adjusted by the position adjustment device 15, and in the case of breast image photographing, the magnification rate of phase contrast photographing is adjusted. Thereafter, the imaging main body unit 4 executes imaging processing, and the power source unit 9 applies a tube voltage and a tube current to the X-ray source 8 to irradiate the subject H with X-rays. When the X-ray dose input from X reaches a preset X-ray dose, the X-ray irradiation from the X-ray source 8 is stopped by controlling the power supply unit 9.

  In this embodiment, the ROM of the control device 22 stores various data used when executing these processing programs, in addition to various programs related to image processing and bone mineral quantification, such as an image processing program and bone mineral quantification program. Has been. The CPU of the control device 22 performs the bone mineral content determination process for measuring the bone mineral content based on the image processing and the image processed data for the acquired image data in cooperation with these control programs and various processing programs. Do.

  Specifically, the control device 22 performs the following process.

First, the control device 22 performs X-ray imaging using an aluminum step in order to correct the device characteristics in a fixed case such as when the bone mineral content measuring device 1 is shipped from the factory or installed at the installation location. Based on the obtained image data, a shading correction value for the deviation of the X-ray irradiation amount on the surface of the subject table 14 is obtained. Then, shading correction is performed on the image data based on the obtained correction value.
Image data shading correction will be described with reference to FIG.
5 (a) and 5 (c) are X-ray images detected by the X-ray detector 11, and FIGS. 5 (b) and 5 (d) are FIGS. c) X-ray detection amount profiles in which the X-ray detection amounts in the Y direction indicated by the arrows are plotted on the vertical axis.
For example, as shown in FIGS. 5A and 5B, there is a variation in the X-ray irradiation amount detected depending on the position on the surface of the subject table 14 before the shading correction, but after the shading correction, As shown in FIGS. 5C and 5D, the variation is alleviated and the bias of the X-ray irradiation amount on the surface of the subject table 14 is corrected.

  Next, a bone mineral amount correction curve calculated based on the detected X-ray dose is generated as a correction coefficient. The generation of the bone mineral correction coefficient will be described by taking aluminum having an X-ray absorption characteristic very similar to that of a human bone as an example.

X-ray imaging using an aluminum step (“Al step” in FIG. 7) detects the X-ray dose that passes through the aluminum step with the thickness changed and the X-ray dose that passes through the acrylic, and the control device 22 attenuates the aluminum. The coefficient μ _Al is obtained.

For example, as shown in FIG. 6, when there is an object in which materials having x-ray absorption coefficients of μ1 and μ2 and thicknesses of x1 and x2, respectively, are overlapped, the thickness “L” of the entire object X1 + x2 = L. Then, if the X-ray dose incident on the object is “I ₀ ” and the X-ray dose transmitted through the object of thickness “L” is “I”, the following equation (1) is established.
I = I ₀ exp (−μ1 × 1−μ2 × 2) (1)
When this equation (1) is modified, the following equation (2) is obtained.
μ1 × 1 + μ2 × 2 = ln (I ₀ / I) (2)
At this time, the following equation (3) is derived.
x1 = {ln (I ₀ / I) −μ2L} / (μ1−μ2) (3)
Thereby, ln (I ₀ / I) can be obtained by X-ray image capturing. If μ2 and L are known at this time, the value of x1 can be obtained if μ1 is obtained.
When measuring the amount of bone mineral, as for μ2, the liquid material 31 such as water is used as described above, and μ1 is a bone of a finger that is the subject H. L can be made constant by immersing a finger in the water tank 30 containing the liquid material 31 such as water and adjusting the amount of the liquid material 31.
Further, in order to accurately obtain this μ1, it is necessary to use, for example, a monochromatic X-ray obtained from a synchrotron radiation X-ray source or the like. In this respect, when a molybdenum anode X-ray tube is used as described above. Can emit a strong characteristic X-ray having an energy of 18 keV, and is excellent in monochromaticity, so that μ1 can be accurately obtained.

Therefore, when the attenuation coefficient μ _Al of aluminum is obtained, in FIG. 7, the thickness of the object is set to “L”, the X-ray dose incident on the object is set to “I ₀ ”, and X attenuated by the object When the dose is “I ₁ ” and the X-ray dose when passing through an object having a thickness “L” is “I”, first, Equation (5) is obtained from Equation (4).
I ₁ = I ₀ exp (−μ _Lu X _Lu ) (4)
μ _Lu = ln (I ₀ / I ₁ ) / X _Lu (5)
Further, energy (Eeff) corresponding to μ _Lu is obtained based on the NIST absorption coefficient database.
Further, an attenuation coefficient (μ _Al ) of aluminum (Al) corresponding to Eeff is obtained based on the absorption coefficient database.
Based on these calculated values, Expression (8) is obtained from Expression (6) and Expression (7), and the theoretical value (X _Al ) of the thickness of the aluminum step is calculated.
μ _Al X _Al + μ _Lu X _Lu = ln (I ₀ / I) X _Al + X _Lu Formula (6)
X _Al + X _Lu = L (7)
X _Al = {ln (I ₀ / I) −μ _Lu L} / (μ _Al −μ _Lu ) (8)

Then, the control device 22 uses the calculation result XAl and the actual measurement value X of the thickness of the aluminum step, as shown in FIG. 8, a correction curve Q indicating a correction coefficient in the bone mineral content measuring device 1. Ask for.
The correction curve Q indicating the correction coefficient indicates that the measured value XAl calculated based on the image data is larger than the actual aluminum thickness X mainly due to the influence of beam hardening as the aluminum thickness X increases. This corrects the decrease. Thus, the correction curve Q for correcting the beam hardening effect has good reproducibility under the same imaging conditions, and the corrected measurement value XAl is plus or minus 1 with respect to the actual aluminum thickness X. A measurement accuracy of ~ 2% is obtained.
The correction coefficient generated as described above is stored in the control device 22.

  Furthermore, the control device 22 performs quantitative measurement of aluminum using this correction coefficient. That is, as shown in FIG. 9, a quantitative image of aluminum can be obtained by performing a profile using a correction coefficient based on image data of an X-ray image when X-rays are irradiated with a tube voltage of 35 kV. .

Similarly, X-ray imaging can be performed using a bone mineral quantification phantom containing calcium carbonate, and the relationship between the attenuation rate of the X-ray dose absorbed and attenuated by the bone mineral quantification phantom and the content of calcium carbonate can be obtained.
For example, the attenuation rate of X-ray dose and calcium carbonate when X-rays are irradiated on an object with an acrylic thickness of 9 mm, 18 mm, and 27 mm using a bone salt quantitative phantom with a calcium carbonate content of 0 to 939 mg / cm ³ The result of having calculated | required the relationship with content of is shown in FIG. FIG. 10 shows the respective results when the irradiation field is set to the normal size and the X-ray tube voltage is set to 30 kV and 35 kV.

  When the relationship between the attenuation rate of the X-ray dose and the calcium carbonate content is obtained in this way, the control device 22 stores the relationship in a storage unit such as a ROM. When imaging for measuring the bone mineral content of the subject H is performed, the bone mineral content is measured based on the relationship between the stored X-ray dose attenuation rate and the calcium carbonate content.

  When performing mammography, the control device 22 performs gradation processing for adjusting image contrast, density adjustment processing, and sharpness for image data of the X-ray image obtained by the X-ray detector 11. Image processing such as frequency processing is performed. Thereby, it is possible to perform image processing suitable for conditions such as an imaging region.

  Next, the effect | action of the bone mineral content measuring apparatus 1 in this embodiment is demonstrated.

  First, an aluminum step is used to obtain a shading correction value for the X-ray dose bias on the surface of the subject table 14, and an X-ray image is taken using a bone mineral quantification phantom. The relationship between the attenuation rate of the weakened X-ray dose and the content of calcium carbonate is obtained.

  Thereafter, the height of the subject table 14 is adjusted to the position of the subject H of the subject, and the X-ray detector 11 and the subject H are adjusted to be separated by a distance of 15 cm or more and 1 m or less. Further, a water tank 30 holding an appropriate amount of the liquid material 31 is placed at a predetermined position on the subject table 14, and the subject H is immersed in the water tank 30. For example, when the subject H is a finger, the finger is sufficiently immersed in the liquid material 31 in the water tank 30. Then, a predetermined amount of X-rays is irradiated from the X-ray source 8 with the fingers immersed in the liquid material 31 of the water tank 30 to perform X-ray image photography.

  The image data of the X-ray image detected by the X-ray detector 11 is sent to the control device 22, and the control device 22 includes the attenuation rate of the X-ray dose and calcium carbonate content obtained previously for the image data. Correction is performed based on a correction coefficient obtained from the relationship with the amount, and a quantitative image capable of obtaining a bone mineral amount value is generated.

  As described above, according to the bone mineral content measuring apparatus 1 in the present embodiment, the bone mineral content can be measured easily and accurately using an apparatus that can be used for general breast image examination. Thereby, the amount of bone mineral can be measured at the time of breast cancer screening, and the measurement can be repeated over time. For this reason, the prevention and early detection of osteoporosis that often occurs particularly in elderly women can be performed without placing a burden on the elderly as much as possible.

  Moreover, since the molybdenum anode X-ray tube excellent in monochromaticity is used as the X-ray source 8, an accurate bone mineral content can be measured.

  In addition, since the tube voltage of the X-ray source 8 is set to 20 kVp or more and 39 kVp or less, the transmitted X-ray dose necessary for measuring the bone mineral content can be obtained, and the bremsstrahlung X other than the characteristic X-ray of 18 keV can be obtained. Since the line does not increase more than necessary, the measurement accuracy does not deteriorate.

  Further, by separating the X-ray detector 11 from the subject H by 15 cm or more, the scattered X-rays can be reduced without reducing the primary X-ray dose by the so-called Gradel effect, and the X-ray detector 11 makes the monochromaticity more uniform. The transmitted X-ray dose by excellent X-rays can be captured. Further, since the distance between the X-ray detector 11 and the subject H is 1 m or less, the X-ray dose to be detected by the X-ray detector 11 is not greatly reduced, and the bone mineral content measurement accuracy is high. Become. Further, since the distance between the X-ray detector and the subject is short, the apparatus can be downsized.

  Furthermore, since the dynamic range of the X-ray detector 11 is as wide as 3 digits or more, it is possible to sufficiently measure X-rays that greatly decrease in the bone portion, and the bone mineral content measurement accuracy can be increased. In addition, since the dynamic range is 7 digits or less, the time spent for image data processing is short, and the apparatus cost can be kept low.

  In addition, since the subject H is a human finger or the like, the subject can simply take a picture with his hands immersed in the water tank 30 placed on the subject table 14 in front of the apparatus. The bone mineral content can be measured by the technique.

  In the present embodiment, the bone mineral content measuring device 1 is configured to be able to also serve as a breast imaging apparatus, but the bone mineral content measuring device 1 may be a dedicated machine for measuring the bone mineral content.

  In the present embodiment, the human hand is used as the subject H as an example, but the subject H is not limited to this, and even if the bone mineral content is measured using the foot or other body part as the subject H. Good.

  Next, referring to FIG. 11 and FIG. 12, a system including the bone mineral content measuring device according to the present invention will be specifically described with reference to examples. The embodiments of the present invention are not limited to these.

  In this example, a digital mammography system manufactured by Konica Minolta MG Co., Ltd. was used. This digital mammography system includes a mammography apparatus (MGU-100B) manufactured by Konica Minolta MG as a bone mineral content measurement apparatus in an example in which the present invention is implemented, and a direct digitizer (REGIUS Vstage MODEL). 190) and a laser imager (DPYPRO MODEL 793). The X-ray tube used in the mammography apparatus is a molybdenum anode, and the focal spot size is nominally 100 μm. The tube voltage of the X-ray source is set to 25 kVp to 35 kVp. A CR cassette, which is a digital X-ray detector, is used as the X-ray detector. The distance from the X-ray tube to the subject table is 65 cm, and the distance from the subject table to the CR cassette table is 49 cm. Note that a scattered X-ray grid is not used. The dynamic range of the used CR cassette photostimulable phosphor plate was almost four digits as shown in FIG.

A digital mammography system including such a mammography apparatus is used to capture an X-ray image of a finger, and an X-ray source of X-ray image data (original image) acquired by a CR cassette. FIG. 12 shows a quantitative image obtained by performing processing based on the correction coefficient obtained from the relationship between the weakness and the calcium carbonate content.
In this embodiment, the X-ray image is taken by irradiating low-energy X-rays. Therefore, in the case of an original image that is not processed, the entire image becomes dark and the image of the finger is clearly displayed. It cannot be determined. On the other hand, in the case of the quantitative image obtained by performing the processing based on the correction coefficient, the finger image can be clearly identified as shown in FIG. It becomes possible to measure the amount of bone mineral.

  The distance from the subject table to the CR cassette table was changed to 15 cm and 65 cm, and similar results were obtained.

It is a side view which shows the principal part structure of the bone mineral content measuring apparatus in this embodiment. It is a schematic diagram which shows the internal structure of the bone mineral content measuring apparatus in this embodiment. It is a figure which shows typically the structural example in the case of using the bone mineral content measuring apparatus in this embodiment for a mammography. It is a block diagram which shows the control structure of the bone mineral content measuring apparatus in this embodiment. 5 (a) and 5 (b) are diagrams showing variations in the X-ray irradiation amount before the shading correction, and FIGS. 5 (c) and 5 (d) are the X-ray irradiations after the shading correction. It is a figure showing the dispersion | variation in quantity. It is a figure explaining the relationship between the X-ray absorption coefficient of the X-rays irradiated to the substance, and the thickness of the substance. It is a figure explaining the attenuation coefficient of the X-ray irradiated to aluminum. It is a graph which shows a correction coefficient. It is explanatory drawing about the quantitative measurement of aluminum. It is a graph which shows the relationship between the attenuation rate of a X-ray, and calcium carbonate content. It is a graph which shows the dynamic range of the X-ray detector in this embodiment. It is a quantitative image obtained by correcting the original image.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Bone salt measuring device 3 Support base 4 Imaging device main-body part 5 Support shaft 7 Holding member 8 X-ray source 9 Power supply part 11 X-ray detector 12 Detector holding part 14 Subject stand 15 Position adjustment apparatus 21 Compression board 22 Control Apparatus 30 Water tank 31 Liquid material H Subject

Claims (6)

A bone mineral content measuring apparatus for taking an X-ray image of a subject and measuring the bone mineral content of the subject based on the obtained image data,
An X-ray source for irradiating the subject with X-rays;
An X-ray detector for recording an X-ray image corresponding to the incident X-ray dose irradiated from the X-ray source;
A liquid material holding member for holding a liquid material containing the subject at the time of imaging,
The X-ray source is a molybdenum anode X-ray tube;
The bone mineral content measuring apparatus, wherein the X-ray detector has a dynamic range of 3 digits or more and is arranged so that a distance from the subject is 15 cm or more.   The bone mineral content measuring apparatus according to claim 1, wherein the X-ray source has a tube voltage set to 20 kVp or more and 39 kVp or less.   The bone mineral content measuring apparatus according to claim 1 or 2, wherein the dynamic range of the X-ray detector is 7 digits or less.   The bone mineral content measuring apparatus according to any one of claims 1 to 3, wherein the X-ray detector is arranged so that a distance from the subject is 1 m or less.   The bone mineral content measuring apparatus according to any one of claims 1 to 4, wherein the subject is a human finger or foot.   The bone mineral content measuring apparatus according to any one of claims 1 to 5, wherein the apparatus can also serve as a breast image capturing apparatus that captures a breast image.