JP2008259688A - Imaging system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging system for visualizing boundaries of a subject with a low X-ray absorption rate clearly and easily, and for acquiring photographed images of high visibility. <P>SOLUTION: The imaging system 1 is equipped with an X-ray tube 2 for emitting X rays and an X-ray detector 31 for recording a radiographic image corresponding to a dosage of X rays emitted from the X-ray tube 2 and performs phase contrast imaging. When a distance from the X-ray tube 2 to a subject H is R1, a distance from the subject H to the X-ray detector 31 is R2, an X-ray tube voltage of the X-ray tube 2 is V, and a focus diameter is D, conditions expressed as 0.10 m≤R1, 0.70 m≤R2, 25 kVp≤V≤40 kVp, and D≤150 μm are satisfied. The X-ray detector 31 satisfies the condition of MTF characteristics expressed as MTF≥0.5 in case of 2 lp/mm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an imaging system, and more particularly to an imaging system that irradiates X-rays and images a subject having a low X-ray absorption rate.

Conventionally, in order to improve the visibility of a photographed image, photographing by a phase contrast method (hereinafter referred to as “phase contrast photographing”) has been performed (see, for example, Patent Document 1). In phase contrast imaging, the focal diameter of the X-ray tube (denoted as D), the distance from the X-ray tube to the subject (denoted as R1), and the distance from the subject to the X-ray detector (denoted as R2) have a predetermined relationship. This is an enlarged shooting. In this phase contrast imaging, X-rays are absorbed when they pass through the subject, and the X-rays are refracted and the edge portion (edge portion) of the object is emphasized. An image in which the boundary portion is emphasized can be obtained, and it is possible to provide a doctor with an optimal image for interpretation that requires detailed examination. This edge enhancement is called an edge effect (phase contrast effect).
In addition, when an X-ray tube used in a medical field or a nondestructive inspection facility is used, geometrical sharpness (blur) according to the focal diameter is generated on the detection surface of the X-ray detector. Since it appears with a size of R2 / R1, it is known that the edge effect by phase contrast imaging is obtained as the focal diameter of the X-ray tube is small (see, for example, Patent Document 2).

  In addition, with the recent digitalization of medical systems, a system for performing the phase contrast imaging using a digital detector has been developed (for example, see Patent Document 3).

  Phase contrast imaging is performed with a mammography system “MGU-100B” manufactured by Toshiba (with a focal point of an X-ray tube of 100 μm) in a mammography system using a breast as an object, and with a Regius system (model 190) manufactured by Konica Minolta. A system that performs data processing and outputs film using the company's imager (Dry Pro model 793) has been put into practical use. Mammography is mainly performed to detect a shadow of a lesion such as a tumor or a microcalcification cluster (hereinafter referred to as an abnormal shadow) that is a finding of breast cancer. Since these detections require close examination, the edge effect by phase contrast imaging is very useful for interpretation.

  By the way, for example, in diseases such as rheumatism, which is now considered a national disease, changes in bone shape are observed as the symptoms progress, but changes in soft tissue such as cartilage wear are observed at the initial symptoms stage. It is supposed to be done. At present, rheumatism is one of the diseases for which there is no fundamental treatment and only the treatment to stop the progression of symptoms. Therefore, the transition to treatment by early detection is important, and the cartilage shape changes. Thus, an X-ray imaging technique capable of observing changes in soft tissue is desired.

  However, in the current X-ray imaging, even in the case of imaging of a hand that performs imaging at a relatively low tube voltage, the tube voltage is reduced only to 40 kVp at most. Therefore, with X-rays of energy irradiated with this tube voltage, It is hardly absorbed by soft tissues such as cartilage, and it is difficult to depict soft tissues such as cartilage. In addition, since the cartilage is covered with a substance whose X-ray absorption coefficient is very similar to that of cartilage whose joint is a joint fluid, it is more difficult to depict the surrounding joint fluid and the cartilage tissue as separate tissues. . In addition, it is considered difficult to obtain an image by X-ray photography in a tissue having low X-ray absorption such as cartilage, and the X-ray image obtained by photographing such a part should be used for diagnosing a disease. I couldn't. For this reason, in X-ray photography, symptoms cannot be found unless the bone part is deformed, and it cannot be said that the technique contributes to early detection of diseases such as rheumatism.

Therefore, in such a tissue, diagnosis has usually been performed based on an image obtained by MRI instead of X-ray imaging. Recently, a technique for taking out synchrotron radiation in which X-rays go straight in parallel and taking the cartilage using this is disclosed.
JP 2001-91479 A Japanese Patent No. 3844830 JP 2001-299733 A

  However, the former MRI imaging has a low spatial resolution and a low resolution of the obtained image. Also, it tends to be avoided because of the high cost. As a result, a disease state such as rheumatism is often discovered by X-ray imaging after the disease has progressed to such an extent that abnormalities in the bone that can be observed by X-ray imaging are observed. At the time of being diagnosed, etc., the patient had to be inconvenient.

  In addition, the latter technique of photographing cartilage using synchrotron radiation is a special imaging method used for experimental research and requires a huge imaging facility, so it is used for diagnosis in general hospitals. It is difficult, and it takes time in the unit of several tens of minutes for diagnosis, and is unsuitable for actual use in a medical field.

  Therefore, the present invention has been made to solve the above problems, and even a subject having a low X-ray absorption rate can easily and clearly depict the boundary of the subject, and has high visibility. It aims at providing the imaging | photography system which can acquire a picked-up image.

In order to solve the above-mentioned problem, the invention described in claim 1 is an X-ray tube that irradiates X-rays and an X-ray image that records an X-ray image corresponding to the X-ray dose of X-rays irradiated from the X-ray tube. An imaging system that performs phase contrast imaging on a subject having a low X-ray absorption rate,
When the distance from the X-ray tube to the subject is R1, the distance from the subject to the X-ray detector is R2, the tube voltage of the X-ray tube is V, and the focal diameter is D,
0.10 m ≦ R1, 0.70 m ≦ R2,
25 kVp ≦ V ≦ 40 kVp,
Satisfying the condition of D ≦ 150 μm,
The X-ray detector has an MTF characteristic at 2 lp / mm, where MTF ≧ 0.5.
It is characterized by satisfying the following conditions.

  According to a second aspect of the present invention, in the imaging system according to the first aspect, a distance R1 from the X-ray tube to the subject and a distance R2 from the subject to the X-ray detector so as to satisfy the condition. And a control means for adjusting the tube voltage V of the X-ray tube.

The invention according to claim 3 is the imaging system according to claim 1 or 2, wherein the focal diameter D of the X-ray tube is
30 μm ≦ D ≦ 150 μm
It is characterized by satisfying the following conditions.

A fourth aspect of the present invention is the imaging system according to any one of the first to third aspects, wherein the distance from the X-ray tube to the X-ray detector is R3 (= R1 + R2). ,further,
0.8m ≦ R3 ≦ (D [μm] / 30 [μm]) m
It is characterized by satisfying the following conditions.

  According to the first aspect of the present invention, the X-ray absorption rate can be improved and the effect of phase contrast can be exhibited even for a subject having a low X-ray absorption rate. As a result, as in conventional X-ray imaging, the apparatus is configured to perform imaging by placing a subject between an X-ray tube and an X-ray detector. The boundary (edge part) of the tissue having a low linear absorption rate can be clearly depicted.

  For this reason, diseases such as rheumatism can be observed over time at regular medical examinations, etc., and it is not necessary to use a special device such as MRI. The effect is that diagnosis is possible.

  According to the second aspect of the present invention, the X-ray absorption rate is improved and the effect of phase contrast is exhibited by adjusting each part so as to perform imaging that satisfies each condition by the control means. You can take a picture that you can. Thereby, there exists an effect that the boundary (edge part) of a structure | tissue with a low X-ray absorptivity can be simply drawn clearly.

  According to the third aspect of the invention, since the focal diameter of the X-ray tube is 30 μm or more, there is an effect that X-rays sufficient to acquire an image can be output from the X-ray tube.

  As the focal spot diameter decreases, the tube current decreases (the output from the X-ray tube decreases). In this regard, according to the invention described in claim 4, when the distance from the X-ray tube to the X-ray detector is R3 (= R1 + R2), 0.8 m ≦ R3 ≦ (D [μm] / 30 [ μm]) m, the X-ray dose sufficient to obtain an image can be obtained, and a clear image can be obtained.

Hereinafter, an embodiment of a photographing system according to the present invention will be described with reference to FIGS. 1 to 7.
First, the configuration of the photographing system 1 in the present embodiment will be described.
As shown in FIG. 1, the imaging system 1 includes an X-ray tube 2, an imaging device 3, and a subject holding member (not shown) disposed between the X-ray tube 2 and the imaging device 3. X-rays emitted toward the subject H from the X-ray tube 2 are received by the imaging apparatus 3, and an X-ray image corresponding to the X-ray dose is detected by an X-ray detector 31 mounted in the imaging apparatus 3. . At the time of imaging, phase contrast imaging at an enlargement ratio M is performed by adjusting the distance R1 between the subject H and the X-ray tube 2 and the distance R2 between the subject H and the X-ray detector 31.

The X-ray tube 2 is an X-ray source that irradiates X-rays. For example, a Coolidge X-ray tube widely used in medical sites and nondestructive inspection facilities, a rotary anode X-ray tube, and the like are applied. The X-ray tube 2 includes a vacuum container (not shown), and includes a cathode and an anode therein. The X-ray tube 2 causes electrons generated from the cathode to collide with the anode to generate X-rays, and at the same time, a window is formed in the vacuum vessel through which X-rays generated inside can be taken out. , X-rays are emitted toward the subject H from the window.
As the cathode of the X-ray tube 2, a thermionic emission type or a field emission type can be applied, and as the anode, for example, tungsten (W), molybdenum (Mo) or the like is used as a main component. It is possible to select and apply the object according to the subject H. Tungsten is used for general radiography, and molybdenum is used for mammography. The window of the X-ray tube 2 may be formed of beryllium (Be), which is a material that hardly absorbs X-rays.

  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 a commonly used X-ray tube, the anode is rotated to prevent a decrease in the life of the anode. An electron beam is made to collide with a surface of a certain size of the anode, and the generated X-rays are emitted toward the subject H from the plane of the certain size of the anode. The size of the plane viewed from the irradiation direction (subject direction) is called the actual focus (focus).

The focal diameter D (μm) is the length of one side when the focal point is a square, the long side or the short side when the focal point is a rectangle or a polygon, and the diameter when the focal point is a circle. The focal diameter D (μm) can be measured by the method defined in (2.2) slit camera of 7.4.1 Focus test of JIS Z 4704-1994.
In the present embodiment, the focal diameter D satisfies the condition of D ≦ 150 μm, and preferably satisfies the condition of 30 μm ≦ D ≦ 150 μm.

  The X-rays emitted from the X-ray tube 2 are divergent light, and when the focal diameter D is 30 μm or more, it becomes an incoherent (incoherent) light source like natural light. However, when a divergent X-ray tube is used, geometrical sharpness (blurring) corresponding to the focal diameter D appears on the detection surface 31a of the X-ray detector 31 with a size of D × R2 / R1. Therefore, as the focal diameter D of the X-ray tube 2 is smaller, the edge effect by phase contrast imaging described later can be obtained. For example, when the focal spot diameter is large, such as 200 μm or more, the edge width due to the phase contrast effect becomes too wide and the sharpness of the edge due to the phase effect cannot be expected. It can be said that it is not suitable. For this reason, in this embodiment, the upper limit of the focal diameter D is defined as 150 μm or less.

The intensity distribution of X-rays emitted from the X-ray tube 2 can be obtained by ray tracing geometrically. FIG. 2 shows the result of calculating the X-ray intensity by ray tracing, assuming that the X-ray tube 2 is a point light source.
As shown in FIG. 2, the X-rays emitted from the X-ray tube 2 are absorbed by the subject H and refracted when passing through the subject H, so that the X-ray intensity at the edge of the subject H increases. . As a result, an edge effect in which the boundary portion of the subject H is emphasized appears.

  However, in a Coolidge X-ray tube (also referred to as a thermionic X-ray tube) widely used in medical sites and nondestructive inspection facilities, the focal diameter D becomes large as shown in FIG. It cannot be regarded as a point source. That is, geometric unsharpness is generated according to the focal diameter D, and this geometric unsharpness can be expressed by D × R2 / R1. In this case, as shown in FIG. 3, the edge enhancement is wider and the intensity is lower than when an ideal point source is assumed.

In the present embodiment, the set value of the tube voltage V of the X-ray tube 2 is adjusted so that the tube voltage V of the X-ray tube 2 satisfies the condition of 25 kVp ≦ V ≦ 40 kVp. For example, when the tube voltage is set to A (kVp), the maximum value of the X-ray energy emitted from the X-ray tube 2 is A (keV), and X-rays of various energies below A (keV) are mixed. For example, the average X-ray energy of multicolor X-rays when A (kVp) is set in a tungsten anode imaging apparatus is about A / 3 to 2A / 3 (keV).
In the present embodiment, the tube voltage V of the X-ray tube 2 is adjusted so as to satisfy the above condition by a main body 33 serving as control means described later.
The lower the tube voltage V of the X-ray tube 2, the lower the average X-ray energy (keV) of X-rays emitted from the X-ray tube 2. The lower the energy X-rays, the greater the absorption by the subject H. Therefore, when photographing a subject H having a low X-ray absorption rate such as soft tissue, the tube voltage V of the X-ray tube 2 is set to a low value. By making the X-ray energy (keV) low, an image with excellent contrast can be obtained. Further, regarding the phase contrast effect, by reducing the irradiated X-ray energy, the effect is increased and a clear image of the edge portion can be obtained.
On the other hand, when the average X-ray energy (keV) is too low, the absorption at the subject H is large, the exposure dose increases, and the burden on the patient increases. In addition, as the average X-ray energy (keV) decreases, the X-ray dose reaching the X-ray detector 31 decreases, so that the image noise increases and the diagnostic ability decreases. Therefore, the average X-ray energy (keV) needs to be adjusted higher than a certain level.
In this regard, when the tube voltage V of the X-ray tube 2 satisfies the condition of 25 kVp ≦ V ≦ 40 kVp, the average X-ray energy (keV) is low enough to improve the soft tissue discrimination ability. In addition, the exposure dose is adjusted to a range that does not cause inconveniences such as an increase in exposure dose and a decrease in diagnostic ability.

The imaging apparatus 3 performs digital imaging, and includes an imaging unit 32 including an X-ray detector 31, a main body unit 33 that is a control unit for performing imaging control, and the like. As described above, the main body 33 can adjust the tube voltage V of the X-ray tube 2. Further, as described later, the main body 33 can adjust the distance R1 from the focal point of the X-ray tube 2 to the subject H and the distance R2 from the subject H to the X-ray detector 31 so as to satisfy predetermined conditions. ing. Specifically, in the present embodiment, the imaging system 1 includes a driving unit (not shown) that moves at least one of the X-ray tube 2, the subject H, and the X-ray detector 31, for example. 33 controls the driving means to adjust at least one of the position of the X-ray tube 2, the position of the subject H, and the position of the X-ray detector 31, and adjusts the subject H from the focal point of the X-ray tube 2. And the distance R2 from the subject H to the X-ray detector 31 are adjusted so as to satisfy a predetermined condition.
The imaging unit 32 includes an X-ray detector 31 and is configured to be able to adjust the height position according to the imaging region.

  The X-ray detector 31 detects X-rays emitted from the X-ray tube 2 and transmitted through the subject H with the detection surface 31a. As the X-ray detector 31, either an analog system using a screen or a film or a digital detector system that detects X-ray dose as digital information may be used. This is preferable because it can be detected as digital information for each pixel and data can be easily processed without using a film or the like.

  Examples of a digital detector system applicable as the X-ray detector 31 include a CR (Computed Radiography) and a two-dimensional image sensor that read information stored in a stimulable phosphor plate or the like with a reader.

  Examples of the two-dimensional image sensor include an FPD (Flat Panel Detector) and a CCD (Charge Coupled Device). An FPD that acquires a signal based on an X-ray irradiation dose for each of a number of pixels arranged two-dimensionally. (Flat Panel Detector) is preferable as a two-dimensional image sensor.

  Such an FPD may be a direct FPD having an array sensor that detects X-rays by directly converting them into electric charges, or a scintillator that converts X-rays into light and light converted by the scintillator. It may be an indirect FPD having an array sensor that detects by converting into electric charge. Indirect FPD scintillators include those having columnar crystal phosphors, those in which phosphors are packed in boxes formed in pixel units of an array sensor described in Japanese Patent No. 3661196, and the like. However, the present invention is not limited to this. The sensitivity increases as the scintillator thickness increases, and the spatial resolution increases as the scintillator thickness decreases. The spectral sensitivity varies depending on the type of scintillator. The phosphor forming the scintillator is preferably an alkali metal halide or alkaline earth metal halide such as CsI: Tl.

  In the present embodiment, the X-ray detector 31 satisfies the condition that MTF ≧ 0.5 when the MTF characteristic at 2 lp / mm. Here, the MTF is a function for evaluating the sharpness of various elements and systems related to image formation (refer to Kyoritsu Shuppan Co., Ltd. edited by the Medical Radiation Dictionary 3rd edition Medical Radiation Dictionary Editorial Committee). In the present embodiment, the MTF performs frequency analysis of a slit image when a slit having a width of 10 μm is captured with R1 = 1.0 meter, R2 ≦ 1.0 cm, at the same tube voltage as when photographing a subject. Assume that it was requested.

  The pixel size S (control unit in the digital detector) of the X-ray detector 31 is preferably 30 μm ≦ S ≦ 150 μm, and more preferably 30 μm ≦ S ≦ 100 μm.

  The main body 33 of the imaging device 3 is connected to the X-ray tube 2, and an operation unit for performing control operations of imaging operations of the X-ray tube 2 and the imaging unit 32 and control for centralized control of each unit of the imaging device 3. And a communication unit (not shown) for communicating with other external devices.

  In the main body 33, it is possible to instruct the X-ray irradiation conditions such as the tube voltage V and the tube current in the X-ray tube 2 and the irradiation timing through the operation unit, and the control unit responds to this instruction operation. Centralized control of the operation of each part of the X-ray tube 2 and the like.

  The subject holding member holds and fixes the subject H at the time of imaging, and here, the distance between the X-ray tube 2 and the form 31 is adjusted so that phase contrast imaging is performed.

Next, a phase contrast imaging method will be described.
FIG. 4 is a diagram for explaining the outline of phase contrast imaging.
As shown in FIG. 4, in the case of a normal imaging method, the subject H is disposed at a position where the X-ray detector 31 is in contact with the subject H (contact imaging position in FIG. 4). In this case, the X-ray image (latent image) recorded in the X-ray detector 31 is approximately the same size as the life size (which means the same size as the subject H).

  On the other hand, the phase contrast imaging provides a distance between the subject H and the X-ray detector 31 and is enlarged with respect to the life size by the X-rays irradiated in a cone beam shape from the X-ray tube 2. A latent image of an X-ray image (hereinafter referred to as an enlarged image) is detected by the X-ray detector 31.

Here, the enlargement ratio M with respect to the life size of the enlarged image is determined by R1 indicating the distance from the focal point of the X-ray tube 2 to the subject H, R2 indicating the distance from the subject H to the X-ray detector 31, and from the focal point of the X-ray tube 2. If the distance to the X-ray detector 31 is R3 (R3 = R1 + R2), it can be obtained by the following equation (1). In view of the effective detection surface size of the X-ray detector 31, an excessively large one is expensive, and therefore M is preferably 3 times or less. In the present embodiment, as described above, the main body 33 as the control means controls the driving means (not shown), so that at least the position of the X-ray tube 2, the position of the subject H, and the position of the X-ray detector 31 are at least. Any one of them is adjusted so that the distance R1 and the distance R2 are suitable for obtaining a predetermined magnification M.
M = R3 / R1 (1)

  In the phase contrast magnified image, the X-rays refracted by passing through the edge of the subject H overlap with each other on the X-ray detector 31 without passing through the subject H, and the X-ray intensity of the overlapped portion is strong. Become. On the other hand, a phenomenon occurs in which the X-ray intensity is weakened in the portion inside the edge of the subject H by the amount of refracted X-rays (see FIG. 2). Therefore, an edge emphasis action (also referred to as an edge effect) in which the X-ray intensity difference is widened at the border of the subject H works, and the edge portion (the border portion or the border) of the subject H is clearly depicted. A high X-ray image can be obtained.

For example, when a stimulable phosphor plate or the like is used as the X-ray detector 31, the laser beam applied when reading the latent image by the X-ray with the reading device is diffused in the phosphor, and thus read out. The X-ray image has a deteriorated sharpness.
In addition, for example, when using an FPD or the like, it is impossible to discriminate a pixel smaller than the pixel size, and the light hitting one pixel is averaged to obtain an image having sharpness deteriorated from an actual X-ray image. It becomes.
As shown in FIG. 5, when close-contact photography is performed, the phase contrast effect does not appear, and the sharpness of the edge portion of the subject is deteriorated by the detector.
When the X-ray absorption rate is low and the difference in X-ray absorption with the surrounding tissue is small, if the edge portion is unclear, it becomes difficult to distinguish from the surroundings, so the detection ability is also low. There is a fear.

However, when there is a difference in refractive index between the subject H and its surroundings, a phase contrast effect is obtained by refraction of X-rays and the edge of the subject H is emphasized, so that the X-ray detector 31 and X It is possible to improve sharpness that is deteriorated and reduced due to the focal diameter of the tube 2.
FIG. 6 shows a simulation of phase contrast imaging with an X-ray tube having a focal diameter of 100 μm (assuming an X-ray energy of 20 keV), R1 = 1.0 m, and R2 = 1.0 m as a subject H. It is an example of a profile. In this case, the refractive index difference δ (described later) of the acrylic rod is about 7E-7. Due to the edge effect, the edge portion (boundary portion) of the subject H can be drawn more clearly than in close-contact photography.

A measure of the phase contrast effect is the refractive index difference between two adjacent objects. The complex refractive index n of the object is given by the following formula (2).
n = 1−δ + iβ (2)

In Expression (2), δ is a term related to X-ray refraction (refractive index difference), and β is a term representing absorption by an object.
For example, microcalcification in mammography is about δ to 1E-6 when the average X-ray energy of X-rays emitted from the X-ray tube 2 is 20 keV, but δ to 5E for soft tissues with low density. In the case where the circumference is covered with joint fluid like cartilage, the refractive index difference between the two objects is further reduced to about δ to 3E-8.
Further, δ becomes larger as the energy of the X-ray to be irradiated is lower.

  When the refractive index difference δ is reduced, the degree of edge enhancement due to X-ray refraction is reduced, and the edge effect becomes clear due to geometrical sharpness caused by the focal diameter D and blurring caused by the X-ray detector 31. Since it becomes difficult to appear, it becomes difficult to detect the effect.

  In order to improve the sharpness of the edge part with the phase contrast effect and to clearly draw the boundary even when shooting a thing with a small refractive index difference with the surroundings, such as the cartilage of the finger, It is necessary to optimize the sharpness of R1, R2 and the X-ray detector 31 and the focal diameter D of the X-ray tube 2.

Therefore, in the present embodiment, as described above, the focal diameter D is D ≦ 150 μm, more preferably 30 μm ≦ D ≦ 150 μm, the tube voltage V of the X-ray tube 2 is 25 kVp ≦ V ≦ 40 kVp, and the X-ray The MTF characteristic of the detector 31 at 2 lp / mm satisfies the condition that MTF ≧ 0.5, and the distance from the X-ray tube 2 to the subject H is R1, and the subject H to the X-ray detector 31 ( When the distance to the detection surface is R2, magnified shooting is performed in an arrangement that satisfies the conditions of 0.10 m ≦ R1 and 0.70 m ≦ R2.
Further, when the distance from the X-ray tube 2 to the X-ray detector 31 is R3 (R3 = R1 + R2), the condition of 0.8 m ≦ R3 ≦ (D [μm] / 30 [μm]) m is satisfied. More preferably. This indicates that the distance R3 between the X-ray tube 2 and the X-ray detector 31 can be increased as the focal diameter D increases. That is, when the focal diameter D is 50 μm, the relationship is 0.8 m ≦ R3 ≦ 1.7 m, and when the focal diameter D is 100 μm, the relationship is 0.8 m ≦ R3 ≦ 3.3 m.

The structure having a low X-ray absorption rate is a structure in which the linear attenuation coefficient k (/ cm) in X-rays of 30 keV satisfies k ≦ 0.5, and particularly, a structure in which k ≦ 0.2 is satisfied. The subject H applied to the imaging system 1 is a tissue having a particularly low X-ray absorption rate in the whole body such as the head, chest, abdomen, spine, and extremities, and includes muscle, skin, fat, and cartilage. These include soft tissues such as blood vessels, and particularly, tissues such as sarcomas and mesothelioma used for diagnosis of articular cartilage and cancer used for diagnosis of rheumatism.
Cartilage itself has a low X-ray absorption rate and is difficult to detect or depict, but the periphery is covered with a substance that is very similar to the X-ray absorption coefficient of cartilage that is joint fluid. Therefore, in order to depict the surrounding synovial fluid and cartilage tissue as separate tissues, it is necessary to express a slight difference in absorption of X-rays with a large contrast. For that purpose, it is effective to photograph with low-energy X-rays. In addition, by performing phase contrast imaging, it is possible to clearly obtain the peripheral portion of the joint fluid and the cartilage tissue.

  In addition, the imaging system 1 may be provided with an irradiation field stop and a housing (both not shown) for preventing excessive exposure between the subject H and the X-ray tube 2.

  In addition, objects that pass through the X-rays emitted from the X-ray tube 2 until they reach the X-ray detector 31 (such as a cassette front plate, protective film, irradiation field stop, housing, and subject holding member) As will be described later, since imaging is performed using low-energy X-rays, these members are preferably formed of a material that hardly absorbs X-rays. In the case of a subject holding member, a structure in which no constituent member is present in a portion through which X-rays are transmitted may be used.

Next, the operation of the imaging system 1 in this embodiment will be described.
In the imaging system 1, when imaging, first, an X-ray detector 31 such as an FPD in which the MTF characteristic when 2 lp / mm satisfies the condition of MTF ≧ 0.5 is installed in the imaging apparatus 3, and the subject H is placed at a predetermined position, and the main body 33 as control means adjusts the magnification R by adjusting the distances R1 and R2 between the subject H and the X-ray tube 2 and between the subject H and the X-ray detector 31. To do. When the X-ray tube 2, the subject H, and the X-ray detector 31 are arranged at a position where a desired magnification M can be obtained, the subject H is fixed to the subject holding member. Then, information on the subject H to be imaged (name, age, imaging region, etc. of the subject providing the subject H) and the tube voltage V of the X-ray tube 2 from the operation unit of the main body 33 and the predetermined value described above. The main body 33 sets and adjusts the tube voltage V of the X-ray tube 2 to a predetermined value. When the X-ray tube 2 and the imaging device 3 are ready, X-rays are emitted from the X-ray tube 2 toward the subject H.
X-rays emitted from the X-ray tube 2 pass through the subject H and form an image on the detection surface 31 a of the X-ray detector 31. At this time, an edge effect in which the edge is emphasized at the border of the subject H by the phase contrast effect works, and the border portion is an image that is sharply depicted. Thereafter, the X-ray image on the X-ray detector 31 is visualized by performing predetermined image processing.

  As described above, in the imaging system 1 in the present embodiment, the focal diameter D of the X-ray tube 2 is D ≦ 150 μm, more preferably 30 μm ≦ D ≦ 150 μm, and the tube voltage V of the X-ray tube 2 is 25 kVp ≦ V ≦. The MTF characteristic when the X-ray detector 31 is 2 lp / mm is 40 kVp, MTF ≧ 0.5, the distance from the X-ray tube 2 to the subject H is R1, and the subject H to the X-ray detector 31 When the distance to (detection surface) is R2, 0.10 m ≦ R1 and 0.70 m ≦ R2, and the distance from the X-ray tube 2 to the X-ray detector 31 is R3 (R3 = R1 + R2) , Phase contrast imaging is performed in an arrangement that satisfies the condition that 0.8 m ≦ R3 ≦ (D [μm] / 30 [μm]) m. As a result, the sharpness of the image deteriorated due to the focal diameters of the X-ray detector 31 and the X-ray tube 2 is improved by the phase contrast effect, and the X-ray tube 2 and the object holding are maintained as in the conventional X-ray imaging. With the device configuration of the member and the X-ray detector 31, a soft tissue such as cartilage of the hand and other boundaries (edge portions) of tissues having a low X-ray absorption rate can be easily and clearly depicted.

  In the present embodiment, the tube voltage V, the distance R1 from the focal point of the X-ray tube 2 to the subject H, and the distance R2 from the subject H to the X-ray detector 31 are adjusted by the main body 33 of the imaging apparatus 3. However, the control means for adjusting these is not limited to the main body 33 of the photographing apparatus 3. Moreover, you may adjust these so that each conditions may be satisfied manually, without using a control means.

  In addition, it is needless to say that the present invention is not limited to the above embodiment and can be modified as appropriate.

  Next, the photographing system according to the present invention will be specifically described with reference to examples.

  In the present embodiment, a substantially cylindrical substance having a diameter of 5 mm is considered as a subject. In imaging with an X-ray tube voltage of 30 kVp, the difference in refractive index δ from the surroundings is 3E-8, and the difference in absorption coefficient β is 2. Each condition (distance from the X-ray tube to the subject R1, distance R2 from the subject to the X-ray detector (detection surface thereof), X-ray detection, assuming a condition equivalent to the finger cartilage of 5E-11 The X-ray intensity distribution detected when performing phase contrast imaging was changed while changing the MTF of the detector and the focal diameter D) of the X-ray tube, and the evaluation of the phase contrast effect was performed for each simulation result. Is. When the difference in absorption coefficient is about 2.5E-11, X-ray absorption of about 2% is observed 1 mm inside from the edge of the cylindrical material.

  In the present embodiment, the case where sharpness is not improved by the phase contrast effect (for example, in the case of a dotted line indicating close-contact photography in FIG. 5) is x, and the X-ray intensity at the edge of the subject is obtained by the phase contrast effect. The case where the X-ray intensity of the portion where there is no subject exceeds the X-ray intensity and the improvement in sharpness is recognized (for example, in the case of a solid line showing phase contrast imaging in FIG. 5) is marked with ◯. In the simulation, the pixel size was calculated as 43.75 μm.

In simulation 1 shown in Table 1 below, when the MTF of 2 lp / mm is 0.6, the focal diameter D is 100 μm, and R2 is 0.75 m, only the distance R1 from the X-ray tube to the subject is changed. A simulation was performed.

  As shown in Table 1, those satisfying all of the conditions of the present invention clearly draw the boundary of the subject because the phase contrast effect causes the X-ray intensity at the edge of the subject to exceed the X-ray intensity at the portion without the subject. The effect was recognized. R1 <0.10 m is not practically preferable in terms of the angle of view.

Simulation 2 shown in Table 2 below changes only the distance R2 from the subject to the X-ray detector when the MTF of 2 lp / mm: 0.6, the focal diameter D: 100 μm, and R1 = 0.75 m. A simulation was performed.

  As shown in Table 2, all the conditions of the present invention are satisfied except for the case of R2 = 0.25 m and the case of 0.5 m (when the condition of the present invention, 0.70 m ≦ R2 is not satisfied). Due to the phase contrast effect, the X-ray intensity at the edge of the subject exceeded the X-ray intensity at the portion without the subject, and the effect of clearly drawing the boundary of the subject was recognized.

The simulation 3 shown in Table 3 below is a simulation performed while changing only the MTF when the focal diameter D is 100 μm, R1 = 0.75 m, and R2 = 0.75 m.

  As shown in Table 3, in the case of MTF = 0.3 and 0.4 of 2 lp / mm (when the MTF characteristic when 2 lp / mm which is the condition of the present invention does not satisfy MTF ≧ 0.5) Other than (), all of the conditions of the present invention are satisfied, and the X-ray intensity at the edge of the subject exceeds the X-ray intensity at the portion without the subject due to the phase contrast effect, so that the boundary of the subject is clearly depicted. The effect was recognized.

The simulation 4 shown in Table 4 below is performed while changing only the focal diameter D of the X-ray tube when MTF of 2 lp / mm is 0.6, R1 = 0.75 m, and R2 = 0.75 m. It is a thing.

  As shown in Table 4, when the focal diameter D = 50 μm and 100 μm, all of the conditions of the present invention are satisfied, and the X-ray intensity at the edge of the subject is the portion where the subject is not present due to the phase contrast effect. The effect of exceeding the X-ray intensity and clearly drawing the boundary of the subject was recognized.

It is a figure which shows the structure of the imaging | photography system in this embodiment. It is a figure explaining the phase contrast effect (edge effect). It is a figure explaining the case where geometric unsharpness with the focus diameter of an X-ray tube arises in a phase contrast effect. It is a figure explaining phase contrast photography. It is a figure which shows the difference between the edge part in the ideal case and the edge part in the case of close_contact | adherence imaging | photography which has not produced the geometrical unsharpness by the focus diameter of an X-ray tube, or the sharpness deterioration by a detector. It is a figure which shows the difference of the edge part in the case of phase contrast imaging | photography, and the edge part in the case of close_contact | adherence imaging | photography.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Imaging system 2 X-ray tube 3 Imaging device 31 X-ray detector 31a Detection surface H Subject

Claims (4)

An imaging system including an X-ray tube for irradiating X-rays and an X-ray detector for recording an X-ray image corresponding to an X-ray dose of X-rays irradiated from the X-ray tube, and performing phase contrast imaging ,
When the distance from the X-ray tube to the subject is R1, the distance from the subject to the X-ray detector is R2, the tube voltage of the X-ray tube is V, and the focal diameter is D,
0.10 m ≦ R1, 0.70 m ≦ R2,
25 kVp ≦ V ≦ 40 kVp,
Satisfying the condition of D ≦ 150 μm,
The X-ray detector has an MTF characteristic at 2 lp / mm, where MTF ≧ 0.5.
An imaging system characterized by being capable of shooting that satisfies the conditions.   2. Control means for adjusting a distance R1 from the X-ray tube to a subject, a distance R2 from the subject to the X-ray detector, and a tube voltage V of the X-ray tube so as to satisfy the condition. The shooting system described in 1. The focal diameter D of the X-ray tube is further
30 μm ≦ D ≦ 150 μm
The imaging system according to claim 1, wherein the imaging system satisfies the following conditions. When the distance from the X-ray tube to the X-ray detector is R3 (= R1 + R2),
0.8m ≦ R3 ≦ (D [μm] / 30 [μm]) m
The imaging system according to any one of claims 1 to 3, wherein the imaging system is satisfied.

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