JPH0793703B2 - Method for energy subtraction of radiographic image and stimulable phosphor sheet laminate used in the method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for energy subtraction of a radiation image and a stimulable phosphor sheet laminate used in the method. More specifically, the present invention uses a stimulable phosphor sheet to store and record a radiation image on the sheet, and then the sheet is scanned with excitation light so that the radiation image is emitted as stimulated emission from the sheet. Energy subtraction method in a radiation image information recording / reproducing system for photoelectrically reading stimulated emission light by a photodetector and reproducing the obtained image signal as a visible image, and a stack of stimulable phosphor sheets used in the method Regarding the body

[0002]

2. Description of the Related Art The same subject is irradiated with X-rays having different energy distributions, and a specific structure (eg, organ, bone, blood vessel) of the subject has a characteristic X-ray energy absorption characteristic. Utilizing this, we obtain two X-ray images in which a specific structure is extracted differently.
A so-called energy subtraction method in which a line image is appropriately weighted and then an image of a specific structure is extracted by performing subtraction (subtraction) between the two images is called a digital subtraction method (also referred to as digital radiography. It is known as one form. Here, as the DR, the existing I.D. I. Digital Fluoroscopy (Digital Fluoroscopy) that digitally processes the output of an X-ray fluoroscopic camera consisting of a tube and TV camera
Fluoroscopy) and Xe-detector such as X-ray detection system used for CT, Scanned Projection Radiograph (Scanned Projection Radiograph)
The one called y) is known.

More specifically, the following various methods are known as conventional energy subtraction methods.

(I) X-rays having different energy distributions are intermittently irradiated to a subject at short time intervals, and X-rays transmitted through the subject are synchronized with the I.V. I. Tube and T.S.
V. A method of obtaining a subtraction image from two or more X-ray images obtained by detecting with an X-ray fluoroscopic camera including a camera or an X-ray detector such as a Xe-detector.
Here, as a method of irradiating X-rays having different energy distributions, i) modifying the X-ray source so that X-rays having such different energy distributions can be emitted, i
i) Emitting X-rays having different energy distributions 2
Such as placing two or more X-ray sources close to each other so that X-rays emitted from them do not interfere with each other, iii) putting a filter for changing the energy distribution of X-rays between the X-ray source and the subject, There is a way.

(II) X having different energy distributions
Two X-ray sources capable of simultaneously radiating rays are arranged close to each other, and a large number of fine slits or fine holes (for example, circles or square holes arranged in a checkered pattern) are arranged. A filter made of an X-ray shielding material such as lead having a structure in which the area ratio between the X-ray shielding portion and the void portion is 1: 1 is provided on one side of the light receiving surface of the X-ray detector. From the X-ray from the other X-ray source
X-rays are inserted between the X-ray source and the subject so that the X-rays do not interfere with each other, and the X-rays are simultaneously emitted from the two X-ray sources to the subject in such a state, whereby different energy distributions are obtained. X on the light receiving surface of the X-ray detector by the X-ray having
A method of forming a line image, detecting the X-ray images by an X-ray detector, separating the detected X-ray images at the time of reading or after reading, and then obtaining a subtraction image from the separated images.

(III) While moving the subject relative to the X-ray source and the X-ray detector, fan-shaped X-rays having different energy distributions are continuously generated from the X-ray source alternately for a certain period of time, Fan-shaped X-rays transmitted through the subject are detected by an X-ray detector installed behind the subject, X-ray images corresponding to X-rays having different energy distributions are obtained from the obtained image signals, and then those X-ray images are obtained. How to get the subtraction image from. As a method of generating X-rays having different energy distributions, the same method as (I) can be considered.

(IV) A filter having the same shape as the filter used in the method (II) is formed of a metal such as copper that absorbs low energy components of X-rays, and this filter is used as one X-ray source and an object. X-rays having different energy distributions which are inserted between the X-rays to generate X-rays having different energy distributions that do not interfere with each other on the light receiving surface of the X-ray detector from the X-rays emitted from the X-ray source. X-ray image is formed on the light receiving surface of the X-ray detector by the X-ray detector, the X-ray image is detected by the X-ray detector, and the detected X-ray image is separated at the time of reading or after the reading, and then the separated image. How to get the subtraction image from.

[0008]

The energy subtraction method can separate and extract images of the structures of the subject having different energy absorption characteristics, and can form an image of only the soft tissue excluding the bones. It is possible.
For example, it is possible to separate and extract an image of a structure such as a bronchus existing in the mediastinum, which has been difficult to be diagnosed by overlapping with a bone in the related art, from the image of the bone. Also, when temporal subtraction is performed in imaging such as abdominal angiography, an artifact due to an abdominal gas image becomes a problem, but the energy subtraction method can erase information related to soft tissue, and a bone image without a gas image can be obtained. It is possible to form only a contrast image. Therefore, the energy subtraction method can obtain diagnostic information that could not be obtained by the conventional method, and is a method that is attracting attention in the medical diagnostic field in principle as a very outstanding method.

However, the above-mentioned conventional energy subtraction method has an essential defect related to DR. That is, the spatial resolution of the subtraction image obtained by DR is generally I.S. I. Tube and T.S.
V. It is determined by the resolution of an X-ray fluoroscopic camera including a camera or an X-ray detector such as a Xe-detector, but the resolution of the X-ray detector used in such a conventional DR is not so high, and The conventional energy subtraction method has a problem that it is impossible to make a fine diagnosis for a specific structure. Further, since the imaging range in the DR is limited by the light receiving area of the X-ray detector, the conventional energy subtraction method has a problem that a subtraction image cannot be simultaneously obtained for a wide range of structures of an object.

Further, the above-mentioned conventional energy subtraction methods (I), (II), (III) and (IV) have the following drawbacks.

1. Require special X-ray source [Method (I)
And method (II)].

2. Misalignment occurs between corresponding pixels of two X-ray images [method when using two X-ray sources]
(I), method (II), method (III) and method (IV)].

3. Only half the resolution can be obtained as compared with the conventional X-ray image forming method [method (II) and method
IV)].

4. Since each X-ray image by X-rays having different energy distributions is formed on the same plane, it is difficult to separate the X-ray images at the time of reading or after the reading [Method (II) And method (IV)].

5. The subject is intermittently irradiated with X-rays having different energy distributions, so that the subject's muscular movement, respiratory movement, peristalsis, etc. cause a positional shift between the images, resulting in a good subtraction image. Cannot be obtained [Method (I)].

6. Since the subject is scanned by the fan-shaped X-rays, it takes time to form one image, and there is a time difference between the beginning and the end of the scan, and each image is caused by the subject's muscular movement, respiratory movement, peristalsis, etc. As a result, a position difference occurs, and as a result, a good subtraction image cannot be obtained, which is particularly unsuitable for extracting an angiographic image [method (III)].

Therefore, an object of the present invention is to provide a radiation image energy subtraction method capable of obtaining a good subtraction image without the drawbacks of the above-mentioned conventional energy subtraction methods. It is in.

Another object of the present invention is the novel X used in the energy subtraction method of the present invention.
To provide a line detector.

[0019]

[Means for Solving the Problems] One of the conventional fluorescent materials
Accumulative phosphors are known as seeds. This stimulable phosphor absorbs and accumulates a part of its radiation energy when it is irradiated with radiation (X-rays, α-rays, β-rays, γ-rays, ultraviolet rays, etc.), and then excited by visible light. When exposed to light,
It refers to a phosphor that exhibits stimulated emission of a quantity of light according to the accumulated energy.By using such a stimulable phosphor, the radiation image of the human body etc. is temporarily stored and recorded on the stimulable phosphor sheet. After that, the stimulable phosphor sheet is scanned with excitation light such as laser light to emit a radiation image as stimulated emission, and the stimulated emission light is photoelectrically read to obtain an image signal, and the image signal is photographed. The applicant has already proposed a radiation image recording / reproducing system for reproducing a visible image on a recording material such as a photosensitive material or a display device such as a CRT (Japanese Patent Laid-Open No. 55-1).
No. 2492, No. 56-11395, etc.). Here, the stimulable phosphor sheet refers to a sheet having a stimulable phosphor layer for accumulating and recording a radiation image. Generally, the stimulable phosphor layer is a suitable binder and a binder. And a stimulable phosphor dispersed in the. When the stimulable phosphor layer is self-supporting, the stimulable phosphor layer itself can be the stimulable phosphor sheet, but in general, the stimulable phosphor layer is provided on a suitable support. To form a stimulable phosphor sheet. Further, usually, a protective film for physically or chemically protecting the stimulable phosphor layer is provided on the surface of the stimulable phosphor layer (the surface opposite to the surface on which the support is provided).

The above radiographic image recording / reproducing system has a very practical advantage that an image can be recorded over an extremely wide radiation exposure region (latitude) as compared with a conventional radiographic system using silver salt photography. . That is, in a stimulable phosphor, it has been recognized that the amount of emitted light stimulated by excitation after storage is proportional to the amount of radiation exposure over a very wide range, and therefore radiation exposure under various imaging conditions is observed. Even if the amount fluctuates considerably, the reading amount of the emitted light is set to an appropriate value and read by photoelectric conversion means to be converted into an image signal, which is used as a recording material such as a photographic light-sensitive material. , A CRT or the like can be output as a visible image to obtain a radiation image that is not affected by variations in radiation exposure.

Further, according to this system, the radiation image stored and recorded on the stimulable phosphor sheet is converted into an image signal, and appropriate signal processing is performed, and the image signal is used to record a recording material such as a photographic light-sensitive material. By outputting the image as a visible image on a display device such as a CRT, it is possible to obtain a very large effect that a radiographic image having excellent observation and interpretation suitability (diagnosis suitability) can be obtained.

In the radiation image information recording / reproducing system using the stimulable phosphor sheet, the reading gain is set to an appropriate value to photoelectrically convert the stimulated emission light and output it as a visible image. Therefore, the variation of the radiation exposure amount due to the variation of the tube voltage or the MAS value of the radiation source, the variation of the sensitivity of the accumulative phosphor sheet, the variation of the sensitivity of the photodetector, the variation of the exposure amount depending on the condition of the subject, Even if the accumulated energy stored in the stimulable phosphor sheet differs due to factors such as different radiation transmittance depending on the subject,
Furthermore, even if the exposure dose of radiation is reduced, it is possible to obtain a radiation image that is not affected by fluctuations in these factors, and the stimulated emission light is converted into an image signal once, and this image signal is converted into this image signal. By performing appropriate signal processing,
It is possible to obtain a radiographic image suitable for a diagnosis site such as the chest and the heart, and it is possible to improve the observation and interpretation suitability.

The radiation image energy subtraction method of the present invention utilizes a radiation image information recording / reproducing system using the above-mentioned stimulable phosphor sheet.
That is, in the radiation image energy subtraction method of the present invention, instead of the X-ray detector used in the conventional energy subtraction method described above, a plurality of stimulable phosphor sheets are arranged in a laminated state. The stimulable phosphor sheet laminate as described above is used as an X-ray detector, and the laminate is irradiated with radiation that has passed through an object including a specific structure having a radiation energy absorption characteristic different from the others. It is characterized in that a subtraction image is obtained from each radiation image read out from a laminated body in which at least two radiation images having different image information of a portion corresponding to an object are accumulated and recorded, and a conventional energy subtraction method is provided. It solves the problems that we had.

That is, in the method of energy subtraction of a radiation image of the present invention, at least radiation having a low energy component absorption characteristic different from that of a radiation transmitted through an object including a specific structure having a radiation energy absorption characteristic different from the others. A stimulable phosphor sheet composed of two types of stimulable phosphor sheets and having a higher low energy component absorption property is placed closer to the subject than a stimulable luminescent sheet having a lower low energy component absorption property. Simultaneously irradiating a plurality of stimulable phosphor sheets arranged in a stacked state with each other by irradiating the stimulable phosphor sheets at a position farther from the photographic subject among the stimulable phosphor sheets. The low-energy component of the radiation is better in the part corresponding to the specific structure than in the accumulative phosphor sheet placed closer to Converting each radiation image stored and recorded in each sheet by exciting light scanning each stimulable phosphor sheet that has stored and recorded the radiation image so that the collected image information is recorded, This stimulated emission light is photoelectrically read to be converted into a digital image signal, and at least 2 from each of the radiation images converted into the digital image signal.
One of the radiation images to be subtracted is obtained, and the digital image signal is subtracted between the corresponding pixels of the at least two radiation images to be subtracted to extract the image of the specific structure.

In the above energy subtraction method, two or three stimulable phosphor sheet laminates are used.
It is preferably composed of a sheet of stimulable phosphor sheet. If the stack is composed of two sheets of stimulable phosphor, then of course the two radiographic images obtained from each sheet are directly two images to be subtracted. In the above energy subtraction method,
The subtraction of the digital image signal between the corresponding pixels of the radiation image means that the digital image signal of the corresponding pixel is multiplied by the weighting coefficient to perform the subtraction to obtain a new image signal. Further, a substance absorbing a low energy component of radiation means a substance which absorbs a low energy component more than a high energy component of radiation.

[0026]

In the energy subtraction method of the present invention, the diameter of the beam of the excitation light (laser light) for scanning the accumulative phosphor sheet at the time of reading is reduced so that the unit area per unit area is reduced. The number of pixels can be increased, and the final output of image data after subtraction processing and various signal processing can be recorded directly on a photosensitive material such as silver salt. A subtraction image having a remarkably high spatial resolution can be obtained, and in principle, a clear subtraction image having a spatial resolution equal to or higher than the discrimination resolution of human vision can be obtained. At the same time, there is no technical problem in increasing the area of the stimulable phosphor sheet, so that a subtraction image can be obtained at once for a large area covering a wide range of structures of the human body. Thus, in the energy subtraction method of the present invention,
The essential problems with the X-ray detector in DR are eliminated.

Further, according to the energy subtraction method of the present invention, since the radiation emitted from the radiation source and transmitted through the subject is simultaneously applied to each stimulable phosphor sheet forming the laminate, at least two radiations to be subtracted. There is no time difference in the formation of the radiation image, and therefore, the subtraction image of the specific structure of the human body, which changes moment by moment, can also be obtained as a good image. Further, in the present invention, since a single radiation source can be used, there is no misalignment between corresponding pixels of at least two radiation images. Further, in the present invention, the conventional general-purpose X-ray generator and imaging apparatus can be used as they are. Further, in the present invention, since the radiation images for the radiations having different energy distributions are recorded on separate stimulable phosphor sheets, there is no problem that the images must be separated at the time of reading or after the reading. Also, obtaining the subtraction image will not reduce the resolution by half. Thus, according to the energy subtraction method of the present invention, the problems of the conventional energy subtraction method can be solved at once. In carrying out the energy subtraction method of the present invention, it is possible to use a cassette used in ordinary X-ray photography as a container for the stimulable phosphor sheet laminate during storage recording. it can. It is one of the advantages of the present invention that the cassette used in ordinary X-ray photography can be used as it is. Furthermore, in the energy subtraction method of the present invention, although the intensity of the radiation energy accumulated in each sheet by stacking a plurality of stimulable phosphor sheets is reduced as the sheet is farther from the subject,
All of them can be obtained as image signals having the same intensity range.

[0028]

Embodiments of the present invention will now be described in detail with reference to the drawings.

In the radiation image energy subtraction method of the present invention, two or more subtractions having different image information due to the difference in the degree of absorption of the low energy component of radiation in the portion corresponding to the specific structure of the subject. In order to store a radiation image so that a desired radiation image is obtained, the following constitution of the stimulable phosphor sheet laminate is specifically adopted.

That is, in the radiation image energy subtraction method of the present invention, the stimulable phosphor sheet laminate is constituted, for example, as any one of the following i), ii) and iii). A radiation image is recorded.

I) Use of a phosphor having a higher absorption characteristic of low energy components of radiation as the stimulable phosphor of the stimulable phosphor sheet located closer to the subject, or positioned closer to the subject The accumulative phosphor layer of the accumulative phosphor sheet, which has a low energy component absorbing substance of radiation dispersedly contained in the accumulative phosphor sheet of the accumulative phosphor sheet, and is thereby positioned closer to the subject. The lower energy component of the radiation transmitted through the subject is absorbed more.

Ii) A support made of a low energy component absorbing material for radiation is used as a support for the stimulable phosphor sheet located closer to the subject, whereby the stimulable phosphor located closer to the subject. The support of the body sheet is made to absorb more low energy components of the radiation transmitted through the subject. In this case, the support composed of the substance absorbing the low energy component of radiation may be composed only of the substance absorbing the low energy component of radiation, or may contain the substance absorbing the low energy component of radiation and this substance in a dispersed state. It may be composed of another substance.

Iii) A combination of i) and ii) above.

As described above, the substance absorbing a low energy component of radiation means a substance that absorbs a low energy component of a radiation more than a high energy component thereof. Specific examples thereof include metals, and particularly Cu, W, Mo, N
i, Pb, Au, Ag, Ba, Ta, Fe, Al, Z
n, Cd, Ti, Zr, V, Nb, Cr, Co and S
It is preferably at least one of n.

In the radiation image energy subtraction method of the present invention, the subtraction is performed between the corresponding pixels of the radiation image to be subtracted in order to obtain the subtraction image. This subtraction is performed as described above. This means to obtain a new image signal by multiplying the image signal of the corresponding pixel of the radiation image to be multiplied by the weighting coefficient and performing subtraction. For example, when there are two images to be subtracted, this subtraction is expressed by an equation: L = mP-nQ (where P and Q are digital image signals of the radiation image to be subtracted, m and n are weighting factors, and L is a new Image signal). To get the subtraction image,
In order to erase the image information other than the image information related to the specific structure to be extracted, it is preferable to match the intensity distributions of the image signals of the portions to be deleted between the images to be subtracted. It is convenient to match the gradations of the parts. In order to realize this, it is preferable that the weighting factors m and n are selected so that the gradations of the portions to be erased of both images match and the subtraction is performed. Depending on the shooting conditions, m = n and further m = n = 1.
Also, in the image to be subtracted, various structures are complicatedly overlapped and recorded as an integral image, so the above weighting coefficient is not necessarily a constant, and in some cases it becomes a function of the thickness of the structure. , It may have non-linearity.

As a concrete method of the above subtraction, a weighting factor is selected so that the intensity distributions of the regions of the structure to be erased (for example, soft tissue such as lung in a chest image) are matched, and all the images to be subtracted. There is a method of multiplying the image signal of the pixel by the weighting coefficient. According to this method, the region of the soft tissue only in each image to be subtracted has a constant intensity, so when subtraction is performed between the images to be subtracted, the image information regarding the soft tissue is lost and only the bone is extracted. The image information is extracted as a difference. Therefore, it is preferable to practice such a simple method.

In the method of energy subtraction of a radiographic image of the present invention, a laminate is formed as in the case of obtaining two radiographic images to be subtracted using a laminate composed of, for example, three accumulative phosphor sheets. When the number of stimulable phosphor sheets to be configured is larger than the number of radiographic images to be subtracted, at least one of the radiographic images to be subtracted is composed of two or more continuous stimulable phosphor sheets. It is obtained from the obtained radiographic image. In order to obtain one radiation image to be subtracted from the radiation images obtained from each of the plurality of continuous stimulable phosphor sheets, it is preferable to use the superposition processing. That is, a radiation image obtained from each of a plurality of continuous stimulable phosphor sheets contains more noise as compared to a radiation image obtained from a stimulable phosphor sheet positioned farther from the subject. However, by multiplying each radiographic image by an appropriate weighting factor and then superimposing the plurality of radiographic images, the radiographic image having a higher S / N ratio than any of the pre-processing radiographic images (that is, subtraction). Radiation image) to be obtained.

The energy subtraction method for a radiation image of the present invention comprises at least two kinds of stimulable phosphor sheets having different low energy component absorption characteristics of radiation and a stimulable phosphor sheet having a higher low energy component absorption characteristic. A laminate is used that is placed closer to the subject than a stimulable phosphor sheet that has a lower energy absorption property. Preferably, a laminate in which two stimulable phosphor sheets having different low energy component absorption characteristics of radiation are arranged as described above is used. In this case, as the means for changing the low energy component absorption characteristics of the radiation of each stimulable phosphor sheet, there are i), ii) and iii) described above. Needless to say, when the laminated body is composed of two stimulable phosphor sheets, the two radiographic images obtained from the respective sheets directly become the two radiographic images to be subtracted.

In the radiation image energy subtraction method of the present invention, the stimulable phosphor sheet is used when the radiation separated into the high energy band and the low energy band by the specific filter is used as the radiation for irradiating the subject. It is preferable because the high-energy component and the low-energy component of the radiation in the laminated body can be more easily separated.

The energy of the radiation image of the present invention
In the subtraction method, making the thickness of the accumulative phosphor sheet or the accumulative phosphor layer closer to the subject smaller means that the accumulative phosphor sheet or the accumulative phosphor sheet placed farther from the subject is stored. It is effective in preventing reduction of the radiation dose reaching the luminescent phosphor layer.

In the energy subtraction method for radiation images of the present invention, the stimulable phosphor used in the stimulable phosphor sheet is such that the wavelength of stimulated emission light emitted by irradiation of excitation light overlaps with the wavelength of excitation light. It is desirable that they are not present in order to improve the S / N ratio. In particular
It is preferable to emit excitation light in the wavelength region of 400 to 800 nm and stimulated emission light in the wavelength region of 300 to 500 nm. As described above, a stimulable phosphor that emits stimulated emission light of 300 to 500 nm and can be preferably used in the present invention is, for example, a rare earth element-activated alkaline earth metal fluorohalide phosphor (specifically, , (Ba _1-xy , Mg, Ca _y ) FX: aEu ²⁺ (where X
Is at least one of Cl and Br, x and y are 0 <x + y ≦ 0.6 and xy ≠ 0, and a is 10 ^−6.
≦ a ≦ 5 × 10 ⁻² ), and (Ba _1-x , M ^II _x ) FX: yA described in JP-A-55-12145, where M ^II is Mg, Ca, Sr, Zn and At least one of Cd, X is at least one of Cl, Br and I
A is Eu, Tb, Ce, Tm, Dy, Pr, Ho,
At least one of Nd, Yb, and Er, x is 0 ≦ x ≦ 0.6, y is 0 ≦ y ≦ 0.2), etc.];
No. 55-12142, ZnS: Cu, Pb,
BaO.xAl ₂ O ₃ : Eu (provided that 0.8 ≦ x ≦ 10) and M ^II O.xSiO ₂ : A (provided that M ^II is Mg, Ca, S)
r, Zn, Cd or Ba, A is Ce, Tb, E
u, Tm, Pb, Tl, Bi or Mn, and x is
0.5 ≦ x ≦ 2.5) and LnOX: xA described in JP-A-55-12144 (where Ln is La, Y, Gd).
And at least one of Lu, X is Cl and B
at least one of r, A is at least one of Ce and Tb, and x is 0 <x <0.1); and the like. Of these, rare earth element activated alkaline earth metal fluorohalide phosphors are preferable, and among them, barium fluorohalide phosphors are particularly preferable from the viewpoint of stimulated emission luminance. This barium fluorohalide fluorescent substance is added with a metal fluoride as disclosed in JP-A-56-2385 and 56-2386, or disclosed in Japanese Patent Application No. 54-150873. As described above, addition of at least one of metal chloride, metal bromide and metal iodide is preferable because the stimulated emission luminance is further improved. In the case of the radiation image energy subtraction method of the present invention, in the case where a laminate is composed of a plurality of stimulable phosphor sheets using different types of stimulable phosphors having different low energy component absorption characteristics of radiation. Is preferably used for each stimulable phosphor sheet by selecting another type of stimulable phosphor having different radiation energy absorption characteristics from the above various stimulable phosphors.

Further, as disclosed in JP-A-55-163500, a phosphor layer of a stimulable phosphor sheet prepared by using the stimulable phosphor as described above is coated with a pigment or a dye. By using and coloring, the sharpness of the radiation image obtained from the sheet can be improved.

In the radiation image energy subtraction method of the present invention, the signal processing used before or after the subtraction is, for example, Japanese Patent Laid-Open No.
-163472, 56-11038, Japanese Patent Application No. 54-151398, 54
-151400, 54-151402, 54-168937, etc., frequency processing, JP-A-55-116339, 55-116
Examples include gradation processing disclosed in No. 340 and No. 55-88740.

FIG. 1 is a schematic sectional view showing an embodiment of a stimulable phosphor sheet laminate used in the energy subtraction method for radiation images of the present invention. The stimulable phosphor sheet laminate shown in FIG. 1 is one in which two stimulable phosphor sheets A and B are arranged in a laminated state.
The stimulable phosphor sheets A and B are prepared by dispersing the stimulable phosphor in a suitable binder on supports 2A and 2B formed of an X-ray permeable material such as polyethylene terephthalate or cellulose acetate. The stimulable phosphor layers 1A and 1B are provided. Here, for the stimulable phosphor sheet A, as the stimulable phosphor of this sheet, is used a luminophore having higher low energy component absorption characteristics than the stimulable phosphor of the stimulable phosphor sheet B. Alternatively, the low-energy component absorbing substance for radiation is dispersedly contained in the stimulable phosphor 1A, so that the low-energy component absorbing property of the radiation is enhanced as compared with the stimulable phosphor sheet B. The surface of the stimulable phosphor layers 1A and 1B of the stimulable phosphor sheets A and B (the surface opposite to the side of the supports 2A and 2B) is provided with the phosphor layer physically or chemically. A protective film such as a polyethylene terephthalate film may be provided for effective protection (the same applies to the storage phosphor sheet laminate shown in FIG. 5 below).

FIG. 2 illustrates how a radiation image is accumulated and recorded on the stimulable phosphor sheet by the energy subtraction method of the radiation image of the present invention using the stimulable phosphor sheet laminate shown in FIG. It is a schematic diagram. A single X-ray source 4 that emits X-rays 6 is arranged in the upper part of the figure, and a stimulable phosphor sheet laminated body shown in FIG. 1 is housed in a cassette 5 behind the subject 7. It is arranged.

When X-rays 6 are radiated from the X-ray source 4, the X-rays 6 pass through the subject 7 including a specific structure having a radiation energy absorption characteristic different from the others, and first, the stimulable phosphor sheet. Reach A. Here, the X-ray image of the subject 7 is accumulated and recorded on the stimulable phosphor sheet A. The phosphor layer of the stimulable phosphor sheet A is formed from a phosphor that absorbs more low energy components of radiation. Therefore, the low energy component is reduced and the high energy component is emphasized in the X-ray transmitted through the stimulable phosphor sheet A. Next, the X-rays transmitted through the stimulable phosphor sheet A reach the stimulable phosphor sheet B, and the stimulable phosphor sheet B is exposed to the subject 7 where the image information related to the low energy component of the radiation is reduced. X
The line image is accumulated and recorded. In this way, the image information is different in the part of the subject 7 corresponding to the specific structure.
One X-ray image is simultaneously stored and recorded on two stimulable phosphor sheets A and B.

An X-ray image is read from the thus obtained two stimulable phosphor sheets A and B by a reading system as shown in FIG. 3 to obtain a digital image signal representing the image. First, while moving the stimulable phosphor sheet A in the direction of arrow Y for sub scanning, the laser light 11 from the laser light source 10 is mainly scanned in the X direction by the scanning mirror 12, and the stimulable phosphor sheet is obtained. X-ray images accumulated and recorded on the sheet from A are sequentially emitted as stimulated emission light 13. The stimulated emission light 13 is incident on the inside of the light collector 14 from one end surface of the light collector 14 made by molding a transparent acrylic plate, and reaches the photomultiplier 15 while repeating total reflection in the light collector 15, and the stimulated emission light is emitted. The light emission amount is output as the image signal S. The output image signal S is a logarithmic converter including an amplifier and an A / D converter.
Digital image signal log S _A logarithmic values by 16 (log S)
Is converted to. The digital image signal log S _A are input to the digital calculator 17 and stored therein. Next, in the same manner, the recorded image of the other one of the stimulable phosphor sheet B is read out, the digital image signal log S _B are stored in the digital arithmetic unit 17. In the digital calculator 17, the image signal of the particular structure desired digital image signal log S _A digital image signal log S _B is subtracted from the extracted between corresponding pixels can be obtained. At this time, the digital image signal log
Each of S _A and log S _B is multiplied by an appropriate weighting factor, but the two weighting factors are preferably selected so that the gray levels of the portions to be erased of both images match. The digital image signal is treated as a logarithmic value here because the image data is band-compressed and unnecessary image information can be completely removed. It is possible to do the same.

In this way, the signal obtained by performing the digital subtraction processing is subjected to image processing such as spatial frequency processing, gradation processing, and averaging processing, if necessary, and then displayed on a CRT or the like. It is reproduced directly on the apparatus or reproduced and recorded on a recording material such as a photosensitive film as described below.

FIG. 4 shows an example of image scanning recording for reproduction. The photosensitive film 20 is moved in the sub-scanning direction of the arrow Y, and the laser beam 21 is applied to the photosensitive film 20.
The main scanning is performed in the X direction, and the laser beam 21 is modulated by the A / O modulator 22 by the image signal from the image signal supplier 23 to form a visible image on the photosensitive film 20. If the output of the digital calculator 17 is used as this image signal, the image of the desired specific structure subjected to the digital subtraction process can be reproduced and recorded on the photosensitive film 20.

The energy of the radiation image of the present invention was measured using the stimulable phosphor sheet laminate shown in FIG.
When performing the subtraction, the subtraction process is performed in the same manner as described above.

FIG. 5 shows the energy of the radiation image of the present invention.
It is a schematic sectional drawing which shows another embodiment of the stimulable phosphor sheet laminated body used for a subtraction method. In this embodiment, one support 2A of the accumulative phosphor sheet A close to the subject is made of a low energy component absorbing material for radiation, and this support 2A absorbs the low energy component of radiation transmitted through the subject. Is performed. The support 2A may be composed of a substance absorbing a low energy component of radiation and another substance containing this substance in a dispersed state, or may be composed only of a substance absorbing a low energy component of radiation. May be.

In the stimulable phosphor sheet laminate shown in FIG. 1, the stimulable phosphor sheet constituting the laminate is a so-called self-supporting stimulable phosphor having no support. It can be replaced with a sheet.

As described in detail above, the method of energy subtraction of a radiation image of the present invention has extremely excellent characteristics, and it greatly contributes to the field of medical diagnosis.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an embodiment of a laminate composed of a stimulable phosphor sheet used in the present invention.

FIG. 2 is a schematic diagram for explaining how a radiation image of a subject is accumulated and recorded on a stimulable phosphor sheet according to the present invention.

FIG. 3 is a schematic diagram illustrating a step of causing a radiation image accumulated and recorded from a stimulable phosphor sheet to emit light as stimulated emission, photoelectrically converting the digital image to obtain a digital signal, and performing a subtraction process on the digital signal.

FIG. 4 is a schematic diagram illustrating a step of reproducing and recording an image subjected to subtraction processing on a photosensitive film using a signal obtained by the subtraction processing.

FIG. 5 is a schematic cross-sectional view showing another embodiment of a laminate composed of a stimulable phosphor sheet used in the present invention.

[Explanation of Codes] A, B Storage phosphor sheet 1A, 1B Storage phosphor layer 2A, 2B Support 4 X-ray source 5 Cassette 6 X-ray 7 Subject 10 Laser light source 11 Laser light 12 Scanning mirror 13 Bright Exhaust emission light 14 Condenser plate 15 Photomul 16 Logarithmic converter 17 Digital calculator 20 Photosensitive film 21 Laser light 22 A / O modulator 23 Image signal supplier

Front page continuation (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location B32B 7/02 103 9268-4F G21K 4/00 L H04N 7/18

Claims (18)

[Claims] 1. Radiation transmitted through an object including a specific structure having a different radiation energy absorption characteristic is composed of at least two kinds of stimulable phosphor sheets having different low energy component absorption characteristics of the radiation, and Accumulative phosphor sheet with higher low energy component absorption characteristics Lower accumulative phosphor sheet with low energy component absorption characteristics Multiple accumulative sheets placed in a stacked state closer to the subject than the fluorescent sheet Due to the simultaneous irradiation of the phosphor sheet, the accumulative phosphors placed closer to the subject on the accumulative phosphor sheet placed farther from the subject among these accumulative phosphor sheets. A radiation image is stored and recorded so that image information in which a low energy component of radiation is absorbed more in a portion corresponding to the specific structure than in a sheet is recorded. The first step of scanning each stimulable phosphor sheet with excitation light to convert each radiation image accumulated and recorded on the sheet into stimulable luminescent light, and photoelectrically reading the stimulable luminescent light A second step of converting into a digital image signal, a third step of obtaining at least two radiation images to be subtracted from each of the radiation images converted into the digital image signal, and the at least two radiation images to be subtracted. A method for energy subtraction of a radiation image, comprising a fourth step of subtracting a digital image signal between corresponding pixels to extract an image of the specific structure. 2. The method of energy subtraction of a radiation image according to claim 1, wherein the laminated body composed of the plurality of stimulable phosphor sheets is composed of two stimulable phosphor sheets. 3. A different type of stimulable phosphor having different absorption characteristics of low energy components of radiation is used for each of the two stimulable phosphor sheets, and the low energy component is applied to a sheet placed near a subject. A sheet with a higher absorption characteristic is used and a sheet placed farther from the subject is used with a sheet with a lower low energy component absorption characteristic, which results in a sheet placed closer to the subject. The method of energy subtraction of a radiation image according to claim 2, wherein the low energy component absorption characteristic is made higher than the low energy component absorption characteristic of the sheet placed at a position far from the subject. 4. A low energy component absorbing substance for radiation is dispersedly contained in a stimulable phosphor layer of a stimulable phosphor sheet placed at a position close to an object, and thereby placed at a position close to an object. 3. The radiation according to claim 2, wherein the low energy component absorption characteristic of the stimulable phosphor sheet is made higher than the low energy component absorption characteristic of the stimulable phosphor sheet placed at a position far from the subject. Image energy subtraction method. 5. A stimulable phosphor sheet placed near a subject comprises a support and a stimulable phosphor layer provided on the support, the support being a low energy component of radiation. The low energy component absorption characteristics of the stimulable phosphor sheet that is made of an absorbing material and is placed closer to the subject than the low energy component absorption characteristics of the stimulable phosphor sheet that is placed far from the subject. The energy of the radiation image according to claim 2, wherein the energy of the radiation image is increased.
Subtraction method. 6. The energy of a radiation image according to claim 5, wherein the support comprises a substance absorbing a low energy component of the radiation and another substance containing the substance in a dispersed state. Subtraction method. 7. The energy subtraction method for a radiation image according to claim 5, wherein the support is composed only of a substance absorbing a low energy component of the radiation. 8. The method of energy subtraction of a radiation image according to claim 4, wherein the substance absorbing the low energy component of radiation is a metal. 9. The metal is Cu, W, Mo, Ni or Pb.
, Au, Ag, Ba, Ta, Fe, Al, Zn, Cd,
The energy subtraction method for a radiation image according to claim 8, wherein the energy subtraction method is at least one of Ti, Zr, V, Nb, Cr, Co and Sn. 10. A plurality of accumulative phosphor sheets having different low-energy component absorption characteristics of radiation are laminated in descending order (or low order) of low-energy component absorption characteristics of radiation. Stimulable phosphor sheet laminate used in the energy subtraction method of radiographic images. 11. The stimulable phosphor sheet according to claim 10, wherein the stimulable phosphor sheet laminate comprises two stimulable phosphor sheets having different absorption characteristics of low energy components of radiation. Sheet laminate. 12. A different type of stimulable phosphor having different low energy component absorption characteristics of radiation is used for each of the two stimulable phosphor sheets, whereby the low radiation of the two sheets is reduced. The stimulable phosphor sheet laminate according to claim 11, wherein the energy component absorption characteristics are changed. 13. A low energy component absorbing substance for radiation is dispersedly contained in a stimulable phosphor layer of one of the two stimulable phosphor sheets, whereby the two stimulable phosphor sheets have a low energy component absorbing substance. The stimulable phosphor sheet laminate according to claim 11, wherein the low energy component absorption property of radiation is changed. 14. One of the two stimulable phosphor sheets comprises a support and a stimulable phosphor layer provided on the support, the support being a low energy component of radiation. A stimulable phosphor sheet laminate according to claim 11, characterized in that it consists of an absorbing material, whereby the low energy component absorption properties of the radiation images of the two sheets are altered. 15. The stimulable phosphor according to claim 14, wherein the support comprises a substance absorbing the low energy component of the radiation and another substance containing the substance in a dispersed state. Sheet laminate. 16. The support according to claim 1, wherein the support is composed only of a substance absorbing a low energy component of the radiation.
4. The stimulable phosphor sheet laminate according to item 4. 17. The stimulable phosphor sheet laminate according to claim 13, wherein the substance absorbing the low energy component of radiation is a metal. 18. The metal is Cu, W, Mo, Ni or P.
b, Au, Ag, Ba, Ta, Fe, Al, Zn, Cd
, Ti, Zr, V, Nb, Cr, Co and Sn.
The stimulable phosphor sheet laminate described.