JPH0786921B2 - Method and apparatus for energy subtraction of radiation image - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for energy subtraction of radiation images.

(Prior Art) In various fields, a recorded radiation image is read to obtain an image signal, the image signal is subjected to appropriate image processing, and then the image is reproduced and recorded. For example, an X with a low gamma value designed for later image processing.
An X-ray image is recorded using a line film, the X-ray image is read from the film on which the X-ray image is recorded, converted into an electric signal, and the electric signal (image signal) is subjected to image processing, and then a copy photograph, etc. A system capable of obtaining a reproduced image having good image quality performance such as contrast, sharpness, and graininess by reproducing the image as a visible image has been developed (see Japanese Patent Publication No. 61-5193).

In addition, according to the applicant, radiation (X-ray, α-ray, β-ray, γ
Ray, electron beam, ultraviolet ray, etc.), a part of this radiation energy is accumulated, and then, when irradiated with excitation light such as visible light, stimulable luminescent light that emits stimulated emission light in a quantity corresponding to the accumulated energy Utilizing the body (stimulable phosphor),
A radiation image of a subject such as a human body is once photographed and recorded on a sheet-shaped stimulable phosphor, and the stimulable phosphor sheet is scanned with excitation light such as laser light to generate stimulated emission light, and the resulting stimulated emission is obtained. A radiation recording / reproducing system that photoelectrically reads emitted light to obtain an image signal and outputs a radiation image of a subject as a visible image to a recording material such as a photographic photosensitive material or a CRT based on the image signal has already been proposed. (Japanese Patent Laid-Open No. 55-12429
No. 56, No. 56-11395, No. 55-0163472, No. 56-164645.
No. 55-116340).

This system has the practical advantage of being able to record images over a very wide radiation exposure area compared to conventional radiographic systems using silver halide photography. That is, it has been confirmed that the amount of stimulated emission light emitted by excitation after storage is proportional to the radiation exposure amount over a very wide range, and therefore the radiation exposure amount varies considerably depending on various imaging conditions. However, the stimulated emission light emitted from the stimulable phosphor sheet is read by the photoelectric conversion means by setting the reading gain to an appropriate value and converted into an electric signal (image signal). By outputting a radiation image as a visible image on a display device such as a photosensitive material or a CRT, it is possible to obtain a radiation image that is not affected by variations in radiation exposure.

In a system using a recording sheet such as an X-ray film or a stimulable phosphor sheet as described above, a plurality of radiation images recorded on the recording sheet are read to obtain an image signal, and then the radiation is calculated based on the image signal. The image may be subjected to subtraction processing.

Here, the subtraction process of the radiographic image means a process of obtaining an image corresponding to a difference between a plurality of radiographic images captured under mutually different conditions, and specifically, the plurality of radiographic images are separated at a predetermined sampling interval. To obtain a plurality of digital image signals corresponding to each radiographic image, and by performing a subtraction process for each corresponding sampling point of the plurality of digital image signals, only a specific subject portion in the radiographic image is obtained. This is a process for obtaining an enhanced or extracted radiographic image.

This subtraction process basically has the following two. That is, by subtracting (subtracting) the radiation image in which the contrast agent is not injected from the radiation image in which a specific portion of the object (for example, a blood vessel when the human body is the subject) is emphasized by injecting the contrast agent, Utilizing so-called time difference subtraction for extracting a specific part (for example, a blood vessel) and the fact that a specific part of a subject has different radiation absorption rates for radiations having different energies, the Radiation having different energies is applied to obtain a plurality of radiation images by the radiations having different energies, and the radiation images are appropriately weighted and the difference is also calculated to extract a specific portion of the subject. There is so-called energy subtraction. The present applicant has also proposed energy subtraction using a stimulable phosphor sheet (JP-A-59).
-83486, JP-A-60-225541).

(Problems to be Solved by the Invention) In the method described in Japanese Patent Laid-Open No. 60-225541, two radiation images obtained by performing radiation imaging twice using radiations having different energies from each other. An image is read, two digital image signals are obtained, and subtraction is performed based on these image signals. However, with this method, there is a time lag between the two shots,
In the meantime, the subject moves, and in the visible image reproduced based on the image signal after subtraction processing, a false image (motion artifact) due to the disagreement of multiple images due to this movement occurs, and the image quality of this visible image is There is a problem of a significant decrease.

Further, in the above-mentioned Japanese Patent Laid-Open No. 59-883486, energy subtraction is performed in one shot by irradiating radiation passing through an object to two recording sheets sandwiching a filter having different absorption rate depending on radiation energy. There have been proposed methods by which the If this method is used, a false image due to the movement of the subject does not occur, but the difference in the radiation energy applied to the two recording sheets is small, so only a low-quality visible image (subtraction image) with a low S / N ratio can be obtained. There is a problem that it cannot be done.

In view of the above problems, the present invention has a high degree of separation of radiation energy without the occurrence of false images due to image mismatch.
An object of the present invention is to provide an energy subtraction method having a high S / N ratio and an apparatus for implementing the method.

(Means for Solving the Problems) An energy subtraction method for a radiation image according to the present invention removes radiation emitted from a radiation source and extending in a predetermined energy range, and removing radiation in an intermediate energy range within the predetermined energy range. Alternatively, the object is irradiated with a first filter that generates radiation separated into low-energy area radiation and high-energy area radiation by attenuating, and the first recording sheet is irradiated with the radiation that has passed through the object. At the same time, by irradiating the second recording sheet with the radiation that has passed through the first recording sheet through a second filter that removes or attenuates the radiation in the low-energy region, a radiation image of the subject can be obtained. Record on the first and second recording sheets, and record the first and second recording sheets after the recording. The first and second image signals representing the radiation image are respectively obtained from the above, and the radiation image is subtracted based on the first and second image signals.

Further, the energy subtraction device of the present invention comprises: a radiation source that emits radiation that spreads in a predetermined energy range; a subject placement portion on which a subject is placed; and a subject placement portion disposed between the subject placement portion and the radiation source. Radiation emitted from a radiation source is incident to generate or separate radiation into radiation in a low energy region and radiation in a high energy region by removing or attenuating radiation in an intermediate energy region within the predetermined energy range. One filter, first and second recording sheets on which a radiation image of the subject is recorded, and radiations in the low energy region, which are inserted between the first and second recording sheets, are removed or attenuated. And a second filter that is disposed, and is provided at a position that faces the radiation source with the subject placement unit interposed therebetween. A recording unit including a sheet holding unit, and first and second image signals representing the radiation image are obtained from the first and second recording sheets on which the radiation image is recorded in the recording unit. It is characterized by comprising a reading unit and a processing unit which subtracts the radiation image based on the first and second image signals.

Here, the "energy" does not mean the intensity of radiation, but the amount proportional to the frequency of radiation.

As the above-mentioned "first filter", for example, a filter or the like to which the absorption characteristic of K-edge whose absorption characteristic changes rapidly with specific energy is applied (for example, "Simulation studies of dual-energy X-" can be used. ray ab
sorptiometry 」Medical Physics, Vol.16, No.1, Jan / Feb
1989).

The K-edge of the "second filter" is preferably on the lower energy side than the K-edge of the "first filter".

Further, the first and second recording sheets need not be separated from each other, and are formed integrally with the second filter on the front surface and the back surface of the second filter, for example. It may be one.

Further, in the case where the first and second recording sheets are stimulable phosphor sheets, the first stimulable phosphor sheet on the subject side that sandwiches the second filter is on the side away from the subject. At least one of a stimulable phosphor having a small radiation absorption rate, a thin stimulable phosphor layer, or a light element stimulable phosphor as compared to a second stimulable phosphor sheet. It is preferable to satisfy one.

(Operation) In the present invention, two recordings sandwiching the filter (corresponding to the second filter of the present invention) having different absorptance depending on the radiation energy described in the above-mentioned JP-A-59-83486. This is a drawback of the method of performing energy subtraction in one shot by irradiating the sheet with radiation, which reduces the difference in the radiation energy applied to the two recording sheets, thus reducing the image quality with a low S / N ratio. This is an improvement in that only a visible image (subtraction image) can be obtained.

In the present invention, the radiation is separated into the radiation in the low energy region and the radiation in the high energy region by passing through the first filter utilizing the absorption characteristic of the K-edge, for example, and the radiation is applied to the subject. The energies of the radiation applied to the first and second recording sheets are well separated, so that a subtraction image with a good S / N ratio can be obtained.

Further, in the present invention, it is possible to perform the energy subtraction by a single photographing as in the one described in the above-mentioned Japanese Patent Laid-Open No. 59-83486, and it is possible to obtain a false image due to the mismatch of the images due to the movement of the subject. (Motion artifact) does not occur either.

When the K-edge of the second filter is on the lower energy side than the K-edge of the first filter,
Radiation in the low energy region which is transmitted without being absorbed in the first recording sheet can be efficiently absorbed by the second filter, and therefore, the second recording sheet is more likely to be mixed with the radiation in the low energy region. A small amount of radiation is emitted, which makes it possible to increase the degree of separation of radiation energy and perform energy subtraction with a higher S / N ratio.

Further, when the first and second recording sheets are sheets of stimulable phosphor, as described above, the first stimulable phosphor sheet on the subject side across the second filter, Compared to the second stimulable phosphor sheet on the side away from the subject, it consists of a stimulable phosphor having a smaller radiation absorption rate, has a thin stimulable phosphor layer, or from a light element stimulable phosphor. When satisfying that, the radiation in the high energy region becomes difficult to be absorbed by the first stimulable phosphor sheet, and similarly to the above, the degree of separation of radiation energy is further increased, and the energy subtraction with a higher S / N ratio is performed. Can be done.

(Examples) Examples of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram of an X-ray imaging apparatus which is an embodiment of a recording unit of the present invention.

The X-rays 3 emitted from the X-ray tube 2 of the X-ray imaging apparatus 1 are transmitted through the first filter 4, and the subject 5 is illuminated by the X-rays 3a transmitted through the first filter 4. The X-rays 3b transmitted through the subject 5 are radiated to the first stimulable phosphor sheet 6, and a part of the energy of the X-rays 3b is accumulated in the first stimulable phosphor sheet 6, whereby the sheet 6 is stored. Subject 5
X-ray images of are stored and recorded. X-ray transmitted through sheet 6
3c further passes through the second filter 7, and the X-ray 3d that has passed through the second filter 7 receives the second stimulable phosphor sheet 8
Is irradiated. As a result, the X of the subject 5 is also displayed on the sheet 8.
The line image is accumulated and recorded.

2A and 2B are diagrams showing the number of photons with respect to the energy of the X-ray 3 immediately after being emitted from the X-ray tube and the energy of the X-ray 3a after passing through the first filter 4, respectively. The horizontal axis of these figures is the X-ray energy (keV), and the vertical axis is the number of photons (relative value) having each energy.

Here, the applied voltage of the X-ray tube 2 is 80 KVp, and the X-ray 3 immediately after being emitted from this X-ray tube 2 is as shown in FIG. 2A.
Except for some peaks, the peaks are almost continuously distributed in the range of 15 keV to 80 keV as an example of the predetermined energy range of the present invention.

The first filter 4 is a 0.2 mm-thick filter made of Sn and has a K-edge in which the absorptance of X-rays changes rapidly at 30 keV. Therefore, the X-ray 3a transmitted through the first filter 4 becomes a spectrum separated into the X-ray 3a 'in the low energy region R1 and the X-ray 3a "in the high energy region R2. The object 5 is illuminated with 3a (3a ', 3a ").

The stimulable phosphor forming the first stimulable phosphor sheet 6 is
It consists of BaFBr: Eu ²⁺ and has a layer thickness of 150 μm. In this first stimulable phosphor sheet 6, among the X-rays 3a transmitted through the first filter 4, especially the X-rays 3a 'in the low energy region R1.
Is absorbed. Therefore, the first stimulable phosphor sheet 6
A first X-ray image of X-rays in which the ratio of the X-rays 3a 'in the low energy region R1 of the subject 5 is considerably high is stored and recorded.

The second filter 7 is a filter made of Cu and having a thickness of 0.5 mm. It cuts X-rays in the low energy region R1 and transmits X-rays in the high energy region R2.

The X-rays 3c transmitted through the first stimulable phosphor sheet 6 also include some X-rays in the low energy region R1 which are not absorbed by the sheet 6 but are transmitted. Therefore, by transmitting the second filter 7, the X-rays 3d transmitted through the second filter 7 become almost only the X-rays in the high energy region R2.

The stimulable phosphor forming the second stimulable phosphor sheet 8 is made of BaFBr: Eu ²⁺ like the stimulable phosphor forming the first stimulable phosphor sheet, but has a double layer thickness. Of 300
μm. Since the layer is thickened in this way, the X-rays 3d that are transmitted through the filter 7 and are formed of the X-rays in the high energy region R2
Are sufficiently absorbed by the second stimulable phosphor sheet 8.
Therefore, the second X-ray image accumulated and recorded on the sheet 8 is an X-ray image by the X-ray of the high energy region R2 of the subject 5.

In this way, the energy is sufficiently separated by the first filter 4 and the second filter 7, and the first and second X-ray images by the X-ray in the low energy region and the X-ray in the high energy region are obtained. The data is stored and recorded in the first and second stimulable phosphor sheets 6 and 8, respectively.

FIG. 3 is a perspective view of an X-ray image reading apparatus which is an embodiment of a reading unit of the energy subtraction apparatus of the present invention.

After imaging is performed by the X-ray imaging apparatus 1 shown in FIG. 1, the first and second stimulable phosphor sheets 6 and 8 are set one by one at a predetermined position of the X-ray image reading apparatus. Here, the case of reading the first X-ray image accumulated and recorded in the first stimulable phosphor sheet 6 will be described.

The stimulable phosphor sheet 6 in which the first X-ray image is stored and recorded, which is set at a predetermined position, is conveyed in the arrow Y direction by the sheet conveying means 15 such as an endless belt driven by a driving means (not shown) ( Sub-scan). On the other hand, a light beam 17 emitted from a laser light source 16 is reflected and deflected by a rotating polygon mirror 19 driven by a motor 18 and rotating at a high speed in the arrow Z direction, and after passing through a focusing lens 20 such as an fθ lens, an optical path is passed by a mirror 21. Then, the light is incident on the sheet 14 and the main scanning is performed in the arrow X direction substantially perpendicular to the sub-scanning direction (arrow Y direction). From the portion of the sheet 14 irradiated with the light beam 17, a stimulated emission light 22 of a light amount corresponding to the accumulated and recorded X-ray image information is emitted, and this stimulated emission light 22 is a light guide 23.
And is photoelectrically detected by a photomultiplier (photomultiplier tube) 24. The light guide 23 is made by molding a light guide material such as an acrylic plate,
The linear incident end face 23a is arranged so as to extend along the main scanning line on the stimulable phosphor sheet 14, and the light receiving face of the photomultiplier 24 is coupled to the emitting end face 23b formed in an annular shape. . The stimulated emission light 22 having the incident end face 23a incident into the light guide 23 travels through the inside of the light guide 23 by repeating total reflection, is emitted from the emission end face 23b, is received by the photomultiplier 24, and is a radiation image. Illuminated luminous light 22
Are converted into electric signals by the photomultiplier 24.

Analog signal S output from photomultiplier 24
Is logarithmically amplified by the log amp 25 and then the A / D converter 2
Input to 6 and sampled to obtain digital image signal S0. The image signal S0 are those representing the first X-ray image stored recorded on the first stimulable phosphor sheet 6, referred to as a first image signal S0 _1. The first image signal S0 ₁ is temporarily stored in the storage unit 27.

Next, in the same manner as described above, the second image signal representing the second X-ray image accumulated and recorded in the second stimulable phosphor sheet 8 is obtained.
S0 ₂ is obtained, the second image signal S0 ₂ is also temporarily stored in the storage unit 27.

The two image signals S0 ₁ and S0 ₂ temporarily stored in the storage unit 27 in this way are read from the storage unit 27 and input to the image processing apparatus 30. In the image processing device 30, based on the two input image signals S0 ₁ and S0 ₂ , S1 = Wa · S0 ₁ −Wb · S0 for each corresponding sampling point of these two image signals S0 ₁ and S0 _{2. 2} + C where Wa and Wb are weighting coefficients and C is a bias component.

According to the above, weighted subtraction, that is, subtraction processing is performed, whereby a first X-ray image by low energy X-rays and a second X-ray image by high energy X-rays carried by the _two image signals S0 ₁ and S0 ₂ respectively. An image signal S1 corresponding to the image of the difference between is generated. This image signal S1
Is sent to the image display device 40, and the visible image (energy subtraction image) based on the image signal S1 is reproduced and displayed on the image display device 40.

FIGS. 4A and 4B are graphs showing the number of photons with respect to the energy of the X-ray immediately after being emitted from the X-ray tube and the energy of the X-ray after passing through the first filter, respectively, in another embodiment. is there. The horizontal and vertical axes of these figures are shown in Figures 2A and 2B.
Similar to the figure, the X-ray energy (keV) and the number of photons (relative value) having each energy are shown. Another embodiment of the present invention will be described below with reference to FIG. 1 and FIGS. 4A and 4B.

Also in this embodiment, the applied voltage to the X-ray tube 2 is 80 KVp, so that FIG. 4A showing the X-ray 3 immediately after being emitted from the X-ray tube 2 is the same as FIG. 2A.

The first filter 4 is made of Nd and has a thickness of 0.
It is a 2 mm filter and has a K-edge with an abrupt change in X-ray absorption rate at about 45 keV. Therefore, the X-ray 3a transmitted through the first filter 4 has a spectrum separated into an X-ray in the low energy region R1 'and an X-ray in the high energy region R2', as shown in FIG. 4B.

Here, as the stimulable phosphor forming the first and second stimulable phosphor sheets 6 and 8, the low energy region
BaFBr: Eu ²⁺ (layer thickness: 150 μm) and GdO so as to efficiently absorb X-rays in R1 ′ and high energy region R2 ′ respectively.
Cl: Ce ³⁺ (layer thickness 200 μm) is used. Further, as the second filter 7, as in the above-described embodiment, Cu (thickness
0.5 mm) is used.

Thus, the stimulable phosphor forming the first stimulable phosphor sheet and the stimulable phosphor forming the second stimulable phosphor sheet need not be the same, and the energy range of the X-ray,
Various optimum ones can be used by combining the first filter and the second filter.

Although the above embodiment is an example in which a stimulable phosphor sheet is used, the recording sheet of the present invention is not limited to the stimulable phosphor sheet, and an X-ray film (generally combined with an intensifying screen at the time of photographing) is used. May be) or the like.

(Effect of the Invention) As described in detail above, the method and apparatus for energy subtraction of a radiation image of the present invention,
For example, since the radiation of the low energy region and the radiation of the high energy region are separated to irradiate the subject by passing through the filter of the first filter utilizing the absorption characteristic of the K-edge, the second filter is inserted. The first and second recording sheets arranged in the above manner are irradiated with radiation having good energy separation. Therefore, energy subtraction with a good S / N ratio can be performed.
In addition, since the energy subtraction can be performed in one shot, no false image such as motion artifact is generated.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an X-ray imaging apparatus which is an embodiment of a recording unit of the present invention, and FIGS. 2A and 2B are an X-ray immediately after being emitted from an X-ray tube and a first image, respectively. FIG. 3 is a diagram showing the spectrum of X-rays after passing through the filter, that is, the number of photons with respect to the energy of X-rays. FIG. 3 is an X-ray image reading apparatus which is an embodiment of the reading unit of the energy subtraction apparatus of the present invention. 4A and 4B are X-ray spectra immediately after being emitted from the X-ray tube and X-ray spectra after passing through the first filter, respectively, in another embodiment of the present invention. . 1 ... X-ray imaging device, 2 ... X-ray tube 3,3a, 3b, 3c, 3d ... X-ray 4 ... First filter, 5 ... Subject 6 ... First stimulable phosphor sheet 7 …… Second filter 8 …… Second stimulable phosphor sheet 16 …… Laser light source, 19 …… Rotating polygon mirror 22 …… Stimulated emission light, 23 …… Light guide 24 …… Photomultiplier 25 ...... Log amplifier, 26 …… A / D converter 30 …… Image processing device, 40 …… Image display device

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location 9163-4C A61B 6/00 350 S

Claims (2)

[Claims] 1. A radiation of a low energy region and a radiation of a high energy region are removed by attenuating or attenuating radiation emitted from a radiation source and spread in a prescribed energy range, in the intermediate energy region within the prescribed energy range. Irradiating a subject through a first filter that generates radiation separated into radiation, irradiating the first recording sheet with the radiation that has passed through the subject, and the radiation having passed through the first recording sheet, By irradiating the second recording sheet through the second filter that removes or attenuates the radiation in the low energy region, the radiation images of the subject are recorded on these first and second recording sheets, and after the recording, First and second image signals representing the radiation image are obtained from the first and second recording sheets, respectively. Et based on the first and second image signals, the energy subtraction method of radiographic images and performing subtraction of the radiographic image. 2. A radiation source that emits radiation that spreads in a predetermined energy range, a subject placement portion on which a subject is placed, and a radiation source that is placed between the subject placement portion and the radiation source. A first filter that produces radiation separated into low-energy region radiation and high-energy region radiation by removing or attenuating radiation in the intermediate energy region within the predetermined energy range by the incident radiation. First and second recording sheets on which a radiation image of the object is recorded, and a second filter inserted between the first and second recording sheets for removing or attenuating radiation in the low energy region. And a sheet holding unit provided at a position facing the radiation source with the subject placement unit interposed therebetween. A reading unit for obtaining first and second image signals representing the radiation image from the first and second recording sheets on which the radiation image has been recorded in the recording unit, and these first and second reading units. A radiation image energy subtraction apparatus comprising a processing unit for performing the subtraction of the radiation image based on the image signal of 1.