JPH07111733B2 - 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 By reproducing it as a visible image, etc.,
A system capable of obtaining a reproduced image with excellent image quality performance such as contrast, sharpness, and graininess 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 irradiation with excitation light such as visible light 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 in a sheet-shaped accumulative fluorescent light, and the accumulative phosphor sheet is scanned with excitation light such as laser light to generate stimulated emission light, and the obtained stimulated emission light is obtained. A radiation recording / reproducing system that photoelectrically reads 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. (JP-A-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 photographed due to 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, after reading a plurality of radiation images recorded on the recording sheet to obtain a plurality of image signals, based on these image signals The subtraction processing of the radiation image may be performed.

Here, the subtraction process of the radiographic image refers to 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 at a predetermined sampling interval. Read to obtain multiple digital image signals corresponding to each radiation image,
This is a process of obtaining a radiation image in which only a specific subject portion in the radiation image is emphasized or extracted by performing a subtraction process for each corresponding sampling point of the plurality of digital image signals.

This subtraction process basically has the following two. That is, by subtracting (subtracting) the radiation image in which the imaging agent is not injected from the radiation image in which a specific portion of the subject (for example, a blood vessel when the human body is the subject) is emphasized by injection of the contrast agent, The so-called time difference subtraction for extracting a specific part of the object (for example, blood vessels) and the fact that the specific part of the object has different radiation absorption rates for radiations having different energies Radiation having different energies is applied to obtain a plurality of radiation images by the radiations having different energies, and a specific portion of the subject is extracted by appropriately weighting the plurality of radiation images and calculating the difference. There is so-called energy subtraction. The present applicant has also proposed energy subtraction using a stimulable phosphor sheet (JP-A-59-59).
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. The image is read to obtain two digital image signals, and subtraction is performed based on these image signals. Also, in the above-mentioned Japanese Patent Laid-Open No. 59-83486, energy subtraction is performed in a single photographing by irradiating radiation passing through an object on two recording sheets sandwiching a filter having different absorption rate depending on radiation energy. There have been proposed methods by which the

When a subtraction operation is performed based on a plurality of radiographic images obtained by using the above various methods, the high-frequency component of the original radiographic image remains in the post-subtraction image as a false image. There is a problem that the image quality of the image deteriorates.

The present invention has been made in view of the above problems, and an object thereof is to provide an energy subtraction method and apparatus that do not generate the false image of the high frequency component.

(Means for Solving the Problem) The first energy subtraction method of the present invention is to irradiate a subject with radiation emitted from a radiation source and spreading in a predetermined energy range, and to cause the radiation transmitted through the subject to have different energies. A plurality of radiation images of the subject are recorded on the recording sheet by irradiating a recording sheet on which a plurality of radiation images of radiation having a distribution are recorded, and the plurality of radiation images of the subject are recorded from the recording sheet after the recording. Obtaining a plurality of image signals representing each of the radiographic images, correcting the difference in the spatial frequency characteristics of the radiographic images of the subject carried by the plurality of image signals, and performing a subtraction operation based on the corrected image signals. It is characterized by performing.

Further, the energy subtraction method of the present invention may use a plurality of recording sheets. That is, the second energy subtraction method of the present invention is to irradiate a subject with radiation emitted from a radiation source and spreading in a predetermined energy range, and to transmit the radiation transmitted through the subject by a plurality of radiations having different energy distributions. The radiation images of the subject are recorded on the plurality of recording sheets by irradiating the plurality of recording sheets on which the radiation images are respectively recorded simultaneously or sequentially, and the plurality of recording sheets after the recording are used to record the subject. Obtaining a plurality of image signals representing a radiographic image, correcting differences in spatial frequency characteristics of the radiographic image of the subject carried by the plurality of image signals, and performing subtraction calculation based on the corrected image signals. It is characterized by.

A first energy subtraction device of the present invention is a device for carrying out the above-mentioned first energy subtraction method of the present invention, in which a radiation source that emits radiation that spreads in a predetermined energy range and a subject are arranged. The object holding section and a recording sheet on which a plurality of radiation images of radiation having different energy distributions are recorded are arranged, and the sheet holding is provided at a position facing the radiation source with the object placing section interposed therebetween. And a reading unit that obtains a plurality of image signals representing each of a plurality of radiation images of the subject from the recording sheet on which the radiation image of the subject is recorded in the recording unit, and The differences in the spatial frequency characteristics of the radiation images carried by each image signal And the spatial frequency characteristic correction calculation unit that, is characterized in that an arithmetic unit composed of a subtraction calculation unit for performing a subtraction operation based on the plurality of image signals after the correction.

The second energy subtraction device of the present invention is a device for carrying out the above-mentioned second energy subtraction method of the present invention, in which a radiation source that emits radiation that spreads in a predetermined energy range and a subject are arranged. And a plurality of recording sheets for respectively recording a plurality of radiation images of radiation having different energy distributions are arranged at the same time or in sequence, and the subject arrangement section is opposed to the radiation source. A recording unit including a sheet holding unit provided at a position; a reading unit that obtains a plurality of image signals representing the radiation image of the subject from the plurality of sheets on which the radiation image of the subject has been recorded in the recording unit ,
And a spatial frequency characteristic correction calculation unit that corrects differences in spatial frequency characteristics of radiation images carried by the plurality of image signals, and a subtraction calculation unit that performs subtraction calculation based on the corrected plurality of image signals. It is characterized by including the following arithmetic unit.

Here, in the first energy subtraction method and the first energy subtraction device, the “recording sheet on which a plurality of radiation images of radiation having different energy distributions are recorded” is, for example, a radiation image is recorded. A plurality of recording layers having a plurality of recording layers, and these recording layers are arranged so as to sandwich a filter for removing or attenuating radiation in a low energy region of radiation spreading in the predetermined energy range, Of the recording layers, one of which has a function as a filter, a recording layer having a relatively high sensitivity to radiation in a low energy region among radiation spread in the predetermined energy range, and a high energy region. Both with a recording layer having a relatively high sensitivity to the radiation of The recording sheet having the BaFX-based phosphor and the SrFX-based phosphor (where X represents halogen) in the stimulable phosphor is one layer as the recording layer, but mainly during the passage of radiation through the recording layer. The radiation on the low energy side is configured to be more attenuated than the radiation on the high energy side, and a radiation image by radiation having different energy distributions near the front surface and the back surface on the radiation incidence side of the recording layer is formed. Recording sheet to record,
Although one recording sheet is used, it refers to a recording sheet configured to record a plurality of radiation images by radiation having different energy distributions on the one recording sheet.

Further, in the second energy subtraction method and the second energy subtraction apparatus, "a plurality of recording sheets on which a plurality of radiation images by radiation having different energy distributions are respectively recorded" means the first energy While the subtraction method and the recording sheet in the first energy subtraction apparatus are configured so that a plurality of radiation images are recorded on one sheet, a plurality of radiation images are recorded by a plurality of recording sheets. Say.

Further, "correcting the difference in spatial frequency characteristics of radiographic images" in each of the above inventions does not necessarily mean that the spatial frequency characteristics of a plurality of radiographic images are completely the same, and an image after subtraction calculation is performed. It is sufficient to bring the spatial frequency characteristics of a plurality of radiographic images close to each other so that the subtraction image has a predetermined image quality or more in view of the purpose of obtaining (subtraction image). The spatial frequency characteristics may be made close to each other over the entire band, or only the spatial frequency characteristics of the band necessary for observing the subtraction image among the spatial frequency bands may be made close to each other. Further, for example, as a result of correcting one image having a poor spatial frequency characteristic, the spatial frequency characteristic may be higher than the other image in a partial band of the spatial frequency band.

(Function) As a result of investigating the cause of the generation of a high frequency component false image, when recording a plurality of radiation images by one shot, a recording layer (or recording sheet) or a filter (existent) near the radiation source is present. In some cases), a part of the radiation is scattered, and the radiation containing the scattered radiation is applied to the recording layer (or recording sheet) on the side away from the radiation source,
Therefore, the radiation image recorded on the recording layer (or recording sheet) on the side remote from the radiation source has a spatial frequency characteristic (MTF; Modulation Tra
nsfer Function, hereinafter MTF. It has been found that one of the causes is that the image has less high frequency components than.

In addition, when recording multiple radiation images in one shot,
Also, when recording a plurality of radiographic images by performing a plurality of radiographs, the difference is due to the difference in the material and thickness of the recording layers (or recording sheets), and only one recording layer. One of the causes is that the MTF differs depending on whether the radiation image is recorded near the front surface or near the back surface of the recording layer, even if it has it. Become.

In the present invention, the difference in MTF of the radiation image carried by the plurality of image signals is corrected, and the subtraction operation of the radiation image is performed based on the corrected image signal. Therefore, the subtraction image has a high frequency of the original image. The component does not remain as a false image, and the image quality of the subtraction image is improved.

(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 an energy subtraction apparatus of the present invention.

The subject 4 is irradiated with the X-rays 3 emitted from the X-ray tube 2 of the X-ray imaging apparatus 1. X-ray 3a transmitted through the subject 4
Is irradiated on the first stimulable phosphor sheet 5, and a part of the energy of the X-ray 3a is accumulated on the first stimulable phosphor sheet 5, whereby an X-ray image of the subject 4 is formed on the sheet 5. Accumulated and recorded. The X-rays 3b that have passed through the sheet 5 further pass through the filter 6, and the X-rays 3c that have passed through the filter 6 are applied to the second stimulable phosphor sheet 7. As a result, the X-ray image of the subject 4 is also stored and recorded on the sheet 7. still,
The subject 4 has two elements for performing subtraction processing.
Two marks 8 are attached.

FIG. 2 is a diagram schematically showing the X-ray images accumulated and recorded on the sheets 5 and 7.

An X-ray image 4a of the subject 4 is accumulated and recorded on almost the entire surfaces of the sheets 5 and 7, and the image 8a of the mark 8 is also recorded together with the X-ray image 4a.
(For simplicity, this image is also called a mark hereinafter). This mark 8a is recorded on the sheets 5 and 7 at the same position as each other.
The relative alignment of the two X-ray images is performed based on the.

FIG. 3 is a perspective view of an X-ray image reading apparatus which is an example of a reading unit of an energy subtraction apparatus of the present invention and an image processing display apparatus which is an example of an arithmetic unit.

After radiography is performed by the X-ray radiographing apparatus 1 shown in FIG. 1, the first and second stimulable phosphor sheets 5 and 7 are set one by one at a predetermined position of the X-ray image reading apparatus 10. here,
First X accumulated and recorded on the first accumulative glory sheet 5
The case of reading a line image will be described.

The stimulable phosphor sheet 5 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 a focusing lens 20 such as an fθ lens, an optical path 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). A portion of the sheet 14 irradiated with the light beam 17 emits stimulated emission light 22 in an amount corresponding to the accumulated and recorded X-ray image information.
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 that has entered the light guide 23 from the incident end face 23a proceeds by repeating total reflection inside the light guide 23, is emitted from the emission end face 23b, is received by the photomultiplier 24, and is a radiation image. Bright light
The photomultiplier 24 converts 22 into an electrical signal.

Analog signal S output from photomultiplier 24
Is logarithmically amplified by the log amp 25 and then the A / D converter 2
6 Input and sampled to obtain digital image signal SO. This image signal SO represents the first X-ray image accumulated and recorded on the first stimulable phosphor sheet 5, and is called the first image signal SO ₁ . The first image signal SO ₁ is temporarily stored in the internal memory in the image processing display device 30.

This image processing display device 30 is loaded with and driven by a keyboard 31 for inputting various instructions, a CRT display 32 for displaying a visible image based on auxiliary information and image signals for instructions, and a floppy disk as an auxiliary storage medium. A drive disk drive device (33) and a main body (34) containing a CPU and internal memory are provided.

Next, in the same manner as above, the second image signal representing the second X-ray image stored and recorded in the second stimulable phosphor sheet 7 is obtained.
SO ₂ is obtained, and this second image signal SO ₂ is also temporarily stored in the internal memory in the image processing display device 20.

When the two image signals SO ₁ and SO ₂ to be subtracted in this way are stored in the internal memory, the second image signal SO ₂ is read out and carried by the _second image signal SO ₂ . The MTF of the X-ray image is corrected. The correction of the MTF is executed by the main body 34 having a built-in CPU or the like, but the combination of the hardware and the software having the function of correcting the MTF is an example of the MTF correction calculation unit of the present invention. Be conceived.

FIG. 4 shows the MTF of the X-ray image carried by the second image signal SO _2.
It is the figure which showed an example. The horizontal axis represents the spatial frequency f, and the vertical axis represents the response to each spatial frequency f.

This X-ray image is preferably the same MTF as the MTF of the first image signal SO ₁ as shown by the broken line 36 in FIG. 4, but the stimulable phosphor arranged on the side remote from the radiation source 2. Since the image is stored and recorded on the sheet 7, the radiation 12 applied to the stimulable phosphor sheet 7 includes the radiation scattered by the stimulable phosphor sheet 5 and the filter 6, and the response on the high frequency side is accordingly increased. MT, as shown by the solid line 35 in FIG.
It is F. Therefore, the second image signal SO ₂
Frequency processing is performed so as to obtain MTF. The broken line 3
6 is obtained in advance by statistically analyzing a large number of X-ray images.

In this embodiment, as this frequency processing method, Japanese Patent Publication No. 62-62
The so-called blur mask processing method proposed in Japanese Patent Application Laid-Open No. 373, JP-A-1-106277, Japanese Patent Application No. 63-66751, etc. is adopted. This blur mask processing method is, for example, as shown in the following formula (1), for each pixel, obtains an average value (blurring mask) Sus of an image signal corresponding to a pixel within a predetermined range around the pixel, This bokeh mask from the image signal SO _{2 of}
By subtracting Sus, and multiplying the subtracted result by a predetermined coefficient β and adding and subtracting with the original image signal SO ₂ ,
This is a method of obtaining the image signal SO ₂ ′ after frequency processing.

SO ₂ ′ = SO ₂ ± β · (SO ₂ −Sus) (1) The MTF of the X-ray image is corrected by scanning each pixel of the X-ray image on the image signal using this arithmetic expression. be able to.

Instead of the above-described unsharp mask processing, the image signal SO ₂ by Fourier-transforming, by inverse Fourier transform performs a filter calculation on Fourier space may be obtained image signal SO ₂ 'after MTF correction.

Although the MTF of the X-ray image carried by the second image signal SO ₂ is corrected in the above embodiment, the MTF of the X-ray image carried by the first image signal SO ₁ is corrected to the second image signal SO ₂ Carried by X
Of course, the correction may be made so as to match the MTF of the line image.

After the MTF is corrected as described above, the relative alignment of the X-ray images carried by the _two image signals SO ₁ and SO ₂ ′ is performed on the image signals (Japanese Patent Laid-Open No. 58-58-58). See 163338). This alignment is performed by relatively linearly moving and rotating the two X-ray images so that the two marks 8a shown in FIG.

After this, subtraction processing, that is, two image signals SO
_{1, SO 2 'S1 = Wa} · SO 1 for each pixel corresponding to each other of -Wb · SO _2' + C However, Wa, Wb are weighting coefficients, C is representative of the bias component.

According to the weighted subtraction according to
The image signals S1 corresponding to the difference image between the first X-ray image and the second X-ray image by the high-energy X-rays are generated by the low-energy X-rays carried by the _two image signals SO ₁ and SO ₂ ′, respectively. It This image signal S1 is the CRT of the image processing display device 30.
A visible image (energy subtraction image) based on the image signal S1 is sent to the display 32 and reproduced and displayed on the CRT display 32. It should be noted that the function (combination of hardware and software) for performing the above subtraction processing executed by the main body unit 34 is considered as an example of the subtraction operation unit of the present invention.

The above embodiment is an example in which a plurality of X-ray images are recorded in one shot using two stimulable phosphor sheets, but the X-ray images are sequentially photographed on the plurality of stimulable phosphor sheets. May be. Alternatively, a single sheet having a plurality of stimulable phosphor layers may be used. Further, as described above, the stimulable phosphor layer may be only one layer and the X-ray image recorded near the front surface and the back surface of the one layer may be read.

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

(Effects of the Invention) As described in detail above, the energy subtraction method and apparatus for radiation images of the present invention corrects differences in MTFs of radiation images carried by a plurality of image signals to perform subtraction processing. Therefore, it is possible to prevent the high frequency component of the image before the subtraction process from remaining as a false image in the image after the subtraction process. Therefore, a high-quality subtraction image can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic view of an X-ray imaging apparatus which is an embodiment of a recording unit of an energy subtraction apparatus of the present invention, and FIG. 2 is a schematic view of X-ray images accumulated and recorded in each stimulable phosphor sheet. FIG. 3 is a perspective view of an X-ray image reading device and an image processing / display device which are examples of a reading unit and an arithmetic unit of the energy subtraction device of the present invention, and FIG. 4 is an X-ray image. It is a figure showing an example of the MTF of. 1 ... X-ray imaging device, 2 ... X-ray tube 3,3a, 3b, 3c ... X-ray, 4 ... Subject 5 ... First stimulable phosphor sheet 6 ... Filter 7 ... Second Accumulative phosphor sheet 8 …… Mark 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 display device

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location 9163-4C A61B 6/00 303 J

Claims (4)

[Claims] 1. A recording sheet for irradiating a subject with radiation emitted from a radiation source and spreading in a predetermined energy range, and recording a plurality of radiation images of the radiation transmitted through the subject by the radiation having different energy distributions. By irradiating the subject with a plurality of radiation images of the subject on the recording sheet, the plurality of image signals representing the plurality of radiation images of the subject are obtained from the recording sheet after the recording. The energy subtraction method for a radiation image, characterized in that the difference in the spatial frequency characteristics of the radiation image of the subject carried by each of the image signals is corrected, and the subtraction calculation is performed based on the corrected image signal. 2. A plurality of radiation images, each of which is formed by irradiating a subject with radiation emitted from a radiation source and spreading in a predetermined energy range, and transmitting the radiation transmitted through the subject by a plurality of radiation images having different energy distributions. The radiation images of the subject are recorded on the plurality of recording sheets by simultaneously or sequentially irradiating the recording sheets of, and a plurality of image signals representing the radiation image of the subject are recorded from the plurality of recording sheets after the recording. The energy of the radiation image is characterized in that the difference in the spatial frequency characteristics of the radiation image of the subject carried by each of the plurality of image signals is corrected, and the subtraction operation is performed based on the corrected image signal. Subtraction method. 3. A radiation source that emits radiation that spreads in a predetermined energy range, and a subject placement unit on which a subject is placed,
A recording unit on which a recording sheet on which a plurality of radiation images by radiation having different energy distributions are recorded is arranged, and a recording unit including a sheet holding portion provided at a position facing the radiation source across the subject placement portion, A reading unit for obtaining a plurality of image signals representing each of the plurality of radiation images of the subject from the recording sheet on which the radiation image of the subject is recorded in the recording unit, and the plurality of image signals are respectively carried. And a subtraction calculation unit that performs a subtraction calculation based on the corrected plurality of image signals. Radiation image energy subtraction device. 4. A radiation source that emits radiation that spreads in a predetermined energy range, and a subject placement unit on which a subject is placed,
A plurality of recording sheets for respectively recording a plurality of radiation images of radiation having different energy distributions are arranged at the same time or sequentially, and the sheet holding is provided at a position facing the radiation source with the object arrangement section interposed therebetween. And a reading unit for obtaining a plurality of image signals representing a radiation image of the subject from the plurality of recording sheets on which the radiation image of the subject has been recorded in the recording unit, and the plurality of images. An arithmetic unit including a spatial frequency characteristic correction arithmetic unit that corrects differences in spatial frequency characteristics of radiographic images carried by the signals, and a subtraction arithmetic unit that performs subtraction arithmetic based on the corrected plurality of image signals. Energy Subtractor of Radiation Image Down apparatus.