JPH04154384A - Method and device for energy subtraction of radiation image - Google Patents

Abstract

PURPOSE:To prevent the shading of a bone part from being mistaken for the shading of a soft part by executing a subtraction processing, based on plural image signals, and deriving an image signal in which the shading of the bone part is expressed in higher density than the shading of the soft part. CONSTITUTION:First of all, a relative alignment of each X-ray image 41, 42 carried by image signals SO1, SO2 accumulated/recorded and converted to cumulative phosphor sheets 5, 7 is executed, and subsequently, addition is executed at every picture element, by which a derived image is smoothed. Also, a subtraction processing is executed in accordance with two prescribed expressions at every picture element, and a soft part image 43 in which shading of a bone part of a body to be photographed is erased and only shading of a soft part is extracted and a bone part image 44 in which shading of the soft part is erased and only shading of the bone part is extracted are derived. Subsequently, the bone part image 44 is subjected to black-and-white inversion, an image in which shading of the bone part is expressed in extremely high density is obtained, and by a fact that the inverted bone part image 45 and the soft part image 43 are superposed, an image signal for showing a superposed image 46 is generated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method and apparatus for energy subtraction of radiographic images.

(Prior Art) It is practiced in various fields to read a recorded radiation image to obtain an image signal, perform appropriate image processing on the image signal, and then reproduce and record the image. For example, a low gamma X
An X-ray image is recorded using a ray film, the X-ray image is read from the film on which the X-ray image was recorded and converted into an electrical signal, and this electrical signal (image signal) is subjected to image processing and then a photocopy is made. By reproducing it as a visible image,
A system that can obtain reproduced images with good image quality performance such as contrast, sharpness, and graininess has been developed (see Japanese Patent Publication No. 81-5193).

The applicant has also discovered that when radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet rays, etc.) is irradiated, part of this radiation energy is accumulated, and when excitation light such as visible light is irradiated, the energy is accumulated. Using a stimulable phosphor (stimulable phosphor) that emits stimulable luminescence light in an amount corresponding to the energy emitted, radiographic images of objects such as the human body are captured on a sheet of stimulable phosphor. The stimulable phosphor sheet is scanned with excitation light such as a laser beam to generate stimulated luminescent light, the resulting stimulated luminescent light is read photoelectrically to obtain an image signal, and based on this image signal, A radiation recording and reproducing system that outputs a radiation image of a subject as a visible image to a recording material such as a photographic light-sensitive material, a CRT, etc. has already been proposed (Japanese Unexamined Patent Publication No. 55-12
No. 429, No. 56-11395.

No. 55-11340, No. 58-164645, No. 5
5-116340 etc.).

This system has the practical advantage of being able to record images over a much wider range of radiation exposure compared to conventional radiographic systems using silver halide photography. In other words, it is recognized that the amount of stimulated luminescence light emitted by excitation after accumulation is proportional to the amount of radiation exposure over an extremely wide range, and therefore the amount of radiation exposure varies considerably depending on various imaging conditions. Even if the stimulable luminescent light is emitted from the stimulable phosphor sheet, the reading gain is set to an appropriate value, the photoelectric conversion means reads it and converts it into an electrical signal (image signal), and this image signal is used to create a photograph. By outputting a radiation image as a visible image to a display device such as a photosensitive material or a CRT, a radiation image that is not affected by fluctuations in radiation exposure can be obtained.

As mentioned above, in a system that uses a recording sheet such as an X-ray film or a stimulable phosphor sheet, after reading multiple radiation images recorded on the recording sheet and obtaining multiple image signals, In some cases, subtraction processing is performed on the radiographic image.

Here, radiographic image subtraction processing refers to processing to obtain an image corresponding to the difference between multiple radiographic images taken under different conditions, and specifically, these multiple radiographic images are collected at a predetermined sampling interval. Read and obtain multiple digital image signals corresponding to each radiographic image,
It is a process of obtaining a radiographic image in which only a specific subject part in the radiographic image is emphasized or extracted by performing subtraction processing on each corresponding sampling point of these plurality of digital image signals.

There are basically two types of subtraccidine treatment: In other words, by subtracting a radiographic image in which no contrast medium has been injected from a radiographic image in which a specific part of the subject (for example, a blood vessel when the human subject is the subject) has been emphasized by injecting a contrast medium, the image of the subject can be visualized. So-called time-difference subtraction that extracts a specific part (for example, a blood vessel), and radiation that has different energies for soft tissue and bone tissue in a specific part of the subject (for example, a human chest) when the subject is the subject. Taking advantage of the fact that each object has a different radiation absorption rate, the same object (for example, the chest of a human body) is irradiated with radiation having different energies, and multiple radiations from each radiation having different energies are generated. A specific part of the subject (
For example, there is so-called energy subtraction that extracts only soft tissue or only bone tissue in the chest. The present applicant has also proposed energy subtraction using a stimulable phosphor sheet (Japanese Patent Laid-Open No. 59-8
(See Japanese Patent Laid-Open No. 80-225541).

Here, the method described in the above-mentioned Japanese Patent Application Laid-open No. 80-225541 involves performing two radiographs using radiation with different energies, and reading the two resulting radiographic images to obtain two images. obtain a digital image signal,
Subtraction is performed based on these image signals. In addition, the above-mentioned Japanese Patent Application Laid-Open No. 59-83488 discloses that energy subtraction can be achieved in one photographing process by irradiating radiation that has passed through the subject onto two recording sheets sandwiching filters that have different absorption rates depending on the radiation energy. A method has been proposed that can do this.

(Problem to be solved by the invention) For example, when obtaining an X-ray image by irradiating an object such as the human chest, which consists of soft parts and bones, the X-rays have a distribution that spreads over a predetermined energy range. When these X-rays pass through the object, the lower-energy X-rays attenuate first, so the X-rays that have passed through the object are higher-energy X-rays than the X-rays that entered the object. A so-called hardening phenomenon occurs in which the components increase. This hardening differs depending on each part of the subject, so for example, if you perform energy subtraction processing to prevent the shadows of bones from interfering with observation and obtain a soft tissue image that only shows the shadows of the soft parts, The soft tissue shadows are not completely erased from the entire image and remain in some parts of the image, for example, in the clavicle region of a chest X-ray image. There is a problem in that the shadows in the bone parts that are exposed give the impression that they are abnormal shadows in soft tissues.

In view of the above circumstances, it is an object of the present invention to provide an energy subtraction method and device that can form an image in which shadows of bone parts do not interfere with observation and are not mistaken for shadows of soft parts. It is something.

(Means for Solving the Problems) The energy subtraction method of the present invention for achieving the above object includes the following steps: A plurality of rays of radiation having different energy distributions from the same subject consisting of bone parts and soft parts. By recording an image on a recording sheet, reading the plurality of radiation images from the recording sheet to obtain a plurality of image signals representing each of the plurality of radiation images, and performing subtraction processing based on the plurality of image signals. , obtaining a processed image signal representing a radiographic image in which shadows of the bone part are expressed with higher density than shadows of the soft part, and outputting a reproduced image based on the processed image signal. .

Here, it is preferable to perform the subtraction processing by changing the parameters of the subtraction processing for each portion of the radiation image.

In addition, the energy subtraction device of the present invention includes a radiation source that emits radiation, a subject placement section where a subject consisting of bones and soft parts is placed, and a plurality of radiographic images using radiation having different energy distributions at the same time. Alternatively, a recording unit comprising a sheet holding section provided at a position facing the radiation source across the subject placement section, in which recording sheets for sequential recording are arranged, and a radiographic image of the subject in the recording unit. 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 recording has been performed; a subtraction process is performed based on the plurality of image signals obtained by the reading unit; A calculation unit that obtains a processed image signal representing a radiation image in which shadows of the bone part are expressed with higher density than shadows of the soft part, and an output unit that outputs a reproduced image based on the processed image signal. It is characterized by the fact that it is equipped with

Here, the above-mentioned "recording sheet" refers to a sheet-like material, such as the above-mentioned stimulable phosphor sheet or X-ray film, on which a radiation image is recorded by irradiating radiation thereon.

Further, the above-mentioned "density" includes the brightness when the image is displayed on, for example, a CRT display, and in this case, "low brightness" corresponds to "high density".

(Function) The energy subtraction method and device of the present invention do not forcibly erase the shadows of bone parts, but instead express the shadows of bone parts with higher density than the shadows of soft parts. This makes the shadows of soft tissue less noticeable, thereby making it easier to observe the shadows of soft tissue.

In addition, since the shadows in the bone area are visible on the image, although they are not noticeable, it is easier to understand the placement relationship between the shadows in the soft tissue and the bone area, compared to when a soft tissue image is the object of observation. There is also an advantage.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. Here, an example using the above-mentioned stimulable phosphor sheet will be described.

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

A subject 4 is irradiated with X-rays 3 emitted from an X-ray tube 2 of this X-ray imaging apparatus 1. X-ray 3 transmitted through object 4
a is irradiated onto the first stimulable phosphor sheet 5, and the X-ray 3a
A part of the energy is stored in the second stimulable phosphor sheet 5, and thereby an X-ray image of the subject 4 is stored and recorded on the sheet 5.

The X-rays 3b that have passed through the sheet 5 further pass through a filter 6, and the X-rays 3C that have passed through the filter 6 are irradiated onto the second stimulable phosphor sheet 7. As a result, the X-ray image of the subject 4 is also accumulated and recorded on the sheet 7. Note that two marks 8 are attached to the subject 4 for performing subtractive image processing.

The above-mentioned X-ray imaging device accumulates and records X-ray images on two stimulable phosphor sheets 5.7 in one imaging session, but the X-ray images are recorded at two consecutive timings with different energies. Each image may be imaged one by one using X-rays.

FIG. 2 is a perspective view of an X-ray image reading device which is an embodiment of the reading unit of the energy subtraction device of the present invention, and an image processing display device which is an embodiment of the arithmetic unit and output unit.

After imaging is performed with the XII imaging device 1 shown in FIG.
The first and second stimulable phosphor sheets 5.7 are set one by one at predetermined positions in the X-ray image reading device IO. Here, the case of reading the first X-ray image accumulated and recorded on the first stimulable phosphor sheet 5 will be described.

The stimulable phosphor sheet 5 on which the first X-ray image has been accumulated and recorded is set at a predetermined position and is transported by a sheet conveying means 15 such as an endless belt driven by a driving means (not shown).
The paper is transported (sub-scanning) in the direction of arrow Y. On the other hand, the light beam 17 emitted from the laser light source 1B is transmitted to the motor 18.
A rotating polygon mirror 19 that is driven by and rotates at high speed in the direction of the arrow.
is reflected and deflected by a focusing lens 20 such as an fθ lens.
After passing through the sheet 1, the optical path is changed by the mirror 21 and the sheet 1
4, and performs main scanning in the direction of arrow X, which is substantially perpendicular to the direction of sub-scanning (direction of arrow Y). A portion of the stimulable phosphor sheet 14 that is irradiated with the light beam 17 emits stimulated luminescence light 22 of an amount corresponding to the accumulated and recorded X-ray image information, and this stimulated luminescence light 22 is It is guided by a guide 23 and photoelectrically detected by a photomultiplier (photomultiplier tube) 24. The light guide 23 is made by molding a light-guiding material such as an acrylic plate, and is arranged so that a linear entrance end surface 23a extends along the main scanning line on the stimulable phosphor sheet 14. The light receiving surface of the photomultiplier 24 is coupled to the annularly formed exit end surface 23b. The stimulated luminescent light 22 entering the light guide 23 from the input end surface 23a travels through the interior of the light guide 23 through repeated total reflection, exits from the exit end surface 23b, is received by the photomultiplier 24, and is converted into a radiographic image. The expressed stimulated luminescent light 22 is converted into an electrical signal by a photomultiplier 24.

The analog signal S output from the photomultiplier 24 is logarithmically amplified by the log amplifier 25, and then the A/D
The signal is input to a converter 26 and sampled to obtain a digital image signal SO. This image signal SO represents the first X-ray image formed by relatively low-energy X-rays accumulated and recorded on the first stimulable phosphor sheet 5, and here this is referred to as the first X-ray image. Image signal S01 of
It is called.

This first image signal S01 is temporarily stored in an internal memory within the image processing display device 30.

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

Next, in the same manner as described above, a second image signal S02 representing a second X-ray image formed by relatively high-energy X-rays stored and recorded on the second stimulable phosphor sheet 7 is generated.
is obtained, and this second image signal SO□ is also temporarily stored in the internal memory within the image processing display device 30.

FIG. 3 shows two image signals s representing first and second X-ray images stored in an internal memory within the image processing and display device.
FIG. 2 is a diagram showing the flow of processing performed within the image processing display device based on ol and so□.

The first and second X-ray image signals SQ, , So□ stored in the internal memory in the image processing and display device are respectively the first X-ray images 41 . A signal carrying a second X-ray image 42. The first X-ray image 41 is an image using relatively low-energy X-rays, and the second X-ray image 42 is an image using relatively high-energy X-rays, although the densities of soft parts and bone parts are different from each other. Both are original images in which both the soft and bone parts are recorded.

These first and second X-ray image signals S01゜SO□
are read out from the internal memory in the image processing display device 30 shown in FIG. 2, and first these two image signals SO! , S
Relative positioning of each X-ray image 41 and 42 carried by o2 is performed on the image signal (Japanese Patent Application Laid-Open No. 1986-
(See Publication No. 183338). This alignment is performed by relatively linearly and rotationally moving the two X-ray images so that the two marks 8 shown in FIG. 1 overlap.

After that, in this embodiment, in order to reduce the effect of hardening of each part on the X-ray image, the parameters of the subtraction process are changed for each part of the X-ray image as follows (patent application 2-89367).

First, a smoothed image in which the low spatial frequencies of the X-ray image are emphasized is obtained. This smoothed image consists of two X-ray images 4
1.42, but in this embodiment, in order to further reduce the noise components included in these X-ray images 41.42, the X-ray images 41.42 that correspond to each other are A superimposed image is obtained by performing calculations for each pixel according to the formula (1), and a smoothed image is obtained by smoothing this superimposed image. Here, for each pixel of the superimposed image, by calculating the average value of a large number of image signals SO corresponding to a large number of pixels surrounding each pixel, this average value is used to generate a smoothed image signal representing a smoothed image. SSM
is required.

After this smoothed image signal SSM is obtained, subtraction processing is performed for each pixel according to the formula % Formula % Soft tissue image 43 in which the shadows of the bone parts of the subject 4 are erased and only the shadows of the soft parts are extracted (See Figure 3) is required. Here, Ka and Kb are two image signals So2. so, the parameter that determines the weighting, Kc is the parameter that determines the bias, and here each parameter Kg, Kb, K
Both c are functions of the smoothed image signal SSM.

FIG. 4 is a diagram showing an example of the functions of the parameters Ka, Kb, and Kc.

As shown in FIG. 4(a), the parameters Ka and Kb are functions that monotonically increase as the value of the smoothed image signal SSM increases, and moreover, Ka>Kb is always satisfied. Further, as shown in FIG. 4(b), the parameter Kc is also a monotonically increasing function.

Parameters Ka, Kb, and Kc as functions with the smoothed image signal SSM as variables are set in advance on the image processing display device M3.
0 (parameter storage means according to the present invention), and is referenced when performing subtraction processing according to equation (2) above.

Also, similar to the above equation (2), equation 52=Ka' for each pixel
・5O2−Kb' ・SOl +Kc'...(
3) Subtraction processing is performed according to the following, and a bone image 44 (see FIG. 3) is obtained in which the shadows of the soft parts of the subject 4 are erased and only the shadows of the bones are extracted. Here, Ka' and Kb' are the above Ka.

Similarly to Kb, Kc' is a parameter that determines the weighting of the two image signals SO2 and SOl, and Kc' is a parameter that determines the bias amount similarly to Kc above, and Ka', Kb',
Kc' is a function of the smoothed image signal SSM like Ka, Kb, and Kc above, but the form of this function is similar to that of Ka, Kb, and Kc shown in FIG. 4, and here, Ka
Examples of the functions of ', Kb', and KC' will be omitted.

Once the bone image is obtained in this way, this image is inverted in black and white to obtain an inverted bone image 45.

In this inverted bone image 45, when the superimposed images are output as a visible image as described later, the shadows of the bones are expressed in an extremely high density so that the shadows of the bones are not noticeable on the visible image. This is an image.

Next, this inverted bone image 45 and the soft tissue image 43 obtained according to equation (2) are superimposed to generate an image signal representing the superimposed image 4B, and a visible image based on this image signal is displayed on the CRT display 32. (See Figure 2). The image displayed on the CRT display 32 is inconspicuous because the shadows of the bones are expressed with high density (low brightness), so the observer can see the soft tissue inside without being bothered by the shadows of the bones. For example, it is possible to observe the shadow of a tumor that appears on the screen.

In the above embodiment, the superimposed image 46 is displayed on the CRT display 32, but a hard copy of the superimposed image 46 may be obtained instead of or in addition to this.

Further, in the above embodiment, in the subtraction processing based on equations (2) and (3), the parameters of the subtraction processing KB, Kb, Kc; Ka'Kb', K
c' was changed according to the value of the smoothed image signal SSM, but since the present invention does not erase the shadow of the bone region, but rather actively (inconspicuously) displays it, the hardware Therefore, the influence of the above parameters Ka and Kb is small.

Kc; Ka', Kb', and Kc' may be fixed regardless of the value of the smoothed image signal S.

Furthermore, in the above embodiment, the soft part image 43 is
is obtained, a bone image 44 is obtained according to equation (3), the bone image 44 is inverted to obtain an inverted bone image 45, and this inverted bone image 45 and soft tissue image 43 are superimposed to form a superimposed image 46. However, instead of following this step, we can use a method that is mathematically equivalent to the second step: Ka /Kb >Ka ' /Kb '> Ka '
/Kb Using Ka' and Kb', S3-Ka'・SO2-Kb' -5O1+Kc
'...(4) By performing the calculation, it is possible to suddenly obtain the image signal s3 representing the superimposed image 46.Therefore, instead of following the above procedure, the calculation based on this formula (5) can be performed. You may also do so.

Furthermore, the present invention is not limited to X-ray images of the chest of a human body, but can also be applied to, for example, X-ray images of human limbs. This method can be widely applied when obtaining line images.

Further, although the above embodiment uses a stimulable phosphor sheet, the present invention is not limited to those using a stimulable phosphor sheet, and the present invention is not limited to those using a stimulable phosphor sheet. It can also be applied to those using (combined) etc.

(Effects of the Invention) As described above in detail, the energy subtraction method and apparatus for radiographic images of the present invention provide a processed image signal representing a radiographic image in which shadows of bone parts are expressed with higher density than shadows of soft parts. Since the reproduced image is output based on the processed image signal, the shadows of the bones do not interfere with the observation and are prevented from being mistaken for shadows of the soft parts. Ru. Furthermore, since the shadows of the bone parts are also displayed, the positional relationships of soft tissues can be accurately grasped.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an X-ray imaging device which is an embodiment of the recording unit of the energy subtraction silane device of the present invention, and FIG. 2 is a reading unit, arithmetic unit, and output unit of the energy subtraction method of the present invention. FIG. 3 is a perspective view of an X-ray image reading device and an image processing display device, which are one embodiment, and FIG. 3 is a diagram showing the flow of processing performed within the image processing display device. FIG. FIG. 2 is a diagram showing an example of a parameter function. 1... X-ray imaging device 2... X-ray tubes 3, 3a
, 3b, 3cm X-ray 4... Subject 5... First stimulable phosphor sheet 6... Filter 7... Second stimulable phosphor sheet 8... Mark 1B... Laser light source 19... Rotating polygon mirror 22... Stimulated luminescent light 23... Light guide 2
4... Photo multiplier 30... Image processing display device 41. 42... Original image 43... Soft part image 44... Bone part image 45... Inverted bone part image 46... Superposition Image Figure ＼ Figure KO, Kb

Claims (3)

[Claims] (1) Record a plurality of radiation images of the same subject consisting of bone parts and soft parts using radiation having different energy distributions on a recording sheet, read the plurality of radiation images from the recording sheet, and identify the By obtaining a plurality of image signals representing each of a plurality of radiographic images and performing subtraction processing based on the plurality of image signals, a radiographic image in which shadows of the bone parts are expressed with higher density than shadows of the soft parts is obtained. 1. An energy subtraction method for radiographic images, characterized in that a processed image signal representing the processed image signal is obtained, and a reproduced image based on the processed image signal is output. (2) The energy subtraction method for a radiographic image according to claim 1, characterized in that the subtraction process is performed by changing the parameters of the subtraction process for each part of the radiographic image. (3) A radiation source that emits radiation, a subject placement section where a subject consisting of bones and soft parts is placed, and a device for simultaneously or sequentially recording multiple radiographic images using radiation having different energy distributions. a recording unit comprising a sheet holding section provided at a position facing the radiation source across the subject placement section, in which a recording sheet is arranged; a reading unit that obtains a plurality of image signals representing each of the plurality of radiographic images of the subject from a recording sheet; performing subtraction processing based on the plurality of image signals obtained by the reading unit; The apparatus is characterized by comprising an arithmetic unit that obtains a processed image signal representing a radiation image in which shadows are expressed with higher density than the shadows of the soft part, and an output unit that outputs a reproduced image based on the processed image signal. Energy subtraction device for radiographic images.