JPH11205682A - Energy subtraction image producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform noise reduction that is equal to one before subtraction processing and to produce an image that has excellent observation appropriateness by performing prescribed processing of image data on which 1st and 2nd systems are recorded based on the original image data that is acquired from mutually different radiation of a energy distribution, and further applying third processing to the image data. SOLUTION: 1st and 2nd X-ray image signals which are stored in internal memory in an image processor have X-rayimages 41 and 42 respectively. The image 41 is an image which is produced by relatively low energy X-ray, the image 42 is an image which is produced by relatively high energy X-ray and both of the images mutually record soft parts and bone parts. An image 45 is acquired through noise reduction processing of an overlapped image 44 with addition processing of both signals and an image 43 of the bone parts which is acquired with subtraction processing. Further, a soft part image 46 that is reduced in noise is acquired through subtraction processing of the images 44 and 45.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for generating an energy subtraction image which reduces noise in an energy subtraction image of a radiation image and obtains an image having excellent observation performance.

[0002]

2. Description of the Related Art In various fields, a recorded radiation image is read to obtain image data, and after appropriate image processing is performed on the image data, the image is reproduced and recorded. For example, an X-ray image is recorded using an X-ray film having a low gamma value designed to be compatible with the subsequent image processing, and the X-ray image is read from the film on which the X-ray image is recorded, and is converted into an electric signal. After conversion, the electric signal (image data) is subjected to image processing, and then reproduced as a visible image in a copy photograph or the like, a reproduced image with good image quality performance such as contrast, sharpness, and graininess can be obtained. A system has been developed (see Japanese Patent Publication No. 61-5193).

Further, a stimulable phosphor sheet (hereinafter, simply referred to as a “sheet”) on which radiation image information is stored and recorded is irradiated with excitation light such as a laser beam, and the radiation image information stored and recorded on the sheet is applied to the stimulable phosphor sheet. Radiation recording / reproduction for detecting a stimulating luminescent light corresponding to the stimulating light to obtain an image signal, and outputting a radiation image of the subject as a visible image to a recording material such as a photographic photosensitive material or a CRT based on the image signal. A system has already been proposed (Japanese Patent Laid-Open No. 55-12429, 56-11395).
No. 55-0163472, No. 56-164645, No. 55-116340).

This system has a practical advantage that an image can be recorded over an extremely wide radiation exposure area as compared with a conventional radiographic system using silver halide photography. That is, it has been recognized that the amount of stimulating light emitted by excitation after accumulation with respect to the amount of radiation exposure is proportional over an extremely wide range. Therefore, the amount of radiation exposure varies considerably depending on various imaging conditions. Even
The stimulated emission light emitted from the stimulable phosphor sheet is read, the gain is set to an appropriate value, read by photoelectric conversion means, and converted into an electric signal (image data). By outputting a radiation image as a visible image to a display device such as a CRT, it is possible to obtain a radiation image that is not affected by fluctuations in radiation exposure.

In a system using an X-ray film, a stimulable phosphor sheet, or the like as described above, a plurality of recorded radiation images are read to obtain a plurality of image data, and based on the image data, the radiation image is obtained. Image subtraction processing may be performed.

Here, the subtraction processing of the radiation images refers to processing for obtaining an image corresponding to the difference between a plurality of radiation images taken under different conditions. By reading at a sampling interval, obtaining a plurality of digital image signals corresponding to each radiation image, and performing a subtraction process for each corresponding sampling point of the plurality of digital image signals,
It refers to a process of obtaining a radiation image in which only a specific subject portion in the radiation image is emphasized or extracted.

This subtraction processing basically has the following two types. That is, by subtracting a radiographic image in which a contrast agent is not injected from a radiographic image in which a specific portion of a subject (for example, a blood vessel when a human body is used as a subject) is enhanced by injection of a contrast agent, the subject is subtracted. Using the so-called time subtraction to extract a specific part (for example, blood vessel) and the fact that the specific part of the subject has different radiation absorptivity for radiation having different energies from each other, By irradiating radiations having different energies, a plurality of radiation images of each radiation having different energies are obtained, and a specific portion of the subject is extracted by appropriately weighting the plurality of radiation images and calculating a difference therebetween. There is so-called energy subtraction. The present applicant has also proposed energy subtraction using a stimulable phosphor sheet (see JP-A-59-83486 and JP-A-60-225541).

The image after the energy subtraction processing is an image obtained by performing a subtraction processing on a plurality of radiation images before the processing (hereinafter, the radiation image before the energy subtraction processing is referred to as an “original image”). There is a problem that the S / N ratio is lower than that of the original image and the image is difficult to see.

For example, a subject composed of a soft part and a bone part, such as the chest of a human body, is irradiated with radiations having different energies to obtain a plurality of radiation images. The plurality of radiation images are read, and the plurality of radiation images are read. Obtain a plurality of image data representing each of them, perform an energy subtraction process based on the plurality of image data, and perform soft-subject image data representing a soft-part image in which mainly a soft part of the subject is recorded, or a bone in which a principal-bone part of the subject is recorded. In some cases, bone image data representing a partial image is obtained, and the obtained soft image or bone image is set as an observation target. Since the soft part image and the bone part image are images in which the shadow of the bone part and the soft part have been eliminated, respectively, the shadow hidden by the bone part or the soft part or the shadow that has become difficult to see due to the influence of the bone part or the soft part. Can emerge,
In some cases, matching is well performed for a predetermined observation purpose. However, as described above, since the soft part image and the bone part image are images obtained by the subtraction processing, the noise component is emphasized as compared with the original image, and the observation suitability is rather deteriorated from this point.

In view of the above, first image data mainly recording a first tissue in a subject is obtained from a plurality of image data obtained by subtraction processing according to the present application,
By obtaining first smoothed image data in which a noise portion has been reduced or removed from this image data, and subtracting the first smoothed image from the original image data to obtain second image data, the original image data is obtained. Noise is reduced to the same degree as that of the original image, and the second image data is processed to further reduce the noise component to obtain new first image data. By repeating these processes, noise is reduced. There has been proposed an energy subtraction image generation method for generating a reduced image with excellent observation suitability (Japanese Patent Laid-Open No. 3-285475).

[0011]

According to the energy subtraction image generation method described in Japanese Patent Application Laid-Open No. 3-285475, an energy subtraction image with reduced noise can be obtained. However, when the noise component included in the original image data is small, the noise is extremely reduced by repeating the above processing,
The result is an unnatural image that looks like a picture, and rather unsightly.

The present invention has been made in view of the above circumstances, and provides a method of reducing a noise to substantially the same level as an original image before subtraction processing and generating a subtraction image having a natural appearance and excellent observation suitability. The purpose is to do so.

[0013]

According to the present invention, there is provided an energy subtraction image generating method, comprising: a plurality of radiations which are transmitted from a subject composed of a plurality of tissues having different radiation absorptances and which are obtained from radiations having mutually different energy distributions; After performing a first process of obtaining first image data representing a first image in which a first tissue in the subject is mainly recorded based on a plurality of original image data representing each of the images, First smoothed image data representing a first smoothed image in which noise components of the first image have been reduced by processing the first image data is obtained, and the first smoothed image data is obtained from the original image data. By performing a subtraction process on the smoothed image data, a second process for obtaining second image data representing a second image of the subject, in which mainly a second tissue is recorded, is performed. After the processing of (2), the second image data is processed to obtain second smoothed image data representing a second smoothed image in which a noise component of the second image is reduced, and the original image is obtained. Subtracting the second smoothed image data from the data to obtain new first image data representing a new first image in which the first tissue of the subject is mainly recorded. After the third processing, the new first image data obtained by the third processing is used as the first image data in the second processing, and the second processing is performed again. By doing so, a new second process for obtaining new second image data representing a new second image in which the second tissue of the subject is mainly recorded, and the new second image data In the third process. By performing the third processing again as image data, a new third processing for obtaining new first image data representing a new first image mainly recording the first tissue of the subject Is repeated a predetermined number of times, the number of repetitions of the new second processing and the new third processing is determined based on the radiation dose when the original image data is obtained. It is characterized by the following.

Further, it is also possible to finally obtain second image data representing a second image in which the second tissue of the subject is recorded by applying the energy subtraction image generation method. That is, the second energy subtraction image generation method of the present invention performs the processing in the energy subtraction image generation method,
The new first image data obtained by the third process or the new third process is used again as the first image data in the second process or the new second process. 2 or the new second processing to obtain new second image data representing a new second image mainly recording the second tissue of the subject. Is what you do.

The "first image" (including the "new first image") and the "second image" (including the "new second image") in each of the energy subtraction image generation methods. Are two images obtained by energy subtraction processing, in which the shadows of different tissues of the same subject are emphasized or extracted, and are not limited to specific ones. It refers to a bone image, an image in which a mammary gland is emphasized and an image in which a malignant tumor is emphasized when a breast of a human body is a subject.

The phrase "repeated a predetermined number of times" means that the number of repetitions includes not only one or a plurality of times but also zero.

Further, "based on the radiation dose" means not only the actual radiation dose when a subject is photographed, but also the reading sensitivity when reading an image recorded on a stimulable phosphor sheet or an X-ray film. This also includes the case where it is based on the center value of standardization (so-called S value) which is an index value to be represented.

[0018]

The applicant of the present invention has noticed that, when obtaining a radiation image, the noise is smaller as the radiation dose when the subject is photographed is larger, and the noise is larger as the radiation dose is smaller. It is what came to mind.

That is, in the energy subtraction image generation method of the present invention, the number of repetitions of the noise reduction processing in the energy subtraction image generation method described in Japanese Patent Laid-Open No. 3-285475 is determined by It is determined based on the radiation dose. For this reason, when the radiation dose is large, the number of repetitions can be small, and when the radiation dose is small, the number of repetitions can be increased. The first and second images can be obtained, and when there is much noise, the first and second images with reduced noise and excellent observation suitability can be obtained.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, an example in which the above-described stimulable phosphor sheet is used will be described.

FIG. 9 is a schematic diagram of an X-ray imaging apparatus.

An X-ray 3 emitted from an X-ray tube 2 of the X-ray imaging apparatus 1 irradiates a subject (the chest of a human body) 4. The X-rays 3a transmitted through the subject 4 are applied to the first stimulable phosphor sheet 5, and X-rays having relatively low energy among the energies of the X-rays 3a are accumulated in the first stimulable phosphor sheet 5, As a result, the 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 that cuts low-energy X-rays, and the high-energy X-rays 3c that have passed through the filter 6 are applied to the second stimulable phosphor sheet 7. Thus, the X-ray image of the subject 4 is also stored and recorded on the sheet 7. The subject 4 is provided with two marks 8 that serve as references for aligning two X-ray images when performing the subtraction processing. It should be noted that the X-ray imaging apparatus 1 uses two
The X-ray images are stored and recorded on the sheets 5 and 7, and each of the X-ray images is recorded at two timings that are temporally successive to each other.
The shooting may be performed one by one.

FIG. 10 is a perspective view of an X-ray image reading apparatus and an image processing and display apparatus for implementing the energy subtraction image generating method of the present invention.

After the radiographing is performed by the X-ray radiographing apparatus 1 shown in FIG. 9, the first and second stimulable phosphor sheets 5 and 7
The sheets are set at predetermined positions of the X-ray image reading apparatus 10 one by one.
Here, the case of reading the first X-ray image stored and recorded on the first stimulable phosphor sheet 5 will be described.

The stimulable phosphor sheet 5 which is set at a predetermined position and on which the first X-ray image is stored is recorded by a sheet conveying means 15 such as an endless belt driven by a driving means (not shown) in the direction of arrow Y. (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 rotated at a high speed in the direction of the arrow Z, passes through a focusing lens 20 such as an fθ lens, and then passes through an optical path by a mirror 21. In the direction of sub-scan (arrow Y direction)
The main scanning is performed in the direction of the arrow X substantially perpendicular to. From the portion of the stimulable phosphor sheet 5 irradiated with the light beam 17, stimulable luminescent light 22 is emitted in an amount corresponding to the stored and recorded X-ray image information. It is guided by a guide 23 and is photoelectrically detected by a photomultiplier (photomultiplier tube) 24. The light guide 23 is formed by molding a light-guiding material such as an acrylic plate. The light guide 23 is arranged such that a linear incident end face 23a extends along the main scanning line on the stimulable phosphor sheet 5. The injection end face 23 formed in an annular shape
The light receiving surface of the photomultiplier 24 is connected to b. The stimulated emission light 22 that has entered the light guide 23 from the incident end face 23a travels through the inside of the light guide 23 by repeating total reflection,
The photostimulable light 22 emitted from the emission end face 23b and received by the photomultiplier 24 and representing a radiation image is converted into an electric signal by the photomultiplier 24.

The analog signal S output from the photomultiplier 24 is logarithmically amplified by a log amplifier 25 and then input to an A / D converter 26 where it is sampled.
A digital image signal S0 is obtained. This image signal S
0 is the first stored and recorded first phosphor sheet 5.
, Which is referred to as a _first image signal S01 here. Image signal S0 ₁ of the first is temporarily stored in the internal memory of the image processing and displaying apparatus 30.

The image processing and display device 30 includes a keyboard (not shown) for inputting various instructions, and a CRT display 32 for displaying auxiliary information for instructions and a visible image based on image signals.

Next, in the same manner as described above, the second X-ray image representing the second X-ray image stored and recorded on the second stimulable phosphor sheet 7 is obtained.
Image signal S0 ₂ is obtained, this second image signal S0 ₂
Is also temporarily stored in an internal memory in the image processing display device 30.

[0029] At the time of the first image signal S0 ₁ and the second image signal S0 ₂ of the read image signal obtained S
0 _1, based on the S0 _2, S value is determined is the sensitivity of the photomultiplier 24. This S value corresponds to the image signal S0
Seeking _1, S0 ₂ of average pixel value, and requests by normalizing the pixel values.

[0030] Figure 1, first and second based on the two image signals S0 _1, S0 ₂ representing an X-ray image, lines in the image processing and displaying apparatus stored in the internal memory of the image processing display device FIG. 6 is a diagram illustrating an example of a basic processing flow to be performed.

The first and second X-ray image signals S 0 ₁ , S 0 stored in the internal memory of the image processing display device
₂ are the first X-ray image 41 and the second X-ray image 41 shown in FIG.
This is a signal carrying the line image 42. The first X-ray image 41 is an image based on relatively low energy X-rays, and the second X-ray image 42 is an image based on relatively high energy X-rays. Both are original images in which both the soft part and the bone part are recorded.

The first and second X-ray image signals S0
_1, S0 ₂ is read from the internal memory of the image processing and displaying apparatus 30 shown in FIG. 10, first, these two image signals S
0 _1, S0 ₂ the relative alignment of the respective X-ray images 41 and 42 carrying each is performed on the image signal (JP 58-1
No. 63338). This alignment is performed as shown in FIG.
This is performed by relatively linearly moving and rotating the two X-ray images so that the two marks 8 overlap. Thereafter, a subtraction process is performed.

Here, the absorption coefficient μ of X-rays is determined as follows for the soft part and the bone part of the subject, and for the low-energy X-rays and the high-energy X-rays.

Μ _L ^T : Absorption coefficient of soft part by low energy X-ray μ _H ^T : Absorption coefficient of soft part by high energy X-ray μ _L ^B : Absorption coefficient of bone by low energy X-ray μ _H ^B : High energy X when this absorption coefficient of the bone along a line, for each pixel corresponding to each other of the two image signals S0 _1, S0 _2, wherein _{S1 = S0 1 - (μ L} T / μ H T) · S0 2 + C ... (1) However, C represents a bias component, and weighted subtraction is performed to obtain a bone image signal S1 representing the bone image 43 (see FIG. 1) from which the shadow of the bone is extracted.

Further, equation ^{_{S2 = (μ L B / μ}} H B) S0 2 -S0 1 + C '... (2) where C' is the soft tissue image signal representing a soft tissue image by performing weighted subtraction accordance represents the bias component Although S2 can be obtained, this calculation is unnecessary in the present embodiment.

Further, an addition process is performed for each pixel corresponding to each other in accordance with the equation S0 = (S0 ₁ + S0 ₂ ) / 2 (3) to generate a superimposed image 44 of the two X-ray images 41 and 42. You. This superimposed image 44 is also an original image in which both the soft part and the bone part are recorded. It is possible to use an X-ray image 41 or an X-ray image 42 instead of the superimposed image 44. However, since the superimposed image 44 is obtained by superimposing two X-ray images 41 and 42, The noise component is reduced as compared with any of the line images, which is advantageous for the subsequent processing.

Next, the noise component included in the bone image 43 is extracted by processing the bone image signal S1.

FIG. 2 is a diagram showing a spectrum with respect to a spatial frequency f of an image obtained by processing a bone image and a bone image signal.

The graph 51 shown in the figure represents the spectrum of the bone image 43, and includes a noise component 53.

First, the bone image signal S1 is subjected to a smoothing process. As this smoothing processing method, for example, for each pixel, an average of image signals corresponding to each pixel in a predetermined area centered on the pixel is obtained, and this average value is used as a simple averaging as an image signal of the central pixel. A processing method, a method using a median filter that sets a median value (median) of image signals in the predetermined area as an image signal of a center pixel, and further divides the predetermined area into a plurality of small areas. A method using an edge-preserving filter (V-filter) that obtains the variance and uses the average value of the small area having the smallest variance as the value of the image signal of the central pixel, Fourier transform of the image signal, and a high spatial Although a method of performing inverse Fourier transform after removing the frequency component can be used, the above-mentioned blur mask processing method has a disadvantage that an edge is blurred, and the above-mentioned median filter is used. Since the method involves exchanging pixels, contour-like artifacts may occur.Furthermore, when the above-mentioned edge-preserving filter is used, honeycomb-like artifacts may occur. There is. Therefore, in the present embodiment, smoothing is performed using the following histogram adaptive filter which is different from any of the above methods. Using this method, it is possible to remove noise without preserving the edges required as image information (stepwise density changes that define the boundaries of the shadows of two different tissues) and without the above-mentioned artifacts.
Another advantage is that noise can be removed in a short time with a simple operation.

First, a histogram of the image signal S1 of a large number of pixels in a predetermined area centering on each pixel of the bone image is created.

FIGS. 3A and 3B show the frequency of appearance of the image signal S1 corresponding to a number of pixels in a predetermined area centered on a certain pixel (image signal S1 ') obtained as described above. FIG. 4 is a diagram showing two different histograms, in which an image signal S1 and an image signal S of a central pixel are plotted.
It is a figure showing an example of the function which made the difference with 1 'a variable.

A function representing a histogram as shown in FIGS. 3A and 3B is generally represented by h (S1), and the absolute value | S1-
As S1 '| increases, it monotonically decreases. For example, FIG.
Let f (S1−S1 ′) be a function as shown in FIG. At this time, a function g (S1) representing the frequency after processing is obtained in accordance with the equation g (S1) = h (S1) × f (S1-S1 ′) (4).
When the function h (S1) has a plurality of peaks as shown in FIG. 3A, the function g (S1) has an effect of extracting only the peak to which the image signal S1 'of the central pixel belongs.

After obtaining the function g (S1) according to the above equation (4), the average value S1h of the image signal S1 weighted by the function g (S1) is obtained. That is, specifically, for example, the first moment of the function g (S1) is obtained according to the following equation.

[0045]

(Equation 1)

The processing according to the above equations (4) and (5) is performed with each pixel of the bone image as the center pixel, whereby the smoothed image signal S1h (for simplicity, the image signal corresponding to each pixel is (The same symbol is used in the image signal representing the entire image). The smoothed image signal S1h is a signal obtained by removing a high spatial frequency component of the original bone image signal S1, as shown in a graph 52 of FIG.
As shown in FIG. 3 (a), the signal near the edge is a signal obtained by calculating an average value after extracting only the mountain to which the pixel belongs, so that the edge in the original bone image is not blurred. Has been saved.

Next, the smoothed image signal S1h is weighted and subtracted from the superimposed image signal S0 (see the above equation (3)) representing the superimposed image 44 for each pixel, that is,

[0048]

(Equation 2)

Here, C ″ represents a bias component.

By performing the above, the image information
A soft part image 46 (see FIG. 1) that carries substantially the same information as the soft part image represented by the equation (2) and has a noise component reduced more than the soft part image represented by the above-described Maki equation is obtained.

In the above embodiment, the bone image signal S1 is smoothed and subtracted from the original image to obtain the soft image signal S1.
This is an example in which 2 ′ is obtained. When a bone image is to be observed, a soft image signal S2 is obtained based on the above equation (2).
What is necessary is just to obtain a bone image in which the noise component is reduced by smoothing this soft part image signal S2 and subtracting it from the original image.

Next, processing that is substantially the same as the basic processing described with reference to FIG. 1 will be described.

FIG. 5 illustrates this substantially identical process.
For the sake of convenience, the
Two image signals S0 representing the first and second X-ray images₁,
S0 _TwoBased on the processing performed in the image processing display device
It is a figure showing another example of the flow of. The same elements as in Fig. 1
Are given the numbers and symbols given in FIG. 1 and explained using FIG.
In order to avoid duplicate explanations,
Description is omitted.

From the two X-ray images 41 and 42, the above equation (1),
Based on equation (2), a bone image 43 (bone image signal S1) and a soft image 47 (soft image signal S2) are obtained.

Next, by processing the bone image signal S1 based on the above equations (4) and (5) in the same manner as in FIG. 1, the noise component contained in the bone image 43 is reduced and smoothed. Then, the noise image 48 (noise signal S _N ) from which only the noise component is extracted is obtained by subtracting the smoothed image signal S1h from the bone image signal S1 for each pixel.

S _N = S 1 −S 1h (7) The noise signal S _N is a signal obtained by extracting the noise component of the bone image as shown in the graph 53 of FIG. Here, since the information on the edge of the bone image is stored in the smoothed image signal S1h even if the spatial frequency is as high as the noise component, the smoothed image signal S1h is smoothed with the bone image signal S1 according to the above equation (7). By calculating the difference from the image signal S1h, the edge information is clearly canceled. Therefore, the noise signal _SN is more purely the noise of the bone image as compared with the case where the smoothing process for losing the edge information is performed. A signal carrying only the component is obtained.

Next, the noise signal S _N obtained in this way and the soft part image signal S2 representing the soft part image 47 (see FIG. 5)
Is weighted and added for each pixel, whereby a soft part image 46 that carries substantially the same information as the soft part image 47 and has a noise component reduced more than the soft part image 47 is obtained. In the present embodiment, the weighted addition is represented by an equation

[0058]

(Equation 3)

According to this, the noise component is further reduced.

Here, it will be described that the example described with reference to FIG. 1 and the example described with reference to FIG. 5 are substantially the same.

In the above equation (8), the soft image signal S2 shown in the above equation (2) and the noise signal S _N shown in the above equation (7)
Is assigned. Note that the bias component (such as C ′ in the above equation (2)) is the density (CRT) of the entire image finally obtained.
(Includes luminance when displaying on a display device etc.)
Therefore, the description is omitted here.

By substituting equations (2) and (6) into equation (8),

[0063]

(Equation 4)

By substituting the bone image signal S1 expressed by the above equation (1) into the equation (9) (ignoring the bias C),

[0065]

(Equation 5)

By rearranging this equation (10),

[0067]

(Equation 6)

Then, if the above equation (3) is substituted,

[0069]

(Equation 7)

Is obtained. This equation (12) is the same as the above equation (6) except for the bias. That is, in the example described with reference to 1 and the example described with reference to FIG. 5, substantially the same processing is performed.

FIG. 6 is a diagram showing the flow of the process of the present embodiment based on the process shown in FIG. 1, and FIG. 7 schematically shows the profile of each image shown in FIG. FIG.

In FIG. 6, elements corresponding to those in FIG. 1 or FIG. 5 are denoted by the same reference numerals and symbols as in FIG. 1 and FIG.

FIGS. 7A and 7B are diagrams schematically showing X-ray images (original images) 41 and 42, respectively.
The image signals S0 ₁ ,
S0 ₂ value is obtained by plotting the change in these image signals S0 _1, S0 to each other although the values are different from the ₂ uniform soft signal component and a step-like representing the (parts of hatched in the figure) And a random noise component are superimposed.

By performing - (represented by the symbol) [0074] Two X-ray image (original image) 41, 42 these two image signals S0 ₁ representing a, S0 ₂ in basis (2) weighted subtraction process on the basis of the equation soft tissue image signal S2 representing the soft tissue image 47 is required, also two image signals S0 _1, S0 ₂ in basis (3)
By performing an addition process (represented by +) based on the equation, a superimposed image signal S0 representing the superimposed image 44 is obtained.

FIG. 7 (c) is a diagram schematically showing the superimposed image signal S0. As in FIGS. 7 (a) and 7 (b), a uniform signal component representing a soft part (the hatched portion in FIG. 7), a signal component representing a bone part changed stepwise, and a random noise component are superimposed. This noise component is represented by two X-rays shown in FIGS. 7 (a) and 7 (b). The image is reduced as compared with the images 41 and 42.

FIG. 7D is a diagram showing the soft part image signal S2 obtained based on the above equation (2). Only signal components representing uniform soft parts are extracted, but random noise components are extracted from the two X-ray images 41 and 42 (FIGS. 7A and 7B).
)).

In the present embodiment, there is no need to determine, but
FIG. 7 (e) shows that the bone image signal S1 is temporarily based on the above equation (1).
FIG. 6 is a diagram showing a bone part image signal S1 in a case where is obtained. Although a signal component representing a bone part changed in a step shape is extracted, like the soft part image signal S2 (FIG. 7 (d)),
The random noise component is larger than any of the two X-ray images 41 and 42 (FIGS. 7A and 7B).

Here, the soft part image 47 (soft part image signal S2, FIG. 7 (d)) is subjected to a smoothing process 51 (see FIG. 6), and the smoothed soft part image signal S2h (FIG. 7) representing the smoothed soft part image 61 is obtained. (f)
) Is required. In this smoothing process 51, the soft part image 47
For example, high spatial frequency components of 1.0 cycles / mm or more are cut.

Next, the smoothed soft part image signal S2h is weighted and subtracted from the superimposed image signal S0, whereby the bone part image signal S1 'representing the bone part image 62 is obtained. As shown in FIG. 7 (g), the bone image signal S1 'has a reduced random noise component compared to the bone image signal S1 (FIG. 7 (e)). The effect of the conversion processing appears, and the high spatial frequency component of the soft part image is slightly mixed.

Next, the bone image obtained as described above
The signal S1 'is subjected to a smoothing process 52. Applied here
In the smoothing process 52, for example, 0.5 cycles /
low-contrast shadows (bone
(The change in the partial image signal S1 'is small)
It is. As this processing method, for example, a predetermined pixel P₀To
Window with an area corresponding to 0.5 cycles / mm
Think, each corresponding to each pixel in this window
In the signal S1 ', a predetermined pixel P₀S1 corresponding to
₀The average value of the signal S1 'within the value of'
The average value is calculated for a given pixel P₀New signal S1 ₀'
A method of scanning the bone image 62 using a filter is adopted.
It is. With this smoothing process 52, a smoothed bone image 63 is displayed.
A smooth bone image signal S1h 'is obtained. This smooth
As shown in FIG. 7 (i), the ossification part image signal S1h '
Noise component and high frequency components of the mixed soft part image are reduced.
However, the rising part has also become dull.

Next, the smoothed bone image signal S1h 'is weighted and subtracted from the superimposed image signal S0, and the soft image 64
Is obtained. This soft part image
As shown in FIG. 7 (h), the noise component 64 is lower in noise component than the soft part image 47 (FIG. 7 (d)), but the rising of the smoothed bone image signal S1h '(FIG. 7 (i)). Since the portion is dull, the information of the bone image of the portion is superimposed as noise. However, the information of the random noise portion and the bone image as noise is considerably small. Therefore, a series of processing is stopped at this stage, and the soft image signal S2 'is displayed on the CRT display 32 (see FIG. 10) of the image processing display device 30. send,
A visible image based on the soft part image signal S2 'may be reproduced and displayed on the CRT display 32 for observation.

However, in this embodiment, the basic processing shown in FIG. 1 or FIG. 5 is further repeated to further improve the image quality.

After obtaining the soft part image signal S2 'representing the soft part image 64, the soft part image signal S2' is subjected to a smoothing process 53,
A smoothed soft part image signal S2h 'representing the smoothed soft part image 65
(FIG. 7 (j)) is obtained. As the smoothing process 53, a process of cutting a spatial frequency component of, for example, 1.5 cycles / mm or more is performed.

The smoothed soft part image signal S2h 'is subjected to a weighted subtraction process from the superimposed image signal S0 to obtain a bone part image signal.
A bone image signal S1 ″ representing the bone image 66 is obtained. This bone image 66 is, as shown in FIG. 7 (k), a bone image 62 (FIG. 7 (g)).
), The information of the random noise and the information of the soft part image mixed as noise are also reduced. When a bone image is to be observed, a visible image based on the bone image signal S1 ″ may be reproduced and displayed on the CRT display 32.

In the present embodiment, a smoothing process 54 is further performed on the bone image signal S1 ″ obtained as described above, and the smoothed bone image signal S1h ″ representing the smoothed bone image 67.
(FIG. 7 (m)) is obtained. As the smoothing process 54, for example, a low contrast component of 1.0 cycle / mm or more is cut.

Next, the smoothed bone image signal S1h "is weighted and subtracted from the superimposed image signal S0 to obtain a soft image signal S2". As shown in FIG. 7 (l), this soft part image signal S2 "
Compared to (FIG. 7 (h)), both the random noise and the information of the bone image as noise are further reduced signals.

By repeating the smoothing process and the weighted subtraction of the superimposed image (original image) in this way, it is possible to alternately obtain a bone image and a soft image in which noise is sequentially reduced.

The number of repetitions is set as follows.

That is, the S value determined based on the image signal as described above is determined with reference to the table shown in FIG.

In the table shown in FIG. 11, the larger the S value, the smaller the radiation dose and the larger the noise. In the above table, N is the number of repetitions for obtaining the soft part image, and M is the number of repetitions for obtaining the bone part image. As shown in the above table, the number of repetitions increases as the S value increases.

As described above, by determining the number of repetitions based on the S value, that is, the radiation dose, the number of repetitions increases as the S value increases, that is, the noise increases. Noise can be further reduced from the bone image. Also,
The smaller the S value, that is, the smaller the noise, the smaller the number of repetitions, so that the noise does not disappear at all from the obtained image. Obtainable.

In this embodiment, the S value is obtained from the image signal. However, the present invention is not limited to this. For example, the radiation amount at the time of photographing the subject may be directly input to the image processing apparatus. Alternatively, the imaging conditions at the time of imaging the subject may be input to the image processing apparatus, and the radiation dose at the time of imaging may be estimated based on the imaging conditions.

FIG. 8 is a flowchart showing another processing flow substantially the same as that of the embodiment described with reference to FIG. 6 and the like are denoted by the same reference numerals and symbols as in FIG. 6 and the like, and description thereof is omitted.

The processing shown in FIG. 8 corresponds to the bone image 62 shown in FIG.
(Process described with reference to FIG. 1 (however, the bone image and the soft image are replaced in FIG. 1))
Is replaced with the processing described with reference to FIG. 5 (however, the bone image and the soft part image are replaced in FIG. 5), and as described above, these are substantially the same processing as each other. is there.

In the processing shown in FIG. 8, only the first stage is replaced with the processing method described with reference to FIG.
This replacement can be performed at any stage of the process that is repeatedly performed, and all are substantially the same process. The present invention includes all substantially the same process modes changed at any one or more of these stages. Included.

In each of the above embodiments, the X of the human chest
Although an example of obtaining a soft part image or a bone part image based on a line image is described, the present invention is not limited to obtaining a soft part image or a bone part image. For example, an image in which a mammary gland is emphasized or a malignant tumor is obtained. May be broadly applied to obtain one or both of two images in which two different tissues in a subject are respectively emphasized or extracted.

Further, the above embodiment is an example in which a stimulable phosphor sheet is used, but the present invention is not limited to the stimulable phosphor sheet, but may be applied to an X-ray film (in general, sensitized when photographing. (Combined with a screen).

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a flow of a basic process performed in an image processing display device.

FIG. 2 is a diagram showing a spatial frequency spectrum of a bone image and an image obtained by processing the bone image signal.

FIG. 3 is a diagram showing two different histograms in which the appearance frequencies of image signals corresponding to a large number of pixels in a predetermined region centering on a certain pixel are plotted;

FIG. 4 is a diagram illustrating an example of a function using a difference between an image signal S1 and an image signal S1 ′ of a pixel at the center of a predetermined area as a variable.

FIG. 5 is a diagram showing a flow of another process substantially the same as the process shown in FIG. 1 performed in the image processing display device.

FIG. 6 is a diagram showing a flow of another basic process of the present invention.

FIG. 7 is a diagram schematically showing a profile of each image shown in FIG. 6 in one predetermined direction.

FIG. 8 is a diagram showing a flow of another process substantially the same as the process shown in FIG. 6;

FIG. 9 is a schematic diagram of an X-ray imaging apparatus.

FIG. 10 is a perspective view of an X-ray image reading apparatus and an image processing and display apparatus implementing the energy subtraction image generation method of the present invention.

FIG. 11 is a table showing a set number of repetition processes;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 X-ray imaging apparatus 2 X-ray tube 3, 3a, 3b, 3c X-ray 4 Subject 5 First stimulable phosphor sheet 6 Filter 7 Second stimulable phosphor sheet 8 Mark 16 Laser light source 19 Rotating polygon mirror 22 Stimulated light 23 Light guide 24 Photomultiplier 25 Log amplifier 26 A / D converter 30 Image processing display 41, 42 X-ray image (original image) 43, 62, 66 Bone image 44 Superimposed image (original image) 45, 63, 67 Smoothed bone image 46, 47, 64 Soft image 48 Noise image 61, 65 Smoothed soft image 51, 52, 53, 54 Smoothing processing

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04N 7/18 G06F 15/70 455B

Claims (2)

[Claims] 1. Based on a plurality of original image data representing each of a plurality of radiation images obtained from radiations having mutually different energy distributions and transmitted through a subject constituted by a plurality of tissues having mutually different radiation absorptances, After performing a first process of obtaining first image data representing a first image in which a first tissue of the subject is mainly recorded, the first image data is processed to perform the first process. By obtaining first smoothed image data representing a first smoothed image in which the noise component of the image has been reduced, and subtracting the first smoothed image data from the original image data, The second where primarily the second organization was recorded
Performing a second process for obtaining second image data representing the image of the second image, and after the second process, processing the second image data to reduce a noise component of the second image. By obtaining second smoothed image data representing the second smoothed image and subtracting the second smoothed image data from the original image data, the first tissue of the subject is mainly recorded. Performing a third process for obtaining new first image data representing the new first image, and after the third process, the new first image data obtained by the third process By performing the second processing again as the first image data in the second processing, a new second image representing a new second image mainly recording the second tissue of the subject is recorded. New second to find image data of
And the third processing is performed again using the new second image data as the second image data in the third processing, whereby the first tissue mainly of the subject is recorded. In an energy subtraction image generation method in which a new third process for obtaining new first image data representing a new first image is repeated a predetermined number of times, the new second process and the new third process Wherein the number of repetitions of is determined based on the dose of the radiation when the original image data is obtained. 2. After performing the processing according to claim 1, the new first image data obtained by the third processing or the new third processing is replaced with the second processing or the new processing. By performing the second process or the new second process again as the first image data in the second process, a new second image mainly including the second tissue of the subject is recorded. An energy subtraction image generation method, wherein new second image data representing an image is obtained.