JPH10105701A - Method and device for radio graph emphasis processing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To subject a radiograph including much noise to a noise sppression- preferrence emphasis processing and to subject a radiograph including less noise to a proper emphasis-preferred emphasis processing with respect to the emphasis processing of radiograph. SOLUTION: Dose information indicating the radiation dose irradiated at the time of photographing a radiograph is acquired by a dose information acquisition means 5, and a conversion function is defined in a conversion function definition mean 4 based on this dose information so that the degree of suppression of a signal to components smaller than a prescribed threshold is larger for the smaller radiation dose at the time of photographing. The defined conversion function is used to performs the conversion processing which suppresses each frequency band component of the picture signal in a nonlinear processing means 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for enhancing a radiographic image signal, and more particularly to suppressing granular noise contained in an enhanced image.

[0002]

2. Description of the Related Art Heretofore, a number of image processing methods and apparatuses have been proposed by the present applicant to perform frequency emphasis processing using an unsharp mask image signal (hereinafter referred to as a blurred image signal) to improve diagnostic performance of a radiation image. It has been proposed (Japanese
-163472 and 55-87953). Here, the blurred image signal is an image signal representing an image having the same number of pixels as the original image signal, but having a lower sharpness than the original image signal, and removing a high-frequency component of a predetermined frequency or more of the original image signal. This is a signal having the frequency response characteristic obtained.

[0003] The above-mentioned frequency emphasis processing is performed by using the original image signal Sorg.
Then, a value obtained by subtracting the blurred image signal Sus from the result and multiplying the result by an enhancement coefficient β is added to the original image signal Sorg to emphasize a predetermined spatial frequency component of the original image signal. This is expressed by the following equation (1).

Sproc = Sorg + β × (Sorg−Sus) (1) (Sproc: an emphasized signal, Sorg: an original image signal,
Sus: blurred image signal, β: emphasis coefficient) At this time, in the above processing, artifacts may be generated by adding signals, but this is due to the frequency response characteristic of the added signal added to the original image signal Sorg. The following method has been proposed as a specific adjustment method for this purpose (Japanese Patent Application 8-
No. 182155).

In this method, first, a plurality of blurred image signals having different sharpness, that is, different frequency response characteristics are created, and a difference between the blurred image signal and two signals in the original image signal is obtained. A plurality of band-limited image signals (hereinafter, referred to as band-pass signals) representing frequency components in a limited frequency band of the original image signal are created, and the band-pass signals each have a desired size by a predetermined function. After the suppression, the added signal is created by integrating the plurality of suppressed bandpass signals. If this processing is expressed as an equation, for example, the following equation (2) Sproc = Sorg + β (Sorg) × Fusm (Sorg, Sus1, Sus2,... SusN) Fusm (Sorg, Sus1, Sus2,... SusN) = f ₁ (Sorg) −Sus1) + f ₂ (Sus1-Sus2) +... + F _k (Susk−1−Susk) +... + F _N (SusN−1−SusN) (2) (where, Sproc: an image signal Sorg in which high-frequency components are emphasized) : Original image signal Susk (k = 1 to N): Blurred image signal f _k (k = 1 to N): Function for converting each bandpass signal β (Sorg): Enhancement coefficient determined based on the original image signal) become that way.

In such processing, functions f _{1 to} f
_N is a function that suppresses the band-pass signal in accordance with the magnitude of the band-pass signal, and by changing the shape of this function, various effects such as prevention of the above-described artifacts can be obtained. Is the above-mentioned patent application
No. 2155. One such effect is
A function is disclosed that has the effect of suppressing the granular noise contained in the enhanced image, but more strongly suppresses components having small absolute values on the basis that the signal value of the noise signal is relatively small. Function is defined as follows.

[0007]

This noise suppression method is a particularly effective method for enhancing a noisy image signal, but it suppresses noise and also suppresses components having a small absolute value of the image signal. It will be.
For this reason, since the noise contained in the image is negligible, it is not an appropriate method when priority is given to the enhancement processing of the original image signal over the removal of the noise.

The present invention has been made in view of the above problems, and has been made in consideration of the above-described embodiments. An object of the present invention is to provide an emphasis processing method and apparatus.

[0009]

According to the radiographic image enhancement processing method of the present invention, a plurality of radiographs having different frequency response characteristics based on an original image signal representing a radiographic image obtained by irradiating a predetermined dose of radiation. Creating a blurred image signal, creating a plurality of bandpass signals representing signals for each frequency band of the original image signal based on the original image signal and the plurality of blurred image signals, and A plurality of converted image signals are created by performing conversion so as to suppress the absolute value of the bandpass signal based on the conversion function, and an integrated signal obtained by integrating the converted image signals is added to the original image signal. A radiographic image enhancement processing method for obtaining a processed image signal in which a predetermined frequency component of the original image signal is enhanced, wherein a component equal to or less than a predetermined threshold value of the plurality of bandpass signals is used. It is characterized in that the degree of the suppression to define the transformation function so the larger the dose is small.

Here, the "blurred image signal" is an image signal representing an image having the same number of pixels as the original image signal but having a lower sharpness than the original image signal. The blurred image signal is first thinned out by performing a predetermined filtering process on the pixels of the original image signal at predetermined intervals, and the same filtering process is repeated on the image signal obtained in this manner. A plurality of image signals with a reduced number of pixels are created, and each of them is created by performing an interpolation process by a predetermined interpolation method so that the number of pixels is the same as that of the original image. Here, for filtering and interpolation, various generally used methods can be applied.

The "plurality of band-pass signals representing signals of each frequency band of the original image signal" may be created by taking a difference between blurred image signals of adjacent frequency bands, for example. And the difference between each blurred image signal. Alternatively, it can be created by taking a difference with another combination of the original image signal and the blurred image signal. The “predetermined conversion function” may be one function or a plurality of functions different for each frequency band. The creation of the converted image signal, the creation of the integrated signal, and the addition of the integrated signal to the original image signal can be represented by, for example, the above equation (2). Also,
"Emphasis a predetermined frequency component" means that a high-frequency component is emphasized, for example, to emphasize an edge portion of an image.

Further, "a component of the band-pass signal which is equal to or less than a predetermined threshold value" means a component which may be regarded as noise in the above-mentioned emphasizing process since the noise signal has a relatively small signal value. . Therefore, the degree of noise suppression can be changed by changing the “degree of suppression for a component of the bandpass signal that is equal to or less than the predetermined threshold”. The “predetermined threshold value” is a value that can be considered as a noise if the component is equal to or less than this value, and a specific value may be determined as a design item.

The reason why the degree of the suppression is set to “increase as the dose is smaller” is that the relative amount of noise contained in the image is affected by the dose of radiation at the time of imaging, that is, the dose is smaller. Noisy (conspicuous)
By becoming an image. That is, the smaller the dose, the greater the degree of suppression, the more noise the image,
This is nothing less than enhancing the effect of noise suppression.

In the case where the predetermined conversion function is different for each of the frequency bands, the degree of suppression of the components below the predetermined threshold is converted to a band-limited image signal in a high frequency band. It is desirable to define the conversion function so that the function becomes larger.

A radiation image enhancement processing apparatus according to the present invention is an apparatus for performing enhancement processing according to the above method, and is based on an original image signal representing a radiation image obtained by irradiating a predetermined dose of radiation. Image signal generating means for generating a plurality of blurred image signals having different frequency response characteristics from each other, and a plurality of signals representing a signal for each frequency band of the original image signal based on the original image signal and the plurality of blurred image signals. A band-pass signal is created, each band-pass signal is converted based on a predetermined conversion function so as to suppress the absolute value of the band-pass signal, and a plurality of converted image signals are created. Non-linear processing means for obtaining a processed image signal in which a predetermined frequency component of the original image signal is emphasized by adding the integrated signal obtained by the integration to the original image signal. Further, based on the dose information, a dose information obtaining unit that obtains dose information representing the dose, and the degree of suppression of components of the plurality of bandpass signals that are equal to or less than a predetermined threshold increases as the dose decreases. And a conversion function defining means for defining the conversion function as described above.

[0016]

According to the image enhancement processing method and apparatus of the present invention, the noise signal is relatively small as a signal value, and the noise of the read image becomes more conspicuous as the radiation dose during imaging is smaller. Focusing on that, the degree of suppression of components below a predetermined threshold of the bandpass signal, which is a component for each frequency band of the image, that is, the degree of noise suppression, is such that the smaller the radiation dose, the larger the bandpass signal. In order to define the function to convert the data, the processing can be performed with priority given to the original enhancement when the dose is large (normal), and with priority given to the noise suppression when the dose is small. Processing can be performed.

Further, since the noise signal is a high-frequency signal, if a function is defined so that the above-mentioned degree of suppression, that is, the noise suppression effect becomes stronger in a higher frequency band, a low-frequency band component with less noise is originally required. Is given priority, and more appropriate enhancement processing can be performed for the entire image.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of an image processing method and apparatus according to the present invention will be described in detail with reference to the drawings. The image processing apparatus described below uses a blurred image signal for an image signal obtained by reading a radiation image of a human body recorded on a stimulable phosphor sheet so that the image becomes an image suitable for diagnosis. The processed image signal is mainly recorded on a film and used for diagnosis.

FIG. 1 is a diagram schematically showing the image processing apparatus. The image processing apparatus 1 includes a blurred image signal creating unit 2 for creating a blurred image signal, and a non-linear processing unit 3 for performing an enhancement process for enhancing a specific frequency. Further, the image processing apparatus 1 includes a conversion function defining unit 4 and a dose information acquiring unit 5. The conversion function defining means 4 is a means for defining a conversion function used for the conversion processing by the non-linear processing means 3, and defines a conversion function by determining, for example, a parameter such as a slope of the function. The dose information acquisition unit 5 is a unit that acquires dose information representing a radiation dose when an image signal to be subjected to enhancement processing is captured. The conversion function definition unit 4 converts the dose information acquired by the dose information acquisition unit 5 into the dose information. The parameters are determined based on the parameters.

Here, the process of creating a blurred image signal will be described in detail. FIG. 2 is a block diagram showing an outline of the blurred image signal creation processing. As shown in FIG. 2, the blurred image signal creating means 2 in FIG. 1 first performs filtering processing on the original image signal Sorg in the x direction and the y direction of the pixels of the original image by the filtering processing means 10. To generate an image signal B ₁ (hereinafter referred to as a low-resolution image signal) having a lower resolution than the original image signal, and then perform the same filtering process on the low-resolution image signal B ₁ to perform the low-resolution image signal B ₁ B ₁ further create a low resolution image signal B ₂ resolution than, those to superimpose successive similar filtering process later. Then, the interpolation processing unit 11, those for low-resolution image signal B _k obtained at each stage of this filtering process, respectively subjected to interpolation enlargement processing to obtain a plurality of different blurred image signal Sus1~SusN sharpness It is.

In the present embodiment, a filter substantially corresponding to a one-dimensional Gaussian distribution is used as a filter for the filtering process. That is, the filter coefficient of the filter is calculated by the following equation (3) for a Gaussian signal.

[0022]

(Equation 1)

Determined according to This is because the Gaussian signal has good localization in both the frequency space and the real space. For example, a 5 × 1 one-dimensional filter when σ = 1 in the above equation (3) is shown in FIG. It will be something like

As shown in FIG. 4, the filtering process is performed on the original image signal Sorg or every other pixel on the low resolution image signal. By performing such filtering processing for every other pixel in the x direction and the y direction, the number of pixels of the low-resolution image signal B ₁ becomes １ ／ of the original image, and the low-resolution image signal obtained by the filtering processing is reduced. By repeatedly performing this filtering process, n low-resolution image signals B _k (k =
1 to n) each have a pixel number of 1/2 ^{2k of the} original image signal.
Image signal.

Next, a description will be given interpolation enlargement process performed on the low-resolution image signal B _k obtained in this manner. As the method of the interpolation operation, various methods such as a method using a B-spline can be mentioned. In the present embodiment, since the low-pass filter based on the Gaussian signal is used in the filtering process, the Gaussian signal is also used for the interpolation operation. Shall be used. Specifically, the following equation (4)

[0026]

(Equation 2)

In the above, an approximation of σ = 2 ^k−1 is used.

When interpolating the image signal B ₁ , σ = 1 because k = 1. In this case, the filter for performing the interpolation processing is a 5 × 1 one-dimensional filter as shown in FIG. In this interpolation processing, first, the low-resolution image signal B ₁
, The low-resolution image signal B ₁ is enlarged to the same size as the original image by interpolating one pixel having a value of 0 every other pixel, and then the interpolated low-resolution image signal B
_This is performed by subjecting 1 to a filtering process using the one-dimensional filter shown in FIG.

Similarly, this interpolation enlargement processing is performed on all the low resolution image signals _Bk . When interpolating the low-resolution image signal B _k , 3 × 2 ^k −1 based on the above equation (4).
By creating a filter of length and interpolating 2 ^k -1 pixels having a value of 0 between each pixel of the image signal B _k ,
Expanding the original image and the same size, the length of 3 × 2 ^k -1 filtering the image signals B _k having pixels interpolated in this value is 0, the interpolation based expansion by performing a filtering process.

Next, the non-linear processing performed using the blurred image signal created as described above will be described.
The following process is a process for emphasizing a specific frequency component, and the original image signal and the blurred image signal are processed according to the above equation (2).

FIG. 6 is a diagram showing an example of an image enhancement processing device for performing such processing. In this image enhancement processing device, the original image signal Sorg
And a blurred image signal Susk, a band pass signal is created based on two adjacent signals. Each bandpass signal converter respectively by the conversion function f ₁ ~f _N at 22 is suppressed to a desired size, further above (2) in accordance with a band-pass signal calculator which is the plurality of suppression The processed image signal Sproc is generated at 23 and added to the original image signal to generate a processed image signal Sproc. Here, the conversion functions f _{1 to} f _N may be all the same function or different functions. FIG. 7 is a diagram illustrating an example of the shape of the conversion function.

Next, the relationship between the shape of the conversion function and the noise suppression effect in the enhancement processing will be described. Generally, a signal component smaller than a predetermined threshold value may be noise, and those skilled in the art can easily estimate the value of the threshold value. Therefore, in order to remove noise, it is only necessary to suppress components smaller than a predetermined threshold. Therefore, in the image enhancement processing apparatus shown in FIG. 6, in order to suppress such a signal in general, instead of the function shown in FIG.
Is used to suppress signal components that can be regarded as noise at the stage of the bandpass signal, and as a result, the processed image signal S obtained by integrating the bandpass signal
The proc does not contain noise.

The present invention improves this noise suppression function. For example, for a high quality (less noise) image, priority is given to emphasis rather than noise suppression, and a signal component having a relatively small value is obtained. On the other hand, it responds to the need to perform the emphasis processing which is the original purpose. As means for this purpose, a conversion function defining means 4 and a dose information obtaining means 5 shown in FIG. 1 are provided.

As a method of acquiring dose information by the dose information acquiring means 5, a radiation dose may be directly measured, and an operator may input the measurement result via an input device,
When the image enhancement processing device of the present invention is a part of a photographing device or a device connected to the photographing device, a measurement result in the photographing device may be transmitted as a signal. Also, since the dose information only needs to know the relative size of the dose,
It is not necessary to be the dose value itself, but may be another parameter correlated with the dose. For example, as a method of reading a radiation image, in order to realize a stable density and contrast of a read image, reading is performed by setting a reading condition most suitable for the read image according to the image or according to a pre-read image. How to do is known. In this method, the reading condition is defined by a parameter (S value) correlated with the radiation dose, but such a parameter can be used as dose information.

The dose information obtained by the dose information obtaining means 5 is transmitted to the conversion function defining means 4. The conversion function defining means 4 employs, for example, a method of approximating a part of a function used for actual conversion as a linear function and defining the function by a parameter representing a slope of the linear function. May be used to define a more complex function, and various other defining methods can be applied. In any case, the parameters defining the function are determined based on the dose information transmitted from the dose information obtaining means 5. For example, in a method of approximating a function as a linear function, the slope of a portion that converts a value smaller than a predetermined threshold value may be increased if the dose is large, and may be decreased if the dose is small.

Here, since the noise signal is generally a high frequency signal, it is considered that the noise signal is included more in the band pass signal in the high frequency band than in the band pass signal in the low frequency band. Therefore, when differentiating the conversion functions based on the dose information as described above, it is not always necessary to enhance the noise suppression effect for the conversion functions that convert all bandpass signals. That is, as shown in FIG. 9, the conversion function used for the conversion of the band-pass signal in the low frequency band is given priority to the emphasis processing and has almost no noise suppression function. The function to be converted may have a stronger noise suppression effect.

As described above, according to the present invention, the emphasis processing is performed with priority on noise suppression for an image containing a lot of noise, and the enhancement of the image is performed with priority on the original emphasis for an image with little noise. Processing can be performed.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing an image processing apparatus according to the present invention.

FIG. 2 is a block diagram illustrating an outline of a blurred image signal creation process.

FIG. 3 is a diagram illustrating an example of a filter used in a filtering process;

FIG. 4 is a diagram showing details of a low-resolution image signal creation process;

FIG. 5 is a diagram illustrating an example of a filter used for interpolation enlargement processing;

FIG. 6 is a diagram illustrating an example of an image enhancement processing device.

FIG. 7 is a diagram illustrating an example of a general conversion function.

FIG. 8 is a diagram illustrating an example of a conversion function having a noise suppression effect.

FIG. 9 is a diagram illustrating an example of a conversion function when a noise suppression effect is changed for each frequency band.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image processing apparatus 2 Unsharp image signal (unsharp mask image signal) creation means 3 Nonlinear processing means 4 Conversion function definition means 5 Dose information acquisition means 10 Filtering processing means 11 Interpolation processing means 21 Subtractor 22 Transformer 23 Arithmetic unit

Claims (4)

[Claims] A plurality of non-sharp mask image signals having different frequency response characteristics based on an original image signal representing a radiation image obtained by irradiating a predetermined dose of radiation; A plurality of band-limited image signals representing signals for each frequency band of the original image signal are created based on the plurality of unsharp mask image signals, and each of the band-limited image signals is subjected to the band limitation based on a predetermined conversion function. A plurality of converted image signals are created by performing conversion so as to suppress the absolute value of the image signal, and an integrated signal obtained by integrating each of the converted image signals is added to the original image signal, thereby obtaining the original image signal. In a radiographic image enhancement processing method of obtaining a processed image signal in which a predetermined frequency component is enhanced, the degree of suppression of a component that is equal to or less than a predetermined threshold value of the plurality of band-limited image signals is Radiographic image enhancement processing method characterized by defining the transformation functions so as to increase as the amount is small. 2. The function for converting a band-limited image signal in a high frequency band, wherein the predetermined conversion function is a function that differs for each of the frequency bands, and the degree of suppression of components of the functions below the predetermined threshold is higher. 2. The radiation image enhancement processing method according to claim 1, wherein the conversion function is defined so as to be large. 3. A non-sharp mask image signal generation method for generating a plurality of non-sharp mask image signals having different frequency response characteristics based on an original image signal representing a radiation image acquired by irradiating a predetermined dose of radiation. Means for generating a plurality of band-limited image signals representing signals for each frequency band of the original image signal based on the original image signal and the plurality of unsharp mask image signals, and A plurality of converted image signals are created by performing conversion so as to suppress the absolute value of the band-limited image signal based on the conversion function, and an integrated signal obtained by integrating the converted image signals is added to the original image signal. A non-linear processing means for obtaining a processed image signal in which a predetermined frequency component of the original image signal is enhanced by performing Dose information acquiring means to be acquired, and based on the dose information, the conversion function is defined such that the degree of suppression of components below a predetermined threshold of the plurality of band-limited image signals increases as the dose decreases. A radiation image enhancement processing device comprising: a conversion function defining unit. 4. The method according to claim 1, wherein the predetermined conversion function is a function different for each of the frequency bands, and the conversion function defining means determines that a degree of suppression of components of each of the functions equal to or less than the predetermined threshold is limited to a high frequency band. 4. The radiation image enhancement processing apparatus according to claim 3, wherein the means for defining the conversion function is such that the conversion function becomes larger as the function of converting the image signal increases.