JPH10146325A - Image correcting processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate excessive correction on an edge part, and ideally correct shading by extracting a boundary of an original image, and performing processing to substitute a signal by isotropically extracting a pixel of a background in the vicinity of this boundary. SOLUTION: A high luminance area of a shading image 200 is extracted as an object, and a low luminance area 202 is extracted as a background, and the areas are divided. Next, after a boundary between an object 201 and a background 202 is extracted on the basis of a binarized image 300, a pixel in the vicinity of a boundary of an original image 200 is isotropically extracted. A boundary image obtained at this time is scanned in order, and in an address where a pixel value is 1, an address of a pixel existing in its peripheral specific range is extracted. Next, among pixels of the original image in this area, a pixel value of the background is substituted with a signal value of the object of the original image in the vicinity of the boundary. Next, a signal value is calculated from a pixel value of the object of the original image in the area, and an image by substituting a pixel value of the background over the whole boundary is finally obtained, and luminance of the original image 200 is corrected by using an obtained shading image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for correcting shading of a gray-scale image, and more particularly to a method for measuring a nuclear magnetic resonance (hereinafter, referred to as "NMR") signal from hydrogen, phosphorus, or the like in a subject and measuring the nuclear magnetic resonance signal. The present invention relates to an image correction processing method applied to a human body cross-sectional image or the like obtained by a nuclear magnetic resonance imaging (MRI) method for imaging a density distribution, a relaxation time distribution, and the like, and an MRI apparatus including such an image correction processing unit. .

[0002]

2. Description of the Related Art In MRI, protons, which are main constituents of a subject, are imaged, and the spatial distribution of the proton density and the spatial distribution of the relaxation phenomenon of the excited state are imaged. Abdomen, extremities, etc., or
The function is photographed two-dimensionally or three-dimensionally.

[0003] Although this MRI image is widely used in clinical practice, there is a case where shading due to the apparatus is strongly generated. In particular, when a local RF receiving coil is used, shading due to the sensitivity distribution of the receiving coil is remarkable,
In some cases, problems such as difficult diagnosis were caused. For example, the profile of the original image of the phantom shown in FIG.
It is originally concave where it should be flat, and reflects the sensitivity distribution of the receiving coil. As means for avoiding this, shading correction has been proposed. As an example, there has been proposed a method of extracting a low-frequency component of an image, treating the extracted low-frequency component as shading caused by a device, and correcting the original image (Axel et al., "Intensity correction in surface coil MR imaging," American Journal of Science,・ Rentgenology, 148, 418-420, 1987).

Although the above method is simple, the extraction of the low-frequency component of the original image causes a deviation from the actual shading near the boundary between the subject and the background. The periphery is overcorrected, and as shown in FIG. 9, there is a disadvantage that the luminance of the peripheral portion of the subject becomes high in the corrected image.

As a method for solving this problem, a method of obtaining a secondary image in which the area outside the boundary between the subject and the background is replaced with the highest pixel value, extracting a low-frequency component of the image, and thereby performing shading correction. (Wald et al., "Phased Array Detector and Automatic Intensity Correction Algorithm for High-Resolution MR Imaging of the Human Brain", Magnetic Resonance in Medicine, 34: 433-439, 1995).

[0006]

In this method, although the effect is shown by a cross-sectional image of the head, it cannot be applied to a subject having a complicated shape such as a complicated boundary between the subject and the background. Has stopped being applied to subjects (circles) with a simple shape such as the head. On the other hand, in MRI, since every part is photographed from every angle, the correction algorithm needs to be applied to an arbitrary image, but there is no algorithm that makes this possible.

Accordingly, an object of the present invention is to provide a shading correction algorithm applicable to an arbitrary image.

[0008]

An object of the present invention is to provide an image correction processing method for correcting shading of an original image which is a two-dimensional or three-dimensional gray image having a subject and a background. Means (101) for dividing the area into two, and means (10) for extracting the boundary between the subject and the background.
2) and means (103) for isotropically extracting picture elements near the boundary from the original image, and replacing the picture element values of the picture elements of the background with the signal values of the subject among the extracted picture elements. Means for forming a second image (104), means for applying a low-frequency pass filter to the second image (105), and use of the third image after the action of the low-frequency pass filter to obtain an original image. This is solved by an image correction processing method according to the present invention including a means (106) for correcting luminance.

By extracting the background near the boundary isotropically, it can be applied to images at all angles. Here, “isotropically extracting” means that if the image is a two-dimensional image, a picture element within a square area or a circular area having a specific size around a boundary point is extracted. In the case of a three-dimensional image, picture elements within a cubic region or a spherical region having a specific size around the boundary point are extracted.

The means for replacing the picture element value of the picture element of the background with the signal value of the subject calculates and obtains the signal value of the subject based on the picture element value of the picture element which is the subject. The pixel value of the background pixel is used. In this case, the signal value of the subject may be obtained based on the pixel value of the entire subject,
It can be obtained by calculating an average value, a median value, or the like based on the picture element value of the picture element which is the subject among the picture elements extracted isotropically.

An image obtained by applying a low-frequency pass filter to the second image obtained by replacing the background picture element in the vicinity of the boundary with the signal value of the subject as described above substantially matches the actual shading in the vicinity of the boundary. Moreover, since such processing is performed isotropically, no matter in which direction the original image has shading, by correcting the original image using this image, an ideal image in which overcorrection is prevented can be prevented. Corrections can be made.

Also, the MRI apparatus of the present invention comprises: a magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field;
A sequencer for controlling a magnetic generating means for irradiating a nuclear magnetic resonance to nuclear spins of a tissue constituting a subject, a means for detecting a nuclear magnetic resonance signal generated from the subject, and a nuclear magnetic resonance The image processing unit includes an image processing unit that performs signal processing on a signal to reconstruct an image, and a unit that displays an image. The image processing unit includes a shading correction unit that executes the image correction processing method.

Since the MRI apparatus of the present invention is provided with such a shading correction means, it is possible to correct the shading of the MR image caused by the sensitivity distribution of the detection means without overcorrection, which is effective for diagnosis. Can provide images.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an image correction processing method according to the present invention will be described below with reference to the flowchart shown in FIG. FIG.
(A) is a diagram schematically showing an MRI image (original image) 200, in which a subject 201 is a high-brightness image with high luminance and a background 202 is a low-luminance image.

In the image correction processing method according to the present invention, a high-luminance area of such a gray-scale image 200 is first extracted as a subject, and a low-luminance area is extracted as a background and divided into regions (means 101). This area division is performed, for example, by setting the maximum pixel value to 5
The image is binarized using 0% as a threshold, with zero as the background and 1 as the subject. The binarized image thus obtained is shown in FIG.
(B). In addition, the threshold value at the time of area division is not limited to 50% of the maximum value.
A known method such as a mode method and a percentage concentration separation method can be adopted.

Next, based on the binarized image 300, the subject 201 and the background
The boundary of 202 is extracted (means 102). As the means 102 for extracting the boundary 401, a differential filter can be applied to the binarized image 300, and a Laplace filter is preferably used. As a result, the pixel value of the boundary 401 as shown in FIG.
Thus, an image (boundary image) 400 having the other (background 402) picture element values of zero is obtained. In this case, a known thinning process may be performed as necessary.

Next, picture elements near the boundary of the original image 200 are isotropically extracted (means 103). To this end, FIG.
As shown in (a), the boundary image 400 obtained by the means 102 is sequentially scanned, and the address where the pixel value is 1 (ie, the address of the boundary).
Then, the addresses of picture elements in a specific range around the pixel are extracted. The specific range extracted here is, for example, 5 ×
It can be a 5 or 7 × 7 square area. FIG.
(A) shows a 5 × 5 extracted area 400a. Here, 24 addresses are extracted around the address [3.3] which is the boundary 301 (diagonal line).

FIG. 5B shows the extracted area (address).
In the following means (104), among the picture elements of the original image in this area, the background picture element value is replaced with the signal value of the subject of the original image near the boundary. This process is hereinafter referred to as edge fill. For the edge fill process, the picture elements in the extracted area are first divided into a subject and a background. In this case, if the pixel value of the binarized image 300 corresponding to each address is 1, it is a subject, and if it is 0, it is a background. FIG. 5 (b)
In the figure, the picture elements of the background are indicated by oblique lines. Next, a signal value is calculated from the picture element value of the subject of the original image in the area. The signal value of the subject is obtained by calculating, for example, a simple average value or a median value of picture element values, and this signal value is used as the value of the picture element value of the background of the original image. In the embodiment shown in FIG. 5B, the picture element values a25, a34, a35, a43, a44,
The signal value of the subject is calculated from a45, a52, a53, a54, and a55, and is replaced with the picture element value of the 15 picture elements that are the background.

The above-described edge fill processing is performed as shown in FIG.
(B) and (c) are performed over the entire boundary, and finally an image (second image) 500 in which the background pixel value is replaced over the entire boundary as shown in FIG. can get. Note that the edge fill processing may be performed on all picture elements that are boundaries, or may be performed discretely.

The second image 500 thus obtained is subjected to a low-pass filter (hereinafter abbreviated as LPF) (means 105). For this reason, the second image is subjected to two-dimensional Fourier transform to obtain a frequency space image, and an LPF such as a two-dimensional Butterworth filter, a two-dimensional Gaussian filter, and a two-dimensional Hanning filter acts on the frequency space image, and the two-dimensional inverse Fourier transform is performed again. In this way, the third image obtained by applying the LPF to the second image 500 performs edge fill on the outside of the original image, so that shading close to the original shading is obtained around the subject.

Finally, the luminance of the original image 200 is corrected using the third image (shading image) obtained by the means 105 (means 106). This correction is performed, for example, by dividing the original image 200 by the shading image. As a result, a corrected image in which shading has been corrected can be obtained.

In the above description, an example in which the correction processing method of the present invention is applied to a two-dimensional image has been described.
The present invention can be similarly applied to a three-dimensional image. In this case, after the contour (boundary) is extracted, a three-dimensional region is extracted isotropically around the picture element that is the boundary, and edge fill processing is performed on this region. Also in this case, the edge fill may be discretely performed instead of the entire picture element serving as the boundary.

The area to be isotropically extracted may be a circular area instead of a square area in the case of two dimensions. In the case of three dimensions, it can be a cubic area or a spherical area.

Further, the correction processing method of the present invention can be applied to the correction of an image having shading due to the apparatus, but the case of applying to the image processing in the MRI apparatus will be described below.

FIG. 6 is a diagram showing an overall outline of an MRI apparatus to which the present invention is applied.
It includes a central processing unit (CPU) 1, a sequencer 2, a transmission system 3, a static magnetic field generating magnet 4, a gradient magnetic field generating system 21, a receiving system 5, and a signal processing system 6.

The CPU 1 inputs photographing parameters from an input device such as the keyboard 22 in advance, and controls each of the sequencer 2, the transmission system 3, the reception system 5, and the signal processing system 6 according to a program. The sequencer 2 operates based on a control command from the CPU 1, and sends various commands necessary for data collection of tomographic images of the subject 7 to the transmission system 3, the gradient magnetic field generation system 21, and the reception system 5. .

The transmitting system 3 has a high-frequency oscillator 8, a modulator 9, and an irradiation coil 11 as a high-frequency coil. The modulator 9 amplitude-modulates a high-frequency pulse from the high-frequency oscillator 8 according to a command from the sequencer 2. The amplitude-modulated high-frequency pulse is amplified through the high-frequency amplifier 10 and supplied to the irradiation coil 11, so that the subject 7 is irradiated with a predetermined pulsed electromagnetic wave.

The static magnetic field generating magnet 4 is for generating a uniform static magnetic field around the subject 7 in an arbitrary direction. Inside the static magnetic field generating magnet, in addition to the irradiation coil 11, a gradient magnetic field coil 13 for generating a gradient magnetic field and a receiving coil 14 of the receiving system 5 are provided. The gradient magnetic field generation system 21 has a configuration in which a gradient magnetic field can be independently applied in directions of Cartesian coordinate axes orthogonal to each other, a three-direction gradient magnetic field coil 13, a gradient magnetic field power supply 12 for supplying current to the gradient magnetic field coil, and a gradient magnetic field power supply 12. And a sequencer 2 for controlling the

The receiving system 5 has a receiving coil 14 as a high-frequency coil, an amplifier 15 connected to the receiving coil 14, a quadrature detector 16 and an A / D converter 17, and an NMR signal from the subject 7 When the receiving coil 14 detects the signal,
The signal is converted into a digital value via an amplifier 15, a quadrature detector 16 and an A / D converter 17, and the quadrature detector 1 is output at a timing specified by the sequencer 2.
6, the data is converted into two series of collected data sampled and sent to the CPU 1.

The signal processing system 6 has an external storage device such as an optical disk 19 and a magnetic disk 20 and a display 18 such as a CRT.
1, the CPU 1 executes processing such as signal processing and image reconstruction, displays a desired cross-sectional image of the subject 7 on the display 18 and displays the desired cross-sectional image on the magnetic disk 20 or the like of the external storage device. Record.

The CPU 1 also has an image correction processing algorithm as shown in FIG. 1, and corrects image shading caused by the sensitivity distribution of the receiving coil 14.

A correction processing method according to the present invention and a conventional correction processing method in which an original image obtained by applying an LPF to shading is used as an example of a phantom image taken using a solenoid type receiving coil in such an MRI apparatus. And will be described in comparison. The graph shown in FIG. 7 is a graph in which the horizontal axis represents the position in the x-direction of the AA ′ cross section of the phantom 600 and the vertical axis represents the signal value. Is concave due to the sensitivity distribution (shading) of the receiving coil 700. In the conventional method, this original image is directly L
Since the sensitivity distribution of the coil is estimated by applying the PF, in this case, the sensitivity distribution is low at the edge portion of the subject (the portion indicated by oblique lines) as shown by the dotted line in the figure, and the original sensitivity distribution is reduced. Is generated. On the other hand, in the correction processing method of the present invention, the edge fill processing is performed on the original image, and the signal value of the subject is isotropically extended to the outside of the subject. In the graph, the estimated sensitivity distribution almost coincides with the actual sensitivity distribution without lowering.

FIG. 8 shows the result of correcting shading of a phantom MR image using the sensitivity distribution estimated by the correction processing method of the present invention. The original image profile is non-uniformity ｛(maximum signal value-minimum signal value)
/ Minimum signal value = b / a｝ was 0.33, but can be improved to 0.11 (b ′ / a ′) by shading correction, and edge enhancement in a conventional corrected image as shown in FIG. Is not seen.

The correction processing method of the present invention is effective for MR images of any part such as a neck image, a shoulder image, a jaw indirect image, a head image, a spine image, and the shading correction of various local coils and multiple coils. Is effective.

[0035]

As can be seen from the above description, according to the correction processing method of the present invention, a boundary of an original image is extracted, and a background picture element near the boundary is isotropically extracted to obtain a signal. By performing the process of replacing values (edge fill process), overcorrection of the edge portion is eliminated, and ideal shading correction becomes possible. Also, since the subject area is expanded in an equal manner, no matter what direction the subject has shading, ideal correction can be performed, and any part,
It can be applied to subjects at all angles.

[Brief description of the drawings]

FIG. 1 is a flowchart showing one embodiment of the present invention.

FIGS. 2A and 2B are diagrams for explaining processing of means 101 in the flowchart of FIG. 1, wherein FIG. 2A shows an original image, and FIG. 2B shows a binarized image.

FIG. 3 is a view for explaining processing of means 102 in the flowchart of FIG. 1;

FIGS. 4A and 4B are diagrams for explaining processing of means 103 and 104 in the flowchart of FIG. 1, in which FIG. 4A illustrates extraction of a region from a boundary image, and FIGS. The figure which shows the replacement of a picture element value.

FIG. 5 is a diagram illustrating an embodiment of an edge fill according to the present invention.

FIG. 6 is a block diagram showing an overall outline of an MRI apparatus to which the present invention is applied.

FIG. 7 illustrates the principle of the present invention.

FIG. 8 is a graph showing an embodiment of processing by the correction processing method of the present invention.

FIG. 9 is a graph showing processing by a conventional correction processing method.

[Explanation of symbols]

 1 ... CPU (image processing means) 2 ... sequencer 21 ... gradient magnetic field generation system (magnetic field generation means) 3 ... transmission system 4 ...・ Static magnetic field magnet (magnetic field generating means) 5 ・ ・ ・ ・ ・ ・ Reception system 6 ・ ・ ・ ・ ・ ・ Signal processing system (image processing means) 200 ・ ・ ・ ・ ・ Original image 201 ・ ・ ・ ・ ・ Subject 202 ・... Background 300... Binarized image 400... Boundary image 401... Boundary 500... Second image

Claims (4)

[Claims] An image correction processing method for correcting shading of an original image which is a two-dimensional or three-dimensional gray image having a subject and a background, means for dividing the original image into a subject region and a background region. 101) means for extracting a boundary between the subject and the background (102); means (103) for isotropically extracting picture elements near the boundary from the original image; A means (104) for forming a second image by replacing a picture element value of a background picture element with a signal value of a subject; and a means (105) for applying a low-frequency pass filter to the second image. ) And means (106) for correcting the luminance of the original image using the third image after the low-pass filter operation. 2. The method according to claim 1, wherein said means for isotropically extracting picture elements in the vicinity of said boundary is a means for extracting picture elements within a square area having a specific size around a boundary point. The image correction processing method described in the above. 3. The method according to claim 1, wherein the signal value of the subject is an average value or a median value of the picture element values of the picture elements which are the subject among the picture elements extracted isotropically. The image correction processing method described in the above. 4. A magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field, and said magnetic field generating means for irradiating a high-frequency magnetic field for generating nuclear magnetic resonance on nuclear spins of a tissue constituting a subject. A sequencer for controlling the means, and means for detecting a nuclear magnetic resonance signal generated from the subject;
In a magnetic resonance imaging apparatus comprising: an image processing unit that performs signal processing on the nuclear magnetic resonance signal to reconstruct an image; and a unit that displays an image, the image processing unit converts the original image into a subject region and a background region. (10)
1) means for extracting a boundary between the subject and the background (1)
02), a means (103) for isotropically extracting a picture element near the boundary from the original image, and a picture element value of a background picture element among the extracted picture elements, Means (104) for forming a second image by replacing with a signal value; means (105) for applying a low-frequency pass filter to the second image;
A magnetic resonance imaging apparatus comprising: a shading correction unit including a unit (106) for correcting the luminance of the original image using the third image after the low-pass filter operation.