JP5638324B2 - Magnetic resonance imaging apparatus and image correction method - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as “NMR”) signals from hydrogen, phosphorus, etc. in a subject, and images nuclear density distribution, relaxation time distribution, etc. The present invention relates to an apparatus (referred to as “MRI”), and more particularly to a technique for correcting non-uniform brightness of an image caused by sensitivity image data of an RF coil used for reception.

  The MRI apparatus measures NMR signals generated by nuclear spins constituting a subject, particularly a human tissue, and images the form and function of the head, abdomen, limbs, etc. two-dimensionally or three-dimensionally. Device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded and measured as time-series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In recent years, a target image can be obtained by combining a plurality of tomographic images obtained for each RF receiver coil by using a multiple coil having a plurality of RF receiver coils to obtain a composite image. It has been broken. Although this method has an advantage of being able to acquire a composite image having a high S / N ratio, the sensitivity non-uniformity of each RF receiving coil is reflected in the composite image, and a composite image with non-uniform luminance is acquired. May end up.

  As a technique for correcting luminance non-uniformity in the composite image, sensitivity image data of the RF receiving coil is calculated, correction coefficient data is calculated using the sensitivity image data, and luminance non-uniformity of the composite image is calculated using the correction coefficient data. There is a technology to correct this.

  As an example of a method for calculating the sensitivity image data of the RF receiving coil, the ratio of the image obtained using the RF receiving coil to the image obtained using the whole body RF coil having uniform sensitivity image data is calculated based on the RF. A method of calculating the sensitivity image data of the receiving coil (Patent Document 1), a method of approximating the low frequency component of the RF receiving coil as the sensitivity image data of the RF receiving coil (Patent Document 2), and imaging a uniform phantom in advance. There are a method (Patent Document 3) that regards the tomographic image as sensitivity image data, and a method (Non-Patent Document 1) that calculates sensitivity image data of the RF receiving coil by a numerical operation by a magnetic field analysis method.

JP-A-8-55928 JP 2002-272705 A Japanese Unexamined Patent Publication No. 7-59750

ISMRM 1405, 1999 DF Kacher, et al

  However, in the above-described prior art, in correcting the luminance non-uniformity of the sensitivity image data of the RF receiving coil, consideration is given to the inside of the slice plane, but to the inside of the slice thickness of the MRI image. Even if there is a sudden change in sensitivity distribution, brightness non-uniformity correction is performed in a state where it is ignored. In addition, even in the 3D TOF (Time of Flight) technique used for blood vessel depiction or the like, the sensitivity distribution at the position where the blood vessel is present cannot be accurately captured.

  Accordingly, an object of the present invention is to provide a technique capable of correcting the uneven brightness of an image even in a thick slice in consideration of the inside of the slice thickness of the MRI image. Note that the description of the thick slice will be described later in the first embodiment.

  In order to achieve the above object, the main configuration of the magnetic resonance imaging apparatus of the present invention is as follows.

  Irradiation means for irradiating a subject placed in a static magnetic field space with high frequency, reception means for receiving a nuclear magnetic resonance signal emitted from the subject, and processing of the received nuclear magnetic resonance signal to process the tomogram of the subject In a magnetic resonance imaging apparatus comprising: processing means for reconstructing an image; auxiliary measurement means for measuring in advance the three-dimensional sensitivity image data in the imaging field of the irradiation means and the reception means, and the processing means have a predetermined slice width Based on the sensitivity measurement data of the main measurement means for acquiring the captured image of the subject with respect to the imaging area and the irradiation means and the receiving means for each block obtained by dividing the imaging area into a plurality of dimensions three-dimensionally. The present invention is characterized in that non-uniform brightness of a captured image caused by non-uniform sensitivity is corrected.

  That is, when the actual measurement positioning is performed, the slice thickness and three-dimensional position coordinate information of the image to be corrected are obtained, and optimum image data is obtained for each pixel from the sensitivity measurement image based on the information, and from the data Accurate sensitivity image data is calculated.

  According to the present invention, accurate luminance non-uniformity correction can be performed even for MRI image data captured with a thick slice width.

1 is a block diagram showing the overall basic configuration of an MRI apparatus according to the present invention. a) is a subject and an imaging range, and b) and c) are diagrams each showing a three-dimensional sensitivity image when a transmission coil is used and when a reception coil is used. a) a subject and an imaging visual field; b) a main measurement tomographic image; and c) a main measurement tomographic information. a) a subject and an imaging field; b) a three-dimensional sensitivity image when a thick slice width is used; and c) a sensitivity image extracted from position information of a main measurement image. a) is a sensitivity image when the transmission coil extracted from the position information of the actual measurement image is used, b) is a sensitivity image when the reception coil is used, c) is a sensitivity image obtained in a) and b). The figure which shows the sensitivity distribution image calculated | required. The flowchart which shows the sensitivity correction process which concerns on 1st embodiment. a) is a subject and imaging field of view, b) a three-dimensional sensitivity image when a thick slice width is used, and c) a tomogram whose sensitivity is corrected by using an average value of the sensitivity of each pixel divided in the slice width direction. The figure which shows how to obtain | require an image. The flowchart which shows the sensitivity correction process which concerns on 2nd embodiment. a) is a subject and imaging field of view, b) is a three-dimensional sensitivity image when a thick slice width is used, and c) is a tomographic image whose sensitivity is corrected using the maximum sensitivity projection for each pixel divided in the slice width direction. The figure which shows how to obtain | require an image. The flowchart which shows the sensitivity correction process which concerns on 3rd embodiment.

Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.
First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall basic configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject. As shown in FIG. 1, the MRI apparatus includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, a transmission system 5, The receiving system 6, the signal processing system 7, the sequencer 4, a central processing unit (CPU) 8, and an operation unit 25 are configured.
Hereinafter, each of the basic configurations described above will be described in detail.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. Thus, a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the subject 1.

  The gradient magnetic field generation system 3 includes a gradient magnetic field coil 9 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil. The gradient magnetic fields Gx, Gy, Gz are applied in the three axial directions of X, Y, and Z by driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the sequencer 4 described later. At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two orthogonal to the slice plane and orthogonal to each other A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded into an echo signal.

  The sequencer 4 is a control means that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence. The sequencer 4 operates under the control of the CPU 8 and collects tomographic image data of the subject 1. Various commands necessary for the transmission are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

  The transmission system 5 irradiates the subject 1 with RF pulses in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and includes a high-frequency oscillator 11, a modulator 12, and a high-frequency amplifier. 13 and a high frequency coil (transmission coil) 14a on the transmission side. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at the timing according to the command from the sequencer 4, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13 and then placed close to the subject 1. By supplying the high frequency coil 14a, an RF pulse is irradiated to the subject 1.

  The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and receives a high-frequency coil (receiving coil) 14b and a signal amplifier 15 on the receiving side. And a quadrature detector 16 and an A / D converter 17. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15, The quadrature phase detector 16 divides the signal into two orthogonal signals at a timing according to a command from the sequencer 4, and each signal is converted into a digital quantity by the A / D converter 17 and sent to the signal processing system 7.

  The signal processing system 7 performs various data processing and display and storage of processing results. The signal processing system 7 includes an external storage device such as an optical disk 19 and a magnetic disk 18 and a display 20 including a CRT. Is input to the CPU 8, the CPU 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display 20, and also the magnetic disk 18 of the external storage device. Record in etc.

  The operation unit 25 is used to input various control information of the MRI apparatus and control information of processing performed by the signal processing system 7 and includes a trackball or mouse 23 and a keyboard 24. The operation unit 25 is disposed close to the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

  In FIG. 1, the high-frequency coil 14a and the gradient magnetic field coil 9 on the transmission side face the subject 1 in the static magnetic field space of the static magnetic field generation system 2 in which the subject 1 is inserted in the vertical magnetic field method. If the horizontal magnetic field method is used, the subject 1 is installed so as to surround it. The high-frequency coil 14b on the receiving side is installed so as to face or surround the subject 1.

At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as widely used clinically. Information on the spatial distribution of the proton density and the spatial distribution of the relaxation time of the excited state is imaged, thereby imaging the form or function of the human head, abdomen, limbs, etc. two-dimensionally or three-dimensionally.
<First Embodiment>
A first embodiment of the present invention will be described with reference to FIGS.

First, sensitivity distribution measurement is performed as in normal imaging (FIG. 2A).
FIGS. 2A to 2C are diagrams for explaining preliminary measurement (preliminary scan) before starting main measurement (main scan). a) shows a subject 1 and an imaging field of view (FOV) 35. That is, the imaging field of view 35 indicates the scan range of the preliminary scan. The image shown in b) is a three-dimensional sensitivity image when received using the transmission coil 14a. As shown in this figure, since the sensitivity distribution is uniform compared to the case shown in c), the brightness of the image is less shaded. Here, the reason why the luminance density is small is that the transmission coil 14a is configured to have a large diameter and axial length so as to cover a wide range of the subject 31.

  On the other hand, the image shown in c) is a three-dimensional sensitivity image when received using the receiving coil 14b. In the region indicated by A in the figure, it can be seen that since the sensitivity distribution is non-uniform compared to the case indicated by b), the brightness of the image is shaded. In other words, the brightness decreases (darkens) as it goes inward compared to the vicinity of the surface of the body 34. This is because the receiving coil is configured to have a small diameter and axial length so as to be disposed in the vicinity of a desired portion of the subject 31. In b) and c), reference numeral 34 schematically shows the body of the subject 31, and reference numerals 32 and 33 schematically show both arms thereof.

  Next, the main measurement (main scan) is performed. At this time, main measurement tomographic image information in the slice plane and the slice plane width direction, that is, three-dimensional position information and slice thickness are acquired (FIG. 2B).

In the present embodiment, the slice width 36 is set to be wider than a normally used width, and a thick slice width (or slab) is used. For example, the slice width is about 50 to 70 mm. Here, the “thick slice width” refers to a width wider than a conventionally used slice width, for example, a width wider by about one digit. b) shows a main measurement tomographic image 37 obtained by the main scan. c) scans the diagnostic region of the subject 1 with the slice width 36 described above, and shows the acquisition of the slice plane of the acquired image data, the three-dimensional position coordinates in the slice plane width direction, and the slice thickness. These data are stored in the magnetic disk 18 of the external storage device.
Here, reference numerals 32 and 33 indicate arms, reference numeral 34 indicates a body part, and reference numeral 38 indicates a part of the built-in part.

  Next, when the slice thickness is equal to or greater than a certain value, sensitivity image data is extracted for each pixel from the data obtained by sensitivity distribution measurement based on the three-dimensional position information of the in-plane pixels (FIG. 2C). Here, the pixel corresponds to each block obtained by dividing one slice 39 in the X, Y, and Z directions as shown in FIG. 2C b).

When the slice width 36 of the diagnostic region acquired in the main scan shown in FIG.
Based on the information of the three-dimensional position coordinates of the pixels in each slice plane stored in the magnetic disk 18 or the like of the external storage device, one slice 39 is sliced in the slice width direction (z direction in the figure) as shown in FIG. And pixels 40a, 40b, and 40c for each block divided and subdivided in the intersecting direction (x, y direction). For the convenience of illustration, the pixels 40a-40c are representatively described, but the pixels are similarly subdivided in the x and y directions.

  On the other hand, the sensitivity distribution corresponding to each of the pixels 40a-40c described above is obtained using the three-dimensional sensitivity image b) obtained in FIG. (See c in FIG. 2C).

  The sensitivity distribution of the measurement tomographic image shown in c) of FIG. 2C is the same as that in FIG. 2C of b), for example, using the MIP method (maximum value projection method) in the pixels 40a, 40b, and 40c. The one having the highest luminance (indicated by “(1)” in the figure) is extracted as a representative value and used as the sensitivity of the position corresponding to the block in the measurement tomographic image. The extracted representative value is made to correspond to the pixel position indicated by “(1)” in the sensitivity image shown in c). A similar extraction method is performed for pixels adjacent in the X direction (indicated by “(2)” to “(4)” in the figure). Further, similar extraction is performed for pixels in the Y direction. With this procedure, a sensitivity distribution image using the transmission coil shown in c) of FIG. 2C is obtained.

  The above procedure is also applied to the three-dimensional sensitivity image, c) of FIG. 2A, and a sensitivity distribution image using the receiving coil is obtained.

  Finally, the sensitivity distribution image using the extracted transmission coil (FIG. 2D a)) and reception coil (FIG. 2D b)) is subjected to a division process, and the sensitivity distribution image based on the transmission coil (FIG. 2D). C)) is obtained (FIG. 2D), and sensitivity correction processing is performed using the obtained sensitivity distribution image.

The obtained sensitivity distribution image based on the transmission coil is used as basic data for sensitivity correction processing.
This sensitivity correction process is usually performed in this field. For example, it is a function of correcting the signal value of the image according to the sensitivity distribution image in order to correct shading caused by the sensitivity distribution of the coil.

Next, the flow of sensitivity correction processing shown in the first embodiment will be described with reference to FIG.
1) In step 301 (S301), sensitivity distribution measurement is performed in the same manner as in normal imaging, and a subject image (a three-dimensional sensitivity image of a subject) is acquired. (See Figure 2A)
2) In step 302 (S302), three-dimensional information of all pixels in the main measurement image plane is acquired from the position information of the main measurement. (See Figure 2B)
3) In step 303 (S303), main measurement is performed in the same manner as in normal imaging, and a subject image (main measurement tomographic image of the subject) is acquired. (See Figure 2C)
On the other hand, while performing step 303, the processing of steps 304 and 305 is performed.
2 ′) In step 304 (S304), sensitivity image data is extracted from the position information obtained in step 302. (See a) and b) in FIG. 2D)
3 ') In step 305 (S305), a sensitivity distribution image is created from the extracted data. (See c in FIG. 2D)
4) Steps 305 and after are the same as normal sensitivity correction processing.

  The above steps are calculated and controlled by the CPU 8 shown in FIG. 1, and the acquired image data and the like are stored in the magnetic disk 18 of the external storage device.

According to the present embodiment, the inside of the slice thickness of the MRI image is taken into consideration, and even if there is a sudden change in sensitivity distribution, luminance nonuniformity correction can be performed without ignoring it. Therefore, according to the present method, for example, the sensitivity distribution at the position where the blood vessel is present can be accurately captured by the 3D TOF (Time of Flight) technique used for blood vessel rendering or the like.
<Second Embodiment>
Next, a second embodiment will be described with reference to FIG. The difference from the first embodiment is that the sensitivity image data is not extracted from the position information of the main measurement image, but the sensitivity image data for the slice tomogram of the main measurement is extracted, and then the slice direction (Z in the figure) is extracted. (Direction) is equally divided and averaged to create sensitivity image data used for correction. Hereinafter, only different portions will be described, and description of the same portions will be omitted.

The corresponding slice tomographic image data is extracted from the three-dimensional sensitivity image data from the slice thickness information and position information of the main measurement, and is divided at equal intervals in the slice direction (Z direction in the figure). Thereafter, the sensitivity image data divided at equal intervals is averaged, and a sensitivity distribution image is created from the data (FIG. 4).
After that, it is the same as the normal sensitivity correction process.

Next, the flow of sensitivity correction processing shown in the second embodiment will be described with reference to FIG. Hereinafter, only portions different from the flow of sensitivity correction processing shown in FIG. 3 will be described, and description of the same portions will be omitted.
In step 306 (S306), slice direction sensitivity distribution data is extracted from the position information and slice thickness information obtained in step 302.
In step 307 (S307), the data extracted in step 306 is equally divided in the slice direction and averaged, and then a sensitivity distribution image is created.
<Third Embodiment>
Next, a third embodiment will be described with reference to FIG. The difference from the second embodiment is that the maximum value projection (MIP method: Maximum Intensity Projection) is not sliced in the slice direction (Z direction in the figure) and averaged, but the slice direction (Z direction in the figure). ) To create sensitivity image data used for correction. Here, the MIP method is a method of performing projection processing in an arbitrary viewpoint direction on data constructed three-dimensionally and displaying the maximum value in the projection path on the projection plane. In the present embodiment, the viewpoint direction is the Z direction in the figure.

Hereinafter, only different portions will be described, and description of the same portions will be omitted.
Based on the slice thickness information and position information of the main measurement, the corresponding slice tomographic image data is extracted from the three-dimensional sensitivity image data, and is divided at equal intervals in the slice direction. Thereafter, the sensitivity image data divided at equal intervals is projected to the maximum value, and a sensitivity distribution image is created from the data (FIG. 6).

Next, the flow of sensitivity correction processing shown in the third embodiment will be described with reference to FIG. Hereinafter, only portions different from the flow of the sensitivity correction processing shown in FIG. 5 will be described, and description of the same portions will be omitted.
In step 308 (S308), the data extracted in step 306 is projected to the maximum value in the slice direction, and a sensitivity distribution image is created from the data.

DESCRIPTION OF SYMBOLS 1 ... Subject, 2 ... Static magnetic field generation system, 3 ... Gradient magnetic field generation system, 4 ... Sequencer, 5 ... Transmission system, 6 ... Reception system, 7 ... Signal processing system, 8 ... Central processing unit (CPU), 9 ... Gradient magnetic field coil, 10 Gradient magnetic field power source, 11 High frequency transmitter, 12 Modulator, 13 High frequency amplifier, 14 a High frequency coil (transmitting coil), 14 b High frequency coil (receiving coil), 15 Signal amplifier, 16 ... Quadrature detector, 17 ... A / D converter, 18 ... Magnetic disk, 19 ... Optical disk, 20 ... Display, 21 ... ROM, 22 ... RAM, 23 ... Trackball or mouse, 24 ... Keyboard, 25 ... Operation unit 31 ... Subject, 32, 33 ... Arm, 34 ... Body part, 35 ... Imaging field, 36 ... Slice width, 37 ... Main measurement tomographic image, 38 ... Built-in, 39 ... 1 slice, 40a, 40b, 40c ... Block Every pixel.

Claims (10)

Irradiation means for irradiating a subject placed in a static magnetic field space with a high frequency; receiving means for receiving a nuclear magnetic resonance signal emitted from the subject; and processing the received nuclear magnetic resonance signal to process the subject. A magnetic resonance imaging apparatus comprising: processing means for reconstructing a tomographic image of a specimen;
Auxiliary measuring means for measuring in advance the three-dimensional sensitivity image data in the imaging field of view of the irradiating means and the receiving means;
The processing means includes a main measurement means for acquiring a captured image of the subject for an imaging region having a predetermined slice width;
Based on the sensitivity image data of the irradiation means and the reception means for each block obtained by dividing the imaging region into a plurality of dimensions in three dimensions, the brightness of the captured image is uneven due to the non-sensitivity of the reception means. We have a correction means for correcting,
The correction means includes a first two-dimensional sensitivity image for the imaging region based on the data for each block in the three-dimensional sensitivity image data of the transmission means, and the three-dimensional sensitivity image data of the reception means. And a second two-dimensional sensitivity image for the imaging region based on the data for each block of the magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1 .
The brightness non-uniformity correction of the image caused by the sensitivity non-uniformity possessed by the receiving means is performed based on data obtained by dividing the second two-dimensional sensitivity image by the first two-dimensional sensitivity image. Magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1 .
A magnetic resonance imaging apparatus that acquires three-dimensional position information and slice width information of the captured image when acquiring the captured image of the subject. The magnetic resonance imaging apparatus according to claim 1 .
The magnetic resonance imaging apparatus characterized in that the processing means calculates the first and second two-dimensional sensitivity images by averaging the sensitivity image data for each block divided in the slice width direction. The magnetic resonance imaging apparatus according to claim 1 .
The magnetic resonance imaging apparatus, wherein the processing means calculates the first and second two-dimensional sensitivity images using a maximum value of sensitivity image data for each block divided in the slice width direction. Irradiation means for irradiating a subject placed in a static magnetic field space with a high frequency; receiving means for receiving a nuclear magnetic resonance signal emitted from the subject; and processing the received nuclear magnetic resonance signal to process the subject. In an image correction method of a magnetic resonance imaging apparatus comprising: processing means for reconstructing a tomographic image of a specimen;
An auxiliary measurement step of measuring in advance the three-dimensional sensitivity image data in the imaging field of view of the irradiating means and the receiving means;
The processing means acquires a captured image of the subject for an imaging region having a predetermined slice width; and
Based on the sensitivity image data of the irradiation means and the reception means for each block obtained by dividing the imaging region into a plurality of dimensions in three dimensions, the brightness of the captured image is uneven due to the non-sensitivity of the reception means. run a correction step of correcting a
The correction step includes: a first two-dimensional sensitivity image for the imaging region based on the data for each block in the three-dimensional sensitivity image data of the transmission unit; and the three-dimensional sensitivity image data of the reception unit. An image correction method for a magnetic resonance imaging apparatus, wherein a second two-dimensional sensitivity image for the imaging region is created based on the data for each block . The image correction method of the magnetic resonance imaging apparatus according to claim 6 ,
The brightness non-uniformity correction of the image caused by the sensitivity non-uniformity possessed by the receiving means is performed based on data obtained by dividing the second two-dimensional sensitivity image by the first two-dimensional sensitivity image. An image correction method for a magnetic resonance imaging apparatus. The image correction method of the magnetic resonance imaging apparatus according to claim 6 ,
An image correction method for a magnetic resonance imaging apparatus, wherein three-dimensional position information and slice width information of the captured image are acquired when acquiring a captured image of the subject. The image correction method of the magnetic resonance imaging apparatus according to claim 6 ,
An image of the magnetic resonance imaging apparatus, wherein the processing means calculates the first and second two-dimensional sensitivity images by averaging the sensitivity image data for each block divided in the slice width direction. Correction method. The image correction method of the magnetic resonance imaging apparatus according to claim 6 ,
The image of the magnetic resonance imaging apparatus, wherein the processing means calculates the first and second two-dimensional sensitivity images using a maximum value of sensitivity image data for each block divided in the slice width direction. Correction method.

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