JP2007190120A - Image processing program and image processor in magnetic resonance imaging - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image without distortion just by one scanning on the basis of K space data obtained from MRI by an EPI method. <P>SOLUTION: By obtaining phase errors which are gradient magnetic field errors in all read directions that cause the distortion in images including errors due to an eddy current and errors in terms of software and correcting linearly and inversely Fourier transformed K space data by using the phase errors, the images for which the distortion is corrected are generated just by one scanning especially in a single shot EPI method. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging method for obtaining a magnetic resonance image of an object from information obtained by EPI (echo planar imaging) and an image processing apparatus using the method.

  Magnetic Resonance Imaging (MRI) is widely used in the medical field as a precision diagnostic means for human bodies and the like, and in recent years, it has also been used as a noninvasive brain function measurement method. Yes. In particular, in the field of brain function research, brain functional imaging (functional MRI; fMRI) is used to image brain activity. In fMRI, echo planar imaging (one of high-speed imaging methods) The Echo Planar Imaging (EPI) method has become an almost indispensable item. In particular, the single-shot EPI method, which is an ultra-high-speed scanning method that collects all data necessary for image construction with a single excitation pulse, is more effective than the multi-shot EPI that collects image data with a plurality of excitation pulses. Although it is inferior in the N ratio (Signal to Noise ratio), it is considered to be advantageous to be able to take images in units of several tens of milliseconds.

  However, in the single shot EPI method, an eddy current magnetic field is generated in order to reverse the readout gradient magnetic field at high speed, and there is a problem in that the image is distorted when the image is reconstructed from the acquired data due to the influence of the magnetic field. . However, it is not practical to limit the generation of eddy currents in terms of hardware. Moreover, the effect of eddy currents is increased by increasing the gradient magnetic field and resolution in order to obtain a more precise image. Nowadays, brain functions are being researched with high magnetic field and high resolution, and it is required to solve the above problems.

As a technique for correcting image distortion due to an eddy current magnetic field caused by high-speed reversal of the readout gradient magnetic field of the EPI method, as shown in, for example, Patent Document 1, preliminary measurement of eddy current magnetic field distribution as a function of time has been performed. Then, a method of correcting the main measurement signal using the pre-measured distribution data and further reconstructing the image is considered.
JP2002-224083

  However, the method disclosed in Patent Document 1 cannot deal with software errors, cannot cope with changes in the eddy current magnetic field distribution after preliminary measurement, and the reconstruction calculation is complicated, and an image can be obtained in real time. In the first place, it is difficult to take advantage of the advantage of single shot EPI that scanning at ultra-high speed is possible because measurement is required multiple times, ie, preliminary measurement and main measurement.

  Therefore, the present invention solves such a problem and is suitable for a magnetic resonance imaging method, particularly a single shot EPI method, and an image processing program that can obtain an image whose distortion is corrected by only one scan. It is an object to provide an image processing apparatus.

  That is, the image processing program according to the present invention is an image processing program for configuring a magnetic resonance image based on K-space data obtained by imaging by an echo planar imaging method using a magnetic resonance imaging apparatus by operating a computer. Based on the K-space data subjected to one-dimensional inverse Fourier transform in the readout direction, the computer cancels the phase change due to the gradient magnetic field applied in the phase encoding direction and the phase difference between adjacent pixels in the readout direction. A phase error calculation step for obtaining a phase error generated in the readout direction by calculation, and a readout gradient magnetic field in the K space that is one-dimensional inverse Fourier transformed in the readout direction based on the phase error obtained in the phase error calculation step. Correct the phase error that occurs in the readout direction when applied. The results is characterized in that to execute the image modification steps constituting the image distortion is modification by one-dimensional inverse Fourier transform in the phase encoding direction.

  As described above, by statistically processing the K space data, which is measurement data obtained from EPI imaging, an image including a linear error of a readout gradient magnetic field, that is, an error due to an eddy current peculiar to the single shot EPI method and a soft error By obtaining phase errors, which are gradient magnetic field errors in all readout directions that cause distortion in the image, correcting the K-space data subjected to one-dimensional inverse Fourier transform using this phase error, and reconstructing the image once. It is possible to obtain an image without distortion at high speed only by scanning by EPI imaging.

  By including the following two specific processing steps in the above-described phase error calculation step, the phase error can be obtained in more detail.

  First, in the phase error calculation step, the phase change due to the gradient magnetic field applied in the phase encoding direction is performed by adding the composite data of each column of the K space data subjected to the one-dimensional inverse Fourier transform in the reading direction. The phase change canceling step for canceling, the phase difference calculating step for calculating the phase difference between pixels adjacent in the readout direction using the calculation result in the phase change canceling step, and the calculation result of the phase difference calculating step And a phase error calculation step of calculating a phase error generated in the reading direction when the reading gradient magnetic field is applied once.

  Next, the second way is a phase difference calculation step of calculating a phase difference between adjacent pixels in the readout direction of each readout line of K space data subjected to one-dimensional inverse Fourier transform in the readout direction in the phase error calculation step. And, based on the calculation result of the phase difference calculation step, the composite data having information on the phase difference between adjacent pixels in each readout line in the readout direction is added to obtain the phase by the gradient magnetic field applied in the phase encoding direction. A phase change canceling step for canceling the change in the phase, and a phase error calculating step for calculating a phase error generated in the reading direction when the read gradient magnetic field is applied once based on the calculation result in the phase change canceling step; This is a program that includes

  Here, in any of these two methods, in the phase error calculation step, the phase change due to the linear error of the gradient magnetic field applied in the reading direction is read out from the central reading line in addition to the cancellation of the phase change. A process of concentrating on a state where a gradient magnetic field is applied can also be executed.

  The image processing apparatus according to the present invention that operates according to the above-described program of the present invention is an image processing that constitutes a magnetic resonance image based on K-space data obtained by an echo planar imaging method using a magnetic resonance imaging apparatus. An apparatus for canceling a phase change due to a gradient magnetic field applied in a phase encoding direction based on the K-space data subjected to a one-dimensional inverse Fourier transform in a reading direction and a phase difference between adjacent pixels in the reading direction. A phase error calculation unit that obtains a phase error generated in the reading direction by calculation, and a read gradient magnetic field is applied to the K space that is one-dimensional inverse Fourier transformed in the reading direction based on the phase error obtained by the phase error calculation unit. Correct the phase error that occurs in the readout direction when the And image modification unit constituting the image distortion is modification by one-dimensional inverse Fourier transform and is characterized in that it comprises an.

  In this image processing apparatus, the phase error can be obtained in more detail by including the following two specific processing functions in the phase error calculation unit, similarly to the above-described program.

  That is, the first is that the phase error calculation unit adds the composite data of each column of the K-space data subjected to the one-dimensional inverse Fourier transform in the readout direction and changes the phase due to the gradient magnetic field applied in the phase encoding direction. A phase change canceling unit that cancels the phase change, a phase difference calculating unit that calculates a phase difference between pixels adjacent to each other in the readout direction using a calculation result by the phase change canceling unit, and a calculation result by the phase difference calculating unit And a phase error calculation unit that calculates a phase error generated in the reading direction when the reading gradient magnetic field is applied once, and the second way is the phase error calculation unit, Phases of pixels adjacent to the readout direction of each readout line of K-space data subjected to one-dimensional inverse Fourier transform in the readout direction Based on the calculation result of the phase difference calculation unit that calculates the difference and the phase difference calculation unit, the composite data having the information regarding the phase difference between the pixels adjacent to each other in the readout direction of each readout line is added to obtain the phase encoding A phase change canceling unit that cancels a phase change due to a gradient magnetic field applied in the direction, and a phase error that occurs in the reading direction when the read gradient magnetic field is applied once based on a calculation result by the phase change canceling unit And a phase error calculation unit for calculating.

  In any of these two modes, the phase error calculation unit cancels the phase change and changes the phase change due to the linear error of the gradient magnetic field applied in the reading direction. It is also possible to execute processing for concentrating on the state in which the readout gradient magnetic field is applied.

  In the present invention, based on K-space data obtained from EPI imaging using an MRI apparatus, phase errors which are gradient magnetic field errors in all readout directions that cause distortion in an image including errors due to eddy currents and soft errors are detected. A method is used in which the phase space is used to correct the K-space data subjected to the one-dimensional inverse Fourier transform to reconstruct the image. As a result, an image without distortion that cannot be suppressed by hardware can be obtained at high speed by only one EPI shooting. Furthermore, compared with the conventional method, the time from photographing to accurate image acquisition can be shortened.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The block diagram shown in FIG. 1 shows an example of a magnetic resonance imaging apparatus (MRI apparatus) used in this embodiment. As shown in FIG. 1, the MRI apparatus 1 includes a static magnetic field generating magnet 2, a high frequency transmission coil 3, a high frequency reception coil 4, and gradient magnetic field coils 5 and 5 as basic components. On the other hand, the control device 7 connected to the computer 6 controls the high frequency signal from the high frequency generator 8 to be modulated into an appropriate signal by the modulator 9. Here, the modulation signal is applied to the high-frequency transmission coil 3 via the amplifier 10. The control device 7 also controls the gradient magnetic field generator 11 to cause the gradient magnetic field coils 5 and 5 to generate appropriate gradient magnetic fields. At the time of MRI measurement, a high-frequency pulse is irradiated from a high-frequency transmission coil 3 to a living body X (for example, a subject if it is a human body) X to cause each magnetic resonance to occur. As a result, an echo signal induced from the subject X by each magnetic resonance is detected as an analog signal by the phase detector 13 via the high frequency receiving coil 4 and the amplifier 12, and this analog detected signal is detected by the AD converter 14. Is converted into a digital signal. The computer 7 has a function as the image processing apparatus according to the present embodiment, and the digital signal output from the AD converter 14 according to the processing procedure by the image processing program of the present embodiment stored in the storage device 15 is received. The processing result is displayed on the display device 16 as each magnetic resonance image (MRI image).

  The image processing apparatus and the image processing program according to the present embodiment are caused by errors caused by eddy currents using K space data that is frequency information obtained by performing echo planar imaging (EPI) scanning with such an MRI apparatus. The distortion is corrected, and an MRI image without distortion (or distortion is extremely small) is generated.

FIG. 2 is a functional block diagram of the computer 6 as the image processing apparatus. That is, the computer 6 includes an input unit 61 that receives an input of a digital signal (frequency information) from the AD converter 14, and a phase error that determines a phase error that occurs in the readout (readout) direction based on the K space data that is the frequency information Calculation unit 62
An image modifying unit 63 that generates an image whose distortion is modified based on the obtained phase error, and an output unit 64 that outputs the generated image data to be displayed on the display device 16 are provided.

  More specifically, the phase error calculation unit 62 can take the following two modes. First, as a first mode, as shown in FIG. 3, the phase error calculation unit 62 adds the composite data of each column of the K space data subjected to the one-dimensional inverse Fourier transform in the reading direction and applies the combined data in the phase encoding direction. A phase change canceling unit 621a for canceling a phase change due to the gradient magnetic field, a phase difference calculating unit 622a for calculating a phase difference between pixels adjacent to each other in the readout direction, using a calculation result by the phase change canceling unit 621a, A mode provided with the phase error calculating part 623a which calculates the phase error which generate | occur | produced in the said reading direction when the reading gradient magnetic field is applied once based on the calculation result by the phase difference calculation part 622a is mentioned. Next, as a second mode, as shown in FIG. 4, the phase error calculation unit 62 calculates the phase of the pixel adjacent to the readout direction of each readout line of the K space data subjected to the one-dimensional inverse Fourier transform in the readout direction. Based on the calculation result by the phase difference calculation unit 621b that calculates the difference and the phase difference calculation unit 621b, the composite data having the information on the phase difference between the pixels adjacent to each readout line in the readout direction is added, A phase change cancel unit 622b that cancels a phase change due to a gradient magnetic field applied in the phase encoding direction, and a read gradient magnetic field applied in the read direction when the read gradient magnetic field is applied once based on a calculation result by the phase change cancel unit 622b. A mode provided with a phase error calculation unit 623c for calculating a generated phase error is mentioned. . These two modes are different in the processing order of the phase change cancellation units 621a and 622b and the phase difference calculation units 622a and 621b, and are common in that the phase error calculation units 623a and 623b finally calculate the phase error. . In detail, as shown in FIGS. 3 and 4, the image modification unit 63 reads the gradient into the K space that is one-dimensional inverse Fourier transformed in the readout direction based on the phase error obtained by the phase error calculation unit 62. A phase error correction unit 631 that corrects a phase error that occurs in the reading direction when a magnetic field is applied, and an image that generates image data in which distortion is corrected by performing a one-dimensional inverse Fourier transform on the correction result in the phase encoding direction A generation unit 632 is provided.

FIG. 5 is a diagram simply showing a processing procedure for creating an MRI image by the EPI method, in which (a) shows a conventional procedure and (b) shows a procedure in the present embodiment. In the conventional procedure, first, a value S ′ _{x, l} obtained by one-dimensional inverse Fourier transform (IFFT (RO)) in the read (readout; RO) direction of the input K space data S _{k , l} is obtained, and this is phase-converted. Image data I _{x, y} is obtained by one-dimensional inverse Fourier transform (IFFT (PE)) in the encoding (PE) direction. The obtained image is usually distorted due to the presence of an error due to an eddy current or a phase error caused by a software error. Therefore, in order to correct the distortion of the image, for example, as described in the section of the prior art, it is necessary to perform image correction by performing at least two scans of the preliminary measurement and the main measurement. On the other hand, in the procedure according to the present embodiment, a step of obtaining a value S ′ _{x, l} obtained by performing one-dimensional inverse Fourier transform (IFFT (RO)) in the read (readout; RO) direction of the input K space data S _{k, l.} Up to this point, the procedure is the same as the conventional procedure, but the phase error is corrected based on the value S ′ _{x, l} (correction), and the corrected value C ′ _{x, l} is set in the phase encoding (PE) direction. Image data I _{x, y} is obtained by performing one-dimensional inverse Fourier transform (IFFT (PE)).

Hereinafter, a specific image processing method in the present embodiment will be described by being divided into two aspects of the phase error calculation unit 62 described above. It is assumed that the matrix size of the K space is MxN. The center point of the K space is (M / 2 + 1, N / 2 + 1). Further, before the following specific processing, the K space data S _{k, l} received at the input unit 61 is one-dimensionally inverse Fourier transformed (S ′ _{x, l} ), and is processed using this value hereinafter. Shall.

  First, in the first aspect, the phase change canceling unit 621a of the phase error calculating unit 62 performs the following calculation.

すなわち上式では、読み出し方向に１次元逆フーリエ変換されたＫ空間データの各列の複合データ（ｃｏｍｐｌｅｘデータ）を合計（ＳＵＭ）する。この処理により、位相エンコード方向にかけられた傾斜磁場による位相の変化がキャンセルされる。さらにこの位相の変化のキャンセルと同時に、読み出し方向にかけられた傾斜磁場のリニアエラーによる位相の変化を、中央読み出し線の読み出し傾斜磁場をかけた状態に集中させる。

That is, in the above equation, the composite data (complex data) of each column of the K space data subjected to the one-dimensional inverse Fourier transform in the reading direction is summed (SUM). This process cancels the phase change due to the gradient magnetic field applied in the phase encoding direction. Further, simultaneously with the cancellation of the phase change, the phase change due to the linear error of the gradient magnetic field applied in the readout direction is concentrated in the state where the readout gradient magnetic field of the central readout line is applied.

Using thus obtained values [rho _x, the phase difference calculating unit 622a, performs the following calculation. Note that “conj ()” in the formula is a function that returns a conjugate complex number in ().

すなわち上式では、読み出し方向に隣接するピクセルの位相の差を算出する。このようにして得られた値ρは、Ｋ空間中、中央の読み出し線を得る際に、初めの読み出し線からの読み出し傾斜磁場エラーによる位相エラーの累積である。この処理により、信号の弱い部分（通常は空気の部分）の影響が抑制されて、信号の強い部分（通常は測定対象の部分）の影響が強調される。

That is, in the above equation, the phase difference between pixels adjacent in the readout direction is calculated. The value ρ obtained in this way is the cumulative phase error due to the read gradient magnetic field error from the first read line when obtaining the central read line in the K space. By this processing, the influence of the weak signal portion (usually the air portion) is suppressed, and the influence of the strong signal portion (usually the portion to be measured) is emphasized.

  In the processing up to this point, in the second aspect, the phase difference calculation unit 621b of the phase error calculation unit 62 performs the following calculation.

すなわち上式では、読み出し方向に１次元逆フーリエ変換されたＫ空間の各読み出し線の読み出し方向に隣接する位相の差を算出する。この処理により、信号の弱い部分（通常は空気の部分）の影響が抑制されて、信号の強い部分（通常は測定対象の部分）の影響が強調される。

That is, in the above equation, the difference in phase adjacent to the readout direction of each readout line in the K space subjected to one-dimensional inverse Fourier transform in the readout direction is calculated. By this processing, the influence of the weak signal portion (usually the air portion) is suppressed, and the influence of the strong signal portion (usually the portion to be measured) is emphasized.

Using the phase difference ρ _l between adjacent pixels of the readout line thus obtained, the phase change canceling unit 622b performs the following calculation.

すなわち上式では、読み出し線の隣接するピクセルの位相の差を有する複合データ（ｃｏｍｐｌｅｘデータ）を合計（ＳＵＭ）する。この処理により、位相エンコード方向にかけられた傾斜磁場による位相の差がキャンセルされる。さらにこの位相の変化のキャンセルと同時に、読み出し方向にかけられた傾斜磁場のリニアエラーによる位相の変化を、中央読み出し線の読み出し傾斜磁場をかけた状態に集中させる。なお、この処理は、上式に代えて次式による演算によっても実現することができる。

That is, in the above expression, the composite data (complex data) having the phase difference between adjacent pixels of the readout line is summed (SUM). By this processing, the phase difference due to the gradient magnetic field applied in the phase encoding direction is canceled. Further, simultaneously with the cancellation of the phase change, the phase change due to the linear error of the gradient magnetic field applied in the readout direction is concentrated in the state where the readout gradient magnetic field of the central readout line is applied. This process can also be realized by an operation according to the following equation instead of the above equation.

上式中、「ａｂｓ（）」は、（）内の実数の絶対値を返す関数である。この場合、数４の式を適用する場合に、磁気横緩和によるＭＲ信号の減少に基づく位相エンコード方向にかけられた傾斜磁場による位相の変化が完全にキャンセルされないという状態を回避することができる。

In the above formula, “abs ()” is a function that returns the absolute value of the real number in (). In this case, when Equation 4 is applied, it is possible to avoid a state in which the phase change due to the gradient magnetic field applied in the phase encoding direction based on the decrease of the MR signal due to the magnetic transverse relaxation is not completely canceled.

  As described above, the accumulated phase error ρ can be obtained in either of the first and second modes. Therefore, the phase error calculation units 623a and 623b perform calculation processing of the following equation to obtain the phase error G.

すなわち上式で得られた位相エラーＧは、読み出し傾斜磁場を１回かけた場合に、その読み出し方向に発生した位相エラーである。

That is, the phase error G obtained by the above equation is a phase error generated in the reading direction when the reading gradient magnetic field is applied once.

  Based on the phase error G obtained in this way, an image in which distortion has been corrected in the image correction unit 63 is generated. Specifically, first, the phase error correction unit 631 performs arithmetic processing according to the following equation.

上式中、「ｅｘｐ（）」は、自然対数の底ｅの（）乗の値を返す関数である。また、式中「ｉ」は、任意の定数である（以下、同じ）。この処理により、読み出し方向に１次元逆フーリエ変換されたＫ空間に読み出し傾斜磁場をかけた場合における、その読み出し方向に発生した位相エラーが修整される。

In the above equation, “exp ()” is a function that returns the value of the base e of the natural logarithm (). In the formula, “i” is an arbitrary constant (hereinafter the same). By this processing, the phase error generated in the reading direction when the reading gradient magnetic field is applied to the K space subjected to the one-dimensional inverse Fourier transform in the reading direction is corrected.

  Subsequently, the image generation unit 632 performs arithmetic processing according to the following expression.

すなわち、数７の式の演算結果である修正後の位相エラーＣ’_x,lを位相エンコード方向に１次元逆フーリエ変換することによって、表示装置１６に表示させるために出力部６４に出力させるべき画像データＩ_x,yを生成する。

That is, the corrected phase error C ′ _{x, l} , which is the calculation result of Equation 7, should be output to the output unit 64 for display on the display device 16 by performing one-dimensional inverse Fourier transform in the phase encoding direction. Image data I _{x, y} is generated.

  A gradient echo (GRE) EPI test was performed using the embodiment of the present invention described above. In this test, a 3T (Tesla) whole body scanner (manufactured by Simens, product name “Trio”) was used as an MRI apparatus. In order to clearly grasp the distortion generated in the MRI image, a K-space matrix size of 128 × 128 is applied. The scanning conditions are: slice thickness; 5 mm, field of view (FOV); 230 × 230 mm, echo time (TE); 50 ms, repetition time (TR); 100 ms, bandwidth: 1862 KHz.

  FIG. 6 shows an MRI image that is a test result when the standard phantom is the measurement target, and FIG. 7 shows an MRI image that is the test result when the subject's head is the measurement target. In both tests using the standard phantom and the subject's head as the measurement target, a is a gradient echo image for comparing an image without distortion, and b is a standard equipment in the MRI apparatus used in this test. An MRI image (EPI image) to which only phase correction is applied, c is an MRI image (EPI image) to which the image correction method according to the above-described embodiment of the present invention is applied in addition to the phase correction that is provided as a standard in the MRI apparatus. ) Respectively. The phase encoding direction in the EPI image is the vertical direction for both b and c.

  As shown in each figure, compared to a, where an accurate image without distortion is obtained, a readout gradient magnetic field error occurs in b, and a large distortion of the image is observed, but error correction is effective in c As a result of the function, it can be seen that an image having an almost accurate shape without distortion as in b can be obtained.

  As described above, by applying the image processing program and the image processing apparatus of the present invention to the single shot EPI method, it is possible to obtain a distortion-free MRI image (EPI image) by only one scan. This indicates that the present invention is extremely useful for obtaining an accurate MRI image in a short time.

  The present invention is not limited to the above-described embodiments and examples. In addition, the specific components of each part and each process are not limited to the above-described embodiments and examples, and various modifications can be made without departing from the gist of the present invention.

It is a block diagram which shows an example of the MRI apparatus used for one Embodiment of this invention. 2 is a basic functional block diagram of an image processing apparatus (computer) in the embodiment. FIG. It is a figure which shows an example of the more detailed functional block diagram of the image processing apparatus. It is a figure which shows an example of the more detailed functional block diagram of the image processing apparatus. It is a figure which compares the image processing procedure in the same embodiment with the conventional image processing procedure. It is a figure which shows an example of the test result which applied the embodiment and set the object as the standard phantom. It is a figure which shows an example of the test result which applied the same embodiment and made the object the head of a test subject.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... MRI apparatus 6 ... Computer (image processing apparatus)
62 ... Phase error calculation unit 63 ... Image modification unit 621a, 622b ... Phase change cancellation unit 622a, 621b ... Phase difference calculation unit 623a, 623b ... Phase error calculation unit 631 ... Phase error correction unit 632 ... Image generation unit

Claims (8)

An image processing program for configuring a magnetic resonance image based on K-space data obtained by imaging by an echo planar imaging method using a magnetic resonance imaging apparatus by operating a computer,
Based on the K-space data subjected to the one-dimensional inverse Fourier transform in the readout direction, the readout direction is calculated by canceling the phase change due to the gradient magnetic field applied in the phase encoding direction and calculating the phase difference between pixels adjacent to the readout direction. A phase error calculation step for obtaining a phase error generated in
Based on the phase error obtained in the phase error calculation step, the phase error generated in the readout direction when the readout gradient magnetic field is applied to the K space that has been subjected to the one-dimensional inverse Fourier transform in the readout direction, and the correction result An image processing program for executing an image modification process for constructing an image whose distortion has been corrected by performing one-dimensional inverse Fourier transform in the phase encoding direction. The phase error calculation step further includes
A phase change cancellation step of adding the composite data of each column of the K space data subjected to the one-dimensional inverse Fourier transform in the readout direction to cancel the phase change due to the gradient magnetic field applied in the phase encoding direction;
Using the calculation result in the phase change canceling step, a phase difference calculating step of calculating a phase difference between pixels adjacent in the reading direction;
The image processing program according to claim 1, further comprising: a phase error calculation step of calculating a phase error generated in the read direction when the read gradient magnetic field is applied once based on a calculation result of the phase difference calculation step. . The phase error calculation step further includes
A phase difference calculating step of calculating a phase difference between pixels adjacent to the readout direction of each readout line of the K space data subjected to one-dimensional inverse Fourier transform in the readout direction;
Based on the calculation result of the phase difference calculation step, the phase change due to the gradient magnetic field applied in the phase encoding direction is performed by adding the composite data having information on the phase difference between adjacent pixels in the readout direction of each readout line. A phase change canceling process for canceling,
The image processing program according to claim 1, further comprising: a phase error calculation step of calculating a phase error generated in the reading direction when the reading gradient magnetic field is applied once based on a calculation result in the phase change canceling step. .   In the phase error calculation step, in addition to canceling the phase change, a process of concentrating the phase change due to the linear error of the gradient magnetic field applied in the readout direction to the state where the readout gradient magnetic field of the central readout line is applied is also executed. The image processing program according to claim 2 or 3. An image processing apparatus for constructing a magnetic resonance image based on K-space data obtained from imaging by an echo planar imaging method using a magnetic resonance imaging apparatus,
Based on the K-space data subjected to the one-dimensional inverse Fourier transform in the readout direction, the readout direction is calculated by canceling the phase change due to the gradient magnetic field applied in the phase encoding direction and calculating the phase difference between pixels adjacent to the readout direction. A phase error calculation unit for obtaining a phase error generated in
Based on the phase error obtained by the phase error calculator, the phase error generated in the readout direction when the readout gradient magnetic field is applied to the K space subjected to the one-dimensional inverse Fourier transform in the readout direction is corrected and the correction result is obtained. An image processing apparatus comprising: an image modifying unit that forms an image whose distortion is modified by performing a one-dimensional inverse Fourier transform in a phase encoding direction. The phase error calculation unit further includes:
A phase change canceling unit that cancels a phase change caused by a gradient magnetic field applied in the phase encoding direction by adding the composite data of each column of the K space data subjected to the one-dimensional inverse Fourier transform in the readout direction;
Using the calculation result by the phase change cancellation unit, a phase difference calculation unit that calculates a phase difference between pixels adjacent in the readout direction;
The image processing apparatus according to claim 5, further comprising: a phase error calculation unit that calculates a phase error generated in the reading direction when the reading gradient magnetic field is applied once based on a calculation result by the phase difference calculation unit. . The phase error calculation unit further includes:
A phase difference calculation unit that calculates a phase difference between pixels adjacent to each other in the readout direction of each readout line of the K space data subjected to one-dimensional inverse Fourier transform in the readout direction;
Based on the calculation result of the phase difference calculation unit, the phase change due to the gradient magnetic field applied in the phase encoding direction by adding the composite data having information on the phase difference between adjacent pixels in the readout direction of each readout line A phase change canceling unit for canceling,
The image processing apparatus according to claim 5, further comprising: a phase error calculation unit that calculates a phase error generated in the reading direction when the reading gradient magnetic field is applied once based on a calculation result by the phase change canceling unit. . The phase error calculation unit also executes a process of concentrating the phase change due to the linear error of the gradient magnetic field applied in the reading direction to the state where the read gradient magnetic field of the central read line is applied, in addition to the cancellation of the phase change. Item 8. The image processing apparatus according to Item 6 or 7.

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