JP4418264B2 - Magnetic resonance imaging apparatus and magnetic resonance image correction method - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance image correction method.

  2. Description of the Related Art A magnetic resonance imaging (MRI) apparatus is known as an apparatus that can take a tomographic image of a subject by using a nuclear magnetic resonance (NMR) phenomenon. Magnetic resonance imaging apparatuses are used in various fields such as medical applications and industrial applications.

  When taking a tomographic image of a subject using a magnetic resonance imaging apparatus, first, the subject is placed in an imaging space where a static magnetic field is formed, and the direction of spin of protons in the subject is determined statically. Align in the direction of the magnetic field to obtain the magnetization vector. Thereafter, by applying a high-frequency magnetic field and a gradient magnetic field, a nuclear magnetic resonance phenomenon is generated to change the magnetization vector of the proton, and a magnetic resonance signal from the proton that returns to the original magnetization vector is received. Then, the magnetic resonance imaging apparatus generates a tomographic image of the subject based on the received magnetic resonance signal.

  In a magnetic resonance imaging apparatus, gradient magnetic fields are applied in three directions of a slice axis direction, a frequency encoding direction, and a phase encoding direction in order to add information on a measurement position to a magnetic resonance signal. The gradient magnetic field is applied by the gradient coil so that the gradient is linear, and information on the measurement position is added to the magnetic resonance signal. However, in the gradient magnetic field, since the gradient coil is limited in shape due to the gradient coil being arranged in a limited narrow space, the linearity of the gradient may deteriorate and the gradient may become nonlinear. In particular, the linearity of the peripheral part of the gradient magnetic field is worse than that of the central part. In this case, position information different from the set value is added to the magnetic resonance signal due to the non-linear gradient of the gradient magnetic field, so that the generated image is distorted and the image quality is degraded. was there.

Conventionally, various methods have been proposed for correcting the occurrence of distortion in an image due to a non-linear gradient of the gradient magnetic field (see, for example, Patent Document 1).
JP 09-122102 A

FIG. 6 is a diagram for explaining that an image is distorted due to a non-linear gradient of the gradient magnetic field and that an image having the distortion is corrected. In FIG. 6, the vertical axis indicates the magnetic field strength G of the gradient magnetic field, the horizontal axis indicates the position x of the gradient magnetic field, and the center of the gradient magnetic field is 0. In FIG. 6, the broken line is setting data G (X) = G ₀ · X for forming a linear gradient in the gradient magnetic field, and the solid line is a nonlinear gradient formed based on the setting data. Actual measurement data J (X).

As shown in FIG. 6, when the imaging region FOV _X0 = 2 · X ₀ [cm] in the position from −X ₀ to + X ₀ is set, the gradient magnetic field G ₀ [G / cm] is expressed by the equation (1). Therefore, ideally, a magnetic resonance signal in the reception band γBW ₀ [kHz] from −γB ₀ to + γB ₀ is received. In Equation (1), γ is a magnetic rotation ratio [kHz / G], and is expressed as γ = 4.257559.

However, as indicated by a solid line and a broken line in FIG. 6, the measured data J (X) of the gradient magnetic field is different from the setting data G (X). Therefore, actually, magnetic resonance signals corresponding to the imaging region FOV _X1 from −X ₁ to + X ₁ are received, and magnetic resonance signals corresponding to −X ₀ to −X ₁ and + X ₀ to + X ₁ are received. Is not received. Nevertheless, it is determined to have been ideally received in the imaging area FOV _X0 positions from -X ₀ to + X _0, for example, a magnetic resonance signal corresponding magnetic resonance signals, the + X ₀ corresponding to + X1 Since the image is reconstructed by using this as an image, an image having pixel positions different from the actual one is generated, and the image is distorted.

Therefore, conventionally, measured data J (X) of a non-linear gradient magnetic field formed based on setting data G (X) for forming a linear gradient magnetic field is measured in advance, and the gradient setting data G ( Based on X) and the actual measurement data J (X), the position information of the pixel data of the image in which the distortion has occurred is corrected to the actual position. For example, a magnetic resonance signal erroneously recognized as corresponding to + X ₀ is corrected as a magnetic resonance signal corresponding to + X ₁ which is the correct position, and an image with corrected distortion is generated.

However, although the imaging region FOV _X0 at the position from −X ₀ to + X ₀ is set, magnetic resonance signals corresponding to −X ₀ to −X ₁ and + X ₀ to + X ₁ are received. Therefore, only an image of a narrow shooting area is generated, and there is a missing portion in the image of the desired shooting area. Therefore, in the past, image distortion was corrected, but it was difficult to generate an accurate image of the imaging region, and image quality was degraded.

  Accordingly, an object of the present invention is to correct image distortion and to easily generate an accurate image of an image area without losing an image in a desired image area, thereby improving image quality. It is an object of the present invention to provide a magnetic resonance imaging apparatus and a magnetic resonance image correction method capable of performing the above.

  To achieve the above object, the magnetic resonance imaging apparatus of the present invention forms a gradient magnetic field and a high-frequency magnetic field in an imaging space in which a subject is accommodated and a static magnetic field is formed, and is based on a magnetic resonance signal from the subject. A magnetic resonance imaging apparatus for generating a tomographic image of the first imaging region of the subject, the storage means for storing the relationship between the setting data of the gradient magnetic field and the measured data corresponding to the setting data, and the storage Based on the relationship between the setting data of the gradient magnetic field stored by the means and the measured data, a second imaging area wider than the first imaging area is set, and the gradient is set so as to correspond to the first imaging area. Setting change means for changing the first setting value for setting a magnetic field to a second setting value for setting the gradient magnetic field so as to correspond to the second imaging region; and the magnetism corresponding to the first setting value. A first image generating means for acquiring a resonance signal as a first magnetic resonance signal and generating a first image based on the first magnetic resonance signal; and the magnetic resonance signal corresponding to the second set value being a second magnetic signal. Based on the relationship between the second image generation means that obtains a resonance signal and generates a second image based on the second magnetic resonance signal, and the setting data of the gradient magnetic field stored in the storage means and the measured data Based on the relationship between the first correction means for correcting the position of the pixel data of the first image and obtaining a third image, and the setting data of the gradient magnetic field stored in the storage means and the measured data, Second correction means for correcting the position of pixel data of the second image to obtain a fourth image; and a defect for calculating a defect region in which pixel data is missing in an area corresponding to the first set value of the third image. Area calculation means An image extracting means for extracting an area corresponding to the first set value in the fourth image as a fifth image and extracting an area corresponding to the missing area in the fifth image as a sixth image; Image synthesizing means for synthesizing the image and the sixth image to generate the tomographic image of the first imaging region;

  According to the magnetic resonance imaging apparatus of the present invention, the storage means stores the relationship between the gradient magnetic field setting data and the actual measurement data corresponding to the setting data. The setting changing unit sets a second imaging region wider than the first imaging region based on the relationship between the gradient magnetic field setting data stored in the storage unit and the measured data, so as to correspond to the first imaging region. The first setting value for setting the gradient magnetic field is changed to the second setting value for setting the gradient magnetic field so as to correspond to the second imaging region. Then, the first image generation means acquires a magnetic resonance signal corresponding to the first set value as the first magnetic resonance signal, and generates a first image based on the first magnetic resonance signal. The second image generation means acquires a magnetic resonance signal corresponding to the second set value as the second magnetic resonance signal, and generates a second image based on the second magnetic resonance signal. The first correction unit corrects the position of the pixel data of the first image based on the relationship between the gradient magnetic field setting data stored in the storage unit and the actual measurement data to obtain a third image. The second correction unit corrects the position of the pixel data of the second image based on the relationship between the gradient magnetic field setting data stored in the storage unit and the actual measurement data, thereby obtaining a fourth image. Then, the missing area calculation unit calculates a missing area in which pixel data is missing in an area corresponding to the first set value of the third image. And an image extraction means extracts the area | region corresponding to a 1st setting value in a 4th image as a 5th image, and extracts the area | region corresponding to a defect | deletion area | region in a 5th image as a 6th image. The image synthesizing unit synthesizes the third image and the sixth image to generate a tomographic image of the first imaging region.

  To achieve the above object, the magnetic resonance image correction method of the present invention forms a gradient magnetic field and a high-frequency magnetic field in an imaging space in which a subject is accommodated and a static magnetic field is formed, and is based on a magnetic resonance signal from the subject. A magnetic resonance image correction method for correcting a tomographic image of the first imaging region of the subject generated in accordance with a relationship between setting data of the gradient magnetic field and actual measurement data corresponding to the setting data, Based on the relationship between the setting data of the gradient magnetic field and the measured data, a second imaging region wider than the first imaging region is set, and the gradient magnetic field is set so as to correspond to the first imaging region A first step of changing the first set value to a second set value for setting the gradient magnetic field so as to correspond to the second imaging region, and the magnetic resonance signal corresponding to the first set value to the first magnetism As a resonance signal A second step of acquiring and generating a first image based on the first magnetic resonance signal; acquiring the magnetic resonance signal corresponding to the second set value as a second magnetic resonance signal; and Based on the third step of generating a second image based on the signal and the relationship between the setting data of the gradient magnetic field and the measured data, the position of the pixel data of the first image is corrected to obtain a third image. A fourth step, a fifth step of correcting the position of the pixel data of the second image based on the relationship between the setting data of the gradient magnetic field and the measured data, and obtaining a fourth image; and A sixth step of calculating a missing region in which pixel data is missing in a region corresponding to the first set value; and extracting a region corresponding to the first set value in the fourth image as a fifth image; 5th image A seventh step of extracting a region corresponding to the missing region as a sixth image, and an eighth step of combining the third image and the sixth image to form the tomographic image of the first imaging region. .

  According to the magnetic resonance image correction method of the present invention described above, in the first step, a second imaging region wider than the first imaging region is set based on the relationship between the gradient magnetic field setting data and the measured data, The first setting value for setting the gradient magnetic field so as to correspond to one imaging region is changed to the second setting value for setting the gradient magnetic field so as to correspond to the second imaging region. In the second step, the magnetic resonance signal corresponding to the first set value is acquired as the first magnetic resonance signal, and a first image is generated based on the first magnetic resonance signal. In a third step, a magnetic resonance signal corresponding to the second set value is acquired as a second magnetic resonance signal, and a second image is generated based on the second magnetic resonance signal. In the fourth step, the position of the pixel data of the first image is corrected based on the relationship between the gradient magnetic field setting data and the actual measurement data, and a third image is obtained. Then, in the fifth step, the position of the pixel data of the second image is corrected based on the relationship between the gradient magnetic field setting data and the actual measurement data to obtain a fourth image. In a sixth step, a missing area where pixel data is missing in the area corresponding to the first set value of the third image is calculated. In a seventh step, an area corresponding to the first set value in the fourth image is extracted as the fifth image, and an area corresponding to the missing area in the fifth image is extracted as the sixth image. In the eighth step, the third image and the sixth image are combined to form a tomographic image of the first imaging region.

  According to the present invention, it is possible to correct image distortion and easily generate an accurate image of an imaging region without losing an image in a desired imaging region, thereby improving image quality. A magnetic resonance imaging apparatus and a magnetic resonance image correction method can be provided.

  Hereinafter, an example of an embodiment according to the present invention will be described with reference to the drawings.

  FIG. 1 is a configuration diagram showing the configuration of the magnetic resonance imaging apparatus of the present embodiment.

  As shown in FIG. 1, the magnetic resonance imaging apparatus includes a static magnetic field magnet unit 12, a gradient coil unit 13, an RF coil unit 14, an RF drive unit 22, a gradient drive unit 23, a data collection unit 24, The control unit 25, the cradle 26, the data processing unit 31, the operation unit 32, and the display unit 33 are included.

  Hereinafter, each component will be sequentially described.

  The static magnetic field magnet unit 12 is provided to form a static magnetic field in the imaging space 11 in which the subject is accommodated. The static magnetic field magnet unit 12 is configured such that the direction of the static magnetic field is along a direction Z perpendicular to the body axis direction of the subject 40.

  The gradient coil unit 13 forms a gradient magnetic field in the imaging space 11 in which a static magnetic field is formed. The gradient coil unit 13 forms a gradient magnetic field in the imaging space 11 in which a static magnetic field is formed, and adds position information to the magnetic resonance signal received by the RF coil unit 14. The gradient coil unit 13 includes a first gradient coil unit that forms a frequency encode gradient magnetic field, a second gradient coil unit that forms a phase encode gradient magnetic field, and a third gradient coil unit that forms a slice selection gradient magnetic field. Has 3 systems.

  As shown in FIG. 1, the RF coil unit 14 is disposed so as to surround the imaging region of the subject 40, and is configured to be used for both transmission and reception. The RF coil unit 14 transmits an RF signal, which is an electromagnetic wave, in order to excite the spin of protons in the imaging region of the subject 40 in the imaging space 11 in which the static magnetic field is formed by the static magnetic field magnet unit 12 to generate a high frequency. Create a magnetic field. The RF coil unit 14 receives electromagnetic waves generated from the excited protons in the subject 40 as magnetic resonance signals. In addition, although the RF coil unit 14 is used for both transmission and reception in the present embodiment, the transmission coil and the reception coil may be provided independently.

  The RF drive unit 22 drives the RF coil unit 14 to form a high-frequency magnetic field in the imaging space 11, and includes a gate modulator (not shown), an RF power amplifier (not shown), and an RF oscillator (not shown). Have Based on the control signal from the control unit 25, the RF drive unit 22 modulates the RF signal from the RF oscillator into a signal having a predetermined timing and a predetermined envelope using a gate modulator. The RF signal modulated by the gate modulator is amplified by the RF power amplifier and then output to the RF coil unit 14.

  The gradient driving unit 23 applies a gradient pulse to the gradient coil unit 13 based on a control signal from the control unit 25 and drives it to generate a gradient magnetic field in the imaging space 11 in which a static magnetic field is formed. The gradient drive unit 23 includes three systems of drive circuits (not shown) corresponding to the three systems of gradient coil units 13.

  The data collection unit 24 collects a magnetic resonance signal received by the RF coil unit 14 based on a control signal from the control unit 25, and a phase detector (not shown) and an analog / digital converter (not shown). And have. The phase detector phase-detects the magnetic resonance signal received by the RF coil unit 14 using the output of the RF oscillator of the RF drive unit 22 as a reference signal, and outputs it to the analog / digital converter. The analog / digital converter converts the analog magnetic resonance signal output from the phase detector into a digital signal and outputs the digital signal to the data processing unit 31.

  The control unit 25 is configured by a computer and controls the RF drive unit 22, the gradient drive unit 23, and the data collection unit 24 based on operation signals input from the operation unit 32 via the data processing unit 31, respectively. Output signal and control. The control unit 25 receives an operation signal based on a predetermined pulse sequence from the operation unit 32 via the data processing unit 31, and sends a control signal to the RF drive unit 22, the gradient drive unit 23, and the data collection unit 24 based on the operation signal. Output the magnetic resonance signal.

  The cradle 26 is a table on which the subject 40 is placed, and can be taken in and out of the imaging space 11 by a cradle driving unit (not shown).

  The data processing unit 31 is configured by a computer. The data processing unit 31 is connected to the operation unit 32 and receives an operation signal from the operation unit 32. The data processing unit 31 is connected to the control unit 25 and outputs an operation signal input to the operation unit 32 by the operator to the control unit 25. The data processing unit 31 is connected to the data collecting unit 24, obtains a magnetic resonance signal collected and output by the data collecting unit 24, performs image processing on the acquired magnetic resonance signal, and outputs image data. Is generated. Then, the data processing unit 31 outputs the generated image data to the display unit 33.

  FIG. 2 is a configuration diagram showing the configuration of the data processing unit 31.

  As shown in FIG. 2, the data processing unit 31 includes a storage unit 301, a setting change unit 302, a first image generation unit 303, a second image generation unit 304, a first correction unit 305, and a second correction. A unit 306, a missing area calculation unit 307, an image extraction unit 308, and an image composition unit 309.

  The storage unit 301 includes a memory, and stores the relationship between the gradient magnetic field setting data and the actual measurement data corresponding to the setting data. The storage unit 301 is connected to a measurement unit (not shown) that measures the relationship between the gradient magnetic field setting data and the actual measurement data corresponding to the setting data, and stores the measurement results of the measurement unit. The storage unit 301 stores the relationship between the gradient magnetic field setting data and the actually measured data corresponding to the setting data in association with the frequency encoding direction, the phase encoding direction, and the slice selection direction.

  The setting changing unit 302 receives an operation signal corresponding to a first set value for setting the first imaging region after the first imaging region as a desired imaging region is input to the operation unit 32 by the operator. Receive from 32. The setting changing unit 302 sets a second imaging region wider than the first imaging region based on the relationship between the gradient magnetic field setting data stored in the storage unit 301 and the measured data, and corresponds to the first imaging region. Thus, the first setting value for setting the gradient magnetic field is changed to the second setting value for setting the gradient magnetic field so as to correspond to the second imaging region. Here, the setting changing unit 302 changes the first setting value in the frequency encoding direction to the second setting value based on the relationship in the frequency encoding direction between the gradient magnetic field setting data stored in the storage unit 301 and the actual measurement data. . The setting changing unit 302 changes the first setting value in the phase encoding direction to the second setting value based on the relationship between the gradient magnetic field setting data stored in the storage unit 301 and the measured data in the phase encoding direction. Then, the setting changing unit 302 generates an operation signal corresponding to the second set value and outputs the operation signal to the control unit 25, and controls the gradient driving unit 23 based on the operation signal to acquire a magnetic resonance signal.

  The first image generation unit 303 first outputs a magnetic resonance signal corresponding to a first set value in which a gradient magnetic field is set so as to correspond to the first imaging region among the magnetic resonance signals output from the data collection unit 24. Acquired as a magnetic resonance signal. Here, the first image generation unit 303 extracts the magnetic resonance signal corresponding to the first set value from the magnetic resonance signal in the gradient magnetic field formed based on the second set value, and obtains it as the first magnetic resonance signal. To do. That is, the first image generation unit 303 thins out the magnetic resonance signals collected by the data collection unit 24 corresponding to the second imaging region, and only the magnetic resonance signal corresponding to the first imaging region that is a desired imaging region is stored in the first. Acquired as one magnetic resonance signal. And the 1st image generation part 303 produces | generates a 1st image based on the acquired 1st magnetic resonance signal.

  The second image generation unit 304 outputs a second magnetic resonance signal corresponding to a second set value in which the gradient magnetic field is set so as to correspond to the second imaging region among the magnetic resonance signals output from the data collection unit 24. Acquired as a magnetic resonance signal. Then, the second image generation unit 304 generates a second image based on the acquired second magnetic resonance signal.

  The first correction unit 305 corrects the position of the pixel data of the first image based on the relationship between the gradient magnetic field setting data stored in the storage unit 301 and the actual measurement data to obtain a third image.

  The second correction unit 306 corrects the position of the pixel data of the second image based on the relationship between the gradient magnetic field setting data stored in the storage unit 301 and the actual measurement data to obtain a fourth image.

  The missing area calculation unit 307 calculates a missing area in which pixel data is missing in an area corresponding to the first set value of the third image that is corrected and generated by the first correction unit 305.

  The image extraction unit 308 extracts a region corresponding to the first setting value as a fifth image in the fourth image that is corrected and generated by the second correction unit 306. Then, the fifth image area corresponding to the defect area calculated by the defect area calculation unit 307 is extracted as the sixth image.

  The image synthesis unit 309 synthesizes the third image corrected and generated by the first correction unit 305 and the sixth image extracted by the image extraction unit 308 to generate a tomographic image of the first imaging region.

  The operation unit 32 is configured by operation devices such as a keyboard and a mouse. The operation unit 32 is operated by an operator and outputs an operation signal corresponding to the operation to the data processing unit 31.

  The display unit 33 is configured by a display device such as a graphic display. The display unit 33 displays a tomographic image of the subject generated based on the magnetic resonance signal from the subject 40. Here, the display unit 33 acquires image data from the data processing unit 31 and displays a tomographic image based on the image data.

  In the present embodiment, the display unit 33 corresponds to the display unit of the present invention. The storage unit 301 of the present embodiment corresponds to the storage unit of the present invention. The setting change unit 302 of this embodiment corresponds to the setting change unit of the present invention. The first image generation unit 303 of the present embodiment corresponds to the first image generation unit of the present invention. The second image generation unit 304 of the present embodiment corresponds to the second image generation unit of the present invention. The first correction unit 305 of the present embodiment corresponds to the first correction unit of the present invention. The second correction unit 306 of the present embodiment corresponds to the second correction unit of the present invention. Further, the missing area calculation unit 307 of the present embodiment corresponds to the missing area calculation means of the present invention. The image extraction unit 308 of the present embodiment corresponds to the image extraction unit of the present invention. The image composition unit 309 of the present embodiment corresponds to the image composition unit of the present invention.

  The operation of the magnetic resonance imaging apparatus in the present embodiment will be described below.

  FIG. 3 is a flowchart showing the operation of the magnetic resonance imaging apparatus of the present embodiment. FIG. 4 is a diagram showing an image generated at each step shown in FIG.

  Prior to the steps shown in FIGS. 3 and 4, the subject 40 is placed on the cradle 26, and the RF coil unit 14 is installed in the imaging region of the subject 40. Then, desired photographing information is input to the operation unit 32 by the operator. For example, a pulse sequence based on the spin echo method and imaging information of a first imaging area that is a desired imaging area are input to the operation unit 32 by the operator.

  Next, as shown in FIG. 3, a setting change step (ST11) is performed. In the setting change step (ST11), a second imaging area wider than the first imaging area is set based on the relationship between the gradient magnetic field setting data and the measured data, and the gradient magnetic field is set so as to correspond to the first imaging area. Is changed to a second setting value for setting the gradient magnetic field so as to correspond to the second imaging region. First, in this step, after a first imaging area, which is a desired imaging area, is input to the operation unit 32 by an operator, an operation signal corresponding to a first setting value for setting the first imaging area is operated. Received from part 32. In this step, the setting changer 302 sets a second imaging region wider than the first imaging region based on the relationship between the gradient magnetic field setting data stored in the storage unit 301 and the measured data. In this step, the setting change unit 302 changes the first setting value for setting the gradient magnetic field so as to correspond to the first imaging region to the second setting value for setting the gradient magnetic field so as to correspond to the second imaging region. Will change. Here, in this step, the first setting value in the frequency encoding direction is changed to the second setting value based on the relationship between the gradient magnetic field setting data stored in the storage unit 301 and the measured data in the frequency encoding direction. 302 changes. The setting change unit 302 changes the first setting value in the phase encoding direction to the second setting value based on the relationship in the phase encoding direction between the gradient magnetic field setting data and the measured data stored in the storage unit 301. In this step, the setting changing unit 302 generates an operation signal corresponding to the second set value and outputs the operation signal to the control unit 25. Based on this, the gradient driving unit 23 is controlled to acquire a magnetic resonance signal.

FIG. 5 illustrates a method for setting a second imaging region wider than the first imaging region, which is a desired imaging region, based on the relationship between the gradient magnetic field setting data and the measured data in the setting change step. FIG. In FIG. 5, the vertical axis indicates the magnetic field strength G of the gradient magnetic field, the horizontal axis indicates the position X of the gradient magnetic field, and the center of the gradient magnetic field is 0. In FIG. 5, the broken line represents linear setting data G (X) = G ₀ · X for forming a gradient magnetic field corresponding to the first imaging region, and the solid line represents the setting data G (X). It is the actual measurement data J (X) of the non-linear gradient formed based on it.

As shown in FIG. 5, when the first imaging area, which is a desired imaging area, is an imaging area FOV _X0 at a position from −X ₀ to + X ₀ , the reception band γBW as shown in the above-described equation (1). _{A zero} magnetic resonance signal is received.

However, as indicated by a solid line and a broken line in FIG. 5, the measured data J (X) of the gradient magnetic field is different from the setting data G (X). Therefore, actually, magnetic resonance signals corresponding to the imaging region FOV _X1 from −X ₁ to + X ₁ are received, and magnetic resonance signals corresponding to −X ₀ to −X ₁ and + X ₀ to + X ₁ are received. Is not received. Nevertheless, it is determined to have been ideally received in the imaging area FOV _X0 positions from -X ₀ to + X _0, for example, a magnetic resonance signal corresponding magnetic resonance signals, the + X ₀ corresponding to + X1 Since the image is reconstructed by using this as an image, an image having a pixel position different from the actual one is generated, and the image is distorted.

Therefore, in this step, a second imaging region FOV _X2 wider than the first imaging region FOV _X0 is set based on the relationship between the gradient magnetic field setting data G (X) and the actual measurement data J (X). That is, in this step, the first imaging area FOV _X0 at the position from −X ₀ to + X ₀ is set to the second imaging area FOV _X2 at the position from −X ₂ to + X ₂ . In this step, the second imaging region FOV _X2 is obtained as shown in Equation (2). Here, in the position _{X 0} of the end portion of the first imaging area _{FOV X0,} since the actual gradient field is J _{(X 0),} the setting data _{G (X) = G 0 ·} X, J (X 0 by obtaining the position corresponding to), it is possible to set the position X ₂ of the end portion of the second imaging area FOV _X2.

In this way, the reception band γBW ₂ when the second imaging region FOV _X2 is set can be calculated as shown in Equation (3).

For this reason, in this step, the setting of the frequency encoding direction is changed so as to correspond to the reception band γBW ₂ in the second imaging region FOV _X2 . Here, the time integration value at the time of reading the gradient magnetic field in the frequency encoding direction corresponding to the second imaging region is set to be the same as that in the first imaging region. Then, the second setting value in the frequency encoding direction corresponding to the second imaging region is obtained so that the number of sampling is increased at a high frequency and the sampling interval is denser than the first setting value in the frequency encoding direction in the first imaging region. .

  In this step, the second set value of the gradient magnetic field in the phase encoding direction corresponding to the second imaging region is further obtained. The amplitude Gy (n) of the gradient magnetic field in the phase encoding direction has a relationship expressed by Equation (4) corresponding to each stage n (n = −128, −127,..., 127, 128) of the phase encoding step. . T indicates the time width of the phase encode pulse.

Here, as in the case of the frequency encoding direction described above, the position X ₂ of the second imaging region with respect to the position X ₀ of the first imaging region is also obtained in the phase encoding direction. (5) and Equation (6) are established. Here, P is the number of samplings corresponding to the first imaging region, and ΔP is the number of samplings that change when the setting is changed from the first imaging region to the second imaging region.

If each Gy (n) is set to the maximum value Gy _max , Equation (7) is established.

  Therefore, ΔP is calculated as shown by the following formula (8) from the formula (5), formula (6), and formula (7).

  Further, the interval ΔGy of each step of the amplitude Gy (n) of the gradient magnetic field in the phase encoding direction is calculated from the equation (1) as shown by the following equation (9).

  In this way, in this step, the second setting value in the phase encoding direction corresponding to the second imaging region is set so that the number of steps of ΔP is increased at intervals of the gradient of ΔGy.

  As described above, in this step, the first setting value in the frequency encoding direction is set to the second setting value based on the relationship between the gradient magnetic field setting data stored in the storage unit 301 and the measured data in the frequency encoding direction. The change unit 302 changes. Then, based on the relationship between the gradient magnetic field setting data stored in the storage unit 301 and the actual measurement data in the phase encoding direction, the setting changing unit 302 changes the first setting value in the phase encoding direction to the second setting value.

  In this step, the setting changing unit 302 generates an operation signal corresponding to the second set value and outputs the operation signal to the control unit 25. Based on this, the gradient driving unit 23 is controlled, and the data collecting unit 24 is magnetically operated. Resonance signals are collected. The magnetic resonance signals collected by the data collection unit 24 are output to the data processing unit 31 and arranged in the k space as shown in FIG.

  Next, as shown in FIG. 3, a first image generation step (ST21) is performed. In the first image generation step (ST21), the magnetic resonance signal corresponding to the first set value is acquired as the first magnetic resonance signal, and the first image I1 is generated based on the first magnetic resonance signal. Here, among the magnetic resonance signals output from the data collection unit 24, the magnetic resonance signal corresponding to the first set value is acquired as the first magnetic resonance signal. That is, as shown in FIG. 4B, only the magnetic resonance signals corresponding to the first imaging region which is a desired imaging region are obtained by thinning out the magnetic resonance signals collected by the data collection unit 24 corresponding to the second imaging region. Is acquired by the first image generation unit 303 as the first magnetic resonance signal. Thereafter, as shown in FIG. 4C, the first image generation unit 303 generates a first image I1 based on the acquired first magnetic resonance signal.

  Next, a second image generation step (ST31) is performed. In the second image generation step (ST31), a magnetic resonance signal corresponding to the second set value is acquired as a second magnetic resonance signal, and a second image I2 is generated based on the second magnetic resonance signal. Here, as shown in FIG. 4C, the second image generation unit 304 acquires the magnetic resonance signals that the data acquisition unit 24 acquires corresponding to the second imaging region, and the acquired second magnetic resonance signals. Based on this, the second image generation unit 303 generates the second image I2.

Next, a first correction step (ST41) is performed. In the first correction step (ST41), the position of the pixel data of the first image I1 is corrected based on the relationship between the gradient magnetic field setting data and the actual measurement data to obtain a third image I3. In this step, as described above, based on the gradient magnetic field setting data G (X) and the actual measurement data J (X), the position information of the pixel data of the image in which the distortion is generated is corrected to the actual position. To do. As shown in FIG. 5, for example, to correct the magnetic resonance signals incorrectly recognized as corresponding to the + X _2, as a magnetic resonance signal corresponding to the + X ₀ which is at a precise location. In this way, as shown in FIG. 4E, a third image I3 that is an image in which the distortion is corrected is generated from the first image I1 that includes the distortion.

  Next, a second correction step (ST51) is performed. In the second correction step (ST51), the position of the pixel data of the second image I2 is corrected based on the relationship between the gradient magnetic field setting data and the actual measurement data to obtain a fourth image I4. Also in this step, as in the first correction step, the position information of the pixel data of the image in which the distortion is actually generated is based on the gradient magnetic field setting data G (X) and the actual measurement data J (X). Correct the position. In this way, as shown in FIG. 4F, a fourth image I4 that is an image in which the distortion is corrected is generated from the second image I2 that includes the distortion.

  Next, a missing area calculation step (ST61) is performed. In the missing area calculation step (ST61), a missing area A1 in which pixel data is missing in the area corresponding to the first set value of the third image is calculated. As shown in FIG. 4E, the third image I3, which is an image whose distortion has been corrected, is corrected in the position information of the pixel data, and thus is displayed in the display area corresponding to the first set value. , There is a missing area A in which pixel data is missing. For this reason, the missing area calculation unit 307 is used to calculate the missing area A1 of the pixel data in the display area displayed corresponding to the first set value by comparing the display area with the third image I3. .

  Next, an image extraction step (ST71) is performed. In the image extraction step (ST71), an area corresponding to the first set value is extracted as the fifth image I5 in the fourth image I4, and an area corresponding to the missing area A is extracted as the sixth image I6 in the fifth image I5. To do. Here, first, as shown in FIG. 4F, a region A2 corresponding to the first set value is cut out in the fourth image I4, and the cut-out region A2 is cut into the fifth image as shown in FIG. 4G. Extract as I5. Thereafter, as shown in FIG. 4H, an area corresponding to the defect area A1 calculated in the defect area calculation step (ST61) and the fifth image I5 extracted in this step is defined as a sixth image I6. Extract.

  Next, an image composition step (ST81) is performed. In the image composition step (ST81), as shown in FIG. 4 (I), the third image I3 and the sixth image I6 are synthesized to obtain a tomographic image I in the first imaging region.

  Next, an image display step (ST91) is performed. In the image display step (ST91), the tomographic image I of the first imaging region synthesized in the image synthesis step (ST81) is converted into display data and displayed on the display unit 33.

  In the present embodiment, the setting change step ST11 corresponds to the first step of the present invention. Further, the first image generation step ST21 of the present embodiment corresponds to the second step of the present invention. Further, the second image generation step ST31 of the present embodiment corresponds to the third step of the present invention. Further, the first correction step ST41 of the present embodiment corresponds to the fourth step of the present invention. Further, the second correction step ST51 of the present embodiment corresponds to the fifth step of the present invention. Further, the missing area calculation step ST61 of the present embodiment corresponds to the sixth step of the present invention. Further, the image extraction step ST71 of the present embodiment corresponds to the seventh step of the present invention. Further, the image composition step ST81 of the present embodiment corresponds to the eighth step of the present invention. Further, the image display step ST91 of the present embodiment corresponds to the ninth step of the present invention.

  As described above, in the present embodiment, first, based on the relationship between the gradient magnetic field setting data G (X) and the actual measurement data J (X), the second imaging area wider than the first imaging area, which is a desired imaging area. Two imaging regions are set, and the first setting value for setting the gradient magnetic field so as to correspond to the first imaging region is changed to the second setting value for setting the gradient magnetic field so as to correspond to the second imaging region. Then, the first image I1 is generated based on the magnetic resonance signal corresponding to the first set value, and the second image I2 is generated based on the magnetic resonance signal corresponding to the second set value. Thereafter, based on the relationship between the gradient magnetic field setting data and the actual measurement data, the positions of the pixel data of the first image I1 and the second image I2 are corrected, and the third image I3 and the fourth image I4 are respectively corrected. obtain. Here, in the third image I3, a missing area A1 in which pixel data is missing in the area corresponding to the first set value is calculated. On the other hand, in the fourth image I4, the region corresponding to the first set value is extracted as the fifth image I5, and the region corresponding to the missing region A1 in the fifth image I5 is extracted as the sixth image I6. Thereafter, the third image I3 and the sixth image I6 are combined to form a tomographic image I in the first imaging region. As described above, in the present embodiment, since the position information of the image is corrected, there is no distortion in the third image I3, and the defect area of the third image I3 that occurs due to the correction of the distortion of the image is represented by the sixth image I6. I make up for it. For this reason, the present embodiment corrects image distortion and can easily generate an accurate image of the shooting area without losing the image in the image of the desired shooting area, thereby improving the image quality. it can.

  In implementing the present invention, the present invention is not limited to the above-described embodiment, and various modifications can be employed.

  For example, in the above embodiment, the setting changing unit changes the first setting value to the second setting value in both the frequency encoding direction and the phase encoding direction, but may be either one.

FIG. 1 is a configuration diagram showing the configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention. FIG. 2 is a configuration diagram showing the configuration of the data processing unit in the magnetic resonance imaging apparatus according to the embodiment of the present invention. FIG. 3 is a flowchart showing the operation of the magnetic resonance imaging apparatus according to the embodiment of the present invention. FIG. 4 is a diagram showing an image generated at each step of the operation in the magnetic resonance imaging apparatus according to the embodiment of the present invention. FIG. 5 illustrates a method of setting a second imaging region wider than the first imaging region based on the relationship between gradient magnetic field setting data and measured data in the magnetic resonance imaging apparatus according to the embodiment of the present invention. FIG. FIG. 6 is a diagram for explaining that an image is distorted due to a non-linear gradient of the gradient magnetic field and that an image having the distortion is corrected.

Explanation of symbols

11: Imaging space 12: Static magnetic field magnet unit 13: Gradient coil unit 14: RF coil unit 15: Search coil unit 22: RF drive unit 23: Gradient drive unit 24: Data collection unit 25: Control unit 26: Cradle 31: Data Processing unit 32: operation unit 33: display unit (display means)
301: Storage unit (storage means)
302: Setting change unit (setting change means)
303: First image generation unit (first image generation means)
304: Second image generation unit (second image generation means)
305: First correction unit (first correction means)
306: Second correction unit (second correction means)
307: Defect region calculation unit (defect region calculation means)
308: Image extraction unit (image extraction means)
309: Image composition unit (image composition means)

Claims (8)

A magnetic field that forms a gradient magnetic field and a high-frequency magnetic field in an imaging space in which a subject is accommodated and a static magnetic field is formed, and generates a tomographic image of the first imaging region of the subject based on a magnetic resonance signal from the subject. A resonance imaging apparatus comprising:
Storage means for storing the relationship between the setting data of the gradient magnetic field and the actual measurement data corresponding to the setting data;
A second imaging area wider than the first imaging area is set based on the relationship between the setting data of the gradient magnetic field stored in the storage means and the measured data, so as to correspond to the first imaging area. Setting changing means for changing the first setting value for setting the gradient magnetic field to a second setting value for setting the gradient magnetic field so as to correspond to the second imaging region;
First image generation means for acquiring the magnetic resonance signal corresponding to the first set value as a first magnetic resonance signal, and generating a first image based on the first magnetic resonance signal;
Second image generation means for acquiring the magnetic resonance signal corresponding to the second set value as a second magnetic resonance signal and generating a second image based on the second magnetic resonance signal;
First correction means for correcting a position of pixel data of the first image to obtain a third image based on a relationship between the setting data of the gradient magnetic field stored in the storage means and the actual measurement data;
Second correction means for correcting a position of pixel data of the second image and obtaining a fourth image based on a relationship between the setting data of the gradient magnetic field stored in the storage means and the actual measurement data;
A missing area calculating means for calculating a missing area in which pixel data is missing in an area corresponding to the first set value of the third image;
An image extracting means for extracting a region corresponding to the first set value in the fourth image as a fifth image, and extracting a region corresponding to the missing region in the fifth image as a sixth image;
An image synthesizing unit that synthesizes the third image and the sixth image to generate the tomographic image of the first imaging region. Magnetic resonance imaging apparatus. First gradient magnetic field forming means for forming the gradient magnetic field in a frequency encoding direction,
The storage means stores a relationship in the frequency encoding direction between the setting data and actual measurement data of the gradient magnetic field,
The setting changing unit is configured to set the first setting value in the frequency encoding direction to the second setting based on a relationship between the setting data of the gradient magnetic field stored in the storage unit and the measured data in the frequency encoding direction. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is changed to a value. Second gradient magnetic field forming means for forming the gradient magnetic field in a phase encoding direction,
The storage means stores a relationship in the phase encoding direction between the setting data of the gradient magnetic field and the measured data,
The setting change unit is configured to set the first setting value in the phase encoding direction to the second setting based on a relationship between the setting data of the gradient magnetic field stored in the storage unit and the measured data in the phase encoding direction. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is changed to a value. The first image generation means extracts the magnetic resonance signal corresponding to the first set value in the magnetic resonance signal in the gradient magnetic field formed based on the second set value, and uses the magnetic resonance signal as the first magnetic resonance signal. The magnetic resonance imaging apparatus according to any one of claims 1 to 3. The magnetic resonance imaging apparatus according to claim 1, further comprising display means for displaying the tomographic image of the first imaging region obtained by the image synthesis means. A gradient magnetic field and a high-frequency magnetic field are formed in an imaging space in which the subject is accommodated and a static magnetic field is formed, and a tomographic image of the first imaging region of the subject generated based on a magnetic resonance signal from the subject is obtained. A magnetic resonance image correction method for correcting based on the relationship between the setting data of the gradient magnetic field and the actual measurement data corresponding to the setting data,
Based on the relationship between the setting data of the gradient magnetic field and the measured data, a second imaging region wider than the first imaging region is set, and the gradient magnetic field is set so as to correspond to the first imaging region. A first step of changing the first set value to a second set value for setting the gradient magnetic field so as to correspond to the second imaging region;
A second step of acquiring the magnetic resonance signal corresponding to the first set value as a first magnetic resonance signal, and generating a first image based on the first magnetic resonance signal;
A third step of acquiring the magnetic resonance signal corresponding to the second set value as a second magnetic resonance signal and generating a second image based on the second magnetic resonance signal;
A fourth step of obtaining a third image by correcting the position of the pixel data of the first image based on the relationship between the setting data of the gradient magnetic field and the measured data;
A fifth step of obtaining a fourth image by correcting the position of the pixel data of the second image based on the relationship between the setting data of the gradient magnetic field and the measured data;
A sixth step of calculating a missing area in which pixel data is missing in an area corresponding to the first set value of the third image;
A seventh step of extracting a region corresponding to the first set value in the fourth image as a fifth image and extracting a region corresponding to the missing region in the fifth image as a sixth image;
An eighth step of synthesizing the third image and the sixth image to form the tomographic image of the first imaging region. In the second step, the gradient magnetic field is formed based on the second set value, and the magnetic resonance signal in the gradient magnetic field formed based on the second set value corresponds to the first set value. The magnetic resonance image correction method according to claim 6, wherein the magnetic resonance signal is extracted and acquired as the first magnetic resonance signal. The magnetic resonance image correction method according to claim 6, further comprising: a ninth step of displaying the tomographic image of the first imaging region synthesized and generated in the eighth step.

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