JP4625677B2 - Magnetic resonance imaging apparatus and image correction evaluation method - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus and an image correction evaluation method for magnetically exciting a nuclear spin of a subject with an RF signal having a Larmor frequency and reconstructing an image from a magnetic resonance signal generated along with the excitation. In particular, the present invention relates to a magnetic resonance imaging apparatus and an image correction evaluation method capable of evaluating whether or not image distortion correction due to magnetic field inhomogeneity has been appropriately performed.

  Conventionally, a magnetic resonance imaging (MRI) apparatus is used as a monitoring apparatus in a medical field.

  The MRI apparatus forms gradient magnetic fields in the X-axis, Y-axis, and Z-axis directions in an imaging region of a subject set inside a cylindrical static magnetic field magnet that forms a static magnetic field by using a gradient magnetic field coil and RF (Radio). By transmitting a radio frequency (RF) signal from a frequency coil, the nuclear spin in the subject is magnetically resonated, and an image of the subject is obtained using a nuclear magnetic resonance (NMR) signal generated by excitation. Is a device for reconfiguring.

  In such an MRI apparatus, there is a problem that the uniformity of the static magnetic field strength is locally disturbed due to the performance limit of the static magnetic field magnet that forms the static magnetic field and the change in magnetic susceptibility due to the subject entering the magnetic field. There is. It is known that when the uniformity of the static magnetic field strength is disturbed, the reconstructed image is distorted.

  As a case that is particularly strongly affected by the non-uniformity of the static magnetic field strength, there is a case where imaging is performed by an EPI (echo planar imaging) method. The EPI method is an ultra-high-speed scanning method that acquires all data for image reconstruction with a single excitation pulse, and is also called a single shot EPI method. Imaging by the EPI method is performed in order to avoid the influence of the body motion and pulsation of the subject at the time of rendering a cerebral infarction in the examination of the head of the subject, for example. The image is strongly affected by the non-uniformity of the static magnetic field strength, and the reconstructed image is distorted if the static magnetic field strength is not sufficiently uniform.

  Therefore, as a method of solving the problem due to the non-uniformity of the static magnetic field strength, the position of the image reconstructed based on the static magnetic field strength distribution obtained by estimating the static magnetic field strength distribution and estimating the distribution of the static magnetic field strength has been conventionally used. And a method of correcting the image value is used (see, for example, Non-Patent Document 1).

  Factors that disturb the uniformity of the static magnetic field intensity and cause distortion in the reconstructed image include errors in the gradient magnetic field, static magnetic field errors due to the single static magnetic field magnet, and static magnetic field due to the subject entering the static magnetic field. There are three factors of error. Therefore, the static magnetic field error can be defined as the sum of the gradient magnetic field error, the static magnetic field error due to the single static magnetic field magnet, and the static magnetic field error due to the subject entering the static magnetic field.

Further, the distortion amount of the image data is obtained for each position in the image space from the error of the static magnetic field, and the image value is corrected for each position in the image space based on the obtained distortion amount of the image data. That is, first, in the image space, the three-dimensional position where the image value is obtained is shifted to the position where there is no distortion. However, since the shifted position is not necessarily on the grid point that should be originally, an image value on the grid point is obtained by interpolation from a plurality of image values at each position after the shift.
J.MICHIELS ET AL. "Geometric distortion in MRI for stereotactic neurosurgery" Magnetic Resonance Imaging, Volume 12, Nimber 5,1994, p.749-765

  In correction of image distortion due to static magnetic field intensity non-uniformity performed in the past, it is important in terms of accuracy to estimate the distribution of the amount of distortion of the static magnetic field according to each error factor of the static magnetic field. In particular, the error of the static magnetic field due to entering the static magnetic field varies depending on the movement of the subject, so there is a possibility that the estimated value of the error distribution of the static magnetic field has an error.

  If the image correction is performed based on the static magnetic field error distribution without accurately estimating the static magnetic field error distribution, the image distortion may not be appropriately improved. Therefore, it is desired to develop a technique for evaluating whether or not the error distribution of the static magnetic field is accurately estimated, or whether or not the image distortion correction due to the non-uniformity of the static magnetic field has been appropriately performed.

  The present invention has been made in order to cope with such a conventional situation. The error distribution of the static magnetic field formed in the imaging region is appropriately estimated, and the image distortion due to the static magnetic field inhomogeneity is appropriately corrected. It is an object of the present invention to provide a magnetic resonance imaging apparatus and an image correction evaluation method capable of evaluating whether or not a failure has occurred.

  In order to achieve the above object, a magnetic resonance imaging apparatus according to the present invention reverses the polarity of a gradient magnetic field in at least one of a phase encoding direction, a readout direction, and a slice encoding direction as described in claim 1. Imaging condition setting means for setting a plurality of imaging conditions so that scanning is executed, and a nucleus of a nucleus by transmitting a high-frequency signal to the imaging area according to each imaging condition set by the imaging condition setting means Data collection means for generating data based on a magnetic resonance signal generated in association with magnetic resonance, and image reconstruction means for reconstructing image data for each of the imaging conditions by performing image reconstruction processing on the data, The distortion of each image data obtained under each imaging condition based on the static magnetic field distribution in the imaging area Image distortion correction means for correcting, and image correction evaluation means for evaluating whether or not the correction of each image data is appropriately performed based on each image data after the correction by the image distortion correction means. It is a feature.

  In order to achieve the above object, the image correction evaluation method according to the present invention provides the polarity of the gradient magnetic field in at least one of the phase encoding direction, the readout direction, and the slice encoding direction as described in claim 4. Reconstructing image data for each of the imaging conditions by performing image reconstruction processing on data collected by executing a scan according to a plurality of imaging conditions in which the image is inverted, and A step of correcting distortion of each of the image data obtained under the respective imaging conditions based on the magnetic field distribution, and whether or not the correction of each of the image data has been appropriately performed based on each of the image data after correction And a step of evaluating.

  In the magnetic resonance imaging apparatus and the image correction evaluation method according to the present invention, whether the error distribution of the static magnetic field formed in the imaging region is appropriately estimated and the image distortion due to the static magnetic field inhomogeneity is appropriately corrected. You can evaluate whether or not.

  Embodiments of a magnetic resonance imaging apparatus and an image correction evaluation method according to the present invention will be described with reference to the accompanying drawings.

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

  The magnetic resonance imaging apparatus 20 includes a cylindrical static magnetic field magnet 21 that forms a static magnetic field, a shim coil 22, a gradient magnetic field coil unit 23, and an RF coil 24 provided inside the static magnetic field magnet 21 in a gantry (not shown). It is a built-in configuration.

  In addition, the magnetic resonance imaging apparatus 20 includes a control system 25. The control system 25 includes a static magnetic field power supply 26, a gradient magnetic field power supply 27, a shim coil power supply 28, a transmitter 29, a receiver 30, a sequence controller 31, and a computer 32. The gradient magnetic field power source 27 of the control system 25 includes an X-axis gradient magnetic field power source 27x, a Y-axis gradient magnetic field power source 27y, and a Z-axis gradient magnetic field power source 27z. In addition, the computer 32 includes an input device 33, a display device 34, an arithmetic device 35, and a storage device 36.

  The static magnetic field magnet 21 is connected to a static magnetic field power supply 26 and has a function of forming a static magnetic field in the imaging region by a current supplied from the static magnetic field power supply 26. In many cases, the static magnetic field magnet 21 is composed of a superconducting coil, and is connected to the static magnetic field power supply 26 at the time of excitation and supplied with current. It is common. In some cases, the static magnetic field magnet 21 is composed of a permanent magnet and the static magnetic field power supply 26 is not provided.

  A cylindrical shim coil 22 is coaxially provided inside the static magnetic field magnet 21. The shim coil 22 is connected to the shim coil power supply 28, and is configured such that a current is supplied from the shim coil power supply 28 to the shim coil 22 to make the static magnetic field uniform.

  The gradient magnetic field coil unit 23 includes an X-axis gradient magnetic field coil 23 x, a Y-axis gradient magnetic field coil 23 y, and a Z-axis gradient magnetic field coil 23 z, and is formed in a cylindrical shape inside the static magnetic field magnet 21. A bed 37 is provided inside the gradient coil unit 23 as an imaging region, and the subject P is set on the bed 37. The RF coil 24 may not be built in the gantry but may be provided near the bed 37 or the subject P.

  The gradient magnetic field coil unit 23 is connected to a gradient magnetic field power supply 27. The X axis gradient magnetic field coil 23x, the Y axis gradient magnetic field coil 23y, and the Z axis gradient magnetic field coil 23z of the gradient magnetic field coil unit 23 are respectively an X axis gradient magnetic field power source 27x, a Y axis gradient magnetic field power source 27y, and a Z axis. It is connected to the gradient magnetic field power supply 27z.

  The X-axis gradient magnetic field power source 27x, the Y-axis gradient magnetic field power source 27y, and the Z-axis gradient magnetic field power source 27z are supplied with currents supplied to the X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z, respectively. In the imaging region, a gradient magnetic field Gx in the X-axis direction, a gradient magnetic field Gy in the Y-axis direction, and a gradient magnetic field Gz in the Z-axis direction can be formed, respectively.

  The RF coil 24 is connected to the transmitter 29 and the receiver 30. The RF coil 24 receives the RF signal from the transmitter 29 and transmits it to the subject P, and receives the MR signal generated along with the excitation of the nuclear spin inside the subject P by the RF signal to the receiver 30. Has the function to give.

  On the other hand, the sequence controller 31 of the control system 25 is connected to the gradient magnetic field power source 27, the transmitter 29, and the receiver 30. The sequence controller 31 has control information necessary for driving the gradient magnetic field power supply 27, the transmitter 29, and the receiver 30, for example, operation control information such as the intensity, application time, and application timing of the pulse current to be applied to the gradient magnetic field power supply 27. And a gradient magnetic field power source 27, a transmitter 29, and a receiver 30 by driving the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 according to the imaging conditions defined by the stored predetermined pulse sequence. It has a function of generating a magnetic field Gy, a Z-axis gradient magnetic field Gz, and an RF signal.

  The sequence controller 31 is configured to receive raw data, which is complex data obtained by detection of the MR signal and A / D conversion in the receiver 30, and supply the raw data to the computer 32.

  For this reason, the transmitter 29 has a function of applying an RF signal to the RF coil 24 based on the control information received from the sequence controller 31, while the receiver 30 detects the MR signal received from the RF coil 24. Then, by executing required signal processing and A / D conversion, a function of generating raw data that is digitized complex data and a function of supplying the generated raw data to the sequence controller 31 are provided.

  That is, the magnetic resonance imaging apparatus 20 uses the static magnetic field magnet 21, the shim coil 22, the gradient magnetic field coil unit 23, the RF coil 24, and the control system 25, so that the magnetic resonance imaging apparatus 20 operates according to each imaging condition set as a pulse sequence. By applying a gradient magnetic field and transmitting an RF signal to the subject P inside, the MR signal generated along with the nuclear magnetic resonance of the nucleus by the RF signal inside the subject P is received and digitized. Generate raw data.

  Further, the computer 32 is provided with various functions by executing the program stored in the storage device 36 of the computer 32 by the arithmetic unit 35. However, the computer 32 may be configured by providing a specific circuit regardless of the program.

  FIG. 2 is a functional block diagram of the computer 32 in the magnetic resonance imaging apparatus 20 shown in FIG.

  The computer 32 uses a sequence controller control device 40, a raw data database 41, an image reconstruction device 42, an image data database 43, a static magnetic field distribution database 44, an image distortion correction device 45, an imaging condition setting device 46, an image by an image correction evaluation program. It functions as a cross-correlation calculation device 47 as an example of a correction evaluation device, a signal intensity comparison device 48 as an example of an image correction evaluation device, and a magnetic field distribution calculation device 49.

  The sequence controller control device 40 receives the raw data from the sequence controller 31 and the raw data received from the sequence controller 31 by providing the sequence controller 31 with a required pulse sequence based on information from the input device 33 or other components. A function of arranging in the k-space (Fourier space) formed in the database 41.

  The imaging condition setting device 46 has a function of setting imaging conditions by generating a pulse sequence and a function of giving the generated pulse sequence to the sequence controller control device 40. Here, the imaging condition setting device 46 generates a pulse sequence for executing a pre-scan for measuring a static magnetic field distribution, as well as a phase encoding (PE) direction, a readout (RO) direction, and a slice encoding (SE). There is provided a function of generating a plurality of pulse sequences so that the scan is executed under a plurality of imaging conditions by reversing the polarity of the gradient magnetic field in at least one direction.

  Therefore, the raw data database 41 stores each raw data generated in the receiver 30 by executing the scanning of the plurality of shooting conditions set by the shooting condition setting device 46 and is formed in the raw data database 41. Each raw data is arranged for each photographing condition in k space. In addition, when a pre-scan for measuring the static magnetic field distribution is executed, raw data for measuring the static magnetic field distribution is stored.

  The image reconstruction device 42 has a function of reconstructing the image data of the subject P by taking the raw data from the raw data database 41 and performing predetermined image reconstruction processing such as Fourier transform processing and maximum value projection processing. And a function of writing the obtained image data into the image data database 43. However, other than the time of diagnosis, for example, phantom image data other than the subject P may be reconstructed.

  Therefore, the image data database 43 stores image data of the subject P and phantom for each imaging condition.

  In the static magnetic field distribution database 44, the static magnetic field distribution is measured or obtained in advance and stored. The static magnetic field distribution database 44 can also store the static magnetic field distribution obtained by the magnetic field distribution calculation device 49.

  The image distortion correction device 45 corrects the distortion of the image data of the subject P and phantom for each imaging condition stored in the image data database 43 based on the static magnetic field distribution read from the static magnetic field distribution database 44, and A function of giving the corrected image data to the cross-correlation calculation device 47 and the signal intensity comparison device 48, respectively.

  The cross-correlation calculation device 47 receives the corrected image data obtained under a plurality of shooting conditions from the image distortion correction device 45, and obtains a cross-correlation coefficient between the plurality of image data. A function as an image correction evaluation apparatus that evaluates whether or not the correction of the image data is appropriately performed based on the correlation coefficient. Further, if necessary, the cross-correlation calculation device 47 gives the display device 34 an evaluation result as to whether or not the correction of the cross-correlation coefficient between the plurality of obtained image data and the image data has been appropriately performed. The function to display is provided.

  The signal intensity comparison device 48 receives the corrected image data obtained under a plurality of shooting conditions from the image distortion correction device 45, compares the signal strength for each pixel between the plurality of image data, and the signal intensity. And a function as an image correction evaluation apparatus that evaluates whether or not the correction of the image data has been appropriately performed based on the comparison result. Further, if necessary, the signal strength comparison device 48 displays the comparison result of the signal strength between the plurality of image data and the evaluation result as to whether or not the correction of the image data is properly performed to the display device 34. A function is provided.

  The magnetic field distribution calculation device 49 has a function of estimating the static magnetic field distribution by an arbitrary method and a function of writing the static magnetic field distribution data obtained by the estimation into the static magnetic field distribution database 44. For estimation of the distribution of the static magnetic field, the raw data for measuring the static magnetic field distribution stored in the raw data database 41 is referred to as necessary. Further, a gradient magnetic field measurement coil 50 for measuring the gradient magnetic field can be provided in the vicinity of the imaging region, and the measurement result of the gradient magnetic field by the gradient magnetic field measurement coil 50 can be read and referenced.

  Next, the operation of the magnetic resonance imaging apparatus 20 will be described.

  FIG. 3 is a flowchart showing an example of a flow when evaluating whether or not the image distortion correction due to the non-uniformity of the static magnetic field has been appropriately performed by the magnetic resonance imaging apparatus 20 shown in FIG. Reference numerals with numbers added to S indicate steps in the flowchart.

  First, in step S 1, the static magnetic field error distribution is estimated in advance by an arbitrary method and stored in the static magnetic field distribution database 44. The error distribution of the static magnetic field may be measured.

  As an example of the estimation method of the static magnetic field error distribution, the cause of the static magnetic field error is the gradient magnetic field error, the static magnetic field error due to the static magnetic field magnet 21 alone, and the static magnetic field due to the subject P entering the static magnetic field. There is a method in which the error is classified into three factors, and the error of the static magnetic field due to each factor is estimated and then added.

  The error of the gradient magnetic field, which is the first factor, is caused by an error from the linearity of the gradient magnetic field that should ideally be linear. The amount of distortion of the static magnetic field based on the gradient magnetic field error is determined independently of the strength of the gradient magnetic field.

  If the coordinates of an arbitrary point of the gantry coordinate system in the imaging region are x, y, and z, the error of the gradient magnetic field becomes a three-dimensional distribution having a gradient in each channel in the x, y, and z directions. Then, assuming that the intensity of the ideal linear gradient magnetic field is Gx, Gy, Gz, the ratio of the gradient magnetic field error in each channel in the x, y, z directions GxErrorRatio (x, y, z), GyErrorRatio (x, y, z), GzErrorRatio (x, y, z) can be expressed as Equation 1-1, Equation 1-2, and Equation 1-3, and ΔGx (x, y, z), ΔGy ( x, y, z) and ΔGz (x, y, z) mean an error amount of the gradient magnetic field in each channel in the x, y, and z directions.

[Equation 1]
GxErrorRatio (x, y, z) = ΔGx (x, y, z) / Gx (1-1)
GyErrorRatio (x, y, z) = ΔGy (x, y, z) / Gy (1-2)
GzErrorRatio (x, y, z) = ΔGz (x, y, z) / Gz (1-3)

  Further, the gradient magnetic field errors ΔGx (x, y, z), ΔGy (x, y, z), ΔGz (x, y, z) and the coordinates x, y, z of the gantry coordinate system are x, y. , Z-direction static magnetic field error distribution ΔBgradx (x, y, z), ΔBgrady (x, y, z), ΔBgradz (x, y, z) due to errors in the gradient magnetic field in each channel in the z and z directions are expressed by the formula (2- 1), equation (2-2), and equation (2-3).

[Equation 2]
ΔBgradx (x, y, z) = ΔGx (x, y, z) × x (2-1)
ΔBgrady (x, y, z) = ΔGy (x, y, z) × y (2-2)
ΔBgradz (x, y, z) = ΔGz (x, y, z) × z (2-3)

  Further, the static magnetic field error due to the static magnetic field magnet 21 alone, which is the second factor, is caused by an error in the actual static magnetic field strength from the ideal static magnetic field strength. The static magnetic field error due to the single static magnetic field magnet 21 has a property that it rapidly deteriorates as the imaging field of view (FOV) increases. Further, the static magnetic field error due to the single static magnetic field magnet 21 depends on the strength of the gradient magnetic field, but is independent of the shape and presence of the subject P in the static magnetic field.

  In addition, the static magnetic field error caused by the subject P entering the static magnetic field, which is the third factor, is a static magnetic field error resulting from the difference in magnetic susceptibility caused by the subject P entering the static magnetic field. Minutes. The error of the static magnetic field due to the subject P entering the static magnetic field depends on the strength of the gradient magnetic field and also depends on the shape and presence of the subject P in the static magnetic field.

  The static magnetic field error distribution can be defined as the sum of static magnetic field error components due to the first, second, and third factors. That is, the error of the static magnetic field at the coordinates x, y, z in the gantry coordinate system is caused by ΔBox (x, y, z), Δ Boy (x, y, z), ΔBoz (x, y, z), and the gradient magnetic field. The error components are ΔBgradx (x, y, z), ΔBgrady (x, y, z), ΔBgradz (x, y, z), the static magnetic field error due to the single static magnetic field magnet 21 is ΔBmag (x, y, z), Assuming that the error of the static magnetic field caused by the subject P entering the static magnetic field is ΔBobj (x, y, z), the following equations (3-1), (3-2), and (3-3) are obtained. Can be represented.

[Equation 3]
ΔBox (x, y, z) = ΔBgradx (x, y, z) + ΔBmag (x, y, z) + ΔBobj (x, y, z) (3-1)
ΔBoy (x, y, z) = ΔBgrady (x, y, z) + ΔBmag (x, y, z) + ΔBobj (x, y, z) (3-2)
ΔBoz (x, y, z) = ΔBgradz (x, y, z) + ΔBmag (x, y, z) + ΔBobj (x, y, z) (3-3)

  Therefore, the distortion amounts ΔX, ΔY, and ΔZ of the static magnetic field in the x, y, and z directions of the gantry coordinate system are expressed by the equation (4-) from the equations (3-1), (3-2), and (3-3). 1), equation (4-2), and equation (4-3).

[Equation 4]
ΔX = ΔBox (x, y, z) / Gx
= GxErrorRatio (x, y, z) × x + {ΔBmag (x, y, z) + ΔBobj (x, y, z)} / Gx (4-1)
ΔY = ΔBoy (x, y, z) / Gy
= GyErrorRatio (x, y, z) × y + {ΔBmag (x, y, z) + ΔBobj (x, y, z)} / Gy (4-2)
ΔZ = ΔBoz (x, y, z) / Gz
= GzErrorRatio (x, y, z) × z + {ΔBmag (x, y, z) + ΔBobj (x, y, z)} / Gz (4-3)

  In equations (4-1), (4-2), and (4-3), the value of {ΔBmag (x, y, z) + ΔBobj (x, y, z)} is prior to the scan for imaging. This can be obtained by executing a pre-scan for measuring the static magnetic field distribution. When the imaging scan is a scan using an EPI sequence, the pre-scan for measuring the static magnetic field distribution can be executed together with the pre-scan for adjusting the reception gain of the EPI sequence and adjusting the delay time of the gradient magnetic field.

  That is, it is possible to collect data for measuring the static magnetic field distribution (distribution map in which the deviation from the center frequency is represented by the phase) in the slice plane set by executing the pre-scan for measuring the static magnetic field distribution. In this case, the in-plane resolution at the time of executing the pre-scan for the static magnetic field distribution measurement can be made coarser than the in-plane resolution at the time of executing the imaging scan.

  As an example of the pre-scan for measuring the static magnetic field distribution, one that collects two image data by changing the echo time (TE: echo time) by the FieldEcho method can be cited. Then, as shown in equation (5), {ΔBmag (x, y, z) from the phase difference Δθ (x, y, z) for each pixel of each image data collected by the pre-scan for measuring the static magnetic field distribution. The value of + ΔBobj (x, y, z)} can be obtained.

[Equation 5]
Δθ (x, y, z) = (TE1−TE2) × {ΔBmag (x, y, z) + ΔBobj (x, y, z)} (5)
However, in Formula (5), TE1 and TE2 are TEs different from each other.

  In other words, ΔBgradx out of the static magnetic field errors ΔBox (x, y, z), ΔBoy (x, y, z), ΔBoz (x, y, z) by differentiating the image data collected by changing TE. The terms (x, y, z), ΔBgrady (x, y, z), and ΔBgradz (x, y, z) are canceled.

  Accordingly, the static magnetic field error distribution ΔBgrad. x (x, y, z), ΔBgrad. y (x, y, z), ΔBgrad. If z (x, y, z) can be obtained, formula (3-1), formula (3-2), formula (3-3), formula (4-1), formula (4-2), formula From (4-3), the static magnetic field errors ΔBox (x, y, z), ΔBoy (x, y, z), ΔBoz (x, y, z) and the static magnetic field distortion amounts ΔX, ΔY, ΔZ are obtained. Can do.

  Error distribution ΔBgrad. Of the static magnetic field due to the error of the gradient magnetic field. x (x, y, z), ΔBgrad. y (x, y, z), ΔBgrad. z (x, y, z) can be measured by the gradient magnetic field measuring coil 50, for example.

  Therefore, the static magnetic field error distribution ΔBgrad. x (x, y, z), ΔBgrad. y (x, y, z), ΔBgrad. z (x, y, z) is measured by the gradient magnetic field measuring coil 50 and given to the magnetic field distribution calculation device 49.

  On the other hand, a pulse sequence for pre-scan for static magnetic field distribution measurement is generated by the imaging condition setting device 46 and given to the sequence controller control device 40. For this reason, the sequence controller control device 40 executes a pre-scan for static magnetic field distribution measurement by giving a pulse sequence for static magnetic field distribution measurement to the sequence controller 31 based on information from the input device 33 or other components. Let

  That is, the subject P or phantom is set on the bed 37 in advance, and a static magnetic field is formed in the imaging region of the static magnetic field magnet 21 (superconducting magnet) excited by the static magnetic field power supply 26. Further, a current is supplied from the shim coil power supply 28 to the shim coil 22, and the static magnetic field formed in the imaging region is made uniform.

  Furthermore, the sequence controller 31 sets the subject P or phantom by driving the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 in accordance with the pulse sequence for measuring the static magnetic field distribution received from the sequence controller control device 40. An X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, and a Z-axis gradient magnetic field Gz are formed in the imaging region, and an RF signal is generated.

  Therefore, an RF signal is given from the transmitter 29 to the RF coil 24 according to the pulse sequence, and the RF signal is transmitted from the RF coil 24 to the subject P. When the RF signal is transmitted to the subject P, the MR signal generated by the nuclear magnetic resonance of the nucleus inside the subject P is received by the RF coil 24 and given to the receiver 30. The receiver 30 receives the MR signal from the RF coil 24 and executes various signal processing such as preamplification, intermediate frequency conversion, phase detection, low frequency amplification, and filtering. Further, the receiver 30 performs A / D conversion on the MR signal to generate raw data that is an MR signal of digital data. Then, the receiver 30 gives the generated raw data to the sequence controller 31.

  The sequence controller 31 gives the raw data received from the receiver 30 to the sequence controller controller 40, and the sequence controller controller 40 inputs the raw data into the raw data database 42. In this way, for example, the pre-scan for measuring the static magnetic field distribution is performed twice by the FieldEcho method with different TE, and the raw data collected by the pre-scan for measuring the static magnetic field distribution is stored in the raw data database 42. Is accumulated.

  The magnetic field distribution calculation device 49 reads the raw data collected by the execution of the pre-scan for the static magnetic field distribution measurement from the raw data database 41, while the static magnetic field due to the gradient magnetic field error from the gradient magnetic field measurement coil 50. Error distribution ΔBgrad. x (x, y, z), ΔBgrad. y (x, y, z), ΔBgrad. In response to z (x, y, z), for example, Formula (3-1), Formula (3-2), Formula (3-3), Formula (4-1), Formula (4-2), Formula ( 4-3) and Equation (5), the static magnetic field errors ΔBox (x, y, z), ΔBoy (x, y, z), ΔBoz (x, y, z) and the static magnetic field distortion amounts ΔX, ΔY, Find ΔZ.

  Further, the magnetic field distribution calculation device 49 writes the obtained static magnetic field distortion amounts ΔX, ΔY, ΔZ in the static magnetic field distribution database 44 as static magnetic field distribution data.

  Note that the static magnetic field error distribution ΔBgrad. x (x, y, z), ΔBgrad. y (x, y, z), ΔBgrad. z (x, y, z) can be obtained by calculation by simulation using a magnetic field model simulating the system, for example, in addition to measurement by the gradient magnetic field measuring coil 50.

  Next, in step S <b> 2, the imaging condition is set by generating a pulse sequence for collecting images of the subject P by the imaging condition setting device 46. At this time, the generated image acquisition pulse sequence reverses the polarity of the gradient magnetic field in at least one of the PE direction, the RO direction, and the SE direction so that, for example, a scan is executed under two imaging conditions. Two pulse sequences are used.

  FIG. 4 is a diagram illustrating an example of a pulse sequence generated for evaluating whether or not the image distortion correction due to the static magnetic field inhomogeneity has been appropriately performed by the magnetic resonance imaging apparatus 20 illustrated in FIG. 1. .

  In FIG. 4, SE1, RO1, and PE1 are first pulse sequences that define gradient magnetic fields in the SE direction, RO direction, and PE direction, respectively. SE2, RO2, and PE2 are SE direction, RO direction, and PE direction, respectively. It is the 2nd pulse sequence which prescribes | regulates the gradient magnetic field. As shown in FIG. 4, for example, first and second two kinds of pulses in which the polarities of the gradient magnetic fields in two directions of the PE direction and the RO direction are reversed with each other and the scan is executed under two kinds of imaging conditions. A sequence is generated by the imaging condition setting device 46.

  Then, the two pulse sequences generated by the imaging condition setting device 46 are given to the sequence controller control device 40.

  Next, in step S3, scanning is executed in accordance with the two shooting conditions set by the shooting condition setting device 46, and image data is reconstructed. That is, the sequence controller control device 40 causes the sequence controller 31 to execute two scan sequences for image acquisition based on information from the input device 33 or other components, thereby executing a scan for image acquisition.

  As a result, each raw data obtained by reversing the polarities of the gradient magnetic fields is arranged in the k space formed in the raw data database 41 for each photographing condition.

  Further, the image reconstruction device 42 reconstructs two pieces of image data by taking each raw data from the raw data database 41 and performing a predetermined image reconstruction process such as a Fourier transform process. Each image data obtained in this way is reconstructed from the raw data collected by inverting the gradient magnetic field, and thus becomes image data having different distortion methods. Each image data is written and stored in the image data database 43.

  Next, in step S <b> 4, each image data stored in the image data database 43 with different distortion is read into the image distortion correction device 45, and each image based on the static magnetic field distribution stored in the static magnetic field distribution database 44. Data distortion correction is performed.

  FIG. 5 is an explanatory diagram showing an example of a method for correcting image data based on the static magnetic field distribution by the magnetic resonance imaging apparatus 20 shown in FIG.

First, the amount of distortion of each image data is obtained for each position in the image space from the static magnetic field distribution. For example, when there is a distortion that cannot be ignored only in the x direction, the distortion amount Δx in the x direction is obtained. Based on the distortion amount Δx, for example, the respective image values P ₁ (X ₁ , Y ₂ ) at the coordinates (X ₁ , Y ₁ , Z ₁ ) and (X ₂ , Y ₂ , Z ₂ ) of the two points after distortion. ₁ , Z ₁ ), P ₂ (X ₂ , Y ₂ , Z ₂ ) are coordinates (X ₁ ′, Y ₁ ′, Z ₁ ′), (X ₂ ′, Y ₂ ′, Z ₂ ′) before distortion. ) Image values P ₁ (X ₁ ′, Y ₁ ′, Z ₁ ′) and P ₂ (X ₂ ′, Y ₂ ′, Z ₂ ′). That is, the image value by distortion amount Δx in the x-direction _{P 1 (X 1, Y 1} , Z 1), the position of _{P 2 (X 2, Y 2} , Z 2) (X 1, Y 1, Z 1), ( X ₂ , Y ₂ , Z ₂ ) are shifted.

Further, the image value P (x, y, z) at the coordinates (x, y, z) of the lattice point that should be originally exists at the position (X ₁ ′, Y ₁ ) in the vicinity of the coordinates (x, y, z) of the lattice point. ', Z ₁ '), (X ₂ ', Y ₂ ', Z ₂ ') image values P ₁ (X ₁ ', Y ₁ ', Z ₁ '), P ₂ (X ₂ ', Y ₂ ', Z ₂ ′) is obtained for each position in the image space by interpolation. At this time, normally, a function higher than the cubic spline is used for interpolation, and the linear is considered to be largely unsatisfactory.

  The two pieces of corrected image data obtained by the image distortion correction device 45 in this way are supplied to the cross-correlation calculation device 47 and the signal intensity comparison device 48, respectively.

  Next, in step S5, the cross-correlation calculation device 47 obtains a cross-correlation coefficient between the corrected image data obtained under the two imaging conditions in which the gradient magnetic field is inverted, and obtains the obtained cross-correlation coefficient. Based on this, it is evaluated whether or not the image data has been properly corrected.

  In other words, the two image data collection conditions are substantially the same except that the polar directions of the gradient magnetic fields are reversed from each other. Therefore, the more appropriately the distortion of the image data is corrected, It is considered that the shape of the object on each image data is more consistent, the cross-correlation between the two image data becomes stronger, and the cross-correlation coefficient becomes larger. On the contrary, if the distortion of the image data is not properly corrected, it is considered that the cross-correlation coefficient between the two image data becomes weak and the cross-correlation coefficient becomes small.

  Therefore, the cross-correlation calculation device 47 compares the cross-correlation coefficient between the two image data with a preset threshold value, and if the cross-correlation coefficient is greater than or equal to the threshold value, the correction is appropriately performed. On the other hand, when the cross-correlation coefficient is less than or less than the threshold value, it is evaluated that the correction is not properly performed.

  Further, if necessary, the cross-correlation calculation device 47 gives the display device 34 an evaluation result as to whether or not the correction of the cross-correlation coefficient between the two obtained image data and the image data has been appropriately performed, and displays it. Let For this reason, the user can easily grasp whether or not the correction of the image data is appropriately performed.

  In step S6, the signal intensity comparison device 48 compares the signal intensity for each pixel between the corrected image data obtained under the two imaging conditions in which the gradient magnetic field is inverted, and uses the signal intensity comparison result. Based on this, it is evaluated whether or not the image data has been properly corrected.

  That is, as described above, the collection conditions of the two image data are substantially the same except that the directions of the polarities of the gradient magnetic fields are reversed, so that the distortion of the image data is appropriately corrected. The closer the shape of the object on each image data, the smaller the difference in signal intensity between the two image data. Conversely, if the distortion of the image data is not properly corrected, the difference in signal strength between the two image data is considered to be large.

  Therefore, the signal intensity comparison device 48 compares the difference value of the signal intensity between the two image data with a preset threshold value, and if the difference value of the signal intensity is equal to or greater than the threshold value or exceeds the threshold value, the correction is performed. On the other hand, it is evaluated that the correction is not properly performed. On the other hand, when the difference value of the signal strength is less than or less than the threshold value, it is evaluated that the correction is appropriately performed.

  Further, the signal strength comparison device 48 gives the display device 34 the evaluation result as to whether or not the correction value of the signal strength difference between the two obtained image data and the correction of the image data has been appropriately performed as necessary. Display. For this reason, the user can easily grasp whether or not the correction of the image data is appropriately performed.

  That is, the magnetic resonance imaging apparatus 20 as described above creates two image data by reversing the gradient magnetic field, and determines whether correction is appropriate based on the comparison result of the correlation coefficient and signal intensity between the two image data after distortion correction. Is to evaluate.

  Therefore, according to the magnetic resonance imaging apparatus 20, it is evaluated whether or not the error distribution of the static magnetic field formed in the imaging region is appropriately estimated and the image distortion correction due to the static magnetic field non-uniformity is appropriately performed. can do.

  Note that some functions and processing of the magnetic resonance imaging apparatus 20 may be omitted, and the processing order may be changed. In addition, the imaging conditions are not limited to two, for example, three imaging conditions including a reference imaging condition, an imaging condition in which the gradient magnetic field only in the SE direction is inverted, and an imaging condition in which the gradient magnetic field only in the RO direction is inverted. As in the case of the above, three or more shooting conditions may be set.

1 is a configuration diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention. The functional block diagram of the computer in the magnetic resonance imaging apparatus shown in FIG. The flowchart which shows an example of the flow at the time of evaluating whether the correction | amendment of the image distortion by the nonuniformity of a static magnetic field was performed appropriately by the magnetic resonance imaging apparatus shown in FIG. FIG. 3 is a diagram showing an example of a pulse sequence generated for evaluating whether or not image distortion correction due to non-uniformity of a static magnetic field has been appropriately performed by the magnetic resonance imaging apparatus 20 shown in FIG. 1. FIG. 3 is an explanatory diagram illustrating an example of a method for correcting image data based on a static magnetic field distribution by the magnetic resonance imaging apparatus 20 illustrated in FIG. 1.

Explanation of symbols

20 Magnetic Resonance Imaging Device 21 Magnet for Static Magnetic Field 22 Shim Coil 23 Gradient Magnetic Field Coil Unit 24 RF Coil 25 Control System 26 Static Magnetic Field Power Supply 27 Gradient Magnetic Field Power Supply 28 Shim Coil Power Supply 29 Transmitter 30 Receiver 31 Sequence Controller 32 Computer 33 Input Device 34 Display Device 35 Computing device 36 Storage device 37 Bed 40 Sequence controller control device 41 Raw data database 42 Image reconstruction device 43 Image data database 44 Static magnetic field distribution database 45 Image distortion correction device 46 Imaging condition setting device 47 Cross correlation calculation device 48 Signal intensity Comparison device 49 Magnetic field distribution calculation device 50 Gradient magnetic field measurement coil P Subject

Claims (4)

An imaging condition setting means for setting a plurality of imaging conditions so that the scan is executed by inverting the polarity of the gradient magnetic field in at least one direction of the phase encoding direction, the readout direction, and the slice encoding direction;
Data collecting means for generating data based on magnetic resonance signals generated along with nuclear magnetic resonance of the nuclei by transmitting a high-frequency signal to the imaging region according to each imaging condition set by the imaging condition setting means;
Image reconstruction means for reconstructing image data for each of the photographing conditions by performing image reconstruction processing on the data;
Image distortion correction means for correcting distortion of each image data obtained by each imaging condition based on the static magnetic field distribution of the imaging region;
Image correction evaluation means for evaluating whether or not the correction of each image data is appropriately performed based on each image data after correction by the image distortion correction means;
A magnetic resonance imaging apparatus comprising: The image correction evaluation unit obtains a cross-correlation coefficient between the image data after correction by the image distortion correction unit, and the correction of each image data is appropriately performed based on the obtained cross-correlation coefficient. The magnetic resonance imaging apparatus according to claim 1, further comprising: a cross-correlation calculating unit that evaluates whether or not The image correction evaluation means is a signal for evaluating whether or not the correction of each image data is appropriately performed by comparing the signal intensity for each pixel between the respective image data after the correction by the image distortion correction means. 2. The magnetic resonance imaging apparatus according to claim 1, further comprising intensity comparison means. The imaging condition is obtained by performing image reconstruction processing on data collected by executing a scan according to a plurality of imaging conditions in which the polarity of the gradient magnetic field in at least one of the phase encoding direction, the readout direction, and the slice encoding direction is reversed Each reconstructing the image data for each,
Correcting each distortion of the image data obtained by the imaging conditions based on the static magnetic field distribution in the imaging area of the image data;
Evaluating whether each of the image data is appropriately corrected based on each of the image data after correction; and
An image correction evaluation method characterized by comprising:

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