JP4812420B2 - 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.

Magnetic resonance imaging apparatus according to one embodiment, the imaging condition setting means for setting a plurality of different imaging conditions, receiving means for receiving magnetic resonance signals from the imaging region according to the plurality of shooting conditions, based on the magnetic resonance signals Image reconstructing means for reconstructing a plurality of image data corresponding to the plurality of photographing conditions, an image distortion correcting means for correcting distortion of the plurality of image data based on a magnetic field distribution of the photographing region, and a post-correction an image correction evaluation means for evaluating whether the correction of at least one of the image data has been properly performed based on the plurality of image data, Ru and a feedback means for feeding back the evaluation result of the image correction evaluation means .

Image correction evaluation method according to an embodiment includes the steps of setting a plurality of different imaging conditions, receiving a magnetic resonance signal from the imaging region according to the plurality of shooting conditions, the plurality of, based on the magnetic resonance signals Reconstructing a plurality of image data corresponding to imaging conditions; correcting a distortion of the plurality of image data based on the magnetic field distribution of the imaging region; and at least one based on the corrected plurality of image data a step one of the image data corrected to assess whether or not properly done, that having a a step of the correction is fed back the evaluation results indicating whether or not properly performed.

  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 a static magnetic field magnet 21, a shim coil 22, a gradient magnetic field coil unit 23, an RF coil 24, and a control system 25 to allow the magnetic resonance imaging apparatus 20 to generate a static magnetic field according to each imaging condition set as a pulse sequence. Applying a gradient magnetic field and transmitting an RF signal to the subject P inside, while receiving and digitizing the MR signal generated along with nuclear magnetic resonance of the nucleus by the RF signal inside the subject P 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 unit 40, a raw data database 41, an image reconstruction unit 42, an image data database 43, a static magnetic field distribution database 44, an image distortion correction unit 45, an imaging condition setting unit 46, an image by an image correction evaluation program. It functions as a cross-correlation calculation unit 47 as an example of a correction evaluation unit, a signal intensity comparison unit 48 as an example of an image correction evaluation unit, a magnetic field distribution calculation unit 49, and a feedback unit 50. The feedback unit 50 includes an approximate curve order change unit 50A, a re-correction instruction unit 50B, and a shimming execution unit 50C.

  The sequence controller control unit 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 unit 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 unit 40. Here, the imaging condition setting unit 46 generates a pulse sequence for executing a pre-scan for static magnetic field distribution measurement, 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.

  In other words, the imaging condition setting unit 46 sets imaging conditions for image acquisition (imaging) for imaging a plurality of images so that a difference in pixel values (or signal intensity, the same applies hereinafter) can be sufficiently obtained. Configured to be able to. Here, the difference in pixel value (or signal intensity) is a difference at corresponding positions between a plurality of images. Imaging conditions for capturing a plurality of images having sufficient pixel value differences can be easily set by inverting the polarity of the gradient magnetic field.

  In addition, multiple imaging conditions with different relaxation times T such as T1 (vertical) relaxation time and T2 (horizontal) relaxation time, and multiple imaging methods such as FE (field echo) method and SE (spin echo) method Even by setting the shooting conditions, a sufficient difference in pixel value or signal value can be obtained at the corresponding position.

  Therefore, if shooting conditions for shooting a plurality of images having a sufficient difference in pixel values are set, it is possible to set shooting conditions by a method other than the method of reversing the polarity of the gradient magnetic field, Hereinafter, a case where a plurality of imaging conditions are set by reversing the polarity of the gradient magnetic field will be described.

  An example of an image acquisition pulse sequence set as an imaging condition is an EPI sequence. It is known that the imaging by the EPI sequence is particularly strongly affected by the non-uniformity of the static magnetic field strength.

  Therefore, the raw data database 41 stores the raw data generated in the receiver 30 by executing the scanning of the plurality of shooting conditions set by the shooting condition setting unit 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 unit 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 unit 49.

  The image distortion correction unit 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 calculating unit 47 and the signal intensity comparing unit 48, respectively.

  The cross-correlation calculation unit 47 receives the corrected image data obtained under a plurality of shooting conditions from the image distortion correction unit 45 and obtains a cross-correlation coefficient between the plurality of image data, A function as an image correction evaluation unit 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 unit 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.

  As a method for evaluating whether correction is appropriate, for example, the cross-correlation coefficient between image data is compared with a preset cross-correlation coefficient threshold value, and the cross-correlation coefficient is equal to or greater than the threshold value or exceeds the threshold value. When the value is a value, it is evaluated that the correction is appropriately performed. On the other hand, when the cross-correlation coefficient is less than the threshold value or less than the threshold value, it is evaluated that the correction is not appropriately performed.

  Then, the cross-correlation calculation unit 47 is configured to give a feedback instruction to the feedback unit 50 so as to optimize the correction when it is determined that the correction is not appropriate. At this time, if the region determined to be inappropriate for correction is local, the region determined to be inappropriate for correction is also notified from the cross-correlation calculation unit 47 to the feedback unit 50. Further, the amount of deviation from the threshold value of the cross correlation coefficient is also notified from the cross correlation calculation unit 47 to the feedback unit 50 as necessary.

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

  As an evaluation method for determining whether correction is appropriate, for example, the difference value of the signal intensity between the image data is compared with a threshold value of the difference value of the preset signal intensity, and the difference value of the signal intensity is equal to or greater than the threshold value or When the value exceeds the threshold value, it is evaluated that the correction is not properly performed. On the other hand, when the difference value of the signal intensity is less than the threshold value or less than the threshold value, it is evaluated that the correction is appropriately performed. It is done.

  Then, when it is determined that the correction is not appropriate, the signal strength comparison unit 48 is configured to give a feedback instruction to the feedback unit 50 so as to optimize the correction. At this time, if the region determined to be inappropriate for correction is local, the region determined to be inappropriate for correction is also notified from the signal intensity comparison unit 48 to the feedback unit 50. Further, the signal intensity comparing unit 48 is configured to notify the feedback unit 50 of the deviation amount from the threshold value of the difference value of the signal intensity as necessary.

  The magnetic field distribution calculation unit 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. Furthermore, a gradient magnetic field measurement coil 51 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 51 can be read and referenced.

  The static magnetic field distribution data estimated by the magnetic field distribution calculation unit 49 is desirably obtained as an approximate curve of an arbitrary order in which the position and the value of the static magnetic field are associated based on the value of the static magnetic field for each position. When obtaining the approximate curve, the error data of the static magnetic field obtained by measurement or design simulation is referred to. Thus, if the static magnetic field distribution data is represented by an approximate curve, the error of the point can be obtained regardless of the position to be corrected.

  However, the static magnetic field distribution data estimated by the magnetic field distribution calculation unit 49 may be a value of the static magnetic field for each position without using an approximate curve.

  The feedback unit 50 has a function of feeding back the evaluation results of correction by the cross-correlation calculation unit 47 and the signal strength comparison unit 48 by an arbitrary method. That is, when feedback unit 50 receives a feedback instruction from cross-correlation calculation unit 47 or signal strength comparison unit 48, feedback unit 50 uses at least one of approximate curve order changing unit 50A, re-correction instruction unit 50B, and shimming execution unit 50C. And is configured to optimize the correction.

  When the approximate curve order changing unit 50A receives a feedback instruction from the cross-correlation calculating unit 47 or the signal intensity comparing unit 48, the instruction to increase the order of the approximate curve stored in the static magnetic field distribution database 44 as the static magnetic field distribution data. And a function of giving an instruction to the image distortion correction unit 45 so as to execute correction of the image data using the static magnetic field distribution data newly obtained by the magnetic field distribution calculation unit 49. Have. In other words, increasing the order of the approximate curve of the static magnetic field distribution data improves the approximation accuracy of the static magnetic field distribution, and as a result, improves the accuracy of image distortion correction based on the static magnetic field distribution data.

  Therefore, the approximate curve order changing unit 50A gives an instruction to increase the order of the approximate curve to the magnetic field distribution calculating unit 49 so that the static magnetic field distribution data is recalculated and the static magnetic field distribution database 44 stores the static magnetic field distribution database 44. Improve the accuracy of magnetic field distribution data. Then, the approximate curve order changing unit 50A is configured to give an instruction to the image distortion correcting unit 45 to perform distortion correction of the image data again using the static magnetic field distribution with improved accuracy.

  However, if the region where it is determined that image data distortion correction is not appropriate is local, in order to reduce the amount of processing, it is limited to the region where image data distortion correction is determined inappropriate. It is also possible to improve the accuracy of the magnetic field distribution data.

  When receiving a feedback instruction from the cross-correlation calculation unit 47 or the signal intensity comparison unit 48, the re-correction instruction unit 50B changes the value of the static magnetic field distribution data used for correction to the image distortion correction unit 45 in a certain direction. It has a function of instructing correction of image data again and a function of determining whether or not the direction in which the static magnetic field distribution data is changed is a direction in which correction is improved.

  Whether or not the direction in which the static magnetic field distribution data is changed is a direction in which correction is improved is determined by re-correction by the image distortion correction unit 45 with respect to each threshold value of the difference value of the cross-correlation coefficient or the signal intensity. This can be done by determining whether or not the amount of deviation has been reduced. If the amount of deviation is reduced, the static magnetic field distribution data is changed again in the same direction. On the other hand, if the amount of deviation is increased, the static magnetic field distribution data is changed in the opposite direction and the image data is corrected again. In this way, the image distortion correction unit 45 can be instructed. Further, the static magnetic field distribution data can be optimized by gradually reducing the amount by which the static magnetic field distribution data is changed again.

  That is, if the static magnetic field distribution data is changed in a certain direction and the image data is corrected again, and the correction evaluation result is improved, the static magnetic field distribution is repeated until it is determined that the correction is appropriate. Convergence calculation in which data change and image data correction are performed can correct image data correction. If the evaluation result of the recorrection is not improved, the recorrection evaluation is attempted by changing the direction of change of the static magnetic field distribution data to the reverse direction. Then, if the determination of the focusing direction and the focusing calculation are repeated, it is considered that the cross correlation coefficient or the difference value of the signal intensity falls within the threshold value and it is determined that the correction has been appropriately performed.

  However, if the region where it is determined that image data distortion correction is not appropriate is local, in order to reduce the amount of processing, it is limited to the region where image data distortion correction is determined inappropriate. It is also possible to change the magnetic field distribution data.

  When receiving a feedback instruction from the cross-correlation calculation unit 47 or the signal strength comparison unit 48, the shimming execution unit 50C has a function of performing shimming by giving a control signal to the shim coil power supply 28. That is, by controlling the current or voltage output from the shim coil power supply 28 to the shim coil 22, shimming conditions can be changed and the uniformity of the static magnetic field distribution can be improved.

  When the uniformity of the static magnetic field distribution is improved, the correction accuracy of the image data is also improved, so that the correction can be made appropriate. When adjusting the shimming conditions, a shimming condition table in which the shimming conditions and the uniformity of the static magnetic field distribution are associated in advance can be referred to. This shimming condition table is configured to be stored or referred to in the shimming execution unit 50C as necessary.

  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 an 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 51, 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 51 and given to the magnetic field distribution calculation unit 49.

  On the other hand, a pulse sequence for pre-scan for static magnetic field distribution measurement is generated by the imaging condition setting unit 46 and given to the sequence controller control unit 40. For this reason, the sequence controller control unit 40 performs a pre-scan for measuring the static magnetic field distribution by giving a pulse sequence for measuring the static magnetic field distribution 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 the 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 unit 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 control unit 40, and the sequence controller control unit 40 inputs the raw data to the raw data database 41. 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 41. Is accumulated.

  Then, the magnetic field distribution calculation unit 49 reads the raw data collected by executing the pre-scan for the static magnetic field distribution measurement from the raw data database 41, while the static magnetic field distribution coil 49 detects the static magnetic field caused by the gradient magnetic field error from the gradient magnetic field measurement coil 51. 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.

  Furthermore, the magnetic field distribution calculation unit 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 the measurement by the gradient magnetic field measuring coil 51.

  Next, in step S2, an imaging condition is set by generating a pulse sequence for collecting an image of the subject P by the imaging condition setting unit 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.

  As an example of the generated pulse sequence for image acquisition, the EPI sequence is mentioned as described above.

  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 unit 46.

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

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

  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 unit 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, image data having different distortion methods stored in the image data database 43 are read into the image distortion correction unit 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. Then, 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 ), (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.

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

  Next, in step S5, the cross-correlation calculating unit 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 unit 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 unit 47 gives the display device 34 the evaluation result as to whether or not the correction of the cross-correlation coefficient between the obtained two 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 unit 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 based on the comparison result of the signal intensity. Thus, 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 unit 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, 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, as necessary, the signal strength comparison unit 48 gives the display device 34 an evaluation value as to whether the obtained signal strength difference value between the two pieces of image data or the correction of the image data has been appropriately performed. Display. For this reason, the user can easily grasp whether or not the correction of the image data is appropriately performed.

  Next, when it is determined in step S7 that at least one of the cross-correlation calculation unit 47 and the signal intensity comparison unit 48 determines that the image data is not properly corrected, the cross-correlation calculation unit 47 and / or the signal intensity comparison is performed. A feedback instruction is given from the unit 48 to the feedback unit 50 so as to optimize the correction.

  At this time, if the region where correction is not appropriate is local, the region where correction is not appropriate is also notified from the cross-correlation calculation unit 47 and / or the signal strength comparison unit 48 to the feedback unit 50. Is done. Further, if necessary, the amount of deviation from the threshold value of the cross-correlation coefficient is notified from the cross-correlation calculation unit 47 to the feedback unit 50, and the amount of deviation from the threshold value of the difference value of the signal strength is sent from the signal strength comparison unit 48 to the feedback unit 50 is notified.

  Then, in step S8, correction feedback processing is performed by an arbitrary method designated in advance. Here, in the feedback unit 50, the method of increasing the order of the approximate curve stored as the static magnetic field distribution data in the static magnetic field distribution database 44, the value of the static magnetic field distribution data is changed in a certain direction, and the image data is corrected again. A case will be described in which any of the methods of performing data collection after changing the method and shimming conditions to improve the uniformity of the static magnetic field distribution is designated as the feedback processing.

  When the method of increasing the order of the approximate curve indicating the static magnetic field distribution data is selected, a feedback instruction is given from the cross-correlation calculation unit 47 or the signal intensity comparison unit 48 to the approximate curve order change unit 50A.

  Then, in step S8A, the approximate curve order changing unit 50A gives an instruction to increase the order of the approximate curve stored in the static magnetic field distribution database 44 as static magnetic field distribution data, while the magnetic field distribution calculating unit 49 The image distortion correction unit 45 is instructed to execute the correction of the image data using the static magnetic field distribution data newly obtained by 49. For this reason, the magnetic field distribution calculation unit 49 increases the order of the approximate curve and calculates the static magnetic field distribution data again.

  In step S4 again, the image distortion correction unit 45 executes correction of the image data using the newly obtained static magnetic field distribution data. At this time, since the order of the approximate curve of the static magnetic field distribution data is increased, the approximation accuracy of the static magnetic field distribution is improved. Therefore, the accuracy of image distortion correction based on the static magnetic field distribution data is improved.

  However, if the region where it is determined that the distortion correction of the image data is not appropriate is local, the region where the distortion correction of the image data is determined not appropriate by the approximate curve order changing unit 50A is the magnetic field distribution calculation unit 49. And / or the image distortion correction unit 45 is notified so that the magnetic field distribution calculation unit 49 and / or the image distortion correction unit 45 performs the calculation of the static magnetic field distribution data and / or the correction process of the image data limited to the region. You may make it perform. The amount of processing can be reduced by such area limitation.

  Next, similarly, in step S5 and step S6, the cross-correlation calculating unit 47 and the signal intensity comparing unit 48 determine whether or not the correction of the image data is appropriate. In step S7, the same feedback processing is repeatedly executed until it is determined that the correction of the image data is appropriate.

  On the other hand, when the method of correcting the image data again by changing the value of the static magnetic field distribution data in a certain direction is selected as feedback processing, a feedback instruction is issued from the cross-correlation calculation unit 47 or the signal intensity comparison unit 48. Is given to the re-correction instruction unit 50B. At this time, the amount of divergence of the cross correlation coefficient and / or the difference value of the signal strength with respect to the respective threshold values is notified from the cross correlation calculation unit 47 and / or the signal strength comparison unit 48 to the recorrection instruction unit 50B.

  Then, in step S8B, the re-correction instruction unit 50B instructs the image distortion correction unit 45 to change the value of the static magnetic field distribution data used for correction in a certain direction and correct the image data again.

  Therefore, in step S4 again, the recorrection instruction unit 50B changes the value of the static magnetic field distribution data in a certain direction and corrects the image data again. In step S5 and step S6, the cross-correlation calculation unit 47 and the signal intensity comparison unit 48 determine whether or not the correction of the image data is appropriate.

  Here, if the direction in which the static magnetic field distribution data is changed is not appropriate, the amount of divergence of the cross-correlation coefficient or the difference value of the signal intensity with respect to the respective threshold values increases, so in step S7, the cross-correlation calculation unit 47 and The signal intensity comparison unit 48 determines that the image data correction is not appropriate.

  In such a case, in step S8B, the re-correction instructing unit 50B corrects the direction in which the amount of divergence of the cross-correlation coefficient or the difference value of the signal strength with respect to each threshold increases and changes the static magnetic field distribution data. Is determined not to be improved. Then, the recorrection instruction unit 50B instructs the image distortion correction unit 45 to change the static magnetic field distribution data in the reverse direction and correct the image data again.

  On the other hand, even if the direction of changing the static magnetic field distribution data is appropriate and the amount of divergence of the difference value of the cross-correlation coefficient or signal intensity with respect to the respective threshold values has decreased, the cross-correlation coefficient or signal intensity is still The difference value may not be within the range of the respective threshold values. Even in such a case, in step S7, the cross-correlation calculation unit 47 and the signal intensity comparison unit 48 determine that the correction of the image data is not appropriate.

  In such a case, in step S8B, the re-correction instruction unit 50B corrects the direction in which the amount of divergence of the cross-correlation coefficient or the difference value of the signal intensity with respect to the respective threshold values decreases and the static magnetic field distribution data is changed. Is determined to be improved. Then, the re-correction instruction unit 50B instructs the image distortion correction unit 45 to change the static magnetic field distribution data in the same direction again to correct the image data.

  In step S7, the same feedback processing is repeatedly executed until it is determined that the correction of the image data is appropriate. If the direction of changing the static magnetic field distribution data changes from the direction in which the correction is improved to the direction in which the correction is not improved as a result of repeating the change of the static magnetic field distribution data, the optimum value of the static magnetic field distribution data Is considered to be between the values before and after the change. Accordingly, in such a case, the change direction of the static magnetic field distribution data is reversed and the change amount is set small by the re-correction instruction unit 50B.

  In addition, when the area determined as the image data distortion correction is not appropriate is local, the area determined by the re-correction instruction unit 50B as the image data distortion correction is not appropriate to the image distortion correction unit 45. The image distortion correction unit 45 may perform the correction process of the image data limited to the area. The amount of processing can be reduced by such area limitation.

  On the other hand, if the method of collecting data again after changing the shimming conditions and improving the uniformity of the static magnetic field distribution is selected as feedback processing, the cross-correlation calculation unit 47 or the signal intensity comparison unit 48 A feedback instruction is given to the shimming execution unit 50C.

  Then, in step S8C, the shimming execution unit 50C refers to a shimming condition table that associates the shimming conditions with the uniformity of the static magnetic field distribution in advance, and sets the shimming conditions so that the uniformity of the static magnetic field distribution is improved. Then, the shimming execution unit 50C controls the current or voltage output from the shim coil power supply 28 to the shim coil 22 by giving a control signal to the shim coil power supply 28 in accordance with the set shimming conditions. Thereby, shimming according to the reset shimming conditions is performed, and the uniformity of the static magnetic field distribution is improved.

  For this reason, if the scan is executed again, not only image data can be collected under a more uniform static magnetic field distribution, but also the static magnetic field distribution data used for correction becomes more uniform. As a result, the correction accuracy of the image data can be improved.

  Then, until it is determined in step S7 that the correction of the image data is appropriate, the processing from step S1 to step S6, scanning, and similar feedback processing are repeatedly executed.

  In this manner, when the cross-correlation calculating unit 47 and the signal intensity comparing unit 48 determine that the image data has been corrected appropriately in step S7, the image data correction is not fed back and distortion is satisfactorily performed. Is displayed on the display device 34. For this reason, the user can make a diagnosis with reference to image data whose distortion has been appropriately corrected.

  In the above-described example, the case where the feedback processing by a single method is designated has been described. However, the feedback processing by a plurality of methods may be used in the designated priority order. For example, if the image data is still not properly corrected even if the order of the approximate curve in the static magnetic field distribution data is increased and the static magnetic field distribution data is changed in a certain direction several times, shimming is performed and data is collected again You may make it do.

  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.

  Further, when an evaluation that the correction is not appropriate is obtained, feedback processing is performed so that appropriate correction is performed.

  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.

  In particular, in the imaging by the EPI sequence that is particularly strongly affected by the non-uniformity of the static magnetic field strength, there is a possibility that the image distortion is not properly corrected. It is possible to evaluate whether or not is properly performed.

  Further, even when the image distortion is not properly corrected, it is possible to obtain image data that is finally corrected more appropriately by the feedback processing.

  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.

  In the above-described embodiment, the case of evaluating the correction of the image distortion caused by the non-uniformity of the static magnetic field intensity has been described. As for correction, the correction result of image distortion can be evaluated based on the same principle. In other words, an image corresponding to the gradient magnetic field intensity distribution based on the cross-correlation between the plurality of images obtained under different imaging conditions and the distortion corrected according to the nonuniformity of the gradient magnetic field distribution and the difference in signal intensity. The distortion correction result can be evaluated.

  Furthermore, when it is evaluated that correction of image distortion is not appropriate, it is possible to obtain appropriately corrected image data by feedback processing.

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 with the magnetic resonance imaging apparatus shown in FIG. The figure which shows an example of the pulse sequence produced | generated in order to evaluate 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. Explanatory drawing which shows an example of the method at the time of correct | amending image data based on static magnetic field distribution by the magnetic resonance imaging apparatus shown in FIG.

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 unit 41 Raw data database 42 Image reconstruction unit 43 Image data database 44 Static magnetic field distribution database 45 Image distortion correction unit 46 Imaging condition setting unit 47 Cross correlation calculation unit 48 Signal intensity Comparison unit 49 Magnetic field distribution calculation unit 50 Feedback unit 50A Approximate curve order change unit 50B Re-correction instruction unit 50C Shimming execution unit 51 Gradient magnetic field measurement coil P Subject

Claims (22)

Shooting condition setting means for setting a plurality of different shooting conditions;
Receiving means for receiving a magnetic resonance signal from the imaging region according to the plurality of imaging conditions;
Image reconstruction means for reconstructing a plurality of image data corresponding to the plurality of imaging conditions based on the magnetic resonance signal;
Image distortion correction means for correcting distortion of the plurality of image data based on the magnetic field distribution of the imaging region;
Image correction evaluation means for evaluating whether or not the correction of at least one of the image data is appropriately performed based on a plurality of corrected image data;
A magnetic resonance imaging apparatus comprising feedback means for feeding back an evaluation result obtained by the image correction evaluation means . 2. The imaging condition setting unit is configured to set a plurality of imaging conditions in which polarities of gradient magnetic fields in at least one of a phase encoding direction, a readout direction, and a slice encoding direction are reversed. The magnetic resonance imaging apparatus described. The image correction evaluation means obtains a cross-correlation coefficient between the corrected plurality of image data, and whether or not the at least one of the image data is appropriately corrected based on the obtained cross-correlation coefficient The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is configured to evaluate the above. The image correction evaluation unit is configured to evaluate whether or not the correction of the at least one image data is appropriately performed by comparing signal intensity for each pixel among the plurality of corrected image data. The magnetic resonance imaging apparatus according to claim 1, wherein: The magnetic resonance imaging apparatus according to claim 1, wherein the image distortion correction unit is configured to correct distortion of the plurality of image data based on a static magnetic field distribution in the imaging region. The magnetic resonance imaging apparatus according to claim 1, wherein the image distortion correction unit is configured to correct distortion of the plurality of image data based on a gradient magnetic field distribution in the imaging region. The magnetic resonance imaging apparatus according to claim 1, wherein the imaging condition setting unit is configured to set a plurality of imaging conditions by an EPI method . Magnetic resonance imaging apparatus according to claim 1, further comprising wherein Rukoto display means for displaying the evaluation result of the image correction evaluation unit. It said feedback means, a magnetic resonance imaging apparatus according to claim 1, wherein Rukoto is configured to correct the distortion of the plurality of image data to the image distortion correction means by changing the magnetic field distribution. It said feedback means, magnetic claim 1, wherein Rukoto configured as said order to increase the approximate curve representing the magnetic field distribution to correct the distortion of the plurality of image data in the image distortion correction means Resonance imaging device. It said feedback means, according to claim 1, wherein Rukoto is configured to correct the distortion of the plurality of image data reconstructed on the basis of the magnetic resonance signal received after shimming the image distortion correction means Magnetic resonance imaging device. A step of setting different shooting conditions;
Receiving a magnetic resonance signal from an imaging region according to the plurality of imaging conditions;
Reconstructing a plurality of image data corresponding to the plurality of imaging conditions based on the magnetic resonance signals;
Correcting distortion of the plurality of image data based on the magnetic field distribution of the imaging region;
Evaluating whether or not the correction of at least one of the image data is appropriately performed based on a plurality of corrected image data;
Feeding back an evaluation result indicating whether or not the correction is appropriately performed;
An image correction evaluation method characterized by comprising: 13. The image correction evaluation method according to claim 12, wherein a plurality of imaging conditions in which the polarities of gradient magnetic fields in at least one of the phase encoding direction, the readout direction, and the slice encoding direction are reversed are set . Obtaining a cross-correlation coefficient between the plurality of corrected image data, and evaluating whether or not the at least one of the image data is appropriately corrected based on the obtained cross-correlation coefficient; The image correction evaluation method according to claim 12 . 13. The method according to claim 12 , wherein whether or not the at least one of the image data has been corrected is evaluated by comparing a signal intensity for each pixel between the plurality of corrected image data . Image correction evaluation method. Image correction evaluation method according to claim 12, wherein that you correct the distortion of the plurality of image data based on the static magnetic field distribution of the imaging area. Image correction evaluation method according to claim 12, wherein Rukoto be configured to correct the distortion of the plurality of image data based on the gradient distribution of the imaging area. The image correction evaluation method according to claim 12, wherein a plurality of photographing conditions by the EPI method are set . Image correction evaluation method according to claim 12, wherein further having a Rukoto the step of displaying the evaluation result indicating whether the correction is properly performed. 13. The image correction evaluation method according to claim 12, wherein distortion of the plurality of image data is corrected by changing the magnetic field distribution . 13. The image correction evaluation method according to claim 12, wherein the distortion of the plurality of image data is corrected by increasing the order of the approximate curve representing the magnetic field distribution . 13. The image correction evaluation method according to claim 12 , wherein distortion of a plurality of reconstructed image data is corrected based on a magnetic resonance signal received after shimming .

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