JP2006262928A - Magnetic resonance imaging system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an MRI system which can improve the picture quality by reducing an artifact and so forth on an image resulting from an irregular magnetic field by an eddy current and a remaining magnetic field which occur accompanying the application of a gradient magnetic field in the diffusing emphasis imaging method. <P>SOLUTION: This system comprises a means which generates a static magnetic field, a means which generates a high-frequency magnetic field pulse in the static magnetic field space, a means which generates the gradient magnetic field in the static magnetic field space, and a measurement control means. In this case, the measurement control means is used to acquire an image in which the diffusing motion of desired molecules included in a desired region is reflected by applying the high-frequency magnetic field pulse and the gradient magnetic field from a specified pulse sequence including an MPG pulse. The measurement control means applies a pre-pulse gradient magnetic field prior to the first high-frequency magnetic pulse in the pulse sequence. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging (hereinafter abbreviated as `` MRI '') apparatus for acquiring a tomographic image of a desired site of a subject using a nuclear magnetic resonance (hereinafter abbreviated as `` NMR '') phenomenon, In particular, the present invention relates to reducing artifacts resulting from application of a gradient magnetic field when acquiring a diffusion weighted image.

  An MRI apparatus is a medical diagnostic imaging apparatus that causes an NMR phenomenon to occur in water molecules in a desired plane that crosses a subject, and obtains a tomographic image in the plane from an NMR signal generated thereby. In general, a high-frequency magnetic field pulse (hereinafter abbreviated as “RF pulse”) that excites magnetization in the plane at the same time as applying a slice gradient magnetic field that specifies the plane from which a tomographic image of the subject is to be obtained. Apply. An NMR signal (echo signal) is obtained by converging the phase of the excited magnetization. Further, in order to give position information to the echo signal, a phase encoding gradient magnetic field and a signal readout gradient magnetic field are applied in a direction perpendicular to each other in the tomographic plane between the excitation and the acquisition of the echo signal.

  An RF pulse for generating an echo signal and each gradient magnetic field are applied based on a preset pulse sequence. Various pulse sequences are known depending on the purpose. Among them, a diffusion-weighted imaging method (hereinafter abbreviated as “DWI method”) that images the degree of diffusion of water molecules and the like is known.

  The general pulse sequence of this DWI method is based on the spin echo pulse sequence, in which an MPG (Motion Probing Gradient) pulse is applied after 90 ° RF pulse and 180 ° inverted RF pulse. Configured. Data for reconstructing one diffusion weighted image is acquired by giving different phase encoding amounts to the echo signals acquired in this way. The main measurement unit for acquiring image data generally uses an SE-based EPI pulse sequence (SE-EPI) or a fast spin echo pulse sequence (FSE).

  However, in the DWI method, it is necessary to apply a phase rotation to the magnetization of water molecules having an extremely slow flow velocity, and thus it is necessary to apply a high-intensity MPG pulse. Therefore, an eddy current resulting from application of an MPG pulse or other gradient magnetic field, or an irregular magnetic field due to residual magnetization, may cause the gradient magnetic field application shape and application amount to deviate from the values defined by the pulse sequence. When such imperfections in the gradient magnetic field occur, it becomes impossible to apply an accurate phase rotation to the excited magnetization. As a result, it is known that artifacts occur on the diffusion weighted image.

  In the DWI method, it is a problem to suppress the above-mentioned artifact, and in particular, it is an important problem to reduce eddy current and residual magnetization caused by application of the MPG pulse. (Patent Document 1) discloses means for correcting eddy currents. That is, it has means to adjust the offset magnetic field of the gradient magnetic field and the current supplied to the shim coil according to the gradient magnetic field application intensity, application time, application timing, etc., and corrects the irregular magnetic field due to the eddy current accompanying gradient magnetic field application is doing.

Further, (Patent Document 2) discloses a method for correcting the influence of residual magnetization. That is, a residual magnetization reset pulse is applied immediately after the MPG pulse to cancel the residual magnetization accompanying the MPG pulse application.
JP-A-10-248822 JP 2002-159462 A

As described in (Patent Document 3), it is known that an eddy current is generated in a nonlinear manner depending on the strength of an applied gradient magnetic field. In addition, it is known that the generation of eddy current is also location dependent. In order to correct eddy currents that exhibit such complex behavior with the means disclosed in the above (Patent Document 1), the eddy current generation pattern is measured in advance in advance in association with the applied gradient magnetic field. It is necessary to keep in mind, and the burden for that is thought to be large. In addition, in order to correct eddy currents having complicated location dependence with a shim coil or the like, it is necessary to prepare a higher-order shim coil. However, there is a case where a realistic compromise is imposed from the technical and economic viewpoint, and as a result, a higher-order component of the eddy current remains.
Japanese Patent Application No. 2004-030577

  In addition, the method disclosed in the above (Patent Document 2) is supposed to cancel the residual magnetization generated by the application of the MPG pulse, but in general, the residual magnetization remains even before the MPG pulse is applied. is there. In other words, the pulse sequence is usually repeated multiple times at the repetition time (TR), and if it is multi-slice measurement, the pulse sequence for measuring the echo signal from each slice is repeated within the TR time, so MPG pulse application There is also a residual magnetization before, and the residual magnetization may be different for each slice. Therefore, only the method disclosed in the above (Patent Document 2) does not cancel the residual magnetization remaining before applying the first MPG pulse from the 90 ° RF pulse, and the phase rotation based on the gradient magnetic field due to this residual magnetization is reflected in the echo signal. There is a possibility of being. (Patent Document 2) does not disclose a solution to this problem.

  Accordingly, an object of the present invention is to improve image quality by reducing artifacts and the like on an image based on an eddy current generated by applying a gradient magnetic field and an irregular magnetic field due to a residual magnetic field in a diffusion weighted imaging method. To provide an MRI device.

In order to achieve the above object, the MRI apparatus of the present invention is configured as follows. That is,
Means for generating a static magnetic field, means for generating a high-frequency magnetic field pulse in the static magnetic field space, means for generating a gradient magnetic field in the static magnetic field space, and a desired region of the subject disposed in the static magnetic field And applying the high-frequency magnetic field pulse and the gradient magnetic field based on a predetermined pulse sequence including an MPG pulse to obtain an image reflecting the diffusion movement of a desired molecule contained in the desired region. Measurement control means, and the measurement control means applies a pre-pulse gradient magnetic field before the first high-frequency magnetic field pulse in the pulse sequence.

In another preferred embodiment of the MRI apparatus of the present invention, the prepulse gradient magnetic field is applied at least in the same direction as the MPG pulse. Alternatively / further, the prepulse gradient magnetic field having substantially the same application intensity and application time as the MPG pulse is applied.
In another preferred embodiment of the MRI apparatus of the present invention, the MPG pulse is applied a plurality of times with the application of the high-frequency magnetic field pulse therebetween, and the prepulse gradient magnetic field and the plurality of MPG pulses are equidistantly spaced. Apply.

  According to the MRI apparatus of the present invention, by adding a pre-pulse gradient magnetic field to the beginning of the pulse sequence, the gradient magnetic field error component based on the eddy current generated by the application of the MPG pulse and the irregular magnetic field due to residual magnetization is always constant. (That is, steady state). For this reason, the phase error applied to the echo signal based on this gradient magnetic field error component can always be kept constant, preferably zero. As a result, it is possible to reduce image artifacts based on fluctuations in the irregular magnetic field and improve image quality.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of the MRI apparatus of the present invention will be described based on the block diagram shown in FIG. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject, and as shown in FIG. 1, a static magnetic field generating magnet 2, a magnetic field gradient generating system 3, a transmitting system 5, and a receiving system 6, a signal processing system 7, a sequencer 4, and a central processing unit (CPU) 8.

  The static magnetic field generating magnet 2 generates a uniform static magnetic field around the subject 1 in the direction of the body axis or in a direction perpendicular to the body axis. In the space having a certain extent around the subject 1 Permanent magnet type, normal conducting type or superconducting type magnetic field generating means is arranged.

  The magnetic field gradient generating system 3 includes a gradient magnetic field coil 9 wound in three axial directions of X, Y, and Z, and a gradient magnetic field power source 10 for driving each gradient magnetic field coil. By driving the gradient magnetic field power supply 10 of each coil in accordance with the command, gradient magnetic fields Gx, Gy, and Gz in three axis directions of X, Y, and Z are applied to the subject 1. The slice plane for the subject 1 can be set by applying this gradient magnetic field.

  The sequencer 4 repeatedly applies an RF pulse that causes an NMR phenomenon to the nucleus of the atoms constituting the living tissue of the subject 1 in a predetermined pulse sequence, operates under the control of the CPU 8, and Various commands necessary for collecting data of one tomographic image are sent to the transmission system 5, the magnetic field gradient generation system 3, and the reception system 6.

  The transmission system 5 irradiates an RF pulse for causing an NMR phenomenon to the atomic nucleus constituting the living tissue of the subject 1, and includes a high frequency oscillator 11, a modulator 12, a high frequency amplifier 13, and a high frequency on the transmission side. And coil 14a. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 according to the command of the sequencer 4, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13 and then placed close to the subject 1. By supplying the high-frequency coil 14a, the subject 1 is irradiated with RF pulses (electromagnetic waves).

  The receiving system 6 detects an echo signal (NMR signal) emitted by the NMR phenomenon of the nucleus of the biological tissue of the subject 1, a receiving side high-frequency coil 14b, an amplifier 15, a quadrature detector 16, And an A / D converter 17. An electromagnetic wave (NMR signal) of the response of the subject 1 due to the RF pulse irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1, and the amplifier 15 and the quadrature phase The signals are divided into two systems orthogonal to each other via the detector 16 and each is input to the A / D converter 17 and converted into a digital quantity, and these digital signals are sent to the signal processing system 7 as echo signal data. It is supposed to be.

  The signal processing system 7 includes a CPU 8, a recording device such as a magnetic disk 18 and an optical disk 19, and a display 20 such as a CRT. The CPU 8 performs processing such as Fourier transform, correction coefficient calculation, image reconstruction, and the like. The signal intensity distribution of the cross section or the distribution obtained by performing an appropriate operation on a plurality of signals is imaged and displayed on the display 20 as a tomographic image.

  In FIG. 1, the high-frequency coils 14a and 14b and the gradient magnetic field coil 9 on the transmission side and the reception side are installed in the magnetic field space of the static magnetic field generating magnet 2 arranged in the space around the subject 1. .

Next, a pulse sequence of the DWI method will be described. This pulse sequence is stored in the magnetic disk 18 as a program, for example, and is read by the CPU 8 as necessary and transmitted to the sequencer 4 for execution. FIG. 2 is an example of a DWI method pulse sequence in which an MPG pulse is applied to an SE-EPI pulse sequence. While applying the slice selection gradient magnetic field 211 in the slice direction Gs, a slice selection 90 ° RF pulse 201 is applied to excite a desired slice region. Thereafter, the first MPG pulse 231 is applied in the reading direction Gr. Next, a gradient magnetic field 212 and a 180 ° inversion RF pulse 202 are applied in the slice direction Gs in order to reverse the magnetization of the excitation region and form an echo signal. Thereafter, the second MPG pulse 232 is applied in the reading direction Gr. After these RF pulses and gradient magnetic field, the phase encode gradient magnetic field 222 for encoding spatial information in the phase encode direction Gp and the frequency encode in the signal readout direction Gr and simultaneously reading the echo signal Applying the gradient magnetic field 234 while reversing the polarity, a plurality of echo signals for image reconstruction are acquired. Before applying the phase encoding gradient magnetic field 222 and the signal reading gradient magnetic field 234, the dephase gradient magnetic fields 221 and 233 are applied to adjust the initial position of the echo signal data arranged in the k space. Since this SE-EPI pulse sequence is described in, for example, (Patent Document 4), further detailed description is omitted.
JP 2003-225223 A

  In order to make the amount of phase rotation applied to the stationary magnetization after the application of the second MPG pulse 232, the application amount of the first MPG pulse 231 and the second MPG pulse 232 {= (application intensity) × (application time)} Are the same. Usually, the application intensity and application time of each MPG pulse are made the same. Thereby, since the phase rotation is applied only to the moving magnetization, the echo signal obtained from the moving magnetization is attenuated. That is, the signal from the diffusing water molecule is attenuated compared to the echo signal from the stationary water molecule. As a result, in the diffusion weighted image, the signal is low in the region where water molecules are diffused.

More specifically, the signal strength S (d) of the spin echo signal acquired by applying the MPG pulse as described above is the signal strength S (d) of the spin echo signal acquired without applying the MPG pulse. se) multiplied by a term whose diffusion coefficient (D) is a variable. That means
S (d) = S (se) · exp (−b · D) (1)
Here, b is a value representing the effect of the MPG pulse and is called b-factor. Based on this b-factor, the applied intensity and duration of the MPG pulse that should be applied to bring about the desired signal degradation in the diffusion-weighted image (i.e. to give the image a contrast reflecting the diffusion of water molecules) Is determined. Thereby, in the DWI method, it is understood that the signal value of the tissue including water molecules that diffuse even in the stationary part is lower than the signal value by the normal spin echo method without the MPG pulse.

  As described above, when an MPG pulse is inserted and a diffusion weighted image is captured, an irregular magnetic field based on eddy current and remanent magnetization generated with the application of the MPG pulse is a problem. In other words, it is necessary to apply a high-intensity MPG pulse in order to reflect the diffusion motion of water molecules having an extremely slow flow velocity in the phase rotation of the magnetization of the water molecules. With the application of such a high-intensity gradient magnetic field, eddy currents are generated in the conductor members constituting the MRI apparatus. Further, based on the hysteresis phenomenon of the ferromagnetic member constituting the MRI apparatus, residual magnetization is generated in the ferromagnetic member. Since the irregular magnetic field based on these eddy currents and remanent magnetization is applied in addition to the gradient magnetic field by the original MPG pulse, the applied amount of the gradient magnetic field by the two MPG pulses is not the same, and the difference is different. It will occur. As a result, artifacts are generated in the obtained diffusion weighted image, or S / N is reduced.

  In multi-slice measurement, the gradient magnetic field applied to measure each slice within the repetition time (TR) is 1 slice depending on the set TR, multi-slice number, measurement echo signal number, echo signal interval, etc. Since the per-measuring time is different, it is not always applied at regular intervals. For this reason, an eddy current that decays with a time constant and an irregular magnetic field due to constant remanent magnetization give a phase rotation amount different in each slice to the intra-slice magnetization, and cause artifacts in the image.

More specifically, the phase rotation amount Φ of the magnetization when the gradient magnetic field strength Gx is applied is an amount proportional to the gradient magnetic field strength Gx and its application time t as shown in Equation (2). That means
Φ = γX ・ ∫Gxdt = γX ・ (gradient magnetic field application amount) (2)
(γ: Magnetic rotation ratio, X: Coordinate value) Since the gradient magnetic field application amount between 90 ° -180 ° RF pulses (hereinafter abbreviated as “time width A”) is ξ1, 180 ° RF pulses − When the gradient magnetic field application amount between the spin echo centers (hereinafter abbreviated as “time width B”) is ξ2, the phase rotation difference ΔΦ is
ΔΦ = γX · (ξ1-ξ2) = γX · Δξ (3)
It becomes. Originally, Δξ = ξ1−ξ2 = 0 and should be ΔΦ = 0, but because of the difference in applied amount due to irregular magnetic field, Δξ ≠ 0, ΔΦ ≠ 0, and MPG pulse These fluctuate according to the state of gradient magnetic field application before application.

A specific influence of the irregular magnetic field and its variation on the spin echo signal acquired at the echo time TE is, for example, that the echo center shifts before and after the echo time TE. In addition, the amount of phase rotation applied to the echo signal varies. Furthermore, when the direction of the irregular magnetic field extends not only to the signal reading direction Gr but also to the phase encoding direction Gp (so-called Cross Term), the echo center shift and the phase rotation fluctuation also extend to the phase encoding direction. Become. This echo center shift and phase rotation variation can have a significant effect, especially on diffusion-weighted images, and can cause severe artifacts on the image.
In order to suppress the fluctuation of the irregular magnetic field, a prepulse gradient magnetic field is applied before the 90 ° RF pulse is applied in the present invention described below.

  Next, an embodiment of a pulse sequence of the DWI method in the MRI apparatus of the present invention will be described. In this embodiment, by applying a pre-pulse gradient magnetic field in the same direction as the MPG pulse application direction before the 90 ° RF pulse, not only the application amount of the MPG pulse but also the application amount of the irregular magnetic field is substantially constant. It is a form to become. Preferably, the prepulse gradient magnetic field has the same shape as the MPG pulse (that is, the gradient magnetic field having the same waveform with the same applied intensity and the same applied time).

  FIG. 3 shows an example of this embodiment. FIG. 3 is an example in which a prepulse gradient magnetic field 301 is further added before the 90 ° RF pulse to the DWI method pulse sequence in which the MPG pulse is applied to the SEE-EPI pulse sequence shown in FIG. Note that the application sequence of the gradient magnetic field in the slice direction Gs and the phase encoding direction Gp is the same as that in FIG. 2, and is omitted in FIG. FIG. 6 shows an overall image of an example of the DWI method pulse sequence of the present invention.

  The prepulse gradient magnetic field 301 and the first and second MPG pulses 231 and 232 are preferably applied in the same amount. That is, it is preferable that the application intensity and application time of these gradient magnetic fields are the same. The application timing is preferably as follows. That is, the time interval 311-1 between the application start times of the prepulse gradient magnetic field 301 and the first MPG pulse 231 and the time interval 311-2 between the application start times of the first MPG pulse 232 and the second MPG pulse 231 are made the same. Accordingly, the time interval 312-1 between the application end time of the prepulse gradient magnetic field 301 and the application start time of the first MPG pulse 231 and the time interval 312 between the application end time of the first MPG pulse 232 and the application start time of the second MPG pulse 231. -2 is the same.

  By applying the pre-pulse gradient magnetic field 301 as described above, the application amount of the irregular magnetic field can be made constant, and preferably substantially the same before and after the 180 ° RF pulse, based on FIGS. Will be described below.

  First, it will be described that the amount of irregular magnetic field applied by eddy current can be made constant. FIG. 4 is a diagram for explaining an irregular magnetic field due to an eddy current generated by application of a gradient magnetic field. Fig. 4 (a) shows that the steep rising and falling parts of the gradient magnetic field 401 should be smooth due to the irregular magnetic field caused by the eddy current generated when the rectangular gradient magnetic field 401 (solid line) is applied. A state where the waveform 402 (dotted line) is distorted is shown.

  FIG. 4 shows an example in which the gradient magnetic field waveforms of the prepulse gradient magnetic field 301 and the first and second MPG pulses 231 and 232 in FIG. 3 are distorted by an irregular magnetic field due to eddy current (dotted line) in accordance with the example of FIG. Shown in (b). From this figure, the applied amount by the distorted gradient magnetic field waveform in TE / 2 time width A between 90 ° RF pulse 201 and 180 ° RF pulse 202, and TE / between the echo center of 180 ° RF pulse 202 and spin echo signal. It is understood that the applied amount by the distorted gradient magnetic field waveform in the two-hour width B is the same. That is, by applying the prepulse gradient magnetic field 301 before the first MPG pulse 231, the application amount 411-1 in the time width A of the irregular magnetic field due to the eddy current generated along with the application of the prepulse gradient magnetic field 301 is changed to the first MPG pulse. It becomes possible to make it equal to the applied amount 411-2 in the time width B of the irregular magnetic field due to the eddy current generated by the application of.

  As a result, for the spin echo signal acquired at the echo time TE, even if there is an irregular magnetic field due to the eddy current generated by the application of two MPG pulses, the amount of gradient magnetic field applied by the irregular magnetic field The phase error based on the error (hereinafter referred to as “gradient magnetic field error component”) is canceled out to be constant, preferably zero.

  Furthermore, by inserting a pre-pulse gradient magnetic field 301 having the same amplitude and application time before the first MPG pulse, it becomes difficult to be affected by the eddy current accompanying the gradient magnetic field applied before that. That is, compared with the applied intensity of the prepulse gradient magnetic field 301, the intensity of the irregular magnetic field due to the eddy current remaining when the prepulse gradient magnetic field 301 is applied is sufficiently small, so that the influence of the irregular magnetic field is relatively negligible.

  This is particularly significant in multi-slice photography. In other words, in the measurement of each slice performed within the repetition time (TR), the measurement time per slice differs depending on the set TR, the number of multi-slices, the number of measurement echo signals, the echo signal interval, etc. The gradient magnetic field is not necessarily applied at regular intervals between the slices. For this reason, an irregular magnetic field caused by an eddy current that decays with a certain time constant gives a different amount of phase rotation to each magnetization. As a result, the phase error fluctuates, resulting in artifacts in the image.

  On the other hand, by inserting the pre-pulse gradient magnetic field 301, the gradient magnetic field error component due to the irregular magnetic field is substantially constant (i.e., steady state) because it is less susceptible to eddy currents associated with the gradient magnetic field previously applied. ). As a result, it becomes possible to reduce the arch fuck of the image.

As long as the time intervals 311-1 and 311-2 are always kept constant, they may be different. By keeping these time intervals 311-1 and 311-2 constant at all times, the gradient magnetic field error component due to the irregular magnetic field can be always constant during the time widths A and B. The phase error applied to the magnetization based on the component can also be made constant at all times.
If a known phase correction technique is used, it is also possible to correct a stationary phase error applied to the echo signal. In particular, the zero-order and first-order phase corrections are easy.

  When there is a cross term effect, application of an MPG pulse may generate a gradient magnetic field error component in a direction different from the application direction. In that case, the gradient magnetic field error component may be made steady by applying the prepulse gradient magnetic field in a direction different from the MPG pulse application direction. Furthermore, the applied intensity of the prepulse gradient magnetic field is not necessarily the same as that of the first MPG pulse and the second MPG pulse. By changing the prepulse gradient magnetic field intensity, the gradient magnetic field error component can be adjusted to adjust the image quality. You may do it.

  As described above, the prepulse gradient magnetic field is inserted before the 90 ° RF pulse, and the time interval between the prepulse gradient magnetic field, the first MPG pulse, and the second MPG pulse is kept constant, resulting in an irregular magnetic field caused by eddy currents. It is possible to make the gradient magnetic field error component to be always constant (preferably the same) between the time width A in which the first MPG pulse is applied and the time width B in which the second MPG pulse is applied. Therefore, the gradient magnetic field error component and the phase error in each time width can always be constant (preferably the same). As a result, artifacts generated in the diffusion weighted image can be reduced.

  As a comparison, in the conventional technique without a prepulse gradient magnetic field, since there is no irregular magnetic field application amount 411-1 in the time width A, the difference in gradient magnetic field error component becomes large, and before the first MPG pulse. It is easy to understand that the phase error amount and the difference thereof also fluctuate and artifacts are likely to occur in the image because it is easily affected by the irregular magnetic field.

  Next, the irregular magnetic field due to the residual magnetization generated with the application of the gradient magnetic field will be described with reference to FIG. FIG. 5 (a) shows a constant irregular magnetic field 502 (shaded part) after the intended gradient magnetic field 501 due to an irregular magnetic field (residual magnetic field) due to residual magnetization generated with the application of the rectangular gradient magnetic field 501 (solid line). ) Is generated. The irregular magnetic field 502 has the same effect as continuing to apply a minute gradient magnetic field. In addition, if there is a gradient magnetic field in the pulse sequence performed immediately before or a gradient magnetic field with a different applied intensity for each TR, such as phase encoding, the residual magnetization immediately before the application of the first MPG pulse fluctuates. These irregular magnetic fields and their variations will cause gradient field error components and resulting phase errors and differences between their 180 ° RF pulses to fluctuate, and these errors and their variations will cause artifacts in the image. Become.

  Therefore, in the present invention, even for an irregular magnetic field due to residual magnetization, by applying a pre-pulse gradient magnetic field before the 90 ° RF pulse, the gradient magnetic field error component is kept constant between 180 ° RF pulses, preferably the same. To do.

  According to the example of FIG. 5 (a), the pre-pulse gradient magnetic field 301 and the gradient magnetic field waveforms of the first and second MPG pulses 231 and 232 in FIG. 3 are distorted by adding an irregular magnetic field due to residual magnetization after them (hatched line) (B) is shown in FIG. Here, as shown in FIG. 5 (b), this prepulse gradient magnetic field 301 is preferably applied in the same direction as the MPG pulse application direction, and an example thereof is shown. In addition, since it is necessary to be equivalent to the influence of the first MPG pulse 231 on the second MPG pulse 232, it is preferable that the application intensity and application time of the prepulse gradient magnetic field 301 are the same as those of the MPG pulse. An example is shown.

  Here, the hysteresis characteristic of the residual magnetization will be described. That is, after the residual magnetization reaches the saturation magnetization, the magnitude of the residual magnetization does not change as long as a gradient magnetic field having the same polarity and the same strength is applied. Therefore, when the polarities of the prepulse gradient magnetic field 301 and the first and second MPG pulses are the same and the applied intensity has a large amplitude, the residual magnetization after application of the prepulse gradient magnetic field 301 reaches saturation magnetization, and then The magnitude of the remanent magnetization does not change even after the two MPG pulses are applied. That is, the residual magnetization becomes almost constant after the prepulse gradient magnetic field 301 is applied.

  From FIG.5 (b), as in FIG.4 (b), the application amount by the gradient magnetic field waveform in the TE / 2 time width A between the 90 ° RF pulse 201 and the 180 ° RF pulse 202, and the 180 ° RF pulse 202 It is understood that the applied amount by the gradient magnetic field waveform in the TE / 2 time width B between the echo centers of the spin echo signal is the same. That is, by applying the prepulse gradient magnetic field 301 to the first MPG pulse 231 before, the application amount 511-1 in the period A of the irregular magnetic field due to the residual magnetization generated with the application of the prepulse gradient magnetic field 301 is changed to the irregularity in the period B. It becomes possible to make it equal to the magnetic field application amount 511-2. That is, in the time width A and the time width B, the gradient magnetic field error component amount based on the inhomogeneous magnetic field due to the residual magnetization and the phase error applied to the magnetization based on the gradient magnetic field error component can always be constant (steady). Furthermore, as shown in FIG. 3, if the mutual time intervals are equal, they can be made the same.

  As a result, for the spin echo signal acquired at the echo time TE, even if there is an irregular magnetic field due to residual magnetization, the phase error based on the gradient magnetic field error component due to the irregular magnetic field is constant (steady). Preferably it can be zero.

  As described above, by inserting a high-intensity pre-pulse gradient magnetic field in front of the 90 ° RF pulse, it is possible to generate residual magnetization, thereby inducing an irregular magnetic field and causing a gradient magnetic field error component. The gradient magnetic field error components in the width A and the time width B can be made the same. In addition, almost constant residual magnetization can be generated by a high-intensity prepulse gradient magnetic field without being affected by the residual magnetization in the measurement of other slices applied immediately before, so that the gradient magnetic field of the immediately preceding pulse sequence can be generated. The gradient magnetic field error component and the phase error based on the gradient magnetic field error component can always be made constant (that is, steady) without being affected. If the phase error does not change, the image quality can be stabilized by performing zero-order and first-order phase error correction on the echo signal. In particular, if the gradient magnetic field error component and the phase error based on this are made the same before and after the 180 ° RF pulse, the phase error based on this gradient magnetic field error component can be made zero in the echo signal. Is also unnecessary.

  When there is a cross term effect, the error component can be made steady or zero by applying the prepulse gradient magnetic field in a direction different from the MPG pulse application direction, as in the case of the irregular magnetic field due to the eddy current. Furthermore, the applied intensity of the prepulse gradient magnetic field does not necessarily have to be the same as that of the first MPG pulse and the second MPG pulse, and the image quality can be adjusted by adjusting the gradient magnetic field error component by changing the prepulse gradient magnetic field intensity. It is desirable to make the same as in the case of the irregular magnetic field due to the eddy current.

  The above is the description of one embodiment of the DWI method pulse sequence in the MRI apparatus of the present invention. However, the present invention is not limited to the contents disclosed in the description of the above-described embodiment, and can take other forms based on the gist of the present invention. For example, the case of the DWI method pulse sequence using the SE-EPI sequence has been described. However, the DWI method of the present invention is not limited to the SE-EPI sequence, and the pulse sequence using the high-speed spin echo method and other 90 ° -180 ° RF pulses. Laws may apply.

  As described above, according to the MRI apparatus of the present invention, by adding a pre-pulse gradient magnetic field to the beginning of a pulse sequence, a gradient magnetic field based on an eddy current generated due to application of an MPG pulse or an irregular magnetic field due to residual magnetization. The error component can always be constant (that is, steady). Therefore, the gradient magnetic field error component, the phase error applied to the magnetization based on the gradient magnetic field error component before and after the 180 ° RF pulse, and the difference between them can be made constant, preferably zero. The phase error applied to the echo signal based on the difference in the gradient magnetic field error component can always be kept constant, preferably zero. As a result, it is possible to improve image quality by reducing image artifacts based on fluctuations in the irregular magnetic field.

The block diagram which shows the whole image of an example of the MRI apparatus with which this invention is applied. The figure showing DW EPI pulse sequence. The figure showing the application timing of the pre-pulse gradient magnetic field and MPG pulse used for an Example. The figure showing the gradient magnetic field error component by an eddy current. The figure showing the gradient magnetic field error component by a residual gradient magnetic field. The figure showing the DW EPI pulse sequence used for an Example.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Subject, 2 ... Magnetic field generator, 3 ... Magnetic field gradient generation system, 4 ... Sequencer, 5 ... Transmission system, 6 ... Reception system, 7 ... Signal processing system, 8 ... CPU, 9 ... Gradient magnetic field coil, 10 ... gradient magnetic field power supply, 14a ... high frequency coil on transmission side, 14b ... high frequency coil on reception side, 15 ... amplifier, 16 ... quadrature phase detector, 17 ... A / D converter, 18 ... magnetic disk, 19 ... magnetic tape, 20 ... Display

Claims (4)

Means for generating a static magnetic field;
Means for generating a high-frequency magnetic field pulse in the static magnetic field space;
Means for generating a gradient magnetic field in the static magnetic field space;
Applying the high-frequency magnetic field pulse and the gradient magnetic field to a desired region of the subject arranged in the static magnetic field based on a predetermined pulse sequence including an MPG pulse, the desired region included in the desired region Measurement control means for obtaining an image reflecting the diffusion movement of the molecule of
In a magnetic resonance imaging apparatus comprising:
The magnetic resonance imaging apparatus characterized in that the measurement control means applies a pre-pulse gradient magnetic field before the first high-frequency magnetic field pulse in the pulse sequence. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus, wherein the prepulse gradient magnetic field is applied at least in the same direction as the MPG pulse. The magnetic resonance imaging apparatus according to claim 1 or 2,
The magnetic resonance imaging apparatus, wherein the pre-pulse gradient magnetic field has substantially the same application intensity and application time as the MPG pulse. The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
The MPG pulse is applied multiple times with the application of the high frequency magnetic field pulse in between,
The magnetic resonance imaging apparatus, wherein the pre-pulse gradient magnetic field and the plurality of MPG pulses are applied at equal time intervals.

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