JP2015202261A - magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the generation of steps or degradation of image quality incident to polarity inversion without losing the advantage of the technique for effectively spoiling the polarity of a spoiled gradient magnetic field in accordance with the polarity of an adjacent gradient magnetic field.SOLUTION: A magnetic resonance imaging apparatus comprises: a static magnetic field generating magnet; a magnetic field generating unit arranged in the static magnetic field generating magnet and for generating a high frequency magnetic field and a gradient magnetic field; a reception unit for receiving a nuclear magnetic resonance signal; a control unit for controlling the operation of the magnetic field generating unit and the reception unit on the basis of a predetermined pulse sequence; and an image creation unit for creating an image using the nuclear magnetic resonance signal received by the reception unit. The pulse sequence contains a spoiled gradient magnetic field pulse for distribution of magnetization distribution. The control unit controls the polarity of the spoiled gradient magnetic field on the basis of a control parameter associated with a position in a k-space in which the nuclear magnetic resonance signal is arranged.

Description

  The present invention relates to a technique for controlling the applied polarity of a spoil gradient magnetic field in a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) equipped with a pulse sequence including a spoil gradient magnetic field pulse.

  An MRI apparatus applies a high-frequency rotating magnetic field (hereinafter referred to as an RF pulse) having a specific frequency to the magnetic moment of a nucleus placed in a static magnetic field, and uses the magnetic resonance phenomenon generated thereby to cause the natural nature of the substance. It is a device that measures scientific information and visualizes it as an image. The signal obtained by the nuclear magnetic resonance phenomenon called echo is due to the energy released in the process of relaxation of the spin excited by the RF pulse, and the longitudinal magnetization and transverse magnetization generated by the precessing spin are echoed. It is deeply related to the characteristics of Among them, the transverse magnetization is important for obtaining echoes. On the other hand, in some cases, it becomes an unnecessary component and may be intentionally dispersed. This is called spoiling. In general, spoiling can be performed by applying a gradient magnetic field or phase fluctuation of an irradiation RF pulse, and the gradient magnetic field amount to be applied and the phase fluctuation amount of the RF pulse are optimized depending on the scene.

  For example, Patent Document 1 describes a technique of inserting a pair of crusher gradient magnetic field pulses having the same applied amount before and after an RF pulse so as not to generate a false echo due to the above-described unnecessary component. Note that the crusher gradient magnetic field used in the technique described in Patent Document 1 disperses slice encoding, and if the gradient magnetic field applied for the purpose of dispersing transverse magnetization is defined as a spoil gradient magnetic field, a spoil gradient magnetic field is defined. Is the same.

JP 2009-201934 A

  As described above, since the spoil gradient magnetic field is a gradient magnetic field for intentionally dispersing unnecessary components of transverse magnetization, the applied amount is generally large, and is affected by the gradient magnetic fields adjacent in time. In the technique described in Patent Document 1, in order to perform spoiling efficiently, the polarity of the spoiling gradient magnetic field applied before and after the RF pulse is made to be the same as the polarity of the gradient magnetic field applied immediately thereafter. It is described that the application is controlled.

  However, when the polarity of the spoil gradient magnetic field is controlled in this way, another problem may occur. For example, the actually applied gradient magnetic field pulse does not have an ideal pulse shape, and thus includes an error magnetic field. When the echo signal is measured while repeating the pulse sequence, if the polarity of the gradient magnetic field pulse changes, the error magnetic field also changes with this change. This change becomes a step in the absolute value or phase of the signal in the k space, and causes artifacts in the image after reconstruction.

  The present invention suppresses generation of a step due to polarity reversal and deterioration of image quality without impairing the advantage of a technique (synchronous control) that effectively spoils the polarity of a spoiling gradient magnetic field in accordance with the polarity of an adjacent gradient magnetic field. This is the issue.

  The present invention solves the above problem by controlling the polarity applied to the spoiling gradient magnetic field in consideration of the k-space arrangement of measurement echoes.

  That is, the MRI apparatus of the present invention includes a static magnetic field generating magnet, a magnetic field generating unit that generates a high-frequency magnetic field and a gradient magnetic field, a receiving unit that receives a nuclear magnetic resonance signal, the magnetic field generating unit, and a receiving unit. A control unit that controls the operation of the unit based on a predetermined pulse sequence, and an image creation unit that creates an image using a nuclear magnetic resonance signal received by the reception unit. Including a spoil gradient magnetic field pulse, the control unit controls the polarity of the spoil gradient magnetic field based on a control parameter related to a position in k-space where the nuclear magnetic resonance signal is arranged.

  According to the present invention, control for improving the spoiling effect efficiency is performed, and even when the image quality is deteriorated due to the control, by considering the arrangement of the measured echoes in the k space, it is determined whether or not the control can be executed. It is possible to reduce the adverse effect on the image quality while maintaining the efficiency of the spoiling effect.

1 is a block diagram showing an overall outline of an MRI apparatus to which the present invention is applied. The figure which shows the basic form of 3D-FSE sequence employ | adopted by 1st embodiment. The figure which shows 3D-FSE sequence changed by 1st embodiment Diagram showing part of the pulse sequence of FIG. 2B Diagram showing another part of the pulse sequence of FIG. 2B The figure which shows the procedure of the spoil gradient magnetic field control of 1st embodiment. The figure which shows an example of the user interface of 1st embodiment. (A)-(d) is a figure which shows echo allocation of 1st embodiment and its modification. The figure which shows 3D-FSE sequence of 2nd embodiment. The figure which shows 2D-FSE sequence of 2nd embodiment. It is a figure which shows the effect of this invention, (a) shows the case of the conventional spoil gradient magnetic field polarity control, (b) shows the case of the spoil gradient magnetic field control by this invention, (c) shows the case of a spoil gradient magnetic field polarity fixed.

Embodiments of the present invention will be described below with reference to the drawings.
First, an outline of an MRI apparatus to which the present invention is applied will be described. FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus to which the present invention is applied. This MRI apparatus uses a magnetic resonance phenomenon to obtain a tomographic image of a subject, and as shown in the figure, a static magnetic field generation magnetic circuit 2, a gradient magnetic field generation unit 3, a transmission unit 5, and a reception unit. 6, a signal processing unit 7, a sequencer 4, a central processing unit (CPU) 8, and an operation unit 25.

  The static magnetic field generating magnetic circuit 2 generates a uniform static magnetic field around the subject 1 in the direction of the body axis or in the direction perpendicular to the body axis. The magnetic field generating means of a permanent magnet system, a normal conducting system, or a superconducting system. Is arranged. The gradient magnetic field generation unit 3 includes a gradient magnetic field coil 9 wound in three axes of X, Y, and Z, and a gradient magnetic field power source 10 that drives each coil. By driving the gradient magnetic field power supply 10, a gradient magnetic field in three axial directions of X, Y, and Z is given to the static magnetic field generated by the static magnetic field generating magnetic circuit 2. By applying this gradient magnetic field, a slice plane for the subject 1 can be set, and an echo signal is encoded to provide position information.

  The transmission unit 5 irradiates a high-frequency signal to cause nuclear magnetic resonance to occur in atomic nuclei constituting the living tissue of the subject 1 by a high-frequency magnetic field pulse (RF pulse) transmitted from the sequencer 4. 11, a modulator 12, a high-frequency amplifier 13, and a high-frequency coil 14 a on the transmission side. The high-frequency pulse output from the high-frequency oscillator 11 is amplified by the high-frequency amplifier 13 and then placed close to the subject 1. By supplying the high-frequency coil 14a on the transmission side, the subject 1 is irradiated with electromagnetic waves (high-frequency signals).

  The receiving unit 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the living tissue of the subject 1. The receiving unit 6 receives a high-frequency coil 14 b on the receiving side, an amplifier 15, and a quadrature detector 16. And an A / D converter 17, the electromagnetic wave (NMR signal) of the response of the subject 1 due to the electromagnetic wave irradiated from the high frequency coil 14 a on the transmission side is a high frequency on the reception side arranged close to the subject 1. The signal is detected by the coil 14b, input to the A / D converter 17 via the amplifier 15 and the quadrature detector 16, and converted into a digital quantity, and further sampled by the quadrature detector 16 at a timing according to a command from the sequencer 4. The two collected data are sent to the signal processing unit 7.

  The signal processing unit 7 performs image reconstruction calculation using the echo signal detected by the receiving unit 6 and displays an image. Processing such as Fourier transform, correction coefficient calculation, image reconstruction, etc., and sequencer for the echo signal 4, a ROM (read-only memory) 21 for storing a program for performing image analysis processing and measurement over time, an invariant parameter used in the execution, and a measurement parameter and reception unit obtained in the previous measurement 6 A RAM (anytime reading / reading memory) 22 for temporarily storing the detected echo signal and an image used for setting the region of interest and storing parameters for setting the region of interest, and image data reconstructed by the CPU 8 Magneto-optical disk 19 and magnetic disk 18 serving as a data storage unit for recording, and these magneto-optical disks 9 or the image data read from the magnetic disk 18 and imaged and a display 20 as a display portion for displaying a tomographic image.

  The sequencer 4 serves as a control unit that repeatedly applies the above-described RF pulse and gradient magnetic field pulse in a predetermined pulse sequence. The sequencer 4 operates under the control of the CPU 8 and performs various commands necessary for collecting tomographic image data of the subject 1. Are sent to the transmitter 5, the gradient magnetic field generator 3 and the receiver 6. The operation unit 25 is used by a user to input control information for processing performed by the signal processing unit 7, and includes operation devices such as a trackball 23 and a keyboard 24.

  Various pulse sequences are programmed in advance according to the imaging method and stored in a memory (ROM). The sequencer 4 reads a predetermined pulse sequence from the memory and uses parameters input from the operation unit 25. The application of the high-frequency magnetic field pulse and the gradient magnetic field pulse and the sampling of the echo signal are controlled in accordance with the pulse sequence.

  In the present embodiment, a pulse sequence including a spoil gradient magnetic field is adopted, and at that time, the polarity of the spoil gradient magnetic field is controlled in consideration of the k-space arrangement of the measured echo signal. The control of the polarity of the spoil gradient magnetic field may be determined, for example, as a default in advance like the parameters of the pulse sequence, and the control parameters stored in the memory may be used when the sequencer 4 operates the pulse sequence. The sequence 4 may use the control parameter value (stored in the temporary memory) set by the operation unit 25.

  Hereinafter, the control of the pulse sequence of the present embodiment, more specifically the control of the spoil gradient magnetic field, will be described using a specific pulse sequence.

<First embodiment>
In the present embodiment, a case will be described in which a spoil gradient magnetic field is controlled in a spin echo type pulse sequence (hereinafter referred to as an SE sequence). 2A and 2B (also collectively referred to as FIG. 2) show a 3D-FSE (Fast SE) sequence to which the present embodiment is applied. FIG. 2A shows a conventional method for controlling the applied polarity of a spoil gradient magnetic field, and FIG. 2B shows a method for controlling the applied polarity of a spoil gradient magnetic field according to this embodiment. 3A and 3B show details of a part (one cycle) of the pulse sequence of FIG. 2, and are two cases in which the application polarity of slice encoding is different. In FIG. 3A, a “positive polarity” gradient magnetic field is applied, and in FIG. 3B, a “negative polarity” gradient magnetic field is applied.

  As shown in FIG. 3A, in the 3D-FSE sequence, first, the slice selection gradient magnetic field pulse 201 is applied together with the excitation RF pulse 101, and then the inverted RF pulse 102 and the slice gradient magnetic field pulse 202 are applied. Next, the slice encode gradient magnetic field pulse 301 and the phase encode gradient magnetic field pulse 401 are applied, and an echo signal (not shown) is acquired while the readout gradient magnetic field pulse 501 is applied. Thereafter, the reversal of the transverse magnetization by the inversion RF pulse 102 is repeated at a predetermined interval (ITE). At this time, while changing the magnitude of the phase encode gradient magnetic field pulse 401, the echo signal is the same as after the first inversion RF pulse 102. To get. Prior to the readout gradient magnetic field pulse 501, a phase gradient magnetic field 504 is applied.

  3A and 3B show only three inversion RF pulses 102, the number of inversion RF pulses applied after one excitation, that is, the number of echo signals (echo train) acquired after one excitation is shown. Normally, more than 3 and up to the set number of encodings are possible.

  Before and after the inversion RF pulse 102, a pair of spoil gradient magnetic field pulses 302a and 302b are applied in order to suppress the generation of false echoes due to the FID signal. The axis of the spoiling gradient magnetic field is the same as the axis of the slice encoding gradient magnetic field. In addition, although the magnitudes (application amounts) of the two spoil gradient magnetic field pulses 302a and 302b are the same, the spoil gradient magnetic field 302a immediately before the inversion RF pulse 102 applied first after the excitation RF pulse 101 is the excitation RF pulse. Since it is applied as the sum of the rewind gradient magnetic field of the slice selection gradient magnetic field 201 applied simultaneously with 101, the size is reduced. In addition, the spoil gradient magnetic field 302b immediately after the inverted RF pulse 102 is applied independently of the adjacent slice encode gradient magnetic field pulse 301 in the figure, but both (302b and 301) may be applied in an overlapping manner. .

  After acquiring the echo signal, before applying the next inversion RF pulse 102, rewind (rephase) gradient magnetic fields 303 and 403 are applied to the slice axis and the phase encode axis.

  A sequence 200 (one cycle in FIG. 2) for measuring a series of echoes (echo trains) while changing the application amount of the phase encode gradient magnetic field 401 with the same application amount of the slice encode gradient magnetic field 301 is shown in FIG. In addition, it repeats while changing the application amount of the slice encode gradient magnetic field 301, and finally obtains echo signals of the necessary number of encodes for both slice encode and phase encode.

  The order (ordering) of encoding given to the echo signal determines the k-space arrangement of the echo signal, and is called echo allocation. In the embodiment shown in FIG. 2, the slice encoding is changed from the maximum value of the positive polarity to the negative polarity. Sequential echo allocation in which the maximum value is sequentially changed is employed (FIG. 6A).

  The applied polarity of the spoiler gradient magnetic field pulses 302a and 302b is basically the same as the applied polarity of the slice encode 301. When the slice encode 301 is positive as shown in FIG. 3A, the spoiler gradient magnetic field pulses 302a and 302b are positive, As shown in FIG. 3B, when the slice encode 301 is negative, the spoil gradient magnetic field pulses 302a and 302b are also negative. The conventional pulse sequence shown in FIG. 2A is this basic form, and the polarity of the spoiling gradient magnetic field is also changed at the m-th cycle in FIG. 3A as the applied polarity of the slice encode gradient magnetic field 301 changes from positive to negative. The polarity of the slice encoding gradient magnetic field and the spoiling gradient magnetic field adjacent to the slice encoding gradient magnetic field are made the same. As a result, since it is not necessary to cancel the application amount due to the encode gradient magnetic fields having different polarities, a stable spoiling effect can be obtained without increasing the application amount of the spoil gradient magnetic field more than necessary.

  However, in this case, the boundary of the applied polarity of the spoil gradient magnetic field is formed during the sequence. This means that the error magnetic field caused by the spoiling gradient magnetic field is also changed at this boundary, and the measurement echo that is affected by the error magnetic field is somewhat affected by the spoiling gradient magnetic field. There is a possibility that the applied polarity changes at the same time. This change forms an absolute value of signal or a phase difference in k-space, and causes artifacts to appear in the reconstructed image. In particular, when the polarity applied to the spoiling gradient magnetic field is the same as that of the phase encoding or slice encoding gradient magnetic field, the polarity applied to the spoiling gradient magnetic field changes at the center encoding where the encoding polarity of the encoding gradient magnetic field changes. It is formed around the center of the space, and the effect on the reconstructed image becomes significant.

  Therefore, in this embodiment, as shown in FIG. 2B, the position of switching the application polarity of the spoil gradient magnetic field is shifted from the position of the center encoding where the application polarity of the slice encode gradient magnetic field 301 changes from positive to negative. Avoid the problem of steps in the periphery. In the embodiment shown in FIG. 2B, the polarity of the spoiling gradient magnetic fields 302a and 302b is reversed to negative in the cycle (nth cycle) after the mth cycle in which the application polarity of the slice encode gradient magnetic field 301 is reversed, and thereafter In the range where the application amount (absolute value) of the slice encode gradient magnetic field 301 is relatively large, the application polarity of the spoile gradient magnetic field is controlled so that the application polarity of the slice encode gradient magnetic field 301 and the spoile gradient magnetic field are the same. Thereby, an efficient spoiling effect can be maintained.

Hereinafter, the operation procedure of the control unit will be described with reference to FIG.
First, when a predetermined pulse sequence, here a 3D-FSE sequence, is selected and imaging is started (step S41), the slice encode gradient magnetic field applied in the first repetition according to the echo allocation set in the pulse sequence. The polarity of the spoil gradient magnetic field is determined so as to be the same polarity as the polarity (step S42). In the example shown in FIG. 2B, since the sequential echo allocation in which the slice encoding is sequentially changed from the positive maximum value to the negative maximum value is adopted, the polarity of the spoiling gradient magnetic field is set to “positive”. . In the case of echo allocation in which the negative value is changed in order from the negative value to the positive value, the polarity of the spoiling gradient magnetic field is set to “negative”. With this setting, the first repetition (first cycle) is executed (step S43). Here, since this embodiment is 3D imaging, the above-described cycle is repeated until all of the slice encoding and phase encoding are completed.

  If the executed sequence cycle is not the last cycle (step S44), it is determined whether or not the polarity of the slice encode gradient magnetic field of the next executed cycle is the same as the polarity of the slice encode gradient magnetic field of the immediately preceding cycle. (S step 45). If the polarity of the slice encode gradient magnetic field of the next sequence 200 is the same as the polarity of the slice encode gradient magnetic field of the immediately preceding sequence, the encode step is advanced (that is, with the slice encode set to the next cycle). Are executed (steps S48 and S43).

  If the polarities are different, the slice encode gradient magnetic field value (encoding amount) of the next cycle is compared with a preset threshold value (step S46). If the slice encode gradient magnetic field value is greater than the threshold value, the inverted RF The polarity of the spoiling gradient magnetic fields 302a and 302b applied before and after the pulse 102 is reversed (step S47). By this inversion, the applied polarity of the spoiling gradient magnetic field becomes the same as the polarity of the slice encode gradient magnetic field. If the value of the slice encode gradient magnetic field is equal to or less than the threshold value in the comparison step S46, the polarity of the spoile gradient magnetic field is not reversed, and the encode step is advanced (that is, with the slice encode set in the next cycle). A cycle is executed (steps S48 and S43). Steps S45 to S48 and S43 are repeated until echo signals of all slice encodings are collected (S44).

The above processing is expressed by mathematical formulas as follows.
The applied intensity G _Encode including the applied polarity of the encode gradient magnetic field is set within the range of the formula (1), the applied polarity G Polarity _Spoil of the spoiler gradient magnetic field is expressed by the formula (2), and the negative polarity is expressed by the formula (2). It is defined as 3).

ここで、エンコード傾斜磁場の印加強度Ｇ_Encodeを指標にスポイル傾斜磁場の印加極性同期制御を行うための制御閾値を式（４）とする。

Here, the control threshold value for performing the application polarity synchronization control of the spoiling gradient magnetic field using the encoding gradient magnetic field application intensity G _Encode as an index is represented by Expression (4).

この場合、本実施形態によるスポイル傾斜磁場の印加極性の制御は、次式（５）、（６）で表される。

In this case, the control of the applied polarity of the spoil gradient magnetic field according to the present embodiment is expressed by the following equations (5) and (6).

Here, when G _Threshold = 0, as shown in FIG. 2A, the application polarity of the spoiling gradient magnetic field is completely synchronized with the application polarity of the encode gradient magnetic field. On the other hand, when G _Threshold = + g, application polarity synchronization is not performed at all, and the spoiling gradient magnetic field is always applied to the positive polarity. Similarly, when G _Threshold = −g, no application polarity synchronization is performed, and the spoiling gradient magnetic field is always applied in a negative polarity. In the example shown in FIG. 2B, G _Threshold has a value in a range excluding 0 and ± g, and does not completely synchronize the applied polarity of the spoiling gradient magnetic field with the applied polarity of the encoded gradient magnetic field, but is shifted from the center encoding. The polarity is reversed.

The threshold (G _Threshold ) for switching the applied polarity of the spoil gradient magnetic field is a value that directly represents the magnitude of the slice encode gradient magnetic field in the above formula, but is easily understood by the user in terms of numerical values and parameters, for example, the amount of shift from the center of the k-space It is preferable that it can be set as (ratio of the threshold to the maximum encoded value). Here, the amount of shift from the center of k-space is a trade-off with the efficiency of the spoiling effect by synchronizing the applied polarity of the spoiling gradient magnetic field with the applied polarity of the adjacent gradient magnetic field. However, it is preferable to determine appropriately according to the imaging method or by looking at the captured image.

  An example of a user interface for setting the shift amount is shown in FIG. FIG. 5 shows a GUI displayed on the display screen of the display 20 (FIG. 1), and the numerical value in the block 51 is configured to be input by the user using the keyboard 24. In the example shown in the figure, a ratio (ratio) with respect to the maximum value of encoding is used as a shift amount parameter (control parameter), and this is numerically input. The present invention is not limited to the example shown in the drawings, and values that are determined in advance at the time of design may be replaced with easy-to-understand words and presented and selected as options.

  According to the present embodiment, the polarity of the spoil gradient magnetic field applied in the vicinity of the slice encode gradient magnetic field is not linked to the polarity of the slice encode gradient magnetic field, but is shifted from the vicinity of the center encode and inverted. It is possible to prevent a step from occurring in the vicinity of the center of the k space, and to prevent occurrence of artifacts and deterioration of image quality.

  Further, according to the present embodiment, by changing the control parameter according to the setting of the user, for example, by setting the control parameter (ratio) described above to 0, complete synchronous control, or by setting the control parameter to 1, It is also possible to use only one polarity (no polarity control). That is, arbitrary control is possible from complete synchronization control to no synchronization control according to the imaging method and the obtained image.

  Although FIG. 2 shows an example in which echo allocation is sequential, the present invention can be applied to other echo allocation. FIG. 6 shows four different echo allocation examples. In the drawing, the diagram on the right side shows the arrangement of the k space (kx-kz space), and the graph on the left side shows the change of the slice encode gradient magnetic field Gs with respect to the time axis. The slice encode Gs actually takes a discrete value, but is simply shown as a linear change in the graph on the left side of FIG. 6A shows sequential, FIG. 6B and FIG. 6C show centric, and FIG. 6D shows echo allocation when there is an echo shift.

In the figure, a time point indicated by a thick arrow is a time point when the polarity of the spoil gradient magnetic field is reversed, and values other than 0 and ± g are set as G _Threshold . Therefore, in the illustrated example, even if the polarity of the slice encode gradient magnetic field is reversed, the polarity of the spoile gradient magnetic field is not reversed immediately, but when the intensity (absolute value) of the slice encode gradient magnetic field exceeds a predetermined threshold value. Inverts the polarity of the spoil gradient.

  For example, in the centric echo allocation shown in FIG. 6C, the polarity of the slice encoding gradient magnetic field changes every time the slice encoding changes, but until the intensity reaches a predetermined threshold, the polarity of the spoiling gradient magnetic field is When the intensity of the slice encode gradient magnetic field exceeds a threshold value while maintaining either “positive” or “negative”, the polarity of the spoiling gradient magnetic field is reversed in accordance with the reversal of the polarity.

  In the case of the echo shift shown in FIG. 6 (d), the polarity reversal of the spoiling gradient magnetic field when the slice encoding changes from positive to negative is performed by shifting the center encoding, but the slice encoding is set to the negative maximum value. Then, when the echo is acquired with the maximum value on the positive side of the slice encoding next time, the polarity of the spoiling gradient magnetic field is also inverted in accordance with the polarity inversion.

  As described above, the present embodiment can be applied to various echo allocations, and can achieve the above-described effects, that is, effective spoiling and image quality prevention.

<Second embodiment>
In the first embodiment, in 3D imaging, when the phase encoding is performed in the echo train and the slice encoding is performed between the echo trains, the polarity control of the spoil gradient magnetic field adjacent to the slice encode gradient magnetic field is performed. However, in this embodiment, the polarity control of the spoiling gradient magnetic field of the present invention is applied to the encoding performed in the echo train.

  Hereinafter, the present embodiment will be described using the 3D-FSE sequence illustrated in FIG. 7. However, the pulse sequence can be applied to any pulse sequence in which a spoiling gradient magnetic field is applied adjacent to the encode gradient magnetic field. It can be applied not only to imaging but also to 2D imaging. In FIG. 7, the same elements as those in FIGS. 2 and 3 are denoted by the same reference numerals, and description thereof is omitted.

  As shown in FIG. 7, after applying the excitation RF pulse 101, a plurality of inversion RF pulses 102 are applied, and an echo signal is measured while applying a readout gradient magnetic field 501 between adjacent inversion RF pulses 102. A pair of spoil gradient magnetic fields 302a and 302b is applied before and after the slice selective gradient magnetic field 201 applied simultaneously with the inversion RF pulse 102, and the slice encode gradient magnetic field 301 is applied immediately after or superimposed on the spoiler gradient magnetic field 302b. In this pulse sequence, the intensity of the slice encode gradient magnetic field 301 is changed for each inverted RF pulse, and a plurality of slice encode echo signals are measured. In the figure, only four inverted RF pulses 102 are simply shown, but for example, echo signals of all slice encodings can be measured with one echo train. In one echo train, the phase encode 401 is fixed.

  The applied polarity of the spoil gradient magnetic field is basically the same as the applied polarity of the adjacent slice encode gradient magnetic field 301, and the applied polarity changes up to the spoil gradient magnetic field 302 before and after the second inversion RF pulse 102. do not do. In the example shown in FIG. 7, the polarity of the slice encode gradient magnetic field 301 is reversed when the number of the inverted RF pulses 102 is changed from the second to the third, but at this time, the polarity of the spoiling gradient magnetic field 302 is not reversed. The polarity of the spoiling gradient magnetic field 302 is reversed after the eye reversal RF pulse to avoid reversing the polarity of the spoiling gradient magnetic field in the vicinity of the center encoding.

  Also in this case, the timing of reversing the polarity of the spoil gradient magnetic field depends on the strength of the slice encode gradient magnetic field, and the polarity of the spoil gradient magnetic field is reversed when the absolute value thereof exceeds a predetermined threshold. The threshold setting method is the same as in the first embodiment, and an encoding value that is a threshold value may be set in advance, or a parameter corresponding to the threshold value, for example, a ratio of the encoding value that is the threshold value to the maximum value is set. An operator may input from the operation unit. Further, a preset threshold value may be changeable.

  As a result, as in the first embodiment, an appropriate threshold value can be set according to the pulse sequence, and a step can be prevented from being formed near the center of the k space.

  In the present embodiment, the polarity of the spoiling gradient magnetic field adjacent to the encode gradient magnetic field is controlled for the pulse sequence to be encoded in the echo train. As shown in FIG. 8, two encodings of phase encode and slice encode are performed. It can also be applied to 2D imaging without one of the above. In FIG. 8, the present embodiment is applied to a spoiling gradient magnetic field 405 applied coaxially with the phase encoding. The phase encoding after the fourth inverted RF pulse 102 is inverted in polarity, and when synchronized with it, the polarity of the spoiling gradient magnetic field is as shown by the dotted line, but here the polarity is shown as shown by the solid line. It shows a state in which a positive polarity is applied without inversion.

  As described above, the embodiments of the present invention have been described by taking the FSE sequence in which the false echo due to the FID signal is particularly problematic as an example. However, the embodiment may be a pulse sequence that is coaxial with a predetermined gradient magnetic field pulse and uses a spoil gradient magnetic field in the vicinity thereof. For example, the present invention can be applied to a pulse sequence other than the FSE sequence, for example, a GrE pulse sequence.

Photographing using a phantom was performed, and the effect of the first embodiment was confirmed. The results are shown in FIG. In the figure, (a), (b), and (c) respectively indicate the case where G _Threshold = 0 (complete synchronization of applied polarity), the case of G _Threshold = 0.2 (partial synchronization of applied polarity), and G _Threshold = + G (no synchronization of applied polarity) is shown, the left side of the figure is kx-kz data (RawData), the center is kx-z data obtained by Fourier transforming kx-kz data in the kz direction ( Hybrid data), the right side shows a profile on a line indicated by a one-dot chain line in the center image.

  As can be seen from FIG. 9A, when the polarity applied to the spoil gradient magnetic field is completely synchronized with the polarity of the slice encode gradient magnetic field, the center in the kz direction, that is, the center in the vertical direction in the figure changes. A boundary has occurred in RawData. When this is subjected to Fourier transform in the kz direction and the profile in the z direction is viewed, false signals are generated like offsets on both sides of the real image portion drawn in the mountain shape. On the other hand, when the spoil gradient magnetic field application polarity synchronization control shown in FIG. 9C is not performed, there is no RAWData boundary, and no false signal is generated in the z-direction profile after Fourier transform. However, in this case, although not shown in FIG. 9, in the image data obtained by further Fourier transforming the hybrid data in the kx direction, another artifact may occur around the image.

On the other hand, in the case of partial synchronization (FIG. 9B), that is, when the boundary of RawData is shifted from the center of k-space by changing G _Threshold , there is no image artifact and FIG. 9B. As shown in FIG. 5, the false signal is greatly reduced in the profile in the z direction after the Fourier transform, and the improvement effect of the present embodiment was confirmed.

  According to the present invention, efficient spoiling according to the imaging method is possible, and an MRI image with good image quality can be obtained.

DESCRIPTION OF SYMBOLS 3 ... Gradient magnetic field generation part, 5 ... Transmission part (magnetic field generation part), 6 ... Reception part, 7 ... Signal processing part (image creation part), 8 ... Control part.

Claims (8)

A static magnetic field generating magnet, a magnetic field generating unit that is arranged in the static magnetic field generating magnet and generates a high-frequency magnetic field and a gradient magnetic field, a receiving unit that receives a nuclear magnetic resonance signal, and operations of the magnetic field generating unit and the receiving unit are predetermined pulse sequences A control unit for controlling based on, and an image creating unit for creating an image using the nuclear magnetic resonance signal received by the receiving unit,
The pulse sequence includes a spoil gradient magnetic field pulse for magnetization dispersion;
The said control part controls the polarity of the said spoil gradient magnetic field based on the control parameter relevant to the position in k space where the said nuclear magnetic resonance signal is arrange | positioned, The magnetic resonance imaging apparatus characterized by the above-mentioned. The magnetic resonance imaging apparatus according to claim 1,
A magnetic resonance imaging apparatus comprising: a memory for storing the control parameter, wherein the control unit reads the control parameter from the memory and performs control based on the pulse sequence. The magnetic resonance imaging apparatus according to claim 1,
It further includes an operation unit that receives input from the operator,
The operation unit receives an input of the control parameter, and the control unit performs control based on the pulse sequence using the control parameter input by the operation unit. A magnetic resonance imaging apparatus according to any one of claims 1 to 3,
The magnetic resonance imaging apparatus, wherein the control parameter is a numerical value indicating a distance or a ratio from the origin of the k space. A magnetic resonance imaging apparatus according to any one of claims 1 to 4,
In the pulse sequence, the k space is scanned from one high frequency side to the other high frequency side from the high frequency side, or the k space is scanned from the high frequency side to one or the other high frequency side across the origin of the k space. A magnetic resonance imaging apparatus characterized by being a pulse sequence. A magnetic resonance imaging apparatus according to any one of claims 1 to 5,
2. The magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence is a two-dimensional pulse sequence including a uniaxial encoding gradient magnetic field or a three-dimensional pulse sequence including a biaxial encoding gradient magnetic field. The magnetic resonance imaging apparatus according to any one of claims 1 to 6,
The magnetic resonance imaging apparatus characterized in that the spoil gradient magnetic field pulse is a gradient magnetic field having the same axis as at least one of a slice selection gradient magnetic field, a slice encode gradient magnetic field, a phase encode gradient magnetic field, and a readout gradient magnetic field. A magnetic resonance imaging apparatus according to any one of claims 1 to 7,
The magnetic resonance imaging apparatus, wherein the pulse sequence is one of a spin echo type pulse sequence and a gradient echo type pulse sequence.

Images