JP2009201934A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an MRI apparatus suppressing an artifact based on a false echo signal without applying a crusher gradient magnetic field pulse with a large amount of application. <P>SOLUTION: The MRI apparatus includes a measurement control means for controlling the measurement of an echo signal by applying a plurality of RF pulses and a plurality of gradient magnetic field pulses to a subject, on the basis of a prescribed pulse sequence. The plurality of gradient magnetic field pulses include the pair of crusher gradient magnetic field pulses equal in the amounts of application, which are applied before/after at least one RF pulse. The measurement control means controls the application of the pair of crusher gradient magnetic field pulses in response to the polarity of the prescribed gradient magnetic field pulse. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. The present invention relates to a technique for suppressing a false echo signal in an apparatus called “MRI”.

  MRI equipment measures NMR signals (echo signals) generated by nuclear spins that make up the body of a subject, especially human tissue, and forms the shape and function of the head, abdomen, limbs, etc. in two or three dimensions. It is a device that images. In imaging, the echo signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded and measured as time-series data. The measured echo signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In such imaging by an MRI apparatus, when three or more high-frequency magnetic field pulses (hereinafter referred to as RF pulses) continue, a stimulated echo is generated in various application patterns of RF pulses and gradient magnetic field pulses in principle. For example, when applying multiple 180 degree RF pulses and measuring multiple echo signals within one repetition time (hereinafter referred to as TR), fast spin echo sequence, multi echo type spin echo sequence, or when TR is short In some cases, even in a single echo type spin echo sequence, a stimulated echo may occur.

  Furthermore, the stimulated echo is measured as a false echo signal by a combination of gradient magnetic field application patterns of a pulse sequence, and the data is arranged at an unfavorable position in the K space, and artifacts may occur on the image. is there. In order to suppress such artifacts, it is common to apply crusher gradient magnetic field pulses to suppress the generation of stimulated echoes and to suppress the mixing of false echo signal data into K-space data. (For example, Patent Document 1).

JP 2002-136498 A

  The conditions under which stimulated echo occurs depend on the combination of various measurement parameters. Therefore, in order to suppress the generation of stimulated echoes in all cases, and to suppress mixing into the K space data as a false echo signal, generally a large amount of crusher gradient magnetic field pulse is applied. There is a need.

  However, in the conventional method in which the crusher gradient magnetic field pulse having the same applied amount and the same polarity is continuously applied in all the encoding patterns, the maximum applied amount of the crusher gradient magnetic field pulse is all set to suppress the false echo signal. It is necessary to apply in the encode pattern. In this case, a larger application amount of the crusher gradient magnetic field pulse is required as compared with the application amount of the crusher gradient magnetic field pulse which is originally necessary for diffusing the FID signal by the 180 degree RF pulse. As a result, the time available for echo signal measurement is shortened and the spatial resolution and S / N of the image are reduced. (Patent Document 1) does not consider this problem.

  Accordingly, an object of the present invention is to realize an MRI apparatus capable of suppressing artifacts based on a false echo signal without applying a crusher gradient magnetic field pulse having a large applied amount.

The MRI apparatus of the present invention that achieves the above object is configured as follows. That is,
A measurement control unit that controls measurement of an echo signal by applying a plurality of pulses and a plurality of gradient magnetic field pulses to a subject based on a predetermined pulse sequence, and the plurality of gradient magnetic field pulses include at least one RF pulse. It includes a pair of crusher gradient magnetic field pulses with the same applied amount applied before and after. The measurement control means controls application of the pair of crusher gradient magnetic field pulses in accordance with the polarity of a predetermined gradient magnetic field pulse.

  According to the MRI apparatus of the present invention, it is possible to suppress artifacts based on a pseudo echo signal without applying a crusher gradient magnetic field pulse having a large applied amount.

  Hereinafter, each embodiment of the MRI apparatus of the present invention will be described with reference to the 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 outline of an example of the MRI apparatus of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an example of the MRI apparatus of the present invention. This MRI apparatus uses a nuclear magnetic resonance (NMR) phenomenon to obtain a tomographic image of a subject, and as shown in FIG. 1, a static magnetic field generation system 1, a gradient magnetic field generation system 2, and a transmission system 3 A receiving system 4, a signal processing system 5, and a sequencer 6.

  The magnetic field generation system 1 generates a uniform static magnetic field around the non-subject 8 in the direction of the body axis or in a direction perpendicular to the body axis. The magnetic field generation system 1 is permanent in a certain space around the subject 8. A magnetic field generating means (not shown) of a magnet system, a normal motor system, or a superconducting system is arranged.

  The gradient magnetic field generation system 2 includes a gradient magnetic field coil 9 that generates a gradient magnetic field in the X, Y, and Z axis directions, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil 9. The gradient magnetic field power supply 10 corresponding to each gradient magnetic field coil 9 is driven according to a command from the sequencer 6 to be described later, so that the gradient magnetic fields Gx and Gz in the three-axis directions of X, Y, and Z are applied to the subject 8. It has become. The slice plane for the subject 8 can be set by applying this gradient magnetic field.

  The transmission system 3 irradiates the RF pulse to cause nuclear magnetic resonance in the atomic nucleus constituting the tissue of the subject 8 by the RF pulse. The high-frequency oscillator 11, the modulator 12, the high-frequency amplifier 13, and the irradiation coil It consists of 14. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by a modulator 12 in accordance with a command from a sequencer 6 to be described later, and the amplitude-modulated high-frequency pulse is amplified by a high-frequency amplifier 13 and placed close to the subject 8 By supplying the irradiation coil 14, the subject 8 is irradiated with an RF pulse that is an electromagnetic wave.

  The receiving system 4 detects an echo signal emitted by nuclear magnetic resonance of the nucleus of the tissue of the subject 8, and includes a high-frequency receiving coil 15, a receiving circuit 16, and an A / D converter 17 on the receiving side. An echo signal, which is an electromagnetic wave in response to the subject 8 due to the RF pulse emitted from the irradiation coil 14 on the transmission side, is detected by the high-frequency receiving coil 15 disposed in the vicinity of the subject 8 and passes through the receiving circuit 16. By being input to the A / D converter 17, it is converted into a digital quantity (hereinafter, the digitized echo signal is referred to as echo data), and the signal is sent to a signal processing system 5 described later.

  The signal processing system 5 includes a CPU 7, a signal processing device 18, a storage device such as a memory 19, a magnetic disk 20, and an optical disk 21, and a display 22 such as a CRT. The CPU 7 reads the pulse sequence application pattern from the memory 19, and instructs the sequencer 6 to be described later to apply the pulse sequence application pattern for measuring the echo signal. The memory 19 applies the pulse sequence application pattern and the reception under any imaging conditions. Information such as echo data obtained by the system 4 is stored under the control of the CPU 7. The signal processing device 18 performs Fourier transform on echo data stored in an area corresponding to the K space of the memory 19, performs correction calculation, image reconstruction, and the like, and outputs the processing result to the memory 19. The processing result output to the memory 19 is displayed on the display 22 as a tomographic image. The magnetic disk 20 and the optical disk 21 can store image data, patient data, and the like obtained by reconstructing echo data and signal data that have been processed by the signal processing device 18 as described above.

  The sequencer 6 repeatedly applies RF pulses and gradient magnetic field pulses that cause magnetic resonance to atomic nuclei constituting the tissue of the subject 8 in a predetermined pulse sequence. The sequencer 6 operates under the control of the CPU 7, Various commands necessary for collecting echo data of the tomographic image of the specimen 8 are sent to the transmission system 3, the gradient magnetic field generation system 2, and the reception system 4. The present invention relates to a pulse sequence pattern executed under the control of the sequencer 6. The present invention may be a two-dimensional measurement or a three-dimensional measurement as a pulse sequence. Further, a radial sequence for acquiring data in the k space radially may be used, or a normal method for acquiring data in the k space in parallel to the kx axis may be used. Furthermore, any pulse sequence may be used as the pulse sequence.

  In FIG. 1, the high-frequency coils 14 and 15 on the transmission side and the reception side and the gradient magnetic field coil 9 are installed in the static magnetic field space of the static magnetic field generation system 2 arranged in the space around the subject 1. .

  At present, the spin species to be imaged by the MRI apparatus are protons, which are the main constituents of the subject, as widely used in clinical practice. By imaging the spatial distribution of the proton density and the spatial distribution of the relaxation phenomenon in the excited state, the form or function of the human head, abdomen, limbs, etc. is photographed two-dimensionally or three-dimensionally.

  Next, an example of a spin echo sequence (hereinafter abbreviated as an SE sequence) provided in the MRI apparatus according to the present invention will be described based on FIG. 2 and the reason for the generation of a false echo signal will be briefly described.

  Fig. 2 shows a sequence chart for one repetition time (TR) of the SE sequence with Flow-Rephase function in the frequency encoding direction, where RF, Gs, Gp, Gf, and signal are RF pulse and slice gradient magnetic field, respectively. Represents the axis of the phase encoding gradient magnetic field, the frequency encoding gradient magnetic field, and the echo signal. The sequencer 6 controls the transmission system 4, the gradient magnetic field system 2, and the reception system 4 based on this sequence chart, and repeatedly executes this sequence at every repetition time (TR), so that it is necessary for image reconstruction. Controls the measurement of spin echo signals.

  Reference numeral 201 denotes a 90 degree RF pulse that applies an RF pulse to the longitudinal magnetization in the imaging slice region, tilts it by 90 degrees, and excites the transverse magnetization. Reference numeral 203 denotes a slice selective gradient magnetic field pulse, which is applied together with the 90-degree RF pulse 201 to excite longitudinal magnetization of only a desired imaging plane by 90 degrees.

  204 is a slice rephase pulse for returning the phase of the transverse magnetization diffused by the slice selective gradient magnetic field pulse. 206 is a phase encoding gradient magnetic field pulse, and its application amount (area surrounded by the waveform and the time axis) is changed at each repetition time (TR), and spatial information in the phase encoding direction is encoded into the echo signal 210, respectively. The

  207 and 208 are frequency dephase gradient magnetic field pulses for dispersing the phase of transverse magnetization in advance and returning the zeroth-order and first-order phase components of transverse magnetization to zero together with the first half of 209. These 207, 208 and 209 form a Flow-Rephase function.

  Reference numeral 202 denotes a 180-degree reversal RF pulse for reversing the magnetization 180 degrees in the imaging plane. The phase of transverse magnetization excited by a 90 degree RF pulse is then dispersed due to inhomogeneous static magnetic field. The 180-degree inversion RF pulse re-converges this phase dispersion by reversing the transverse magnetization by 180 degrees in the opposite direction, thereby generating a spin echo.

  205 is a slice selective gradient magnetic field pulse, and when applied together with the 180-degree RF pulse 202, the magnetization in the same area as the imaging slice area excited by the 90-degree RF pulse is inverted by 180 degrees. .

  Reference numeral 209 denotes a frequency encoding gradient magnetic field pulse that encodes spatial information in the frequency encoding direction into an echo signal. When the application amount of the frequency encoding gradient magnetic field pulse 209 becomes equal to the sum of application amounts of the frequency dephase gradient magnetic field pulses 207 and 208, the amplitude of the echo signal 210 becomes maximum. In other words, the echo signal 210 is the time when the time between the 90-degree RF pulse and the 180-degree RF pulse is the same as the time between the 180-degree RF pulse and the peak time of the echo signal 210, and the frequency encoding is performed. A peak is reached at the center of the gradient magnetic field pulse 209. The sequencer 6 controls the application amount of the frequency dephase gradient magnetic field pulses 207 and 208, the application amount of the frequency encode gradient magnetic field pulse 209, and the application timing of each RF pulse 201, 202 in this way.

  Further, after measuring the echo signal 210, spoiler gradient magnetic fields 211 and 212 for phase dispersion of the residual transverse magnetization are applied in the slice direction and the frequency encoding direction, respectively. 222 is a crusher gradient magnetic field pulse for dispersing the FID signal generated by the application of the 180-degree RF pulse 202.

  Reference numeral 221 denotes a gradient magnetic field pulse applied in advance for canceling the application amount of the crusher gradient magnetic field pulse 222, and has the same application amount as the crusher gradient magnetic field pulse 222. In the present invention, a pair of 221 and 222 is referred to as a crusher gradient magnetic field pulse. In the SE sequence shown in FIG. 2, a pair of crusher gradient magnetic field pulses are inserted only in the phase encoding direction, but 180 degrees in at least one of the three axes including other slice directions and frequency encoding directions. A pair of crusher gradient magnetic field pulses may be inserted before and after the RF pulse 202. It should be noted that the application amount of the crusher gradient magnetic field pulse before and after the 180-degree RF pulse is preferably limited to the minimum application amount necessary for diffusing the FID signal immediately after the 180-degree RF pulse.

  The sequencer 6 repeats the above sequence for one repetition time (TR) while changing the application amount of the phase encoding gradient magnetic field, and controls the measurement of echo signals necessary for image reconstruction.

  Next, the reason why the false echo signal is generated in the above SE sequence will be described. If the excitation angle by the 90-degree RF pulse 201 and 180-degree RF pulse 202 in the SE sequence shown in FIG. 2 is uniformly and ideally 90 degrees and 180 degrees, respectively, in the imaging slice region, An echo signal is generated at the center of the frequency encoding gradient magnetic field 209. However, the excitation profile of the imaging slice area is uniform and ideally not 90 degrees or 180 degrees, especially on both sides of the imaging slice area, even if 180 RF pulses are applied, the excitation angle is less than 180 degrees Thus, an incomplete excitation profile is obtained in the entire imaging slice region.

  For this reason, even with a 180-degree RF pulse, the magnetization component that felt only energy equivalent to a 90-degree RF pulse has its transverse magnetization returned to longitudinal magnetization. The magnetization component that has returned to the longitudinal magnetization is excited again to the transverse magnetization by the next RF pulse, and this transverse magnetization becomes a stimulated echo. That is, when three or more RF pulses are continued, imperfection of the excitation profile cannot be avoided, and thus a stimulated echo is generated.

  Furthermore, in the process of generating the stimulated echo, when the application amount of the gradient magnetic field is balanced, the stimulated echo is measured as a false echo signal. A specific example will be described with reference to FIG. FIG. 3 shows the pulse sequence of FIG. 2 for two repetition times (TR) in the nth and n + 1th phase encode sequences. The slice selection gradient magnetic field Gs is the same as in FIG.

  During the phase encoding n sequence, transverse magnetization is generated by the 90-degree RF pulse 201-1 (subscripts 1 and 2 represent the codes for the phase encoding n and n + 1 sequences, respectively). In the 180-degree RF pulse 202-1, the magnetization component that only felt energy equivalent to the 90-degree pulse changes its transverse magnetization to longitudinal magnetization. At this stage, the longitudinal magnetization is not affected by the phase diffusion caused by the gradient magnetic field pulses 208-1,209-1,212-1,222-1. The longitudinal magnetization is again changed to transverse magnetization by the 90-degree RF pulse 201-2 in the phase encoding n + 1 sequence, and phase reconvergence is performed by the next 180-degree RF pulse 202-2.

With respect to the stimulated echo generated in the above 90 ° -180 ° -90 ° path, first pay attention to the phase encoding axis (Gp), and the amount of gradient magnetic field pulse applied (absolute value) is represented by []. The relationship of magnetic field pulse application amount is

-[206-1] + [221-1]-[206-2] + [221-2] ≒ [222-2]

Thus, the application amount of the gradient magnetic field pulse on the phase encode axis is balanced, and the state becomes close to the state without phase encode.

Next, pay attention to the lead-out axis.

[207-1] + [207-2] <-[208-2] + [209-2] / 2

Thus, the gradient magnetic field pulse application amount is balanced before the center of the frequency encoding gradient magnetic field 209-2 by the application amount [207-1].

  As a result, the stimulated echo generated on the 90-180 degree-90 degree path is measured as the false echo signal 301 before the center of the frequency encoding gradient magnetic field 209-2.

  Next, FIG. 4 shows a state of the K space in which echo data measured by the SE sequence with the Flow-Rephase function is arranged. As shown in FIG. 4, in addition to normal echo data 401, since a false echo signal is generated by balancing the gradient magnetic field pulse application amount during a certain phase encoding, the measured false echo signal data 402 is K Arranged in space. Artifacts occur on the image obtained by reconstructing the K space data including the pseudo echo signal data.

In such a case, in order not to generate the false echo signal, it is only necessary to make it impossible to balance the application amount of the gradient magnetic field pulse as described above. To that end, in the conventional method,

-[206-1] + [221-1]-[206-2] + [221-2] >> [222-2]

The application amounts of [221-1] and [221-2] were increased so as to be greatly different from the above. However, to set the same echo time (TE) to obtain the desired contrast and increase the amount of [221-1] and [221-2], the measurement matrix is lowered or the bandwidth is increased. Each will result in a reduction in the spatial resolution or S / N of the image.

  Based on the above outline, a method for suppressing a false echo signal without increasing the application amount of the crusher gradient magnetic field pulse more than necessary according to the present invention will be described.

(First embodiment)
A first embodiment of the MRI apparatus of the present invention will be described. In the present embodiment, the application amount of the pair of crusher gradient magnetic fields is controlled in accordance with the polarity of the phase encode gradient magnetic field in the SE sequence.

  Specifically, in the present embodiment, paying attention to the fact that the false echo signal is generated only on one side in the ky direction of the K space, that is, only on the polarity on one side of the phase encoding, The polarity of the crusher gradient magnetic field pulse is changed.

  In the pulse sequence controlled in this way, if a crusher gradient magnetic field pulse having a minimum application amount is applied, the echo signal can be measured in the same state as the phase encoding in which no false echo signal is generated. .

Hereinafter, the present embodiment will be described in detail with reference to FIG. FIG. 5 shows a sequence chart of the SE sequence according to the present embodiment, where the phase encoding is m-th (the subscript of each pulse is m) and n-th (the subscript of each pulse is n). The sequence chart at the time of repetition is shown. As in FIG. 3, the slice selection gradient magnetic field is the same as in FIG. The RF pulse (RF), the frequency encoding gradient magnetic field (Gf), and the echo signal (Sg) are the same as in FIG. The difference between the sequence shape in each repetition and the sequence shape shown in FIGS. 2 and 3 is that the polarity of the crusher gradient magnetic field pulse and the polarity of the phase encoding gradient magnetic field pulse are the same. That is, in the phase encode m-th repetition, the polarity of the phase encode gradient magnetic field pulse 206-m is positive, so that the polarity of the crusher gradient magnetic field pulses 221-2m and 222-m is also the same positive. On the other hand, in the phase encode n-th repetition, the polarity of the phase encode gradient magnetic field pulse 206-n is negative, so that the polarities of the crusher gradient magnetic field pulses 221-n and 222-n are the same negative. The sequence 6 controls the phase encode gradient magnetic field pulse and the crusher gradient magnetic field pulse in this way, so that the sum of the applied amounts of the gradient magnetic field pulses 206-m, 221-m, 206-n, and 221-n Different from the applied amount of 222-n, that is,

[206-m] + [221-m]-[206-n]-[221-n] ≠-[222-n]

It becomes. As a result, the application amount of the gradient magnetic field pulse is not balanced, and the stimulated echo is not measured as a false echo signal.

  In the above description, it has been explained that the polarity of the crusher gradient magnetic field pulse is changed in accordance with the polarity of the phase encode gradient magnetic field pulse. Control of the polarity or application amount of the crusher gradient magnetic field pulse may be performed only when the vicinity of the phase encoding is repeated in the vicinity (2 or 3 phase encodings before and after).

  As described above, according to the MRI apparatus of the present embodiment, the polarity of the phase encoding gradient magnetic field and the polarity of the crusher gradient magnetic field are the same, that is, corresponding to the polarity of the phase encoding gradient magnetic field, By controlling the application amount of the crusher gradient magnetic field, it is possible to avoid the balance of the gradient magnetic field application amount. As a result, it is possible to suppress the measurement of the stimulated echo as a false echo signal with the minimum necessary crusher gradient magnetic field, thereby reducing artifacts on the image based on the false echo signal.

(Second embodiment)
Next, a second embodiment of the MRI apparatus of the present invention will be described. In this embodiment, a false echo signal suppression technique similar to that of the first embodiment is applied to a multi-spin echo type radial sequence. The difference from the first embodiment described above is the sequence type and crusher gradient magnetic field application control based on the sequence type. Hereinafter, only differences from the first embodiment will be described with reference to FIG. 6, and description of the same points will be omitted.

  FIG. 6 (a) is a diagram showing a sequence chart of a multi-spin echo type radial sequence. FIG. 6A shows an example of a sequence for measuring two spin echo signals, but this embodiment is not limited to two echo signals. In the spin echo type radial sequence, the phase encode gradient magnetic field 206 and crusher gradient magnetic fields 221 and 222 in the phase encode direction are deleted from the SE sequence shown in FIG. 2, and the Flow Rephase pulses 207 and 208 in the frequency encode direction are deleted. The basic sequence is a sequence in which a pair of spoiler gradient magnetic field pulses are inserted in front of the frequency encoding gradient magnetic field 209 with the 180-degree RF pulse interposed therebetween.

  Further, in order to measure the second echo signal, the sequence after the 180-degree RF pulse for measuring the first echo signal is repeated. Then, while rotating the application axes of the two gradient magnetic fields other than the slice gradient magnetic field direction in this sequence around the coordinate origin of the imaging space in a predetermined angular step within the slice plane, a spin echo signal is generated at each angle. measure. In the radial sequence, since there is no distinction between the phase encoding direction (Gp) and the frequency encoding direction (Gf), G1 and G2 are used instead of Gp and Gf.

  In this embodiment, the frequency encoding gradient magnetic field applied when measuring the echo signal is referred to as a radial sequence encoding gradient magnetic field. The specific application sequence of each pulse is as follows.

  A 90 degree RF pulse 601 is applied together with a slice selective gradient magnetic field pulse 602 to excite longitudinal magnetization in a predetermined imaging slice region. Thereafter, the slice rephase pulse 603 is applied to return the phase of the transverse magnetization diffused by the slice selective gradient magnetic field pulse 602. On the G2 axis, a frequency dephase gradient magnetic field pulse 607 is applied in which the phase of transverse magnetization is dispersed in advance.

  Then, a 180 ° inversion RF pulse 605 for reversing the transverse magnetization by 180 ° is applied together with the slice selection gradient magnetic field pulse 606. Further, the crusher gradient magnetic field pulses 608 and 609 for phase dispersion of the residual transverse magnetization are applied so as to have the same application amount with the 180-degree RF pulse 605 interposed therebetween. The echo signal from the transverse magnetization refocused by the 180 degree RF pulse 605 is measured as the first echo signal 611 under application of the frequency encoding gradient magnetic field pulse 610.

  Thereafter, in order to measure the second echo signal 619, the 180-degree RF pulse 614 is applied again together with the slice selection gradient magnetic field pulse 615. Further, the crusher gradient magnetic field pulses 613 and 617 for phase dispersion of the residual transverse magnetization are applied so as to have the same application amount with the 180-degree RF pulse 614 interposed therebetween. Then, the second echo signal 619 is measured under the application of the frequency encoding gradient magnetic field pulse 618. Finally, spoiler gradient magnetic field pulses 620 and 621 for phase dispersion of the residual transverse magnetization are applied to the slice gradient magnetic field (Gs) axis and the G2 axis, respectively.

  Two spin echo signals are measured at each angle while rotating the gradient magnetic field pulses of the G1 axis and G2 axis in such a basic sequence around the coordinate origin of the imaging space by a predetermined angle step within the slice plane. The At this time, the amplitude of each gradient magnetic field pulse of the G2 axis in this basic sequence is applied in a distributed manner to the G1 axis and the G2 axis according to each rotation angle. The sequencer 6 thus controls the application of each RF pulse and each gradient magnetic field pulse, and controls the measurement of echo signals necessary for image reconstruction.

  FIG. 6 (b) shows the state of the K space where the echo data measured by the radial sequence is arranged. Each arrow corresponds to one echo signal, and the direction of the arrow indicates the scanning direction of each echo signal in the K space. As shown in FIG. 6 (b), the radial sequence measures data on each locus obtained by rotating a radial locus passing through the origin of the K space around the origin of the K space by a predetermined angle step.

Next, the reason why a stimulated echo occurs in such a multi-spin echo type radial sequence will be described based on the sequence shown in FIG. FIG. 7 shows an RF pulse (RF) and a gradient magnetic field pulse (G1, G2) when the basic sequence shape shown in FIG. 6 (a) is rotated to a certain angle. The other axes are the same as those in FIG. The sign of each gradient magnetic field pulse is distinguished by attaching 1 to the G1 axis and 2 to the G2 axis. In FIG. 7, the applied amount of the gradient magnetic field pulse for both the G1 axis and the G2 axis is

-[607] + [608] ≒ [617] + [618] / 2

When the rotation angle satisfies the above relationship, a stimulated echo is measured as a false echo signal when the gradient magnetic field pulse 618 is applied. This is because the excitation angle felt by transverse magnetization is 90 degrees in the first 90 degree RF pulse 601, 90 degrees in the second 180 degree RF pulse 605, and 90 degrees in the third 180 degree RF pulse 614. This is because the application amount of the gradient magnetic field pulse is balanced within one repetition time (TR). In the conventional method, there is no method for suppressing the stimulated echo rather than increasing the application amount of the crusher gradient magnetic field pulse.

  Next, crusher gradient magnetic field pulse application control according to the present embodiment will be described with reference to FIG. FIG. 8 is a sequence chart showing the application control of the crusher gradient magnetic field pulse according to the present embodiment, and FIG. 8 (a) shows a case where the encode gradient magnetic field pulses 610-1 and 618-1 have a positive rotation angle. FIG. 8 (b) shows a case where the encode gradient magnetic field pulses 610-2 and 618-2 have a rotation angle at which the polarity is negative.

  In the present embodiment, the application amounts of the crusher gradient magnetic field pulses 608, 609 and 613, 617 applied with the 180-degree RF pulse interposed therebetween are controlled in accordance with the polarities at the respective rotation angles of these encode gradient magnetic field pulses. Specifically, the applied amount of the crusher gradient magnetic field pulses 608, 609, 613, 617 so that the polarity of the encode gradient magnetic field pulses 610, 618 and the polarity of the crusher gradient magnetic field pulses 608, 609, 613, 617 are the same regardless of the rotation angle. To control.

  In the case of FIG. 8 (a), since the encode gradient magnetic field pulses 610, -1 618-1 are positive, the polarities of the rusher gradient magnetic field pulses 608-1, 609-1 and 613-1, 617-1 are also positive. . On the other hand, in the case of FIG. 8 (b), since the encode gradient magnetic field pulses 610-2 and 618-2 are of negative polarity, the polarities of the rusher gradient magnetic field pulses 608-2 and 609-2 and 613-2 and 617-2 are also negative. And By controlling the polarity of the encode gradient magnetic field pulse and the crusher gradient magnetic field pulse in this way, the sequencer 6 prevents the application amount of the gradient magnetic field pulse from being balanced within one repetition time (TR), and stimulates the stimulation. Suppresses a delayed echo from becoming a false echo signal.

  As described above, in the MRI apparatus of this embodiment, in the multi-spin echo type radial sequence, by controlling the application amount of the encoding gradient magnetic field corresponding to the polarity of the crusher gradient magnetic field pulse, at all rotation angles. Since it is possible to prevent the application amount of the gradient magnetic field pulse from being balanced within one repetition time (TR), it is possible to suppress the stimulated echo from becoming a false echo signal. As a result, artifacts can be suppressed with the minimum necessary crusher gradient magnetic field pulse application amount.

(Third embodiment)
Next, a third embodiment of the MRI apparatus of the present invention will be described. In the first and second embodiments described above, the sequencer 6 controls the application pattern of the crusher gradient magnetic field pulse. However, in this embodiment, the operator can set the application pattern of the crusher gradient magnetic field pulse. To do. The difference from the above-described embodiments is that the operator provides a UI (user interface) for setting the application pattern of the crusher gradient magnetic field pulse, and the rest is the same as the above-described embodiments. Only the different points will be described based on 8, and the description of the same points will be omitted.

  FIG. 8 shows an example of a UI for setting a crusher gradient magnetic field pulse application pattern. Various parameters displayed on the display 20 and displayed on the UI by the operator via the mouse or keyboard are selected and set. Is done. FIG. 8 shows only parameters according to the present embodiment, and the display and description of other parameters are omitted.

The item “Crusher Polarity” is a parameter according to the present embodiment, and is a UI for selecting an application pattern of a crusher gradient magnetic field pulse. This “Crusher Polarity” has the following options.
“AUTO” is an option to automatically apply the crusher gradient magnetic field pulse control of the known anti-stimulated echo countermeasure incorporated in the pulse sequence control software.
“NORMAL” is an option in which crusher gradient magnetic field pulse control is applied with a certain polarity without applying known stimulated echo control.
“REVERSE” is an option in which crusher gradient magnetic field pulse control is applied with the reverse polarity of NORMAL.
“RANDOM” is an option to which control is applied in which the polarity of the crusher gradient magnetic field pulse is alternately changed between positive polarity and negative polarity for each TR or for each slice.
“STATIC” is an option in which control that applies the same crusher gradient magnetic field pulse as in non-oblique imaging is applied to AUTO control.

  When the operator selects a desired application pattern, the sequencer 6 applies a crusher gradient magnetic field pulse of the selected application pattern.

  As described above, according to the MRI apparatus of the present embodiment, the operator can select the application pattern of the crusher gradient magnetic field pulse, so that an unexpected artifact is generated on the image when a certain measurement condition is combined. When you do this, you can avoid such artifacts by changing the Crusher Polarity.

The block diagram which shows the whole structure of an example of the MRI apparatus which concerns on this invention The pulse sequence figure which shows an example of SE sequence with a Flow-Rephase function. Pulse sequence diagram illustrating the stimulated echo formation path, showing Figure 2 in 2TR The figure which shows the k space data which show the position of the false echo which generate | occur | produces in conventional embodiment. FIG. 5 is a pulse sequence diagram showing the SE sequence according to the first embodiment of the present invention over 2TR. (a) A pulse sequence diagram showing an example of a radial sequence, and (b) a diagram that does not occupy an example of K-space data acquired by the radial sequence. The sequence diagram explaining the reason for generating a false echo signal in the conventional radial sequence. An example of a pulse sequence diagram illustrating the radial sequence according to the second embodiment of the present invention over 2TR, (a) is an encode gradient magnetic field pulse is positive, (b) is an encode gradient magnetic field pulse is negative If. The figure which shows UI which concerns on the 3rd Embodiment of this invention.

Explanation of symbols

  1 Static magnetic field generation system, 2 Static gradient magnetic field generation system, 3 Transmission system, 4 Reception system, 5 Signal processing system, 6 Sequencer, 7 Central processing unit, 8 Sample, 9 Gradient magnetic field coil, 10 Gradient magnetic field power supply, 11 High frequency Transmitter, 12 modulator, 13 high frequency amplifier, 14 irradiation coil, 15 high frequency receiving coil, 16 receiving circuit, 17 A / D converter, 18 signal processing device, 19 memory, 20 magnetic disk, 21 optical disk, 22 display

Claims (6)

Measurement control means for controlling the measurement of the echo signal by applying a plurality of RF pulses and a plurality of gradient magnetic field pulses to the subject based on a predetermined pulse sequence;
The plurality of gradient magnetic field pulses are a magnetic resonance imaging apparatus including a pair of crusher gradient magnetic field pulses having the same applied amount applied before and after at least one RF pulse,
The measurement control means controls the application of the pair of crusher gradient magnetic field pulses in accordance with the polarity of a predetermined gradient magnetic field pulse. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus, wherein the measurement control means controls the polarity of the predetermined gradient magnetic field pulse and the polarity of the pair of crusher gradient magnetic field pulses to be the same. The magnetic resonance imaging apparatus according to claim 2.
The magnetic resonance imaging apparatus, wherein the measurement control unit changes the polarity of the pair of crusher gradient magnetic field pulses at a timing at which the polarity of the predetermined gradient magnetic field pulse is switched. The magnetic resonance imaging apparatus according to claim 2.
The pulse sequence is a spin echo sequence;
The RF pulse sandwiched between the pair of crusher gradient magnetic field pulses is a 180 degree RF pulse,
The magnetic resonance imaging apparatus, wherein the predetermined gradient magnetic field pulse is a phase encode gradient magnetic field pulse. The magnetic resonance imaging apparatus according to claim 2.
The pulse sequence is a multi-spin echo type radial sequence,
The RF pulse sandwiched between the pair of crusher gradient magnetic field pulses is a 180 degree RF pulse,
The magnetic resonance imaging apparatus, wherein the predetermined gradient magnetic field pulse is an encode gradient magnetic field pulse. The magnetic resonance imaging apparatus according to claim 1,
Display means for displaying choices of application patterns of the crusher gradient magnetic field pulses;
The measurement control unit controls application of the crusher gradient magnetic field pulse in accordance with a selected application pattern.

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