JP2004073893A - Mri system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the degradation of image quality due to eddy currents and residual magnetization caused by a phase encode gradient. <P>SOLUTION: In an MRI system, an excitation RF (radio frequency) pulse is transmitted, then a reversal RF pulse is transmitted, a phase encode gradient is then applied on a warp axis, and finally an NMR signal is received while applying a read gradient on a read axis. After the processes from transmitting the reverse RF pulse to receiving an NMR signal are repeated M times, a killer gradient is applied. The processes from transmitting the excitation RF pulse to applying the killer gradient are repeated N times. Then NMR signals processed by different phase encodes of (M×N) times are collected. The polarity of the killer gradient is switched so as to be identical with the polarity of the sum of phase encode gradients of M times to be applied when processes from transmitting the reverse RF pulse to receiving the NMR signal are repeated M times. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to an MRI (Magnetic Resonance Imaging) apparatus, and more particularly, to an MRI apparatus capable of preventing deterioration in image quality of an image due to the influence of eddy current and residual magnetization caused by a phase encoding gradient.

FIG. 27 shows a pulse sequence of the conventional high-speed SE (Spin Echo) method.
In this pulse sequence Kp, an excitation pulse R and a slice gradient ss are applied. Next, the first inversion pulse P1 and the slice gradient ss are applied, the phase encoding gradient gy (i) is applied to the warp axis, and then the NMR signal is received from the first echo SE1 while applying the read gradient rr. Thereafter, a rewind gradient gyr (i) having the same time integral as the encode gradient gy (i) and the opposite polarity is applied to the warp axis. Next, the second inversion pulse P2 and the slice gradient ss are applied, the encode gradient gy (i + 1) is applied to the warp axis, and then the NMR signal is received from the second echo SE2 while applying the read gradient rr. Thereafter, a rewind gradient gyr (i + 1) having the same time integral as the encode gradient gy (i + 1) and the opposite polarity is applied to the warp axis. Next, an NMR signal is received from the third echo SE3 while applying a third inversion pulse P3 and a slice gradient ss, applying an encode gradient gy (i + 2) to the warp axis, and then applying a read gradient rr. Thereafter, a rewind gradient gyr (i + 2) having the same time integral as the encode gradient gy (i + 2) and the opposite polarity is applied to the warp axis. After repeating the transmission of the inverted pulse and the reception of the NMR signal M times, the killer gradient Kil is applied. Hereinafter, this is repeated N times with a repetition time TR to collect NMR signals with (M × N) times different phase encoding performed on the warp axis.
Note that p = 1, 2,..., N. Although M = 3 in FIG. 27, M ≧ 4 is often used in order to shorten the scan time. Further, there is a relation of i = (p−1) M + 1, and (i), (i + 1), (i + 2) in the encoding gradients gy (i), gy (i + 1), gy (i + 2) in FIG. 27 are phase encoding numbers. Is represented.

The killer gradient Kil is a gradient pulse for spoiling transverse magnetization, and its polarity is conventionally fixed.

FIG. 28 shows a data collection trajectory on the k-space by the pulse sequence Kp.
In the case of an echo train of M = 3, the k-space Ksp is divided into three segments Sg1, Sg2, and Sg3. Then, data of the first segment Sg1 (for example, tc1) is collected by the first echo SE1, data of the second segment Sg2 (for example, tc2) is collected by the second echo SE2, and data of the third segment Sg3 is collected by the third echo SE3. Collect data (for example, tc3).
FIG. 28 shows p = 1, and the position on the phase axis of the data is determined by the phase encoding gradients gy (1), gy (2), and gy (3), and the rewind gradients gyr (1), gyr (2). , Gyr (3), the phase encoding amount is returned to “0”.
Since the data near the phase encoding amount “0” determines the contrast of the image, the time TEeff up to the second echo SE2 shown in FIG. 27 is the effective echo time.

FIG. 29 shows a pulse sequence of the conventional 3D (three-dimensional) fast SE method.
In this pulse sequence Kp ′, an excitation pulse R and a slice gradient ss are applied. Next, a rewind gradient gzr (0) having the same time integration value as that of the phase encode gradient applied to the slice axis and having the opposite polarity is applied to the slice axis while superimposed on the crusher gradient. P1 and the slice gradient are applied, and the phase encode gradient gz (i) is applied to the slice axis while being superimposed on the crusher gradient. Further, a phase encoding gradient gy (i) is applied to the warp axis. Next, the NMR signal is received from the first echo SE1 while applying the read gradient rr, and then the rewind gradient gyr (i) having the same polarity as the encode gradient gy (i) of the warp axis and the opposite polarity is applied. A rewind gradient gzr (i) having the same time integration value and the opposite polarity as the phase encode gradient gz (i) of the slice axis is applied while being superimposed on the crusher gradient. Next, a second inversion pulse P2 and a slice gradient are applied, and the phase encode gradient gz (i + 1) is applied to the slice axis while being superimposed on the crusher gradient. Further, a phase encoding gradient gy (i + 1) is applied to the warp axis. Next, the NMR signal is received from the second echo SE2 while applying the read gradient rr, and thereafter, the rewind gradient gyr (i + 1) having the same time integration value as the encode gradient gy (i + 1) of the hoop axis and the opposite polarity is applied. Then, a rewind gradient gyr (i + 1) having the same time integration value and the opposite polarity as the phase encode gradient gz (i + 1) of the slice axis is applied while being superimposed on the crusher gradient. Next, a third inversion pulse P3 and a slice gradient are applied, and a phase encode gradient gz (i + 2) is superimposed on the crusher gradient and applied to the slice axis. Further, a phase encoding gradient gy (i + 2) is applied to the warp axis. Next, the NMR signal is received from the third echo SE3 while applying the read gradient rr, and thereafter, the rewind gradient gyr (i + 2) having the same time integration value as the encode gradient gy (i + 2) of the warp axis and the opposite polarity is applied. . After repeating the process from transmission of the inversion pulse P to reception of the NMR signal M times (M = 3 in this case), the killer gradient Kil is applied. Hereinafter, this is repeated at a repetition time TR to collect NMR signals having undergone phase encoding on the slice axis and the warp axis.

In the high-speed SE method such as the pulse sequences Kp and Kp ′ in FIGS. 27 and 29, the amplitude of the phase encoding gradients gy (i), gy (i + 1), and gy (i + 2) of the warp axis is reduced in order to shorten the repetition time TR. Is made as large as possible, and the time width tgy is shortened accordingly. In the 3D fast SE method like the pulse sequence Kp ′ in FIG. 28, similarly, the amplitudes of the phase encode gradients gz (i), gz (i + 1), and gz (i + 2) on the slice axis are increased as much as possible. The time width tgz is shortened accordingly.
However, an eddy current is generated as the amplitude of the gradient pulse is increased and the time width is shortened. Further, as the amplitude is increased, residual magnetization is generated. When an eddy current or residual magnetization is generated, an artifact is generated on an image due to the influence of the eddy current or residual magnetization, and the image quality is deteriorated.
In particular, in an MRI apparatus using a permanent magnet, there is a problem of deterioration of image quality due to residual magnetization generated in the magnetic shunt plate.

On the other hand, for example, Japanese Patent Application Laid-Open No. 6-245917 proposes a technique for canceling the effect of residual magnetization by applying an offset gradient and a technique for correcting a phase shift due to residual magnetization of a magnetic shunt plate by calculation. ing.

The effect of the residual magnetization caused by the phase encoding gradient becomes remarkable especially when the sequence of the high-speed SE method is executed in the MRI apparatus using the permanent magnet.
This will be described with reference to FIGS.
As shown in FIG. 30, the phase encoding gradient gy (i) of the warp axis causes a residual magnetization having an intensity ΔGY (i), and the residual magnetization causes a phase shift in the first echo SE1. A phase shift is similarly generated for the second echo SE2 and subsequent echoes.
Further, the phase encoding amount cannot be returned to “0” due to the rewind gradient gyr (i), which affects the phase of the second echo SE2 and the phase of the third echo SE3. The same applies to the rewind gradient gyr (i + 1) or less, and the phase encoding amount cannot be returned to “0”, which affects the phase of the subsequent echo.
Furthermore, the phase of a stimulated echo STE (Stimulated Echo) generated simultaneously with the spin echo needs to match the phase of the spin echo, but these phases do not match. FIG. 30 shows the second echo SE2, which is the most important for determining the contrast of the image, and the second stimulated echo STE2, which is generated at the same time. The intensity ΔGY due to the phase encoding gradient gy (i) is shown. The phase of the second echo SE2 is shifted due to the influence of the residual magnetization in (i), while the second stimulated echo STE2 is not affected (from the first inversion pulse P1 to the second inversion pulse P2). Since the section is longitudinal magnetization), the phases do not match. As a result, interference occurs, and ghost artifacts (Shading Artifacts) and ringing artifacts (Ringing Artifacts) occur on the image.

In addition, as shown in FIG. 31, a residual magnetization of intensity ΔGZ (i) is generated by the phase encode gradient gz (i) of the slice axis, and the residual magnetization causes a phase shift in the first echo SE1. A phase shift is similarly generated for the second echo SE2 and subsequent echoes.
Also, the phase encoding amount cannot be returned to “0” due to the rewind gradient gzr (i), which affects the phase of the second echo SE2 and the phase of the third echo SE3. The same applies to the rewind gradient gzr (i + 1) or less, and the phase encode amount cannot be returned to “0”, which affects the phase of the subsequent echo.
Further, the phase of the second echo SE2, which is the most important for determining the contrast of the image, and the phase of the second stimulated echo STE2 generated at the same time are out of phase with each other, causing artifacts on the image.

However, the prior art proposed in the above-mentioned Japanese Patent Application Laid-Open No. 6-245917 has a problem that it is not enough to prevent the image quality of an image from deteriorating due to the influence of eddy current and residual magnetization caused by the phase encoding gradient. .
SUMMARY OF THE INVENTION It is an object of the present invention to provide an MRI apparatus capable of preventing deterioration of image quality due to eddy current and residual magnetization caused by a phase encoding gradient.

According to a first aspect, the present invention provides an MRI apparatus including an RF pulse transmitting unit, a gradient magnetic field applying unit, and an NMR signal receiving unit, wherein an RF pulse is transmitted by the RF pulse transmitting unit, and the gradient magnetic field is applied. Means for applying a phase encoding gradient to the warp axis, receiving the NMR signal by the NMR signal receiving means, and then applying the phase encoding gradient to the fundamental component having the same time integral and the opposite polarity as the phase encoding gradient by the gradient magnetic field applying means. Applying a rewind gradient to the warp axis to which a warp axis correction component for correcting the effect of eddy current or residual magnetization caused by the gradient is applied, or a rewind gradient corresponding to the basic component and a warp axis correction component. An MRI apparatus characterized in that an auxiliary rewind gradient is applied to a warp axis. In addition, at the time of “adding”, the addition may be performed as an amplitude, a time width, or both the amplitude and the time width.
In the MRI apparatus according to the first aspect, "(a fundamental component having the same phase integration gradient and time integration value and opposite polarity) + (a warp axis correction component for correcting the influence of eddy current or residual magnetization caused by the phase encoding gradient)" Is applied to the warp axis, or the effect of eddy current or residual magnetization caused by the rewind gradient consisting of (a fundamental component having the same time integration value as the phase encoding gradient and the opposite polarity) and Is applied to the warp axis.
The amount of phase encoding is returned to "0" by the basic component (original rewind), and the effect of eddy current or residual magnetization caused by the phase encoding gradient is canceled by the warp axis correction component. Therefore, it is possible to prevent the image quality of the image from deteriorating due to the influence of the eddy current or the residual magnetization caused by the phase encoding gradient.
According to a second aspect, the present invention relates to an MRI apparatus including an RF pulse transmitting unit, a gradient magnetic field applying unit, and an NMR signal receiving unit, wherein the RF pulse transmitting unit transmits an RF pulse, and Means for applying a phase encode gradient to the warp axis by adding a warp axis correction component for correcting the influence of eddy current or residual magnetization caused by the phase encode gradient to a basic component of the phase encode gradient of the warp axis determined by the scan parameter. Alternatively, a phase encoding gradient corresponding to the basic component and an auxiliary phase encoding gradient corresponding to the warp axis correction component are applied to the warp axis, and after the NMR signal is received by the NMR signal receiving means, the gradient magnetic field is applied. Means that the basic component of the phase encoding gradient and the time integration value are equal and opposite in polarity. Provides an MRI apparatus wherein applying a gradient to the warp axis. In addition, at the time of “adding”, the addition may be performed as an amplitude, a time width, or both the amplitude and the time width.
In the MRI apparatus according to the second aspect, the phase consisting of “(basic component of phase encoding gradient determined by scan parameter) + (warp axis correction component for correcting the influence of eddy current or residual magnetization caused by phase encoding gradient)”. An encoding gradient is applied to the warp axis, or a phase encoding gradient composed of (a fundamental component of the phase encoding gradient determined by scan parameters) and a warp axis correction for correcting the influence of eddy current or residual magnetization caused by the phase encoding gradient. Component) is applied to the warp axis.
Phase encoding is performed by the basic component (original phase encoding), and the warp axis correction component cancels the influence of eddy current or residual magnetization caused by the phase encoding gradient. Therefore, it is possible to prevent the image quality of the image from deteriorating due to the influence of the eddy current or the residual magnetization caused by the phase encoding gradient.

According to a third aspect of the present invention, in the MRI apparatus having the above-described configuration, an RF pulse is transmitted by the RF pulse transmitting unit, and a read gradient is read by the gradient magnetic field applying unit without applying a phase encoding gradient to the warp axis. An NMR signal is received by the NMR signal receiving means while applying to the axis, and first phase information obtaining means for obtaining first phase information from the received NMR signal, and an RF pulse is transmitted by the RF pulse transmitting means. The gradient encoding means applies the equivalent phase encoding gradient equal to the phase encoding gradient and the time integral to the lead axis without applying the phase encoding gradient to the warp axis, and the equivalent phase encoding gradient and the time integral are equal and opposite to each other. A polarity equivalent rewind gradient is applied to the lead axis, and then the read gradient is applied to the read axis by the gradient magnetic field applying means. Second phase information acquisition means for receiving an NMR signal by the NMR signal reception means while applying the voltage, and acquiring second phase information from the received NMR signal; and the first phase information and the second phase information And a warp axis correction component calculating means for obtaining the warp axis correction component from the above.
In the MRI apparatus according to the third aspect, the warp axis correction component is obtained as follows.
(1) Transmit an RF pulse, receive an NMR signal while applying a read gradient to a lead axis without applying a phase encoding gradient to a warp axis, and obtain first phase information from the received NMR signal. .
(2) transmitting an RF pulse, applying no phase encoding gradient to the warp axis, applying an equivalent phase encoding gradient having the same time integration value as the phase encoding gradient to the lead axis, and applying the equivalent phase encoding gradient and the time integration value to the read axis; Apply an equivalent rewind gradient of the same polarity to the lead axis, and then receive the NMR signal while applying the read gradient to the lead axis, and acquire the second phase information from the received NMR signal.
(3) A warp axis correction component is obtained from the first phase information and the second phase information.
The first phase information is phase information when there is no influence of eddy current or residual magnetization due to a phase encoding gradient. The second phase information is phase information when an influence of eddy current or residual magnetization caused by a phase encoding gradient is equivalently applied to a lead axis. Therefore, by comparing the first phase information with the second phase information, it is possible to quantitatively know the influence of the eddy current or the residual magnetization caused by the phase encoding gradient. Therefore, a warp axis correction component for canceling it can be obtained quantitatively.

According to a fourth aspect of the present invention, in the MRI apparatus having the above configuration, the RF pulse transmitting unit transmits an RF pulse, the gradient magnetic field applying unit applies a phase encoding gradient to a warp axis, An NMR signal is received by the NMR signal receiving unit while a read gradient is applied to a lead axis by an applying unit, and current phase information obtaining unit for obtaining current phase information from the received NMR signal is provided by the RF pulse transmitting unit. Transmitting an RF pulse, applying a phase encoding gradient having an increased time width and reduced amplitude to the warp axis by the gradient magnetic field applying means, and then applying the read gradient to the read axis by the gradient magnetic field applying means. An ideal phase in which an NMR signal is received by signal receiving means and ideal phase information is obtained from the received NMR signal. Providing a distribution obtaining unit, an MRI apparatus wherein from the phase information and phase information of the ideal that it has and a warp-axis correction component calculating means for determining the warp-axis correction component of the current.
In the MRI apparatus according to the fourth aspect, the warp axis correction component is obtained as follows.
(1) Transmit an RF pulse, apply a phase encoding gradient to a warp axis, receive an NMR signal while applying a read gradient to a lead axis, and acquire current phase information from the received NMR signal.
(2) Transmit an RF pulse, apply a phase encoding gradient with an expanded time width and reduced amplitude to the warp axis, receive an NMR signal while applying a read gradient to the lead axis, and ideally use the received NMR signal. To obtain the phase information.
(3) A warp axis correction component is obtained from the current phase information and the ideal phase information.
The current phase information is phase information including the influence of eddy current or residual magnetization due to the phase encoding gradient. Further, the ideal phase information is phase information when there is no influence of eddy current or residual magnetization caused by a phase encoding gradient. Therefore, by comparing the current phase information with the ideal phase information, it is possible to quantitatively know the influence of the eddy current or the residual magnetization caused by the phase encoding gradient. Therefore, a warp axis correction component for canceling it can be obtained quantitatively.

In a fifth aspect, the present invention provides an RF pulse transmitting means for transmitting an excitation RF pulse, an RF pulse transmitting means for transmitting an inverted RF pulse, and a gradient magnetic field applying means for applying a phase encoding gradient to a warp axis. After receiving the NMR signal by the NMR signal receiving means while applying a read gradient to the lead axis by the gradient magnetic field applying means, and repeating the transmission from the transmission of the inverted RF pulse to the reception of the NMR signal M times, An MRI in which a killer gradient is applied by the gradient magnetic field applying means, and a process from transmission of the excitation RF pulse to application of the killer gradient is repeated N times to collect (M × N) times of NMR signals subjected to different phase encoding. In the apparatus, the gradient magnetic field applying means applies the magnetic field when the process from transmission of the inverted RF pulse to reception of the NMR signal is repeated M times. Times of to have the same Gokusei the polarity of the sum of the phase encoding gradient, to provide an MRI apparatus characterized by switching the polarity of the killer gradient.
In the MRI apparatus according to the fifth aspect, the polarity of the killer gradient is switched so as to be the same as the polarity of the sum of a plurality of phase encoding gradients applied by the corresponding pulse train.
Conventionally, since the polarity of the killer gradient was constant, it only functioned as a spoiler. However, if the polarity of the killer gradient is switched so as to be the same as the polarity of the sum of the multiple phase encoding gradients applied in the corresponding pulse train, the function of canceling the influence of the eddy current or residual magnetization caused by the phase encoding gradient Can also be played. Therefore, it is possible to prevent the image quality of the image from deteriorating due to the influence of the eddy current or the residual magnetization caused by the phase encoding gradient.

In a sixth aspect, the present invention provides an RF pulse transmitting means for transmitting an excitation RF pulse, an RF pulse transmitting means for transmitting an inverted RF pulse, and a gradient magnetic field applying means for applying a phase encoding gradient to a warp axis. In the MRI apparatus, the NMR signal is applied to the warp axis and the slice axis, and then the NMR signal is received by the NMR signal receiving means while the read gradient is applied to the read axis by the gradient magnetic field applying means. The transmission phase of the inverted RF pulse by the RF pulse transmission means is changed or the detection phase of the phase detection by the NMR signal reception means is changed so as to suppress the zero-order phase component caused by the influence of the residual magnetization. An MRI apparatus is provided.
The effects of the residual magnetization caused by the phase encoding gradient include a zero-order phase component that is uniformly applied without depending on the location and a first-order phase component that changes linearly depending on the location. The zero-order phase component mainly generates ghost artifacts, and the first-order phase component mainly generates shading artifacts.
Here, since the zero-order phase component appears as a shift in the overall phase of the NMR signal, the transmission phase of the inverted RF pulse by the RF pulse transmitting unit is changed, or the phase detection by the NMR signal receiving unit is detected. By changing the phase, it can be restored. Thus, ghost artifacts can be suppressed.

According to a seventh aspect of the present invention, in the MRI apparatus having the above configuration, the RF pulse transmitting unit transmits an RF pulse, and does not apply a phase encoding gradient to the warp axis and the slice axis. Applying an equivalent phase encoding gradient having the same time integral as the encoding gradient to the lead axis, applying an equivalent rewind gradient having the same polarity as the time integral and the opposite polarity to the lead axis, and then applying the gradient magnetic field; A NMR signal is received by the NMR signal receiving means while applying a read gradient to the lead axis by means, phase information is obtained from the received NMR signal, and a zero-order phase component is obtained from the phase information to obtain the zero-order phase component. An MRI apparatus characterized by comprising an acquisition unit is provided.
In the MRI apparatus according to the seventh aspect, the zero-order phase component is obtained as follows.
(1) transmitting an RF pulse, applying no phase encoding gradient to the warp axis and the slice axis, applying an equivalent phase encoding gradient having a time integral value equal to the phase encoding gradient to the lead axis, and An NMR signal is received while applying an equivalent rewind gradient having the same integral value and the opposite polarity to the lead axis, and then applying a read gradient to the lead axis, and acquiring phase information from the received NMR signal.
(2) A zero-order phase component is obtained from the phase information.
The phase information is the phase information when the influence of the eddy current or the residual magnetization caused by the phase encoding gradient is equivalently applied to the lead axis. Therefore, if the phase at the center of the gradient is obtained, the zero-order phase component of the influence of the eddy current or the residual magnetization caused by the phase encoding gradient can be quantitatively known.

According to an eighth aspect, the present invention provides an MRI apparatus configured as described above, wherein the NMR signal obtained when the phase encoding gradient and the rewind gradient are applied to the warp axis and the phase encoding gradient and the rewind gradient are not applied to the warp axis. An MRI apparatus further comprising a zero-order phase component obtaining means for obtaining the zero-order phase component from the obtained NMR signal.
In the MRI apparatus according to the eighth aspect, the zero-order phase component is obtained as follows.
(1) Apply the phase encode gradient and the rewind gradient to the warp axis to obtain the current NMR signal.
(2) An ideal NMR signal is obtained without applying a phase encode gradient and a rewind gradient to the warp axis.
(3) The zero-order phase amount is obtained from the phase at the gradient center of the 1D Fourier transform in the read direction of the current NMR signal, and the zero-order phase amount is obtained from the phase at the gradient center of the 1D Fourier transform in the read direction of the ideal NMR signal. And the zero-order phase component of the influence of the eddy current or residual magnetization is obtained from the difference between the two.
By taking the difference between the zero-order phase component when the phase encoding gradient and the rewind gradient are applied and when it is not applied, the zero-order phase component of the influence of the eddy current or residual magnetization caused by the phase encoding gradient and the rewind gradient is quantitatively determined. You can know.
Since the phase encoding gradient and the rewind gradient are added to the warp axis, the influence of the eddy current or the residual magnetization is superimposed on the zero-order phase component. Therefore, in order to prevent this, it is preferable to use the phase encoding gradient and the rewind gradient in which the influence of the eddy current or the residual magnetization is corrected according to the first or second viewpoint.

According to a ninth aspect, the present invention provides an MRI apparatus comprising an RF pulse transmitting unit, a gradient magnetic field applying unit, and an NMR signal receiving unit, wherein the RF pulse transmitting unit applies an excitation pulse, and then applies the RF pulse. The inversion pulse is transmitted by the transmitting means, the phase encoding gradient is applied to the slice axis by the gradient magnetic field applying means, the NMR signal is received by the NMR signal receiving means, and the phase encoding gradient and time integration are received by the gradient magnetic field applying means. A rewind gradient obtained by adding a slice axis correction component for correcting the influence of the eddy current or residual magnetization caused by the phase encoding gradient to a basic component having the same and opposite polarity is applied to the slice axis, or corresponds to the basic component. And the auxiliary rewind gradient corresponding to the slice axis correction component are marked on the slice axis. It provides an MRI apparatus characterized by repeating only encode the number of slice axis direction to.
In the MRI apparatus according to the ninth aspect, "(a basic component having the same phase integration gradient and time integration value and opposite polarity) + (a slice axis correction component for correcting the influence of eddy current or residual magnetization caused by the phase encoding gradient)" Is applied to the slice axis, or the effect of eddy current or remanent magnetization caused by the rewind gradient consisting of (a fundamental component having the same phase integration gradient and time integration value and opposite polarity). Is applied to the slice axis.
The phase encoding amount is returned to “0” by the basic component (original rewind), and the effect of the eddy current or residual magnetization caused by the phase encoding gradient is canceled by the slice axis correction component. Therefore, it is possible to prevent the image quality of the image from deteriorating due to the influence of the eddy current or the residual magnetization caused by the phase encoding gradient.

According to a tenth aspect, the present invention provides an MRI apparatus having the above-described configuration, wherein the RF pulse is transmitted by the RF pulse transmitting unit, and a read gradient is read by the gradient magnetic field applying unit without applying a phase encoding gradient to a slice axis. An NMR signal is received by the NMR signal receiving means while applying to the axis, and first phase information obtaining means for obtaining first phase information from the received NMR signal, and an RF pulse is transmitted by the RF pulse transmitting means. A phase encoding gradient is not applied to the slice axis, but a differential gradient corresponding to the difference between the phase encoding gradient and the basic component of the subsequent rewind gradient is applied to the read axis by the gradient magnetic field applying means, and the phase encoding gradient and time integration are applied. Equivalent phase encoding gradient with equal value and equivalent with time integral value equal to the basic component of the rewind gradient An NMR signal is received by the NMR signal receiving means while a read gradient is applied to the lead axis by the gradient magnetic field applying means, and second phase information is obtained from the received NMR signal. And a slice axis correction component calculation unit for obtaining the slice axis correction component from the first phase information and the second phase information. An MRI apparatus is provided.
In the MRI apparatus according to the tenth aspect, the slice axis correction component is obtained as follows.
(1) Transmit an RF pulse, receive an NMR signal while applying a read gradient to a read axis without applying a phase encode gradient to a slice axis, and obtain first phase information from the received NMR signal. .
(2) transmitting an RF pulse, applying no phase encoding gradient to the slice axis, and applying a difference gradient corresponding to the difference between the phase encoding gradient and the subsequent basic component of the rewind gradient to the read axis, And applying an equivalent rewind gradient having the same time integral value as the basic component of the rewind gradient to the lead axis, and receiving the NMR signal while applying the read gradient to the lead axis. The second phase information is obtained from the obtained NMR signal.
(3) A slice axis correction component is obtained from the first phase information and the second phase information.
The first phase information is phase information when there is no influence of eddy current or residual magnetization due to a phase encoding gradient. The second phase information is phase information when an influence of eddy current or residual magnetization caused by a phase encoding gradient is equivalently applied to a lead axis. Therefore, by comparing the first phase information with the second phase information, it is possible to quantitatively know the influence of the eddy current or the residual magnetization caused by the phase encoding gradient. Therefore, a slice axis correction component for canceling it can be obtained quantitatively.

According to the MRI apparatus of the present invention, it is possible to prevent the image quality of the image from deteriorating due to the influence of eddy current or residual magnetization caused by the phase encoding gradient. In particular, it is useful for improving image quality in an MRI apparatus using a permanent magnet.

Hereinafter, the present invention will be described in more detail with reference to the embodiments shown in the drawings. It should be noted that the present invention is not limited by this.

-1st Embodiment-
FIG. 1 is a block diagram of an MRI apparatus according to a first embodiment of the present invention.
In the MRI apparatus 100, the magnet assembly 1 has a space portion (hole) for inserting a subject inside, and a permanent magnet that applies a constant main magnetic field to the subject so as to surround the space portion. 1p, a gradient magnetic field coil 1g for generating a gradient magnetic field of a slice axis, a warp axis, and a lead axis, a transmission coil 1t for applying an RF pulse for exciting spins of nuclei in the object, and a signal from the object. A receiving coil 1r for detecting an NMR signal is arranged. The gradient magnetic field coil 1g, the transmission coil 1t and the reception coil 1r are connected to a gradient magnetic field drive circuit 3, an RF power amplifier 4 and a preamplifier 5, respectively.

The sequence storage circuit 8 operates the gradient magnetic field drive circuit 3 based on the stored pulse sequence in accordance with a command from the computer 7 to generate a gradient magnetic field from the gradient magnetic field coil 1g of the magnet assembly 1 and perform gate modulation. The circuit 9 is operated to modulate the carrier wave output signal of the RF oscillation circuit 10 into a pulse signal having a predetermined timing and a predetermined envelope shape. The modulated pulse signal is applied to the RF power amplifier 4 as an RF pulse, and the power is amplified by the RF power amplifier 4. After that, a voltage is applied to the transmission coil 1t of the magnet assembly 1 to selectively excite a target slice area.

The preamplifier 5 amplifies the NMR signal from the subject detected by the receiving coil 1 r of the magnet assembly 1 and inputs the amplified NMR signal to the phase detector 12. The phase detector 12 uses the carrier output signal of the RF oscillation circuit 10 as a reference signal, performs phase detection on the NMR signal from the preamplifier 5, and supplies the NMR signal to the A / D converter 11. The A / D converter 11 converts the analog signal after the phase detection into a digital signal and inputs the digital signal to the computer 7.
The computer 7 reads data from the A / D converter 11 and performs an image reconstruction operation to generate an image of a target slice area. This image is displayed on the display device 6. Further, the computer 7 is responsible for overall control such as receiving information input from the console 13.

FIG. 2 is a flowchart showing a warp axis correction component acquisition process in the MRI apparatus 100.
In step S1, data sy0 (k) is collected by the pulse sequence A0 shown in FIG. In this pulse sequence A0, an excitation pulse R and a slice gradient ss are applied, then an inversion pulse P and a slice gradient ss are applied, and then an NMR signal is received from the echo SE while a read gradient gxw is applied, and data sy0 is received. Collect (k). Note that no phase encoding gradient is applied to the warp axis.
In step S2, data sy0 (k) is subjected to one-dimensional Fourier transform in the read direction, and the result is set to SY0 (x).
In step S3, a first-order gradient dy0 of the phase term (Arctan {SY0 (x)}) of the Fourier transform result SY0 (x) is obtained (calculated by applying the least square method or the like). Further, a phase amount b0 = Arctan {SY0 (0)} at the gradient center is obtained. This phase amount b0 is the phase amount at the center of the gradient when there is no residual magnetization.

The above steps S1 to S3 are processes for obtaining the effect (deviation of the echo center of the echo signal SE) that the gradient magnetic fields gx1 and gxw are mainly caused by the eddy current.

In step S4, data syi (k) is collected by the pulse sequence Aj shown in FIG. In this pulse sequence Aj, an excitation pulse R and a slice gradient ss are applied, and then an equivalent phase encode gradient gy (i) whose time integral is equal to the phase encode gradient gy (i) of the encode number i determined by the scan parameter is read. Applied to the lead axis, and after a time ty (as long as possible to improve accuracy), an equivalent rewind gradient gyr (i) having the same time integration value and the opposite polarity as that of the equivalent phase encoding gradient gy (i) is applied to the lead axis. I do. Next, the NMR signal is received from the echo SE while applying the inversion pulse P and the slice gradient ss, and then the read gradient gxw is applied, and the data syi (k) is collected. Note that no phase encoding gradient is applied to the warp axis. Here, step S4 may be repeated for all the encoding numbers i (j = i in this case), but step S4 is repeated for the encoding number i appropriately selected to reduce the time (in this case, j ≠). i), and the data of the encoding number not selected may be obtained by interpolation. In the simplest case, the step S4 is executed only for the encode number giving the maximum phase encode gradient (in this case, j = 1 only), and the data of the other encode numbers are calculated in proportion to the encode gradient. Is also good.
In step S5, the data syi (k) is subjected to one-dimensional Fourier transform in the read direction, and the result is set as SYi (x).
In step S6, the first-order gradient dyi of the phase term (Arctan {SYi (x)}) of the Fourier transform result SYi (x) is obtained (by applying the least square method or the like). Further, the phase amount bi = Arctan {SYi (0)} at the gradient center is obtained, and the phase amount difference (bi−b0) is calculated. This phase amount difference (bi-b0) is a zero-order phase component that is uniformly applied without depending on the location due to the influence of the residual magnetization caused by the phase encoding gradient gy (i).
The above steps S4 to S6 are processes for obtaining the effect of the phase encoding gradient gy (i) mainly caused by the residual magnetization.

In step S7, the magnitude ΔGY (i) of the influence of the eddy current and the residual magnetization caused by the equivalent phase encoding gradient gy (i) is calculated from (dyi-dy0).
dy0 represents the phase rotation when there is no phase encoding gradient. On the other hand, dyi represents the phase rotation when there is an equivalent phase encoding gradient gy (i). Originally, since the equivalent phase encoding gradient gy (i) is canceled by the equivalent rewind gradient gyr (i), (dyi-dy0) = 0 should be satisfied. However, if (dyi-dy0) ≠ 0, the magnitude of the difference indicates the magnitude of the influence of the eddy current and the residual magnetization caused by the equivalent phase encoding gradient gy (i). Therefore, the magnitude ΔGY (i) of the influence of the eddy current and the residual magnetization caused by the equivalent phase encoding gradient gy (i) can be calculated from (dyi-dy0).
That is, when the separation of the read gradient gxw is a_gxw, and the shift time of the echo center due to the eddy current or residual magnetization caused by the equivalent phase encoding gradient gy (i) is ty0,
γ · ΔGY (i) · ty = γ · a_gxw · ty0 (where γ is the gyromagnetic ratio)
Relationship
ΔGY (i) = a_gxw · ty0 / ty
Holds.

In step S8, the correction coefficient αi (the magnitude of the influence of the eddy current or the residual magnetization per unit phase encoding amount) is obtained by the following equation.

In step S9, from the basic component gy (i) of the phase encoding gradient determined by the scan parameter and the correction coefficient αi,
qi = αi · gy (i) (however, qi ≦ 1.0)
To obtain the warp axis correction component qi.

FIG. 5 is a flowchart showing an imaging data collection process of the MRI apparatus 100.
In step S10, a new rewind gradient gyr (i) 'is obtained from the basic component gy (i) of the phase encoding gradient determined by the scan parameter and the warp axis correction component qi.
gyr (i) ′ = 0.01 · gy (i) + qi
In step S11, imaging data is collected by the pulse sequence Bp of the fast SE method using the new rewind gradient gyr (i) ′ shown in FIG. At this time, the detection phase in the phase detector 12 is adjusted so as to cancel the phase amount difference (bi-b0), or the transmission phases of the corresponding inverted pulses P1, P2, and P3 are adjusted.
By performing imaging using the data for imaging collected as described above, it is possible to prevent the image quality of the image from deteriorating due to the influence of eddy current or residual magnetization caused by the phase encoding gradient of the warp axis.

-2nd Embodiment-
The second embodiment is a modification of the first embodiment, and acquires the warp axis correction component qi in the same manner as the first embodiment, but instead of the pulse sequence Bp of FIG. Imaging data is collected by the pulse sequence Cp.
That is, as shown in FIG. 7, while applying the rewind gradient gyr (i) corresponding to the basic component gy (i), the auxiliary rewind gradient gqr (i) corresponding to the warp axis correction component qi is applied to the warp axis. Apply.
As a result, even when imaging is performed using the collected imaging data, it is possible to prevent the image quality of the image from deteriorating due to the influence of eddy current or residual magnetization due to the phase encoding gradient.

-Third embodiment-
The third embodiment is a modification of the first embodiment, and acquires the warp axis correction component qi in the same manner as the first embodiment, but obtains the basic component gy (i) of the phase encoding gradient determined by the scan parameter. A new phase encoding gradient gy (i) ′ is obtained from the above and the warp axis correction component qi, and as shown in FIG. 8, the pulse sequence Dp of the fast SE method using the new phase encoding gradient gy (i) ′ is used. Collect data for imaging.
As a result, even when imaging is performed using the collected imaging data, it is possible to prevent the image quality of the image from deteriorating due to the influence of eddy current or residual magnetization due to the phase encoding gradient.

-Fourth embodiment-
The fourth embodiment is a modification of the first embodiment, and acquires the warp axis correction component qi in the same manner as the first embodiment, but as shown in FIG. 9, the basic component gy (i) Is applied, and an auxiliary phase encoding gradient gq (i) corresponding to the warp axis correction component qi is applied to the warp axis. collect.
As a result, even when imaging is performed using the collected imaging data, it is possible to prevent the image quality of the image from deteriorating due to the influence of eddy current or residual magnetization due to the phase encoding gradient.

-Fifth embodiment-
In the fifth embodiment, a warp axis correction component acquisition process of FIG. 10 is performed instead of the warp axis correction component acquisition process of FIG.
FIG. 10 is a flowchart illustrating a warp axis correction component acquisition process according to the fifth embodiment.
In step V1, the i-th phase encoding gradient gy (i) B having a phase encoding amount equal to the i-th phase encoding gradient gy (i) determined by the scan parameter and having an amplitude that can ignore the influence of the eddy current and the residual magnetization is used. Data sBi (k) is collected from the first echo by the pulse sequence F1p shown in FIG.
In step V2, the data sBi (k) is subjected to one-dimensional Fourier transform in the read direction, and the result is set as SBi (x).
In step V3, the value SBi (0) of the Fourier transform result SBi (x) at x = 0 is set as ideal phase information φi. The ideal phase information φi represents a phase offset component (zero-order phase component) when there is no influence of eddy current or residual magnetization due to the phase encoding gradient.

In step V4, data sBi '(k) is collected from the first echo by the pulse sequence F1p' shown in FIG. 12 using the i-th phase encoding gradient gy (i) determined by the scan parameter.
In step V5, the data sBi '(k) is subjected to one-dimensional Fourier transform in the read direction, and the result is set to SBi' (x).
In step V6, the value SBi '(0) of the Fourier transform result SBi' (x) at x = 0 is set as the current phase information φi '. The current phase information φi ′ represents a phase offset component (a zero-order phase component) when there is an influence of an eddy current or residual magnetization caused by a phase encoding gradient.

In step V7, a warp axis correction component qi to be added to match the current phase information φi ′ with the ideal phase information φi is calculated based on the magnet characteristics of the permanent magnet type MRI apparatus.

In step V8, q (i + M-1) is obtained from the warp axis correction components q (i + 1) by the second to Mth echoes in the same manner as steps V1 to V7.
FIGS. 13 and 14 show pulse sequences F2p and F2p ′ at the time of the second echo.
FIGS. 15 and 16 show pulse sequences F3p and F3p ′ at the time of the third echo.

The data for imaging shown in FIGS. 5 to 9 are collected using the warp axis correction component qi obtained as described above, and the imaging is performed using the data for imaging. Deterioration of image quality due to the influence of residual magnetization can be prevented.

-Sixth embodiment-
The sixth embodiment is an embodiment in which the polarity of the killer gradient is switched to suppress the influence of eddy current or residual magnetization caused by the phase encoding gradient.
FIGS. 17 and 18 show a pulse sequence according to the sixth embodiment.
In the pulse sequence Gp of FIG. 17, an excitation pulse R and a slice gradient ss are applied.
Next, the first inversion pulse P1 and the slice gradient ss are applied, the phase encoding gradient gy (i) is applied to the warp axis, and then the NMR signal is received from the first echo SE1 while applying the read gradient rr. Thereafter, a rewind gradient gyr (i) having the same time integral as the encode gradient gy (i) and the opposite polarity is applied to the warp axis. Next, the second inversion pulse P2 and the slice gradient ss are applied, the encode gradient gy (i + 1) is applied to the warp axis, and then the NMR signal is received from the second echo SE2 while applying the read gradient rr. Thereafter, a rewind gradient gyr (i + 1) having the same time integral as the encode gradient gy (i + 1) and the opposite polarity is applied to the warp axis. Next, an NMR signal is received from the third echo SE3 while applying a third inversion pulse P3 and a slice gradient ss, applying an encode gradient gy (i + 2) to the warp axis, and then applying a read gradient rr. Thereafter, a rewind gradient gyr (i + 2) having the same time integral as the encode gradient gy (i + 2) and the opposite polarity is applied to the warp axis. Thus, the process from transmission of the inversion pulse to reception of the NMR signal is repeated M times (here, M = 3). Next, a killer gradient Kil is applied, and the polarity (positive here) is the same as the polarity of the sum of the phase encoding gradients gy (i), gy (i + 1), and gy (i + 2).

{Next, in the pulse sequence Gq in FIG. 18, from the transmission of the excitation pulse R to the rewind gradient gyr (j + 2), as in FIG. Next, a killer gradient Kil is applied, and has the same polarity (here, negative) as the sum of the phase encoding gradients gy (j), gy (j + 1), and gy (j + 2).

As described above, when the polarity of the killer gradient Kil is switched so as to be the same as the polarity of the sum of the M phase encoding gradients applied when the process from transmission of the inverted RF pulse to reception of the NMR signal is repeated M times, The effect of the eddy current or residual magnetization caused by the phase encoding gradient can be canceled by the killer gradient Kil. Therefore, it is possible to prevent the image quality of the image from deteriorating due to the influence of the eddy current or the residual magnetization caused by the phase encoding gradient.

-Seventh embodiment-
FIG. 19 is a flowchart showing a zero-order phase component acquisition process in the MRI apparatus 100.
In step R1, as shown in FIG. 20, data sy0 (k) of the second echo SE2 is collected by a pulse sequence H0 in which no phase encoding gradient and no rewind gradient are applied to the warp axis.
In step R2, the data sy0 (k) is subjected to one-dimensional Fourier transform in the read direction, and the result is set to SY0 (x).
In step R3, the zero-order phase amount b0 = Arctan {SY0 (0)} is obtained. The zero-order phase amount b0 is the phase amount at the center of the gradient when there is no residual magnetization.

In step R4, as shown in FIG. 21, data syj (k) of the second echo SE2 is collected by a pulse sequence Hj in which a phase encoding gradient gy (i) and a rewind gradient gyr (i) ′ are applied to the warp axis. Here, the rewind gradient gyr (i) ′ is a new rewind gradient gyr (i) ′ obtained from the basic component gy (i) of the phase encoding gradient determined by the scan parameter and the warp axis correction component qi.
In step R5, the data syi (k) is subjected to one-dimensional Fourier transform in the read direction, and the result is set as SYi (x).
In step R6, a zero-order phase amount bj = Arctan {SYi (0)} is determined. The zero-order phase amount bj is the phase amount at the center of the gradient when there is residual magnetization.
In step R7, a zero-order phase component bi = b0-bj is obtained.
The ghost artifact can be suppressed by using the zero-order phase component bi obtained as described above in step S12 of FIG.

-Eighth embodiment-
The eighth embodiment is intended to prevent deterioration in image quality of an image due to the influence of eddy current or residual magnetization caused by the phase encode gradient of the slice axis in the 3D fast SE method.

FIG. 22 is a flowchart showing a slice axis correction component acquisition process in the MRI apparatus 100.
In step L1, data sz0 (k) is collected by the pulse sequence I0 shown in FIG. In this pulse sequence I0, an excitation pulse R and a slice gradient ss are applied, then an inversion pulse P and a slice gradient ss are applied, and then an NMR signal is received from the echo SE while applying a read gradient gxw, and the data sz0 is received. Collect (k). Note that no phase encoding gradient is applied to the warp axis and the slice axis.
In step L2, the data sz0 (k) is subjected to one-dimensional Fourier transform in the read direction, and the result is set as SZ0 (x).
In step L3, a first-order gradient dz0 of the phase term (Arctan {SZ0 (x)}) of the Fourier transform result SZ0 (x) is determined (determined by applying a least square method or the like). Further, a phase amount b0 = Arctan {SZ0 (0)} at the gradient center is obtained. This phase amount b0 is the phase amount at the center of the gradient when there is no residual magnetization.
The above-described steps L1 to L3 are processes for obtaining the effect (deviation of the echo center of the echo signal SE) that the gradient magnetic fields gx1 and gxw are mainly caused by eddy current.

In step L4, data szi (k) is collected by the pulse sequence Ij shown in FIG. In this pulse sequence Ij, a slice gradient ss is applied to the excitation pulse R and the read axis, and then the difference between the phase encode gradient gz (i) and the rewind gradient gzr (i) of the slice axis of the encode number i determined by the scan parameter is calculated. A corresponding differential gradient dd is applied to the read axis, then an equivalent phase encode gradient gz (i) equal to the slice axis phase encode gradient gz (i) is applied to the read axis, and an inversion pulse P and a slice gradient ss are applied. Then, an equivalent rewind gradient gzr (i) equal to the rewind gradient gzr (i) of the slice axis is applied to the read axis, and after time tz, the NMR signal is received from the echo SE while applying the read gradient gxw, and the data szi (k Collect). Note that no phase encoding gradient is applied to the warp axis. Here, step L4 may be repeated for all the encode numbers i (j = i in this case), but step L4 is repeated for the encode number i appropriately selected in order to reduce the time (in this case, j ≠). i), and the data of the encoding number not selected may be obtained by interpolation.
In step L5, the data szi (k) is subjected to one-dimensional Fourier transform in the read direction, and the result is set as SZi (x).
In Step L6, the primary gradient dzi of the phase term (Arctan {SZi (x)}) of the Fourier transform result SZi (x) is obtained (by applying the least square method or the like). Further, a phase amount bi = Arctan {SZi (0)} at the gradient center is obtained, and a phase amount difference (bi−b0) is calculated. This phase amount difference (bi-b0) is a zero-order phase component that is uniformly applied irrespective of the location due to the influence of the residual magnetization caused by the phase encoding gradient gz (i).
The above steps L4 to L6 are processes for obtaining the influence of the phase encoding gradient gz (i) mainly caused by residual magnetization.

In step L7, the magnitude ΔGZ (i) of the influence of the eddy current and the residual magnetization caused by the phase encoding gradient gz (i) is calculated from (dzi−dz0).
dz0 represents the phase rotation when there is no phase encoding gradient. On the other hand, dzi represents the phase rotation when there is a phase encoding gradient gz (i). Originally, the phase encode gradient gz (i) is canceled by the rewind gradient gzr (i), so (dzi-dz0) = 0. However, if (dzi-dz0) ≠ 0, the magnitude of the difference indicates the magnitude of the influence of the eddy current and the residual magnetization caused by the phase encoding gradient gz (i). Therefore, the magnitude ΔGZ (i) of the influence of the eddy current and the residual magnetization caused by the phase encoding gradient gz (i) can be calculated from (dyi−dy0).
That is, when the amplitude of the read gradient gxw is a_gxw and the shift time of the echo center due to the eddy current or residual magnetization caused by the phase encode gradient gz (i) is tz0,
γ · ΔGZ (i) · tz = γ · a_gxw · tz0 (where γ is the gyromagnetic ratio)
Relationship
ΔGZ (i) = a_gxw · tz0 / tz
Holds.

In step L8, the correction coefficient βi (the magnitude of the influence of the eddy current or the residual magnetization per unit phase encoding amount) is obtained by the following equation.

In step L9, from the basic component gz (i) of the phase encoding gradient determined by the scan parameter and the correction coefficient βi,
wi = βi · gz (i)
To determine the slice axis correction component wi.

FIG. 25 is a flowchart illustrating the data collection processing for imaging.
In Step L10, a new rewind gradient gzr (i) ′ is obtained from the basic component gz (i) of the phase encoding gradient determined by the scan parameter and the slice correction component wi.
gzr (i) ′ = gz (i) + wi
In step L11, imaging data is collected by the pulse sequence Jp of the 3D fast SE method using the new slice axis rewind gradient gzr (i) ′ shown in FIG. At this time, it is preferable to simultaneously use the rewind gradient gyr (i) ′ of the new warp axis in the first embodiment described above. Further, the detection phase in the phase detector 12 is adjusted so as to cancel the phase amount difference (bi-b0), or the transmission phases of the corresponding inverted pulses P1, P2, and P3 are adjusted.
As described with reference to FIGS. 7 to 9, an auxiliary rewind gradient or an auxiliary phase encode gradient of the slice axis may be used.

イ メ ー ジ ン グ By performing imaging using the data for imaging collected as described above, it is possible to prevent the image quality of the image from deteriorating due to the influence of eddy current or residual magnetization caused by the phase encode gradient of the slice axis.

FIG. 1 is a block diagram illustrating an MRI apparatus according to a first embodiment of the present invention. 5 is a flowchart illustrating a warp axis correction component acquisition process according to the first embodiment. FIG. 3 is a view illustrating one example of a pulse sequence used in the warp axis correction component acquisition processing of FIG. 2. FIG. 3 is another example diagram of a pulse sequence used in the warp axis correction component acquisition processing of FIG. 2. 5 is a flowchart illustrating an imaging data collection process according to the first embodiment. FIG. 6 is a view showing an example of a pulse sequence used in the imaging data acquisition processing of FIG. 5. FIG. 9 is an exemplary diagram of a pulse sequence used in an imaging data acquisition process according to the second embodiment. It is an illustration figure of the pulse sequence used in the data acquisition processing for imaging of a 3rd embodiment. It is an illustration figure of the pulse sequence used in the data acquisition processing for imaging of a 4th embodiment. It is a flow chart which shows warp axis correction ingredient acquisition processing in a 5th embodiment. FIG. 11 is an exemplary diagram illustrating a first echo of a pulse sequence used in the warp axis correction component acquisition processing in FIG. 10. FIG. 11 is another example of the first echo of the pulse sequence used in the warp axis correction component acquisition processing of FIG. 10. FIG. 11 is an illustration of a second echo of a pulse sequence used in the warp axis correction component acquisition processing of FIG. FIG. 11 is another example of the second echo of the pulse sequence used in the warp axis correction component acquisition processing in FIG. 10. FIG. 11 is an illustration of a third echo of a pulse sequence used in the warp axis correction component acquisition processing of FIG. 10. FIG. 11 is another example of the third echo of the pulse sequence used in the warp axis correction component acquisition processing in FIG. 10. It is an illustration figure of a pulse sequence used in data collection processing for imaging in a 6th embodiment. It is another example figure of the pulse sequence used in the data acquisition process for imaging in 6th Embodiment. It is a flow chart of the 0th order phase component acquisition processing in a 7th embodiment. FIG. 20 is an illustration of one example of a pulse sequence used in the zero-order phase component acquisition processing of FIG. 19. FIG. 20 is another exemplary diagram of a pulse sequence used in the zero-order phase component acquisition processing in FIG. 19. It is a flow chart which shows a slice axis correction ingredient acquisition processing in an 8th embodiment. FIG. 23 is an illustration of a pulse sequence used in the slice axis correction component acquisition processing of FIG. 22. FIG. 23 is a diagram illustrating another example of the pulse sequence used in the slice axis correction component acquisition processing of FIG. 22. It is a flow chart which shows data collection processing for imaging in an 8th embodiment. FIG. 26 is a view showing an example of a pulse sequence used in the imaging data acquisition process of FIG. 25. It is a pulse sequence diagram of the conventional high-speed SE method. It is explanatory drawing of the data collection locus in k-space. It is a pulse sequence figure of the conventional 3D high-speed SE method. It is an explanatory view of a problem of the conventional high-speed SE method. It is an explanatory view of a problem of the conventional 3D fast SE method.

Explanation of reference numerals

Explanation of reference numerals

Reference Signs List 100 MRI apparatus 1 Magnet assembly 1p Permanent magnet 7 Computer 8 Sequence storage circuit

Claims (4)

The excitation RF pulse is transmitted by the RF pulse transmission unit, the inverted RF pulse is transmitted by the RF pulse transmission unit, the phase encoding gradient is applied to the warp axis by the gradient magnetic field application unit, and then the gradient magnetic field application unit is applied. The NMR signal is received by the NMR signal receiving means while the read gradient is applied to the lead axis, and the transmission from the transmission of the inverted RF pulse to the reception of the NMR signal is repeated M times, and then the killer gradient is applied by the gradient magnetic field applying means. An MRI apparatus that repeats from transmission of the excitation RF pulse to application of the killer gradient N times to collect (M × N) times of NMR signals subjected to different phase encodings,
The gradient magnetic field applying unit is configured to polarize the killer gradient so that the polarity of the killer gradient is the same as the polarity of the sum of the M phase encode gradients applied when the process from transmission of the inverted RF pulse to reception of the NMR signal is repeated M times. An MRI apparatus characterized in that the MRI apparatus is switched. An excitation RF pulse is transmitted by the RF pulse transmitting means, then an inverted RF pulse is transmitted by the RF pulse transmitting means, and then a phase encoding gradient is applied to the warp axis by the gradient magnetic field applying means, or is applied to the warp axis and the slice axis. In the MRI apparatus, the NMR signal is received by the NMR signal receiving means while applying the read gradient to the lead axis by the gradient magnetic field applying means.
Either change the transmission phase of the inverted RF pulse by the RF pulse transmitting means or suppress the phase at the NMR signal receiving means so as to suppress the zero-order phase component caused by the influence of the residual magnetization caused by the phase encoding gradient. An MRI apparatus characterized by changing a detection phase of detection. The MRI apparatus according to claim 2,
An RF pulse is transmitted by the RF pulse transmitting means, and a phase encoding gradient is not applied to the warp axis and the slice axis, and an equivalent phase encoding gradient having a time integral equal to the phase encoding gradient is applied to the read axis by the gradient magnetic field applying means. Then, an equivalent rewind gradient having the same time integration value as the equivalent phase encoding gradient and the opposite polarity is applied to the lead axis, and then, while the read gradient is applied to the lead axis by the gradient magnetic field applying means, the NMR signal receiving means applies the read gradient to the read axis. An MRI apparatus comprising: a zero-order phase component acquisition unit that receives an NMR signal, acquires phase information from the received NMR signal, and obtains the zero-order phase component from the phase information. 3. The MRI apparatus according to claim 2, wherein the NMR signal obtained when the phase encoding gradient and the rewind gradient are applied to the warp axis and the NMR signal obtained when the phase encoding gradient and the rewind gradient are not applied to the warp axis. An MRI apparatus further comprising a zero-order phase component obtaining means for obtaining the zero-order phase component from the MRI.

Images