JP3382868B2 - Phase shift measuring method and MR imaging apparatus - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to an MR imaging method, a phase shift measuring method and an MR (Magnetic Res).
onance) imaging device. More specifically, an MR imaging method by a pulse sequence capable of preventing the influence of the fluctuation of the residual magnetization due to the change of the gradient magnetic field pulse, and the subsequent by the influence of the eddy current and the residual magnetization caused by the preceding phase encoding pulse in the pulse sequence. Shift measuring method for measuring the phase shift of the echoes of the same and an MR imaging apparatus for implementing those methods.

[0002]

2. Description of the Related Art Japanese Unexamined Patent Publication No. 10-75940 discloses the following prior art. (1) Send excitation pulse, send inverted pulse, apply phase encoding pulse to phase axis, apply read pulse to read axis, apply rewind pulse to phase axis, and then send inverted pulse Then, a dephasor pulse is applied to the phase axis, a pre-scan sequence is performed to collect data from the echo while applying a read pulse to the phase axis, and the phase data obtained by performing a one-dimensional Fourier transform on the collected data is obtained. Based on this, a phase shift measurement method for measuring the phase shift of the subsequent echo due to the effects of eddy currents and residual magnetization caused by phase encoding pulses. (2) After transmitting the excitation pulse, the inversion pulse is transmitted,
Applying the phase encoding pulse to the phase axis, collecting the data from the echo while applying the read pulse to the lead axis, and applying the rewind pulse to the phase axis is repeated multiple times while changing the phase encoding pulse. In the pulse sequence of the high-speed spin echo method that collects data of multiple echoes by excitation, the above (1)
Compensation pulse for compensating the amount of phase shift measured by the phase shift measurement method of, is added to the phase encoding pulse, added immediately before or after the phase encoding pulse, or both, incorporated into the rewind pulse, or the rewind pulse. An MR imaging method that is applied immediately before or after, or both.

[0003]

The above-mentioned conventional technique is used when the compensation pulse is not added by the phase shift amount measured by the phase shift measuring method of (1) and the pulse sequence of the fast spin echo method of (2). The precondition is that the amount of phase shift that occurs is equal.

However, in the permanent magnet type MR imaging apparatus, the two are not always equal due to the magnetic hysteresis characteristics of the magnetic compensating plate and the like, and the above-mentioned precondition of the prior art does not hold. This will be described with reference to FIGS. 11 to 13.

FIG. 11 is a pulse sequence diagram of the conventional fast spin echo method. This FSE (Fast Spin E)
In the cho) sequence SQ, first, the excitation pulse R and the slice gradient ss are applied. Next, the first inversion pulse P1
And a slice gradient ss is applied. Next, the phase encoding pulse gy1i is applied to the phase axis. Next, while applying the read pulse gxw, the first echo echo1
Receive the signal. Then, a rewind pulse gy1ri having the same time integrated value and the opposite polarity as the phase encoding pulse gy1i is applied to the phase axis. Note that i is a repetition number, i = 1 to I (for example, I = 128). Next, the second inversion pulse P2 and the slice gradient ss are applied, the phase encoding pulse gy2i is applied to the phase axis, and the second echo echo2 is applied while applying the read pulse gxw.
Then, a rewind pulse gy2r having the same time integral value as the phase encoding pulse gy2i and the opposite polarity and gy2r is applied to the phase axis. Similarly, the jth inversion pulse Pj and the slice gradient ss are applied, the phase encoding pulse gyji is applied to the phase axis, and the read pulse gxw is applied while the jth echo echoj to the NMR.
Receiving a signal and then applying to the phase axis a rewind pulse gyjri having the same time integral value and the opposite polarity as the phase encoding pulse gyji, is repeated for j = 3 to J (for example, J = 8).

In the permanent magnet type MR imaging device, due to the magnetic hysteresis characteristic of the magnetic compensating plate and the like, as shown in FIG. 12, the residual magnetization before applying the phase encoding pulse gy11 (i = 1) is m1. If there is,
By applying the phase encoding pulse gy11, the residual magnetization traces the history a1 and a2 and reaches m1 again, and by applying the rewind pulse gy1r1 the residual magnetization traces history a3 and a4 and reaches m2. Further, as shown in FIG. 13, since the residual magnetization before applying the phase encoding pulse gy21 is m2, by applying the phase encoding pulse gy21, the residual magnetization has a history a5.
A6 reaches m3, and by applying the rewind pulse gy2r1, the residual magnetization traces histories a7 and a8 and reaches m2 again. As described above, in the permanent magnet type MR imaging device, due to the magnetic hysteresis characteristics of the magnetic shunt plate,
The residual magnetization varies depending on the history of changing the gradient magnetic field pulse such as the phase encoding pulse or the rewind pulse.

However, the prescan sequence is F
Since the SE sequence is cut out up to the first echo collection, the application history of the gradient magnetic field pulse is matched and the residual magnetization is matched for the first echo, so the above-mentioned precondition is satisfied. Since the application history of the gradient magnetic field pulse does not match after two echoes, the residual magnetization does not match and the above-described precondition is not satisfied. Therefore, with respect to the second and subsequent echoes, there is a problem that the phase shift of the subsequent echoes due to the influence of the residual magnetization caused by the gradient magnetic field pulse cannot be sufficiently corrected. Regarding the eddy current,
Since a linear relationship is established between the area of the gradient magnetic field pulse and the generated phase error, it is considered that the above problem does not occur.

Therefore, a first object of the present invention is to provide an MR imaging method capable of preventing the influence of the fluctuation of the residual magnetization due to the change of the gradient magnetic field pulse. A second object of the present invention is to measure the phase shift of the subsequent echo due to the effect of eddy current and remanent magnetization caused by the preceding phase encoding pulse in the FSE pulse sequence, in agreement with the actual FSE sequence. An object of the present invention is to provide a phase shift measuring method capable of performing the above. Furthermore, a third object of the present invention is to provide an MR imaging apparatus for implementing the above method.

[0009]

SUMMARY OF THE INVENTION In a first aspect, the invention involves transmitting an excitation pulse followed by an inversion pulse, applying a phase encoding pulse to a phase axis and a read pulse to a lead axis. While collecting data from the echo and applying a rewind pulse to the phase axis, changing the phase encoding pulse multiple times,
A fast spin-echo MR imaging method that collects data of multiple echoes with one excitation, wherein the gradient axis is positive or negative with an amplitude equal to or greater than the maximum amplitude used in that axis. The MR imaging method is characterized in that the reset pulse is inserted before and after the inversion pulse. In the MR imaging method according to the first aspect, since the amplitude of the reset pulse is not smaller than the amplitudes of other gradient magnetic field pulses,
Regardless of the application history of other gradient magnetic field pulses, the residual magnetization after applying the reset pulse is always constant. Since the reset pulse is inserted after the inversion pulse, that is, immediately before the application of the phase encoding pulse, the residual magnetization before the application of the phase encoding pulse is always constant. Therefore, it is possible to prevent the influence of the fluctuation of the residual magnetization due to the change of the gradient magnetic field pulse. Since the reset pulse is inserted before the inversion pulse, the reset pulse does not affect the phase encoding.

In a second aspect, the invention transmits an excitation pulse, an inversion pulse, a phase encoding pulse applied to the phase axis, a read pulse applied to the read axis, and a rewind pulse applied to the phase axis. Apply, then send an inversion pulse, apply a dephaser pulse to the phase axis, apply a read pulse to the phase axis, collect data from the echo, and obtain the data by a one-dimensional Fourier transform. A phase shift measuring method for obtaining the amount of phase shift due to the influence of the phase encoding pulse based on the obtained phase data, wherein the phase axis has a positive or positive amplitude having an amplitude equal to or larger than the maximum amplitude used in the axis. A negative reset pulse before and after the inversion pulse,
Provided is a phase shift measuring method characterized by insertion. In the phase shift measuring method according to the second aspect, since the amplitude of the reset pulse is not smaller than the amplitude of the other gradient magnetic field pulse, the residual magnetization after the reset pulse is applied is irrespective of the application history of the other gradient magnetic field pulse. It will always be constant. Since the reset pulse is inserted after the inversion pulse, that is, immediately before the application of the phase encoding pulse, the residual magnetization before the application of the phase encoding pulse is always constant. That is, it is possible to match the residual magnetization before the application of the phase encoding pulse in the FSE sequence. Therefore, the phase shift of the subsequent echo due to the influence of the eddy current and the residual magnetization due to the preceding phase encoding pulse in the FSE sequence can be measured in conformity with the actual FSE sequence. Since the reset pulse is inserted before the inversion pulse, the reset pulse does not affect the phase encoding.

According to a third aspect of the present invention, in the MR imaging method according to the first aspect, the compensation pulse for compensating the phase shift amount measured by the phase shift measuring method according to the second aspect is a phase encoding pulse. The present invention provides a method for MR imaging, characterized in that the MR imaging method is incorporated into a pulse, a pulse is added immediately before or after a phase encoding pulse, or both, a pulse is incorporated into a rewind pulse, and a pulse is added immediately before or after a rewind pulse, or both. . In the MR imaging method according to the third aspect, since the phase shift amount that can be measured by matching with the actual FSE sequence is compensated, the influence of the eddy current and the residual magnetization caused by the preceding phase encoding pulse in the FSE pulse sequence and the like. It is possible to accurately correct the phase shift of the subsequent echo due to.

In a fourth aspect, the present invention provides an excitation pulse, an inversion pulse, a read pulse applied to the lead axis, followed by an inversion pulse and a dephasor pulse on the phase axis. Applying and applying read pulse to the phase axis, data is collected from the echo, and the data is one-dimensional Fourier transformed to obtain the first phase data, and the phase axis is set to the maximum amplitude used in the axis. A positive or negative reset pulse with equal or greater amplitude,
The second phase data is acquired before and after the inversion pulse, and otherwise the same, and the influence of the reset pulse is obtained based on the difference between the first phase data and the “second phase data”. There is provided a phase shift measuring method characterized by obtaining a phase shift amount by As described in the first to third aspects, by using the reset pulse, the residual magnetization before the application of the phase encoding pulse can be made constant at all times. However, the effect of having a reset pulse must be taken into consideration (typical one is eddy current). In the phase shift measuring method according to the fourth aspect, the amount of phase shift due to the influence of the reset pulse can be obtained from the difference between the phase data when the reset pulse is not inserted and the phase data when the reset pulse is inserted.

According to a fifth aspect of the present invention, in the MR imaging method of the third aspect, the present invention provides a reset pulse or a compensation pulse so as to compensate the phase shift amount measured by the phase shift measuring method of the fourth aspect. An MR imaging method characterized by changing the area of a pulse is provided. In the MR imaging method according to the fifth aspect, it is possible to execute the FSE sequence in which the phase shift due to the influence of the reset pulse is corrected.

In a sixth aspect, the present invention comprises an RF pulse transmitting means, a gradient pulse applying means, and an NMR signal receiving means, controlling each of these means to transmit an excitation pulse and then inversion. Sending a pulse, applying a phase-encoding pulse to the phase axis, collecting data from the echo while applying a read pulse to the read axis, and applying a rewind pulse to the phase axis is performed multiple times while changing the phase-encoding pulse. MR by high-speed spin echo method that repeatedly collects multiple echo data with one excitation
An MR imaging device for performing imaging, wherein a positive or negative reset pulse having an amplitude equal to or larger than a maximum amplitude used in the axis is inserted into an arbitrary gradient axis before and after the inversion pulse. MR equipped with reset pulse applying means for
An imaging device is provided. M according to the sixth aspect
The R imaging apparatus can preferably carry out the MR imaging method according to the first aspect.

According to a seventh aspect of the present invention, in the MR imaging apparatus of the sixth aspect, the RF pulse transmitting means, the gradient pulse applying means and the NMR signal receiving means are controlled to generate an excitation pulse. Send, invert pulse, phase encoding pulse on phase axis, read pulse on lead axis, rewind pulse on phase axis, followed by inversion pulse and dephasor pulse. The phase shift amount due to the influence of the phase encoding pulse is applied based on the phase data obtained by applying data to the echo while applying the read pulse to the phase axis while applying the read pulse to the phase axis. Then, a compensation pulse for compensating for the phase shift amount is obtained and the phase encoding pulse of the pulse sequence of the fast spin echo method is used. The phase encoding pulse, the phase encoding pulse is added immediately before or after the phase encoding pulse, or both, the rewind pulse is included, or the phase shift correction means is added immediately before or after the rewind pulse, or both, and A reset pulse application in which a positive or negative reset pulse having an amplitude equal to or larger than the maximum amplitude used in the axis is inserted into the phase axis of the pulse sequence for obtaining the shift amount before and after the inversion pulse. There is provided an MR imaging apparatus comprising the means. The MR imaging apparatus according to the seventh aspect can preferably implement the MR imaging method according to the third aspect.

According to an eighth aspect of the present invention, in the MR imaging apparatus of the seventh aspect, the RF pulse transmitting means, the gradient pulse applying means and the NMR signal receiving means are controlled to generate an excitation pulse. Send the inversion pulse, apply the read pulse to the lead axis, then send the inversion pulse, apply the dephasor pulse to the phase axis, and apply the read pulse to the phase axis while the echo echoes the data. Collecting and one-dimensional Fourier transforming the data to obtain first phase data, a positive or negative reset pulse having an amplitude equal to or greater than the maximum amplitude used on that phase axis, Inserted before and after the inversion pulse, the second phase data is obtained in the same manner other than the above, and based on the difference between the first phase data and the “second phase data”, An MR characterized in that a phase shift amount due to the influence of a set pulse is obtained, and an area of a reset pulse or a compensation pulse to be added to a pulse sequence of the fast spin echo method is changed so as to compensate the phase shift amount. An imaging device is provided. In the MR imaging apparatus according to the eighth aspect,
The MR imaging method according to the fifth aspect can be suitably implemented.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail with reference to the embodiments shown in the drawings. The present invention is not limited to this. FIG. 1 is a block diagram of an MR imaging apparatus according to an embodiment of the present invention. This M
In the R imaging apparatus 100, the magnet assembly 1 has a space portion (hole) for inserting the subject therein, and a permanent magnet that applies a constant main magnetic field to the subject by surrounding the space portion. 1p, a gradient magnetic field coil 1g for generating a gradient magnetic field of a slice axis, a phase axis, and a read axis, a transmission coil 1t for giving an RF pulse for exciting spins of nuclei in a subject, and a A receiving coil 1r for detecting an NMR signal is arranged. The gradient magnetic field coil 1g and the transmission coil 1t
The receiving coil 1r is connected to the gradient magnetic field driving circuit 3, the RF power amplifier 4, and the 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
The gate modulation circuit 9 is operated to modulate the carrier wave output signal of the RF oscillation circuit 10 into a pulsed signal having a predetermined timing and a predetermined envelope shape, which is applied as an RF pulse to the RF power amplifier 4, and the power is supplied by the RF power amplifier 4. After amplification, it is applied to the transmitter coil 1t of the magnet assembly 1 to selectively excite a desired slice area. The preamplifier 5 amplifies the NMR signal from the subject detected by the receiving coil 1r of the magnet assembly 1 and inputs it to the phase detector 12. The phase detector 12 uses the carrier wave output signal of the RF oscillation circuit 10 as a reference signal, phase-detects the NMR signal from the preamplifier 5, and supplies it to the A / D converter 11. The A / D converter 11 converts the analog signal after phase detection into a digital signal and inputs it to the computer 7. Calculator 7 is A /
Data is read from the D converter 11 and image reconstruction calculation is performed to generate an image of a desired 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 pulse sequence diagram of the fast spin echo method according to the present invention. In this reset pulse application FSE sequence SQwr, first, the excitation pulse R and the slice gradient ss are applied. Next, a first inversion pulse P1 and a slice gradient ss are applied, but before and after the first inversion pulse P1, a positive reset pulse gysl1 and a positive reset pulse gysl1 equal to the maximum amplitude used in the phase axis Insert gysr1. Next, the phase encoding pulse gy1i is applied to the phase axis. Next, read pulse gxw
While applying the signal, the NMR signal is received from the first echo echo1. Then, the phase encoding pulse gy1i
And repolarization pulse with the same time integration value and opposite polarity gy1ri
Is applied to the phase axis. Note that i is the repetition number,
i = 1 to I (for example, I = 128). Next, the second inversion pulse P2 and the slice gradient ss are applied, but before and after the second inversion pulse P2, a positive reset pulse equal to the maximum amplitude used in the phase axis is provided on the phase axis.
Insert gysl2 and gysr2. Next, the phase encoding pulse gy2i is applied to the phase axis, and the read pulse gxw
While receiving the NMR signal from the second echo echo2, a rewind pulse gy2r having the same time integrated value and the opposite polarity as the phase encoding pulse gy2i is applied to the phase axis. Similarly, the j-th inversion pulse Pj and the slice gradient ss are applied, the reset pulses gysli and gysri are inserted before and after the j-th inversion pulse Pj, and the phase encoding pulse gyji is applied to the phase axis. The jth echo echoj while applying the read pulse gxw
From the phase encoding pulse gyji and applying a rewind pulse gyjri having the same time integration value and the opposite polarity to the phase axis.
Repeat for J (eg J = 8). Although not shown,
Finally, a dephasing pulse (grk) with a large area is inserted to disperse the nuclear spin phases. This amplitude matches the amplitude of the phase encoding pulse.

A compensating pulse (gypn) for compensating the phase shift amount measured by a phase shift measuring method using a gradient magnetic field pulse (FIG. 6) which will be described later is used as a phase encoding pulse gy1i to gyJI or / and a rewind pulse gy1r.
It is incorporated into i-gyJrI (the area is adjusted).
This compensating pulse (gypn) is added to either or both of the phase encoding pulses gy1i to gyJI immediately before or after (the total area is one), or the rewind pulse gypn
It may be added immediately before or after 1ri to gyJrI, or both of them (the area is one in total). In addition, a compensation pulse (gyps) that compensates for the amount of phase shift measured by the phase shift measurement method using a reset pulse (FIG. 8) described later.
A reset pulse pair (gysl1 / gysr1 to gyslJ / gysr
For each J), one is installed (area is adjusted). The compensating pulse (gyps) may be added to each of the reset pulse pairs (gysl1 / gysr1 to gyslJ / gysrJ) immediately before or after the reset pulse or both of them (the total area is one).

Further, reset pulses gysl1 and gysr1
May be a pulse with an amplitude greater than the maximum amplitude used in the phase axis. Alternatively, a negative pulse may be used.

As shown in FIG. 3, in the MR imaging apparatus 100, if the residual magnetization before applying the reset pulse gysl1 is m1, it is reset because of the magnetic hysteresis characteristics of the magnetic compensating plate of the permanent magnet 1p. Pulse gy
By applying sl1, the residual magnetization traces the history b1 and b2 and returns to m1 again. Next, when the reset pulse gysr1 is applied, the residual magnetization traces the history b1 and b2 and returns to m1 again. As shown in FIG. 4, since the remanent magnetization before applying the phase encoding pulse gy11 (i = 1) is m1, the remanent magnetization follows the history a1 and a2 by applying the phase encoding pulse gy11, and is again m1. When the rewind pulse gy1r1 is applied, the residual magnetization traces the history a3, a4 and reaches m2. Next, by applying the reset pulse gysl2, the residual magnetization traces the history b5, b6 and returns to m1 again. Next, when the reset pulse gysr2 is applied, the residual magnetization is
Following the history b1 and b2, the process returns to m1 again. Also, FIG.
As shown in FIG. 4, the residual magnetization before applying the phase encoding pulse gy21 is m1, so that the residual magnetization is history a by applying the phase encoding pulse gy21.
Reaching m1 through 5 and a6, rewind pulse gy2r1
By applying, the residual magnetization traces the history a7, a8 and reaches m4. Next, by applying a reset pulse gysl3, the residual magnetization traces the history b9, b10 and is again m1.
Return to. Next, when the reset pulse gysr3 is applied, the residual magnetization traces the history b1 and b2 in FIG.
Return to. As described above, the MR imaging apparatus 100
Then, even if the gradient magnetic field pulse such as the phase encoding pulse or the rewind pulse is changed, the remanent magnetization immediately before the application of the phase encoding pulse is always constant without depending on the history thereof.

FIG. 6 is a flow chart showing the phase shift correction value determination processing by the gradient magnetic field pulse according to the present invention.
In step E1, the initial value of the amplitude agypn of the compensation pulse gypn is set to an appropriate value. Here, n is the number of the phase encoding pulse whose phase shift amount is to be measured (correction value is to be determined). In step E2, FIG.
The data is collected from echoes echo1 and echo2 by the pulse sequence of.

FIG. 7 is a pulse sequence diagram of the prescan according to the present invention. In this reset pulse application pre-scan sequence PSQwr, first, the excitation pulse R
And a slice gradient ss is applied. Next, a first inversion pulse P1 and a slice gradient ss are applied, but before and after the first inversion pulse P1, a positive reset pulse gysl1 and a positive reset pulse gysl1 equal to the maximum amplitude used in the phase axis Insert gysr1. The reset pulses gysl1 and gysr1 are the same pulses as the reset pulses gysl1 and gysr1 in the reset pulse application FSE sequence SQwr of FIG. A large pulse (broken line) for removing FlD (Free Induction Decay) is usually inserted on both sides of the slice gradient ss, but its role is omitted by the reset pulses gysl1 and gysr1. This is important in that the hardware restrictions of the device are not tightened. Next, a phase encoding pulse gyn for measuring the amount of phase shift (for determining a correction value) is applied to the phase axis. Next, the read pulse gxr which is the first half of the normal read pulse gxw is applied. After that, the read pulse is set to "0". Next, a pulse in which a compensation pulse gypn (described later) is incorporated into a rewind pulse gyrn having the same time integration value as the phase encoding pulse gyn but a time integration value and the opposite polarity is applied to the phase axis.

Next, the second inversion pulse P2 and the slice gradient ss are applied, but before and after the second inversion pulse P2, a positive reset equal to the maximum amplitude used in the phase axis is applied to the phase axis. Insert the pulses gysl2 and gysr2. Next, a dephasor pulse gywdn1 equal to the rewind pulse gyrn is applied to the phase axis. Next, while applying the read pulse gywn1 to the phase axis, echoes echo1 to N
After receiving the MR signal, the dephasor pulse gyw is then received.
A rephasor pulse gywrn1 equal to dn1 is applied to the phase axis.

Next, the third inversion pulse P3 and the slice gradient ss are applied, but before and after the third inversion pulse P3, a positive reset equal to the maximum amplitude used in the phase axis is applied to the phase axis. Insert the pulses gysl3 and gysr3. Next, a dephasor pulse gywdn2 equal to the rewind pulse gyrn is applied to the phase axis. Next, while applying the read pulse gywn2 to the phase axis, echoes echo2 to N
After receiving the MR signal, the dephasor pulse gyw is then received.
A rephasor pulse gywrn2 equal to dn2 is applied to the phase axis. Finally, a dephasing pulse grk having a large area is put in to disperse the phases of the nuclear spins. This amplitude matches the maximum amplitude of the phase encoding pulse.

The prescan sequence of FIG. 7 is basically the prescan sequence of FIG. 32 of Japanese Unexamined Patent Publication No. 10-75940 with a reset pulse added, but it is more resistant to residual magnetization. Has become.

Returning to FIG. 6, in step E3, the echo ec
The data obtained from ho1 and echo2 are subjected to one-dimensional Fourier transform, and the obtained phases are φ1 and φ2. In step E4, (φ1−φ2) / 2 is obtained, and the result is fitted by a linear function using the least square method or the like to obtain the primary term dn of the result. At step E5, the first-order phase shift amount φn is calculated by the following equation. φn = dn · Xres · 10 ⁶ / (γ · fov) Here, Xres is the number of echo sampling points. Further, γ is a gyromagnetic ratio. Also, fov is
The size (cm) of the imaging visual field.

At step E6, the amplitude agypn of the compensation pulse gypn is updated. new_agypn = (1 + φn / gypnarea) old_agypn where new_agypn is the amplitude after updating, and old_agypn
Is the amplitude before update, and gypnarea is the area (corresponding to the dephase amount) of the compensation pulse gypn before update. At step E7, steps E2 to E6 are repeated a set number of times. Then, it progresses to step E8. At step E8, the above steps E1 to E7 are repeated for necessary n. Then, the process ends. From the above, the phase shift amount for the phase encoding pulse (gyn) (φ
n) can be measured and the correction value (amplitude agypn of the compensation pulse gypn) can be determined.

FIG. 8 is a flow chart showing the phase shift correction value determination processing by the reset pulse according to the present invention.
In step F1, the initial value of the amplitude agyps of the compensation pulse gyps is set to an appropriate value. In step F2off,
Data is collected from the echo echo_off by the pulse sequence of FIG.

FIG. 9 is a pulse sequence diagram of the prescan for phase error detection by the reset pulse according to the present invention. In the phase error detecting prescan sequence PSQoff by the reset pulse, first, the excitation pulse R and the slice gradient ss are applied. Next, the first inversion pulse P1 and the slice gradient ss are applied. No reset pulse is applied. Next, the compensating pulse gyps is applied to the phase axis. Next, the read pulse gxr which is the first half of the normal read pulse gxw is applied. After that, the read pulse is set to "0". Next, the second inversion pulse P2 and the slice gradient ss are applied. Next, the dephasor pulse gywd1 is applied to the phase axis. Next, receiving the NMR signal from the echo echo_on while applying the read pulse gyw1 to the phase axis,
Then, a rephasor pulse gywr1 equal to the dephasor pulse gywd1 is applied to the phase axis.

Returning to FIG. 8, in step F2on, as shown in FIG.
Data is collected from echo echo_on by the pulse sequence of.

FIG. 10 is a pulse sequence diagram of the prescan for phase error detection by the reset pulse according to the present invention. In the phase error detecting prescan sequence PSQon by the reset pulse, first, the excitation pulse R and the slice gradient ss are applied. Next, the first inversion pulse P1 and the slice gradient ss are applied, but a reset pulse gysl1 is applied to the phase axis before and after the first inversion pulse P1.
And insert gysr1. The reset pulses gysl1 and gysr1 are the same pulses as the reset pulses gysl1 and gysr1 in the reset pulse application FSE sequence SQwr of FIG. Next, the compensating pulse gyps is applied to the phase axis. Next, the read pulse gxr which is the first half of the normal read pulse gxw is applied. After that, the read pulse is set to "0". Next, the second inversion pulse P2 and the slice gradient ss are applied. Next, the NMR signal is received from the echo echo_on while applying the read pulse gyw1 to the phase axis, and then the rephasor pulse gywr1 equal to the dephasor pulse gywd1 is applied to the phase axis.

Returning to FIG. 8, in step F3, the echo ec
Data obtained from ho_off and echo_on are one-dimensional Fourier transformed, and the obtained phases are set to Φ1 and Φ2. In step F4, (Φ1−Φ2) is obtained, and the result is fitted by a linear function using the method of least squares or the like to obtain the linear term D of the result. In step F5, the first-order phase shift amount Φ is calculated by the following equation. Φ = D · Xres · 10 ⁶ / (γ · fov) Here, Xres is the number of echo sampling points. Further, γ is a gyromagnetic ratio. Also, fov is
The size (cm) of the imaging visual field.

In step F6, the amplitude agyps of the compensation pulse gyps is updated. new_agyps = (1 + Φ / gypsarea) old_agyps where new_agyps is the updated amplitude, old_agyps
Is the amplitude before updating, and gypsarea is the area of the compensation pulse gyps before updating. In step F7, the above steps F2off to E6 are repeated a set number of times. Then, the process ends. From the above, the phase error (Φ) of the reset pulse can be measured and the correction value (compensation pulse gyps
Can be determined.

[0035]

According to the MR imaging method of the present invention, it is possible to prevent the influence of the fluctuation of the residual magnetization due to the change of the gradient magnetic field pulse. Further, according to the phase shift measuring method of the present invention, the phase shift of the subsequent echo due to the effect of the eddy current or the residual magnetization caused by the preceding phase encoding pulse in the FSE sequence is made to coincide with the actual FSE sequence, It can be measured. Furthermore, according to the MR imaging apparatus of the present invention,
The above method can be suitably implemented.

[Brief description of drawings]

FIG. 1 is a block diagram showing an MR imaging apparatus according to an embodiment of the present invention.

FIG. 2 is a pulse sequence diagram of a reset pulse application FSE sequence according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a change in residual magnetization due to a reset pulse in the reset pulse application FSE sequence of FIG.

FIG. 4 is an explanatory diagram showing a change in residual magnetization due to a first phase encoding pulse and a rewind pulse in the reset pulse application FSE sequence of FIG. 3.

5 is an explanatory diagram showing a change in residual magnetization due to a second phase encoding pulse and a rewind pulse in the reset pulse application FSE sequence of FIG.

FIG. 6 is a flowchart of a phase shift correction value determination process using a gradient magnetic field pulse.

FIG. 7 is a pulse sequence diagram of a reset pulse application pre-scan sequence.

FIG. 8 is a flowchart of a phase shift correction value determination process using a reset pulse.

FIG. 9 is a pulse sequence diagram of a pre-scan sequence for phase error detection using a reset pulse (preset pulse OFF).

FIG. 10 is a pulse sequence diagram of a pre-scan sequence for phase error detection by a reset pulse (preset pulse ON).

FIG. 11 is a pulse sequence diagram of an example of a conventional FSE sequence.

FIG. 12 is an explanatory diagram showing a change in residual magnetization due to a first phase encoding pulse and a rewind pulse in the FSE sequence of FIG. 11.

13 is an explanatory diagram showing a change in remanent magnetization due to a second phase encoding pulse and a rewind pulse in the FSE sequence of FIG.

[Explanation of symbols]

100 MR Imaging System 1 Magnet assembly 1g gradient magnetic field coil 1t transmitter coil 1p permanent magnet 7 calculator 8 Sequence memory circuit 12 Phase detector

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) A61B 5/055

Claims (5)

(57) [Claims] 1. An excitation pulse is transmitted, an inversion pulse is transmitted, a phase encoding pulse is applied to the phase axis, a read pulse is applied to the lead axis, a rewind pulse is applied to the phase axis, and then an inversion pulse is applied. , The dephasor pulse is applied to the phase axis, the read pulse is applied to the phase axis, and the data is collected from the echo.
A phase shift measuring method for obtaining the amount of phase shift due to the influence of the phase encoding pulse based on the phase data obtained by the dimensional Fourier transform, wherein the phase axis is equal to or larger than the maximum amplitude used in the axis. A phase shift measuring method, characterized in that a positive or negative reset pulse having a large amplitude is inserted before and after the inversion pulse. 2. An excitation pulse is transmitted, an inversion pulse is transmitted, a read pulse is applied to a lead axis, an inversion pulse is subsequently transmitted, a dephasor pulse is applied to a phase axis, and a read pulse is applied to the phase axis. Data from the echo while applying it to the one-dimensional Fourier transform of the data to obtain the first phase data, the phase axis having an amplitude equal to or greater than the maximum amplitude used in that axis Positive or negative reset pulse is inserted before and after the inversion pulse, the second phase data is acquired in the same manner except that the difference between the first phase data and the second phase data is obtained. A phase shift measuring method characterized in that a phase shift amount due to the influence of the reset pulse is obtained based on the above. 3. An RF pulse transmitting means, a gradient pulse applying means, and an NMR signal receiving means are provided to control the respective means to transmit an excitation pulse and then an inversion pulse to transmit a phase encoding pulse. Is applied to the phase axis,
High speed to collect data from echo while applying read pulse to read axis and repeat to apply rewind pulse to phase axis multiple times while changing phase encoding pulse. M for performing MR imaging by spin echo method
R-imaging device, wherein a positive or negative reset pulse having an amplitude equal to or larger than a maximum amplitude used in the axis is inserted into an arbitrary gradient axis before and after the inversion pulse. An MR imaging apparatus comprising means. 4. The MR imaging apparatus according to claim 3, wherein the RF pulse transmitting means, the gradient pulse applying means and the NMR signal receiving means are controlled to transmit an excitation pulse and an inversion pulse. , Phase encoding pulse is applied to the phase axis, read pulse is applied to the read axis, rewind pulse is applied to the phase axis, then inversion pulse is transmitted, dephasor pulse is applied to the phase axis, and read pulse is applied. Is applied to the phase axis, data is collected from the echo, the amount of phase shift due to the influence of the phase encoding pulse is obtained based on the phase data obtained by one-dimensional Fourier transforming the data, and the amount of phase shift is compensated. Compensating pulse to be incorporated into the phase encoding pulse of the pulse sequence of the fast spin echo method, or For adding the phase shift correction means for adding to the rewind pulse immediately before or after the rewind pulse, or for adding the phase rewind pulse to the rewind pulse, or for adding the phase shift amount to the rewind pulse. The phase axis of the pulse sequence is provided with reset pulse applying means for inserting a positive or negative reset pulse having an amplitude equal to or larger than the maximum amplitude used in the axis before and after the inversion pulse. Characteristic MR imaging device. 5. The MR imaging apparatus according to claim 4, wherein the RF pulse transmitting means, the gradient pulse applying means and the NMR signal receiving means are controlled to transmit an excitation pulse and an inversion pulse. , Apply read pulse to read axis, then send inverted pulse, apply dephasor pulse to phase axis, collect read data from echo while applying read pulse to phase axis Fourier transform to obtain first phase data, and insert a positive or negative reset pulse having an amplitude equal to or larger than the maximum amplitude used in the axis into the phase axis before and after the inversion pulse. In the same manner except for the above, the second phase data is acquired, and the position due to the influence of the reset pulse is obtained based on the difference between the first phase data and the second phase data. An MR imaging apparatus characterized in that a phase shift amount is obtained, and an area of a reset pulse or a compensation pulse to be added to a pulse sequence of the fast spin echo method is changed so as to compensate the phase shift amount.