JPH06205754A - Preparation pulse applying method for mr system - Google Patents

Abstract

PURPOSE:To provide the T2 preparation pulse applying method for an MR device (nuclear magnetic resonance system) causing no loss of the vertical magnetization intensity even apart from the magnetic field center and capable of reducing the effect of the nonuniform magnetic field. CONSTITUTION:A rephase gradient pulse (a) is added to the gradient pulse synchronous with the first 90 deg.-pulse in synchronization with the application of preparation pulses consisting of 90 deg., 180 deg., 90 deg.-series to an RF coil, and a de-phase gradient pulse (b) having a half-area of the selection exciting gradient for the second 90 deg.-pulse is added before the second 90 deg.-pulse.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a preparation pulse applying method for an MR device (nuclear magnetic resonance device).

[0002]

2. Description of the Related Art An MR apparatus has been recently used as a means for visualizing anatomical information of a human body in the same manner as an X-ray CT apparatus. The MR device is a physics that emits a part of the absorbed RF energy when a specific atomic nucleus forming a biological tissue is excited by absorbing a specific radio frequency (hereinafter abbreviated as RF) under the influence of a magnetic field. It is based on a phenomenon (NMR phenomenon).

FIG. 7 is a conceptual diagram of an MR device. In the figure, reference numeral 1 is a magnet for generating a magnetic field, and 2 is an RF coil provided in the magnet 1 to which an electric power in an RF band is applied to obtain a detection signal thereof. In the case of the whole body magnet as shown in the figure, the patient 3 is placed so as to be wrapped inside the RF coil 2.

In such a state, the transmitter 4 applies power of a frequency in the RF band to the RF coil 2. The power applied to the transmitter 4 at this time is, for example, 10 KW to 20 KW.
Is used. As a result, a signal based on the NMR phenomenon from the body of the patient 3 is induced in the RF coil 2. Receiver 5
Receives the detection signal, amplifies it, and sends it to the A / D converter 6. The A / D converter 6 converts the input signal into digital data.

The computer 7 inputs the output data of the A / D converter 6, performs image processing, and displays the result on the display 8. The operator examines the lesion area of the patient while looking at the image displayed on the display 8.
Reference numeral 9 is an interface that acts as an intermediary between the computer 7 and other components (the transmitter 7, the receiver 5, and the A / D converter 6).

By the way, in this type of MR device, when performing high-speed scanning with a repetition time of several ms, prior to a series of data acquisition in order to add image contrast,
A pulse sequence consisting of various RF pulses and gradient pulses called preparation pulses is often added. Among the preparation pulses, there is a T2 preparation pulse (T2 is a lateral relaxation time) consisting of a 90 ° -180 ° -90 ° sequence for giving T2 contrast.

This T2 preparation pulse is generally of a non-selective type in which a magnetic field gradient for slice selection is not added when RF is applied, as shown in FIG.
(A) is an RF application pulse, (b) is a slice axis.
No gradient magnetic field is applied. Here, the first 90 ° is the inclination with respect to the static magnetic field direction (z-axis direction), the next 180 ° is the inclination of 90 ° to 180 °, and the next 90 ° is the inclination of 90 ° from this inclination. The repetition time is an interval between excitations that are repeatedly performed for sampling data, after the sampling of data is performed following the T2 preparation pulse.

This non-selective excitation method has no problem near the center of the magnet (center of the magnetic field). However,
If it is separated from the center of the magnet, it is easily affected by the non-uniformity of the magnetic field, and it appears as shading artifacts in the image. For this reason, it is desirable that the selective excitation method be used for the gradient magnetic field for the T2 preparation pulse.

In general, selective excitation by 90 ° pulse is
Since the phase of transverse magnetization shifts during RF application, a method of aligning the phase of transverse magnetization by adding a rephase gradient of the opposite sign, which is half the area of the slice gradient, to the slice gradient is used as shown in FIG. To be In the figure, S1 is 1/2 of the area of the slice gradient, and the area S2 of the rephase gradient a of the opposite sign following this S1 is equal to S1.

This selective excitation method is not affected by magnetic field inhomogeneity even if it is far from the center of the magnet, and z
An axial sliced image can be obtained.

[0011]

However, if this rephasing method is simply applied to the T2 preparation pulse, the transverse magnetization after the first 90 ° pulse will be corrected, but during the second 90 ° pulse. The phase shift of the generated magnetization causes strength loss of the longitudinal magnetization at the end of application of the second 90 ° pulse, and the S / N ratio of the image deteriorates.

The present invention has been made in view of the above problems, and an MR device 90 capable of reducing the influence of magnetic field inhomogeneity without causing a strength loss of longitudinal magnetization even if it is separated from the magnetic field center. It is an object of the present invention to provide a preparation pulse applying method consisting of a ° -180 ° -90 ° series.

[0013]

According to the present invention for solving the above-mentioned problems, the first 90 ° pulse is synchronized with applying a preparation pulse consisting of a 90 ° -180 ° -90 ° sequence to an RF coil. A rephase gradient pulse is added to the synchronized gradient pulse, and a dephase gradient pulse having an area half the selective excitation gradient for the second 90 ° pulse is added before the second 90 ° pulse. Is characterized by.

[0014]

The gradient magnetization for selective excitation is applied in order to give a slice gradient in synchronization with the application of the T2 preparation pulse of 90 ° -180 ° -90 ° to the RF coil.
At this time, a rephase gradient pulse is added to the slice axis at the previous 90 ° pulse. Generally, 9 for selective excitation
The spins excited by the 0 ° pulse are out of phase during the application of the RF pulse. The rephase gradient pulse is used to restore this shift.

FIG. 1 shows the principle of the present invention. In this figure, (a) is an RF pulse and (b) is a slice axis.
A rephase gradient pulse a is added to restore the above-mentioned phase shift of the spin. A feature of the invention is that the dephasing gradient pulse b of half the area of the selective excitation gradient for the second 90 ° pulse precedes the second 90 ° pulse.
Has been applied.

The phase of the spins excited during the application of the second 90 ° pulse is out of phase, but the transverse magnetization does not remain after the second 90 ° pulse, so the phase cannot be returned by the usual method. To this end, a gradient of half the area of the slice gradient and of opposite polarity is added before the 90 ° pulse. By taking such a sequence, it is possible to return the phase of the spin with a phase shift, and the selective excitation by the T2 preparation pulse does not cause loss of longitudinal magnetization intensity even if the magnetic field is separated from the center of the magnetic field, and the influence of magnetic field inhomogeneity. Can be made smaller.

[0017]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 2 is a diagram showing a specific operation sequence of the present invention. (A) is an RF pulse, (b) is a slice axis, (c) is time, (d) is a rotating coordinate system (x, y, z).
The states of spin magnetization in are respectively shown. Macroscopic magnetizations are represented by white arrows, and the individual magnetizations that make them are represented by black arrows.

At the time a before application of T2 preparation, the magnetization is in an equilibrium state as shown in (a), with the z-axis being the main magnetic field (static magnetic field) direction. During the first 90 ° pulse, the phase of the magnetization is out of phase, and the intensity of macroscopic magnetization (white arrows) becomes small as a result of dispersion of the individual magnetizations, as shown in (b).

When the rephase pulse a is applied at time b, the phase shift returns at time c, and the intensity of transverse magnetization becomes large along the y-axis as shown in (c). Here, when the 180 ° pulse shown in (a) is applied, y is applied until the second 90 ° pulse is applied.
The transverse magnetization oriented in the axial direction is inverted at time d as shown in (d) and oriented in the -y axis direction. The magnetization of T2 which is different during this time is attenuated by each T2 value (transverse relaxation time), and the difference in T2 value is left as the difference in transverse magnetization intensity.

Before the second 90 ° pulse, the phase of the lateral magnetization is shifted by the dephase gradient pulse b, and the intensity of the macroscopic magnetization is as shown in (e) at time e immediately before the second 90 ° pulse. It is getting smaller. However,
During this period from time 2 to time e, the phase shift that the magnetization receives is
It is offset by the phase shift of the magnetization that the second 90 ° pulse receives during application. Therefore, the phases are aligned by the time point after the second 90 ° pulse until the time point, and no attenuation of the longitudinal magnetization intensity occurs as shown in (f).

This allows selective excitation of the T2 preparation pulse. Since selective excitation is strong against magnetic field inhomogeneity, shading artifacts such as those seen with the non-selective excitation type T2 preparation pulse are improved.

FIG. 3 shows NMR in which selective excitation according to the present invention is performed.
2 is a block diagram showing the configuration of an imaging device (MR device) of FIG. In the figure, the magnet assembly 11 has a space portion (hole) for inserting a subject (not shown) therein, and a constant static magnetic field is applied to the subject so as to surround the space portion. A static magnetic field coil and a gradient magnetic field coil for generating a gradient magnetic field (the gradient magnetic field coils are x, y,
(provided with a coil for each z axis), and RF for applying a high-frequency pulse for exciting spins of nuclei in the subject.
A transmission coil, a reception coil that detects an NMR signal from the subject, and the like are arranged.

The static magnetic field coil, the gradient magnetic field coil, the RF transmitting coil and the receiving coil are the static magnetic field power source (main magnetic field power source) 1.
2, the gradient magnetic field driving circuit 13, the RF power amplifier 14, and the preamplifier 15 are connected to each other. The sequence storage circuit 18 is a feature of the present invention, and operates the gate modulation circuit 19 (modulates the high frequency output signal of the RF oscillation circuit 20 at a predetermined timing) with an arbitrary view in accordance with a command from the computer 17, RF to pulse
It is applied from the power amplifier 14 to the RF transmission coil.

The operation of the sequence storage circuit 18 will be described in more detail. The sequence storage circuit 18 stores the timing of applying the preparation pulse composed of the 90 ° -180 ° -90 ° series and the timing of applying the gradient magnetic field in synchronization with the timing. The gradient magnetic field also includes a sequence for applying a rephase pulse as shown in FIG. Then, based on the command from the computer 17, the RF pulse and the slice gradient pulse (including the rephase pulse) having the timings shown in FIG. 1 are synchronously output from the gradient magnetic field drive circuit 13 and the RF power amplifier 14. Function.

That is, the sequence storage circuit 8 operates the gradient magnetic field drive circuit 13, the gate modulation circuit 19 and the A / D converter 21 by a sequence signal based on the spin warp method. Further, the sequence storage circuit 18 has the gate modulation circuit 19 before starting the series of sequence operations described above.
And the gradient magnetic field drive circuit 13 is operated to selectively excite in a desired direction.

The phase detector 22 phase-detects the output signal of the preamplifier 15 (the NMR signal detected by the receiving coil) by using the output of the RF oscillation circuit 20 as a reference signal to perform A / D.
It is given to the converter 21. The A / D converter 21 performs analog / digital conversion of the NMR signal obtained via the phase detector 22 and inputs it to the computer 17.

The computer 17 exchanges information with the operation console 23 and implements various scan sequences, switches the operation of the sequence storage circuit 18, rewrites the memory, and performs A / D conversion. The image reconstruction calculation is performed using each data from the device 21.
The image processing result is displayed on the display device 16.

In FIG. 1, which is the sequence of the present invention,
Rephase gradient pulse a after the first 90 ° pulse
And the dephasing gradient pulse b before the second 90 ° pulse have the same area if the slice gradients for the two 90 ° pulses have the same magnitude, and these two gradients are 18
Since the 0 ° pulse is sandwiched, a shape having only a slice gradient is possible as shown in FIG. In this method, generally, there is no rephase gradient pulse attached to the selective excitation.

In the above embodiment, the 90 ° pulse and the 180 ° pulse have the same phase in the T2 preparation pulse. This is expressed as 90 ° (x) -180 ° (x) -90 ° (x), but 90 ° (x) -1 obtained by shifting the 180 ° phase from the 90 ° phase by π / 2.
It may be an 80 ° (y) -90 ° (-x) series.

Further, a preparation pulse for a diffusion image in which a relatively large gradient called a motion probe gradient is added to arbitrary axes on both sides of 180 ° is added to a preparation pulse in the form of 90 ° -180 ° -90 °. is there. This preparation pulse is also used for selective excitation. 5 and 6 are 90 ° -180 ° according to the present invention
It is a figure which shows the other method of preparation excitation which consists of a -90 degree series. In each case, (a) is an RF pulse, (b)
Indicates the slice axes, respectively. FIG. 5 shows the case with a rephase gradient, and FIG. 6 shows the case without a rephase gradient.

[0031]

As described above in detail, according to the present invention, even if the distance from the center of the magnetic field is eliminated, the longitudinal magnetization intensity is not lost and the influence of the magnetic field nonuniformity can be reduced to 90 °. It is possible to provide a preparation pulse applying method including a -180 ° -90 ° series.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a diagram showing a specific operation sequence of the present invention.

FIG. 3 is a configuration block diagram of an MR device in which selective excitation of the present invention is performed.

FIG. 4 is an explanatory diagram of another method of the present invention.

FIG. 5 is a diagram showing another method of preparation excitation including a 90 ° -180 ° -90 ° sequence according to the present invention.

FIG. 6 is a diagram showing another method of preparation excitation including a 90 ° -180 ° -90 ° series according to the present invention.

FIG. 7 is a conceptual diagram of an MR device.

FIG. 8 is an explanatory diagram of a non-selective T2 preparation method.

FIG. 9 is an explanatory diagram of selective excitation.

[Explanation of symbols]

 11 magnet assembly 12 main magnetic field power supply 13 gradient magnetic field drive circuit 14 RF power amplifier 15 preamplifier 16 display device 17 computer 18 sequence storage circuit 19 gate modulation circuit 20 RF oscillation circuit 21 A / D converter 22 phase detector 23 operation console

Claims (2)

[Claims] 1. A rephase gradient pulse is added to a gradient pulse for selective excitation synchronized with the first 90 ° pulse in synchronism with application of a preparation pulse consisting of a 90 ° -180 ° -90 ° sequence to an RF coil. , A selective excitation of an MR device characterized in that a dephase gradient pulse having an area half that of the gradient pulse for selective excitation for the second 90 ° pulse is added before the second 90 ° pulse Preparation pulse application method. 2. The same area as the gradient pulse for selective excitation generated in synchronization with the first 90 ° pulse is synchronized with the application of the preparation pulse consisting of the 90 ° -180 ° -90 ° series to the RF coil. A selective excitation preparation pulse applying method for an MR apparatus, wherein a gradient pulse is generated in synchronism with a second 90 ° pulse and a rephase gradient is not required.