JPS6263847A - Effective magnetic field variable type nuclear magnetic resonance image pickup equipment - Google Patents

Abstract

PURPOSE:To shorten the recovery time to a thermal equilibrium state of magnetization by making the magnetization in a relaxation process follow up an effective magnetic field by the effective magnetic field which is converted from the direction to which the magnetization is oriented, to the Z direction, after a nuclear magnetic resonance signal is observed. CONSTITUTION:A gradient magnetic field use coil is driven through a gradient magnetic field controlling circuit 8 from a controller 17, and a z gradient magnetic field is applied. At the same time, an exciting coil 4 is driven. In such a state, an observation is executed. After a nuclear magnetic resonance observation period is ended, it is stopped to apply all gradient magnetic fields Gz, Gx and Gy, and a resonance signal from a resonance frequency variable oscillator 9 is applied from the direction (y). In such a case, its frequency is lowered gradually from a resonance frequency of an object atomic nucleus, or form an appropriate frequency of a degree a little higher than its resonance frequency. As a result, an effective magnetic field converts its direction slowly in the direction Z, and the magnetization follows up this effective magnetic field and converts its direction in the direction Z.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is directed to nuclear magnetic resonance (nuclear magnetic resonance).
ic resonance) (hereinafter referred to as f"N
It is abbreviated as MRJ. ) using the phenomenon to inject t into the subject.
The present invention relates to a nuclear magnetic resonance examination apparatus (hereinafter referred to as an NMR imaging apparatus) in which the nuclear distribution of a specific atom can be determined from outside a subject, and particularly relates to a method of applying an effective magnetic field.

(Prior Art) As methods for applying an RF magnetic field to a spin system to make it resonate, there are conventionally a pulse method and a CW method. Usually, N.M.
Rljtlffi uses pulses (9o° pulses and 180° pulses).
NMR is performed by imparting a series of positional information to a population of spins excited by a pulsed magnetic field.
A tomographic image of the subject is obtained by observing the signals. When a series of operations in which a pulsed magnetic field is applied to excite a spin system and an NMR signal is observed is repeated over multiple views, the magnetization (magnetic moment) M of the once excited spin system is oriented in the direction of the static magnetic field and reaches an equilibrium state. The next excitation is performed one time after the current is reached. The interval between excitation pulses is usually defined as τ.

(Problems to be Solved by the Invention) By the way, as shown in FIG. 5, the longitudinal relaxation time T1, which represents the speed at which the magnetization M of the excited spin system is oriented in the direction of the static magnetic field and spontaneously recovers to an equilibrium state, is: Approximately 500m for living organisms
s ~ 2s, and when Tτ ~ TI, there was a problem that imaging took time (saturation recovery method (
This natural recovery method is used in pulse sequences such as the SR method (SR method) and the inversion recovery method (IR method).

One way to shorten the recovery time is to apply an RF pulse (906 pulses) as shown in Figure 6 to force the magnetization M in the xy plane to move in the l-axis direction (direction of the static magnetic field).
) There is a way to do it. This method is usually called the DEFT method. This method has problems such as that the RFIa field intensity distribution is not uniform depending on the location and that the timing of pulse application is difficult.

In view of these points, an object of the present invention is to force the excited magnetization in the xy plane to efficiently direct the magnetization in the direction of the static magnetic field (two axes) with less unevenness depending on the location, and to shorten the recovery time. An object of the present invention is to provide a variable effective magnetic field type nuclear magnetic resonance Ill apparatus that can shorten the time and speed up imaging.

(Means for Solving the Problems) In order to achieve such an object, in the present invention, a high frequency pulse and a magnetic field are applied to the object to generate a nuclear magnetic resonance signal, and this signal is used to detect the counterstain. In a nuclear magnetic resonance imaging device designed to obtain images of tissues, the magnetization during the relaxation process is detected using an effective magnetic field that changes direction from the direction in which the petrification is facing. The device is equipped with an effective magnetic field generating means that has the function of following this effective magnetic field and forcibly directing it in two directions, thereby shortening the time required for magnetization to recover to a thermal equilibrium state after observing a nuclear magnetic resonance signal. It is characterized by

<Example> The present invention will be described in detail below using the drawings. FIG. 1 is a block diagram of essential parts of an embodiment of an NMRI imaging apparatus according to the present invention. In the figure, 1 is a static magnetic field coil for generating a uniform static magnetic field Ho (two directions in this case), and 7 is a static magnetic field control circuit that drives this static magnetic field coil 1. Contains a regulated power supply. 6 is a shim coil control circuit for driving a shim coil (not shown) attached to the static magnetic field coil in order to increase the uniformity of the uniform static magnetic field 110.

3 generally shows the gradient magnetic field coil. 8
is a gradient magnetic field control circuit that drives the gradient magnetic field coil.

4 is an excitation coil that applies R pulses as electromagnetic waves to the subject (placed in a space surrounded by static magnetic field coils, gradient li! field coils, etc.). N.M.
This is a detection coil for detecting the R resonance signal, and is arranged with an axis in a direction rotated by 90 degrees with respect to the axis of the excitation coil 4.

Reference numeral 9 denotes an RF variable oscillator capable of generating a frequency (ffi) around the frequency fo corresponding to the NMR resonance condition of the atomic nucleus to be measured. is applied to the excitation coil 4 via the gate circuit 10 and the power amplifier 11 that control the current.

12 is an amplifier that amplifies the NMR resonance signal obtained from the detection coil 5, 13 is a phase detection circuit, and 14 is a wave memory circuit that stores the phase-detected waveform signal from the amplifier 12. Reference numeral 15 denotes II which receives the signal from the wave memory circuit and performs predetermined signal processing to obtain a tomographic image of the subject.

16 is a display device for displaying the obtained tomographic image and the like.

The sequence controller 17 is configured to be able to output signals (analog signals) necessary for controlling the gradient magnetic field and control signals necessary for applying the RF pulse and receiving the NMR signal.

The operation of such a configuration will be explained below. - A sequence in which an NMR signal is generated by applying a magnetic field and an RF pulse, and this is observed, and a tomographic image of the subject is reconstructed from the NMR signal obtained in this way. The operations of the components are similar to those of conventional devices, and will be briefly described below. The present invention is characterized by an operation devised to shorten the waiting time, that is, the recovery time of the spin system, before entering the afterfire sequence of NMR signal observation.

There is, and its operation will be described later.

Here, a case of a sequence in which a tomographic image is obtained using a projection restoration method (PR method) will be described.

1) Static magnetic field control, I! ! 1 circuit 7 connects the static magnetic field coil 1
In a state where a static magnetic field HO is applied to the subject by driving the
The sequence controller 17 drives the gradient fishing field coil (ZI[lI gradient magnetic field coil) via the gradient magnetic field υ control circuit 8 to apply two gradient magnetic fields Gz+ as shown in FIG. 2(b). Under this condition, the J-Goo 8 circuit 10 is opened to allow the RFI field (in this case, a 90° pulse) output from the RF variable oscillator 9 to pass through, input it to the power amplifier 11, and drive the excitation coil 4. , the magnetization of a particular slice becomes parallel to the y-axis direction of the rotating coordinate system.

(Let the coordinates of the laboratory system be the X, Y, Z coordinates, and the rotating coordinate system be x, y, z. However, the Z axis and the two axes are coincident.) 2 Following the gradient magnetic field Gz+ < G This is to match the phases of the NMRfi signals generated from different parts of the subject to which z""" is applied.

Next, the application of the Z gradient magnetic field is stopped, and an X gradient magnetic field sGX and a y gradient magnetic field Gy of a predetermined magnitude are applied.

As a result, the NMR signal (FI
D signal) is generated. This NMR signal is detected by the detection coil 5, guided to the phase detection circuit 13 via the amplifier 12, where the phase is detected, and then the wave memory circuit 1
It is stored in 4. The stored data is read at appropriate timing by the computer 15, where it is Fourier transformed and becomes a signal of one projection.

In this case, the projection direction is
This direction is perpendicular to the direction of the composite magnetic field of Gy and the y gradient magnetic field Gy. Therefore, by appropriately changing the X gradient magnetic field SGX and the y gradient magnetic field Gy, it is possible to change the J31 field by 1q to different projections. In this way, a tomographic image of the subject is reconstructed using the fft8 machine 15 using data from a large number of projections (or views).

The 1q reconstructed image is displayed on the display device 16.

Next, the operation when forcibly recovering magnetization after observing the NMR signal in each view will be described.

As shown in Figure 3, the effective magnetic field in the rotating system -ff
is expressed as old eff=10+qd10H1, and when it changes under the following conditions, γ: gyromagnetic ratio) 10: static magnetic field strength in the z-axis direction Hl: rotating magnetic field strength ω: resonance frequency The direction of magnetization is It is known that it follows the direction of the old eff (for example, C, P, 5lichter, p.
rinciples of magnetic
“Resonance”.

p16-22. )larper, NeXv Y
ork +' 1963 or A, △bragam
, ”The prineiples of
"NucIear Magnetism",
p 65 66, ○xford IJ ni
v.

Press, London and New
"y'ork, 1961).

The present invention utilizes this theory. First, burst wavy R
The RF magnetic field generated by applying the F signal is controlled to apply a rotating magnetic field in the spin direction to keep I/14eH- parallel to the direction of magnetization. The RF multiplied signal frequency is then increased from or higher than the resonant frequency of the target nucleus.
Slowly lower the frequency from an appropriate frequency (slightly higher frequency if appropriate) and change the direction of the old c++ in two axes. This allows the magnetization in the process of relaxation to follow this effective magnetic field 1l-1eff and be directed in the Z-axis direction.

As a method of changing the direction of the rotating magnetic field from the y-axis direction to the two-axis directions, for example, there is a method of using II tall of the RF magnetic field.aRF coil is fixed in a laboratory system; however, changing the phase of the RF magnetic field It is possible to apply an RF magnetic field from any direction within the xy plane of the rotating system. Therefore, when applying in the X direction, RF
A rotating magnetic field can be applied in the X direction by giving the doubled signal to the RF coil with a 90" phase shift. In this case, the rotational speed ω for the experimental system of the rotating system is set to R", which is the frequency of the magnetic field. When set equal to ωRF, an equivalent magnetic field −/′γ is produced in one of two directions as shown in FIG. 4.
1s magnetic field (effective magnetic field) is old. +18. Shigaru'r becomes. In the resonant state, the helical rotation /γ = - interest, which cancels out the static magnetic field, mJ in the X direction! 1l=l, only.

If the RF frequency is changed here, the frequency of the rotating system will also change (in this case, it is kept in the y-axis force direction), and therefore the Vγ will change and ILA)s/γ~-
-H. This produces a magnetic field in the Z direction. Therefore, the effective magnetic field is the vector sum of the magnetic fields in these two directions and the rotating magnetic field in the y7-i direction. 1 period 7・γ+old. As the two components of are increased, the effective magnetic field changes direction in the Z direction.

The operation based on this principle is as follows. N
The total gradient Ii field Gz after the end of the MR observation period.

Gx, Gy application is stopped, and RF from RF variable oscillator 9
Apply the double signal from the X direction (Whether the RF double signal is applied from the X direction or the X direction is determined by controlling the RF double signal phase.
``IJ'' is sufficient). In this case, the frequency is gradually lowered from the resonant frequency of the target nucleus, or from a suitable frequency slightly higher than the resonant frequency. follows this effective magnetic field and changes direction in two directions.The sequence controller 22 controls the phase and frequency of the RF magnetic field as described above.

Although the rotational speed of the effective magnetic field in two directions is limited by the above equations (1) and (2>), the above-mentioned forced recovery time TE is extremely short compared to the natural relaxation time of magnetization.

In addition, as a method of rotating the effective magnetic field in two directions from the X direction, a method using control of the RF magnetic field is shown in the embodiment, but the following method may be used without being limited to this.

After applying a rotating magnetic field in the direction of magnetization, static m jiJ 1
- A method to change IQ.

In this case, there are the following methods for changing the static magnetic field.

■Change the driving current of the static magnetic field.

■Install an auxiliary magnetic field coil and change its drive current.

■Use a gradient RJI field coil.

Further, in the embodiment, an example was described in which NMR signals are observed using the PR method, but the method is not limited to the PR method, and other methods such as the saturation recovery method and the spin echo method can also be applied.

(Effects of the Invention) As explained above, the present invention provides the following effects.

Since the excitation repetition time Tr can be made sufficiently shorter than the wL relaxation time T+, it is possible to achieve faster f-wave acquisition and to make the imaging time much shorter than in the prior art.

In addition, since it is not easily affected by non-uniformity of the RF transmitting coil, the spin can be directed uniformly in two axial directions.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a block diagram showing an embodiment of a right-handed variable magnetic field nuclear magnetic resonance imaging apparatus according to the present invention, FIG. 2 is a diagram showing a pulse sequence, and FIGS. The figure is a diagram for explaining the effective magnetic field applied to a rotating system, FIG. 5 is a diagram for explaining changes in magnetization, and FIG. 6 is a diagram for explaining a conventional magnetization recovery method. 1... Static magnetic field coil, 3... Gradient magnetic field coil, 4
... Excitation coil, 5... Detection coil, 6... Shim coil control circuit, 7... Static magnetic field control circuit, 8... Gradient magnetic field control circuit, 9... RF variable oscillator, 10...・
Gate circuit, 11... Power amplifier, 12... Amplifier, 13... Phase detection circuit, 14... Wave memory circuit, 15... Total n devices, 16... Display device, 17
...Sequence controller. Figure 2 Figure 3 Figure 4, etc. Figure 5 Figure 6

Claims (1)

[Claims] 1) A nuclear magnetic resonance imaging apparatus that applies high-frequency pulses and a magnetic field to an object to generate a nuclear magnetic resonance signal, and uses this signal to obtain an image of the tissue of the object, After observing the nuclear magnetic resonance signal, an effective magnetic field that changes direction from the direction in which the magnetization is oriented to the direction of the static magnetic field is used to force the magnetization during the relaxation process to follow this effective magnetic field and to be directed in the direction of the static magnetic field. What is claimed is: 1. A variable effective magnetic field type nuclear magnetic resonance imaging apparatus, comprising an effective magnetic field generating means having a function of generating a magnetic field, and shortening the time for recovery of magnetization to a thermal equilibrium state after observing a nuclear magnetic resonance signal. 2) The effective magnetic field generating means applies R in the form of a burst wave.
2. The variable effective magnetic field type nuclear magnetic resonance imaging apparatus according to claim 1, wherein the effective magnetic field is generated by controlling the phase and frequency of the F magnetic field. 3) The effective magnetic field generating means applies R in the form of a burst wave.
The variable effective magnetic field type nucleus according to claim 1, characterized in that the effective magnetic field variable type nucleus is configured to apply a rotating magnetic field in the direction of magnetization by an F magnetic field and generate the effective magnetic field by changing a static magnetic field. Magnetic resonance imaging device.