JPH04307031A - Magnetic resonance imageing apparatus - Google Patents

Abstract

PURPOSE:To enable reduction in stimulation of peripheral nerves by alternating time variation components within a rising or falling time of an inclined magnetic field between positive and negative to achieve a hourly alternation of an eddy current in vivo to be inducted by an hourly change in a far or near direction of the nerves. CONSTITUTION:An electrostatic magnetic field magnet 1, X- to Z-axis inclined magnetic field coils 2 and a transmitting/receiving coil 3 are arranged in a gantry 20 and X- to Z-axis inclined magnetic field power sorces 7-9 and a transmitter 5 are driven and controlled with a computer system 11 through a sequencer 10. An echo signal is brought in as an MR signal received with a receiver 6 and undergoes a, specified signal processing to generate a tomographic image of an object to be inspected and the image is shown on a display section 12. In the control with the computer system 11. A waveform of a triangular wave, since wave or the like which turns to zero in time integration while an inclined magnetic field read out normally varies hourly is made to overlap an inclined magnetic field waveform thereby achieving hourly alternation of an eddy current in vivo in a far or near direction of the nerve.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrafast magnetic resonance imaging (MRI) apparatus.

0002

[Prior Art] Generally, an MRI apparatus is used for X-ray tomography (
It is non-invasive compared to X-ray CT (CT) equipment, has excellent ability to visualize soft tissues in particular, and can capture a wide variety of biological information (morphology, shape, etc.).
It has the characteristics of a comprehensive image diagnostic device in that it can obtain information on blood flow, metabolism, etc.). However, its biggest drawback is the longer imaging time compared to other modalities. Therefore, it is difficult to image a moving part. Another problem is that the patient is restrained for a long time. For this reason, despite having high clinical potential,
Currently, its application is limited to the brain, central nervous system, and parts of the body that move relatively little, and it has not reached the point where it can be called a device for the whole body. Recently, high-speed scanning and its application technology have significantly shortened the imaging time, but instantaneous imaging of the heart is still difficult.

In contrast, in recent years, an imaging method called ultra-high-speed scanning has been developed that allows data collection to be performed in several tens of milliseconds. According to this, it is possible to instantaneously image moving organs such as the heart and abdomen. Therefore, until now, MRI has been mainly limited to morphological diagnosis of the brain and central nervous system.
It is expected that the device will become widespread as a true whole-body MRI device that includes the abdominal and cardiac regions.

[0004] However, in order to put ultra-high-speed MRI equipment into practical use, it is necessary to overcome many technical issues, and among them, high-speed control of strong gradient magnetic fields is not only a technical aspect, but also a safety issue. Consideration must be given to this point. In other words, the time-varying components of the gradient magnetic field and the cross magnetic field perpendicular to it, which were slight and did not pose a problem in conventional MRI equipment, are induced by the time-varying components in ultra-high-speed MRI equipment because the magnetic field strength is extremely strong. There was a risk of eddy currents stimulating the subject's peripheral nerves, posing a safety problem.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to address the above-mentioned circumstances, and its purpose is to significantly reduce the stimulation of peripheral nerves due to in-vivo eddy currents induced by temporal variations in gradient magnetic fields; Alternatively, it is an object of the present invention to provide a highly safe magnetic resonance imaging apparatus that can avoid such problems.

[0006]

[Means for Solving the Problems] A magnetic resonance imaging apparatus according to the present invention includes means for generating a gradient magnetic field having a time-varying component, and controlling the gradient magnetic field generating means so that the time-varying component alternates between positive and negative. and means for alternating the eddy current induced by the time-varying component in the distal and proximal directions of the nerve.

[0007]

[Operation] According to the magnetic resonance imaging apparatus of the present invention, by alternating the time-varying magnetic field components between positive and negative during the rise and fall times of the gradient magnetic field, in-vivo eddy currents induced by time-varying This can be temporally alternated in the lateral and proximal directions, and peripheral nerve stimulation caused by in-vivo eddy currents can be significantly reduced or avoided.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a magnetic resonance imaging apparatus according to the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of an embodiment. Inside the gantry 20, there are a static magnetic field magnet 1, X-axis, Y-axis, and Z-axis gradient magnetic field coils 2,
and a transmitting/receiving coil 3. The static magnetic field magnet 1 as a static magnetic field generator is configured using, for example, a superconducting coil or a normal conducting coil. The X-axis, Y-axis, and Z-axis gradient magnetic field coils 2 include an X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy,
This is a coil for generating a Z-axis gradient magnetic field Gz. The transmitter/receiver coil 3 is used to generate high frequency pulses and detect magnetic resonance (MR) signals generated by magnetic resonance. The subject P on the bed 13 is inserted into an imaging possible area (a spherical area in which an imaging magnetic field is formed, and diagnosis is possible only within this area) in the gantry 20.

The static magnetic field magnet 1 is driven by a static magnetic field control device 4. The transmitter/receiver coil 3 is driven by a transmitter 5 during magnetic resonance excitation and is coupled to a receiver 6 during MR signal detection. The X-axis, Y-axis, and Z-axis gradient magnetic field coils 2 are driven by an X-axis gradient magnetic field power supply 7, a Y-axis gradient magnetic field power supply 8, and a Z-axis gradient magnetic field power supply 9.

X-axis gradient magnetic field power supply 7, Y-axis gradient magnetic field power supply 8
, Z-axis gradient magnetic field power supply 9, and transmitter 5 are driven by a sequencer 10 according to a predetermined sequence, and
x, Y-axis gradient magnetic field Gy, Z-axis gradient magnetic field Gz, high frequency (R
F) Generating pulses in a predetermined pulse sequence described below. In this case, the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy
, Z-axis gradient magnetic field Gz are mainly used, for example, as a reading gradient magnetic field Gr, a phase encoding gradient magnetic field Ge, and a slicing gradient magnetic field Gs, respectively. The computer system 11 drives and controls the sequencer 10, captures echo signals as MR signals received by the receiver 6, performs predetermined signal processing, and generates a tomographic image of the subject, which is displayed on the display unit 12. indicate.

Next, the principle of ultrahigh-speed MRI will be explained with reference to the pulse sequence shown in FIG. Although not shown, during the period when the sequence shown in FIG. 2 is executed, a static magnetic field is statically applied to the subject P by the static magnetic field magnet 1 driven by the static magnetic field control device 4. ing.

[0012] By applying an RF pulse (90 degree pulse) consisting of a selective excitation pulse while applying a slice gradient magnetic field Gz to the subject P, magnetic resonance is generated in the nuclear spins of a specific region (region of interest) of the subject P. It is excited and the magnetization vector of the nuclear spin of a specific atomic nucleus within the slice is tilted by 90 degrees. Thereafter, by applying a 180-degree pulse, which is a high-frequency magnetic field, to reverse the nuclear magnetization vector, the rotational phase of the nuclear magnetization vector, which was dephased and dispersed by the application of the 90-degree pulse, is rephased and converged.

[0013] After applying a 180 degree pulse,
By applying the readout gradient magnetic field Gx while inverting it many times, a multi-echo echo signal is generated, and by advancing phase encoding for each echo signal, all the data necessary for image reconstruction can be generated with one excitation. can be collected.

As shown by the broken line in FIG. 2, the phase encoding can be carried out by continuously applying the phase encoding gradient magnetic field Gy while the readout gradient magnetic field Gx is being applied.
As shown by the solid line in , there is a method of applying a phase encode gradient magnetic field Gy in a pulsed manner every time the readout gradient magnetic field Gx is switched (rise and fall). The former is called the echo planar method, and the latter is called the ultrafast Fourier transform method.

In the case of the echo planar method, the collected data is as shown in FIG. 3, with the horizontal axis kx in the Gx direction and the vertical axis k
This is a zigzag scan of k space (spatial frequency domain) with y as the Gy direction. On the other hand, in the ultra-fast Fourier transform method, the collected data is on equidistant grid points in k-space, as shown in Figure 4, so by directly performing two-dimensional Fourier transform, it is possible to reconstruct the data without artifacts. An image is obtained.

The waveform of the readout gradient magnetic field Gx is the rectangular wave shown in FIG. An alternative method is to use a sine wave. In this case, with normal evenly spaced data sampling, the sampling points in the Gx direction are unevenly spaced on k-space, so uneven sampling and data interpolation are required to obtain data on equally spaced grid points. becomes.

[0017] Here, in the ultrahigh-speed MRI apparatus, since the readout gradient magnetic field Gx is extremely large compared to other gradient magnetic fields,
When discussing the influence of gradient magnetic fields on living organisms, it is sufficient to discuss only the readout gradient magnetic field Gx. When the human body is positioned as shown in FIG. 5 with respect to the center of the gradient magnetic field, the magnetic field Bg due to the readout gradient magnetic field Gx is expressed as x·Gx, where x is the position in the X-axis direction. On the other hand, according to Maxwell's theory of electromagnetic waves,
As a cross magnetic field Bc orthogonal to the readout gradient magnetic field Gx, z
・Gz will exist. Here, z is the position in the Z-axis direction. FIG. 5 shows a case where the center of the gradient magnetic field is located at the heart; in this case, x<z, and it can be seen that the time-varying magnetic field component dBc/dt at the head tends to be a problem.

As the readout gradient magnetic field Gx changes over time in a trapezoidal or sinusoidal waveform, the magnetic fields Bg and Bc (hereinafter simply referred to as B) also change over time, as shown in FIG. According to the time-varying component dB/dt, as is known,
Eddy currents are induced in living organisms, which increase the potential within nerve cells, and when that potential exceeds a certain threshold, it is felt as nerve stimulation. For example, when a sine wave is used as the readout gradient magnetic field Gx, the cross magnetic field Bc is 61 (
There is a report that a slight muscle contraction (twitch) was felt in the area between the eyebrows when T/s) (rms).

Consider the eddy current waveform induced by the time-varying component dB/dt of this gradient magnetic field. Eddy current J generated when the current waveform flowing through the gradient magnetic field coil is a trapezoidal wave
The waveform of is shown in FIG. Here, periods T1 and T3 are 100
μs, T2 is 0 to 100 μs. Physiological experiments on nerve stimulation have shown that when an eddy current J is induced in the distal direction of the nerve during period T1 in Fig. 7, and an eddy current J is induced in the proximal direction of the nerve during period T3, the difference between the two eddy currents is For example, time T2 is 30μ
It has been confirmed that within s, as shown in FIG. 8, electromyograms from innervated muscles do not appear. Therefore, if the eddy current is alternated in the distal and proximal directions of the nerve during the rising and falling times of the gradient magnetic field waveform (trapezoidal waveform) (corresponding to T2 = 0), nerve stimulation by the time-varying magnetic field component dB/dt can be achieved. can be significantly reduced or avoided.

[0020] Here, in the readout gradient magnetic field of ultrafast MRI, what contributes to the spin echo signal is the diagonally shaded part shown in FIG. The readout gradient magnetic field may be controlled so that the direction of the eddy current changes over time, that is, the time-varying magnetic field component dB/dt alternates between positive and negative. Therefore, controlling the gradient magnetic field does not affect the spin echo signal.

As a specific method, as shown in FIG. 10, the time-varying component d of a normal readout gradient magnetic field (trapezoidal waveform) is
A gradient magnetic field for avoiding nerve stimulation is generated by superimposing a superposed gradient magnetic field such that the B/dt value becomes 0, that is, the in-vivo eddy current alternates between positive and negative. The superimposed gradient magnetic field waveform for superimposition is a waveform such as a triangular wave or a sine wave whose time integral is 0. FIG. 10 shows the case of a triangular wave. The time integral of the in-vivo eddy current induced by the superimposed gradient magnetic field waveform becomes 0 from the above conditions. As shown in FIG. 10, the in-vivo eddy current induced by the gradient magnetic field for avoiding nerve stimulation, which is the result of addition of the trapezoidal wave and the triangular wave, alternates between positive and negative during the rising and falling periods of the trapezoidal wave. In addition, the superposed gradient magnetic field waveform is shown in Figure 1.
As shown in FIG. 1, a triangular wave train or a sine wave train having multiple periods may be used.

Here, there are the following two methods for realizing the gradient magnetic field for avoiding nerve stimulation. First, when the gradient magnetic field coil is composed of a single-turn coil, this can be realized by making the current flowing through the coil itself have the same waveform as the gradient magnetic field for avoiding nerve stimulation shown in FIG. In addition, when the gradient magnetic field coil consists of a multi-turn coil, a trapezoidal wave,
Alternatively, by passing a first coil current with a sinusoidal normal gradient magnetic field waveform through a part of the multiplex windings, and flowing a second coil current with a superimposed gradient magnetic field waveform through the remaining multiplex windings, spatially Superposition of gradient magnetic fields can be realized.

As described above, according to this embodiment, by alternating the time-varying magnetic field components at the rise and fall of the gradient magnetic field, the eddy currents induced by the time-varying magnetic field components are directed toward the distal and proximal directions of the nerve. Can be alternated,
The eddy current can suppress the excitation of the nerve membrane. As a result, nerve stimulation due to time-varying magnetic field components can be significantly reduced or avoided.

It should be noted that the present invention is not limited to the above-described embodiments, but can be implemented with various modifications. For example, although only the readout gradient magnetic field has been described in the embodiment, the waveforms of other gradient magnetic fields may be similarly controlled. Further, although the multi-echo spin echo method has been described as a pulse sequence for ultra-high-speed MRI, the present invention is not limited to this.

[0025]

As explained above, according to the present invention, a waveform such as a triangular wave or a sine wave whose time integral becomes 0 within the rise and fall times of the readout gradient magnetic field is read out, and the gradient magnetic field waveform is read out. By superimposing the two, the in-vivo eddy current alternates in time in the distal and proximal directions of the nerve, thereby providing an ultra-high-speed MRI apparatus that can avoid nerve stimulation.

[Brief explanation of the drawing]

FIG. 1 is a block diagram schematically showing an embodiment of an ultrahigh-speed MRI apparatus according to the present invention.

FIG. 2 is a diagram showing a pulse sequence of the entire ultrahigh-speed MRI apparatus according to the present embodiment.

FIG. 3 is a diagram showing how data is collected using the echo planar method.

FIG. 4 is a diagram showing how data is collected using the ultrafast Fourier method.

FIG. 5 is a diagram showing the relationship between gradient magnetic fields and cross magnetic fields.

FIG. 6 is a diagram showing gradient magnetic field waveforms and their time-varying components.

FIG. 7 is a diagram showing waveforms of coil current and eddy current.

FIG. 8 is a diagram showing changes in evoked myoelectric potential versus time between eddy currents.

FIG. 9 is a diagram showing the relationship between readout gradient magnetic field and spin echo in ultrafast MRI.

FIG. 10 is a diagram showing an example of a gradient magnetic field waveform for avoiding nerve stimulation according to the present embodiment and an in-vivo eddy current induced thereby.

FIG. 11 is a diagram showing another example of a gradient magnetic field waveform for avoiding nerve stimulation according to the present embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1... Static magnetic field magnet, 2... X-axis, Y-axis, Z-axis gradient magnetic field coil, 3... Transmission/reception coil, 4... Static magnetic field control device, 5... Transmitter, 6... Receiver, 7, 8, 9... Gradient magnetic field Power supply, 10... sequencer, 20... gantry.

Claims (3)

[Claims] 1. Means for generating a gradient magnetic field having a time-varying component; and means for controlling the gradient magnetic field generating means such that the time-varying component alternates between positive and negative, the magnetic field being induced by the time-varying component. A magnetic resonance imaging device characterized by alternating eddy currents generated in the nerve distal and proximal directions. 2. The gradient magnetic field generating means includes a single-turn gradient magnetic field coil, and the control means causes a first coil current having a time-varying component to flow through the gradient magnetic field;
2. The magnetic resonance imaging apparatus according to claim 1, further comprising means for superimposing a second coil current, which alternates in positive and negative directions, on the first coil current during a period in which the first coil current varies over time. 3. The gradient magnetic field generating means includes a multi-turn gradient magnetic field coil, and the control means includes means for causing a first coil current having a time-varying component to flow through some windings of the gradient magnetic field coil. , means for passing a second coil current alternating between positive and negative through other windings of the gradient magnetic field coil during a period in which the first coil current varies over time;
The magnetic resonance imaging apparatus according to claim 1, wherein the coil currents are spatially superimposed.