JPH09154830A - Inspection method using nuclear magnetic resonance and its device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain information on a chemical shift of a living body at high speed with a high image quality by changing its pass area almost in proportion to impressing speed of a signal reading out inclined magnetic field in a band- pass filter. SOLUTION: When a nuclear magnetic resonance signal is measured, a reading-out inclined magnetic field whose gradient polarity is periodically reversed is impressed, and an echo signal containing both X axis directional space information and chemical shift information is continuously generated. A series of echo train signal obtained in this way is converted into a digital signal by an A/D converter 12 after passing through a band-pass filter 11. Here, the band-pass filter 1 changes its pass area according to intensity of the continuously changing signal reading out inclined magnetic field when the nuclear magnetic resonance signal is detected. Therefore, a signal from outside of a photographing visual field (a noise) can be reduced from the nuclear magnetic resonance signal, and a chemical shift image having a high image quality can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inspection method and apparatus using nuclear magnetic resonance, and in particular, it uses nuclear magnetic resonance suitable for acquiring information on a chemical shift of a living body at high speed and high image quality. An inspection method and an apparatus.

[0002]

2. Description of the Related Art In the nuclear magnetic resonance phenomenon in an inspection method using nuclear magnetic resonance, even if substances containing atoms of the same kind have different molecular structures, the resonance frequency of the detected signal changes slightly. It has been known.

This is called a chemical shift, and by utilizing this phenomenon, it becomes possible to separate the resonance signal of each substance.

For example, a magnetic resonance spectroscopy method (Magnetic Resonance Spectroscopic I
maging: MRSI) is known as a method of imaging the spatial distribution of each substance corresponding to the resonance signals separated in this way.

Generally, the chemical shift information thus obtained is extracted from a time-series signal, but the spatial information of the signal is added by stepwise changing the amount of gradient magnetic field applied in each spatial coordinate axis direction. Is performed and the signal measurement is repeated (phase encoding).

From this, two-dimensional phase encoding is required when measuring a two-dimensional image with a matrix number of m × m, and signal measurement must be repeated m × m times.
An extremely long measurement time will be required.

Therefore, a high-speed MR for shortening this measurement time
As the SI method, for example, the multiple encoding method proposed by Matsui et al. Has come to be known (see Japanese Patent Publication No. 5-85172).

[0008] This method, when detecting the resonance signal,
Echo signals are continuously generated by periodically inverting the gradient polarities of the signal reading gradient magnetic field (hereinafter, such echo signals are referred to as echo train signals).

In this case, since the echo train signal obtained by one-time excitation / measurement includes chemical shift information and one-dimensional spatial information, the number of matrices m ×
When measuring a two-dimensional image of m, one-dimensional phase encoding is sufficient. That is, the number of times of signal measurement is repeated m times, the dimension of measurement can be reduced by one dimension, and the measurement time can be greatly shortened.

When the gradient polarity of the signal reading gradient magnetic field is periodically inverted, it is necessary to switch the gradient magnetic field having a large applied intensity at a high speed.
A sine (cosine) wave is usually used rather than a wave.

Further, when the nuclear magnetic resonance signal is detected from the object, the set band (Δf0) of the band pass filter for limiting the signal band of the nuclear magnetic resonance signal is irrespective of the applied intensity of the readout gradient magnetic field. , A constant value Δf0 represented by the following equation (1) is used.

[0012]

(Equation 1)

Here, γ is the gyromagnetic ratio of the nucleus to be measured, Grmax is the maximum applied intensity of the gradient magnetic field for signal readout, and Lx is the imaging field of view.

[0014]

However, in the above-described high-speed MRSI method, when a sine (cosine) wave is used as the gradient magnetic field for signal reading, the signal band fs in the field of view is actually It has been clarified that the signal band fs (t) at time t changes according to the applied strength of the read gradient magnetic field and that the signal band fs (t) follows the following equation (2).

[0015]

(Equation 2)

Here, Gr (t) represents the applied intensity of the signal reading gradient magnetic field at time t.

Since this is | Gr (t) | ≦ Grmax, Δfs (t) ≦ Δf0, and as long as the set band of the bandpass filter is constant, signals (noise) from outside the field of view of the image pickup are obtained. Is also passed, which means that these signals deteriorate the image quality.

The present invention has been made under such circumstances, and an object thereof is to provide an inspection method and apparatus using nuclear magnetic resonance capable of acquiring information on a chemical shift of a living body at high speed and with high image quality. There is.

[0019]

SUMMARY OF THE INVENTION Among the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows.

That is, when the nuclear magnetic resonance signal is generated, the gradient polarity of the gradient magnetic field for signal reading is periodically inverted and applied, and it is continuously generated by reversing the gradient polarity of the gradient magnetic field for signal reading. In an inspection method using nuclear magnetic resonance to reconstruct a chemical shift image from an echo signal after passing through a bandpass filter, the bandpass filter has its passband made substantially proportional to the applied intensity of the gradient magnetic field for signal readout. It is characterized by changing it.

According to the inspection method using nuclear magnetic resonance configured as described above, the signal band in the imaging field of view that changes depending on the applied intensity of the read gradient magnetic field is changed according to the applied intensity of the read gradient magnetic field. Since it is compatible, signals (noise) from outside the field of view can be shut out.

Therefore, it becomes possible to acquire information on the chemical shift of the living body at high speed and with high image quality.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of an inspection method using nuclear magnetic resonance according to the present invention will be described below.

Example 1. First, FIG. 2 is a configuration diagram showing an outline of a magnetic resonance diagnostic apparatus to which an examination method using nuclear magnetic resonance according to the present invention is applied.

In the figure, the subject 1 is arranged in a uniform static magnetic field generated by the static magnetic field generating magnet 2.

Further, in this static magnetic field, a gradient magnetic field generating coil 3 to which a gradient magnetic field for slice selection is applied is arranged, and a probe 4 for irradiating the subject 1 with a high frequency pulse is arranged. Has been done.

The probe 4 can also detect a nuclear magnetic resonance signal generated from the sliced surface of the subject 1 selectively excited by the gradient magnetic field generating coil 3.

The nuclear magnetic resonance signal is generated with the application of the read gradient magnetic field for periodically inverting the gradient polarity, and is detected by the probe 4.

The nuclear magnetic resonance signal detected by the probe 4 is converted into a digital signal through the analog-digital (A / D) converter 12 after the signal band is limited by the bandpass filter 11. It has become.

Here, in the present embodiment, the bandpass filter 11 changes its passband substantially in proportion to the applied strength of the readout gradient magnetic field, and the details will be described later. .

The nuclear magnetic resonance signal thus converted into a digital signal is imaged by a computer 5, and the imaged information is visualized by a display 6.

The drive power supply unit 7 for driving the gradient magnetic field generating coil 3, the transmitter 8 and the band pass filter 11 are each controlled by the sequence controller 10.

FIG. 3 shows the magnetic resonance diagnostic apparatus,
It is explanatory drawing which shows one Example of the pulse sequence of the high-speed MRSI method which measures the in-vivo distribution of the information regarding the chemical shift of the subject 1 at high speed.

This figure shows, for example, 2 using spin echo.
6 illustrates a multi-encoding method for three-dimensional image measurement.

In the figure, first, by applying a gradient magnetic field Gs ₁ for slice selection and irradiating an exciting high-frequency pulse RF ₁ modulated with a sinc waveform or the like,
Nuclear spins in a predetermined rice of the subject 1 are tilted to 90 ° to be in an excited state.

[0036] Then, in order to align the nuclear spin phase dispersed upon application of gradient magnetic field Gs _1, applying a gradient magnetic field for returning the phase Gs ₁ '. Further, by applying a gradient magnetic field Ge for phase encoding, spatial information in the y-axis direction is added to the nuclear magnetic resonance signal generated by the excitation.

From the irradiation of the exciting high-frequency pulse RF ₁ to TE
After the time of / 2, a gradient magnetic field Gs ₂ for slice selection is applied and a 180 ° high-frequency pulse RF ₂ modulated with a sinc waveform or the like is applied to invert the nuclear spin in the excited state to 180 °.

When measuring the nuclear magnetic resonance signal (Sig), a gradient magnetic field Gr (sinusoidal wave) for reading in which the gradient polarity is periodically inverted is applied, and spatial information in the X-axis direction and chemical shift are applied. An echo signal containing both information is continuously generated (hereinafter, the signal thus obtained is referred to as an echo train signal).

In this case, the gradient magnetic field Grq
By setting the area equal to the area of 1/4 cycle of the gradient magnetic field Gr (shown by the shaded area in the figure), the generation intervals of the echo signals can be adjusted to equal intervals.

Such an operation is repeated at a repeat time TR for the number of times corresponding to the number of pixels in the y-axis direction while changing the applied strength of the gradient magnetic field Ge stepwise.

Further, the series of echo train signals obtained in this way are converted into digital signals by the A / D converter 12 after passing through the band pass filter 11 as described above.

Here, as described above, the band pass filter 11 has its pass band according to the intensity of the gradient magnetic field Gr for signal readout which is continuously changed when the nuclear magnetic resonance signal is detected. Is being changed.

That is, the set band (Δf (t)) of the bandpass filter 11 at the time t is changed with time according to the following equation (3).

[0044]

(Equation 3)

As a result, it becomes possible to reduce the signal (noise) from the outside of the field of view from the nuclear magnetic resonance signal, and it becomes a signal capable of obtaining a high quality chemical shift image.

Then, the nuclear magnetic resonance signals thus obtained are grouped into odd-numbered echoes and even-numbered echoes, and then three-dimensional real space (chemical shift (δ) axis, X axis) And the Y axis) and the three-dimensional k-space (kδ axis, kx axis and ky axis) connected by the Fourier transform.

Thereafter, a chemical shift image can be obtained by subjecting this three-dimensional data array to a three-dimensional inverse Fourier transform, and two chemical shift images created from odd-numbered and even-numbered echo groups are: After phase correction, they are added.

Example 2. In the first embodiment, as the bandpass filter 11, a filter whose passband can be adjusted over time is used, but an example in which a filter that cannot be adjusted is used will be described below.

As shown in FIG. 1, the output from the receiver 9 is a plurality of band pass filters 11-having different pass bands.
1, 11-2, ..., 11-n, respectively.

Each band pass filter 11-1, 11-2,
, 11-n are output to the A / D converter 12 at appropriate timings by the multiplexer 13.

Here, the multiplexer 13 is controlled by the output from the pulse generator 15 which is sequentially input at a predetermined time interval based on the sequencer table 14 created based on the above-mentioned pulse sequence, for example. ing.

When the nuclear magnetic resonance signal is detected, when digitization is performed at a constant sampling rate Δt0 (Δt0 = 1 / Δf0), the sampling rate Δt (t)
When changing the value to perform digitization (Δt (t) = 1 /
Δfs (t)).

When digitization is performed at a constant sampling rate, as shown in FIG. 4, since k space (kx axis direction) is scanned at unequal intervals, interpolation is performed before performing Fourier transform. Equidistant points (kx
It is necessary to estimate the value of (axial direction).

When digitization is performed by changing the sampling rate, the k-space (kx axis direction) is scanned at equal intervals as shown in FIG. 5, so that the above correction is not necessary. .

In either case, since sinusoidal scanning is performed in the kδ-axis direction, kδ is obtained in order to obtain accurate values at two-dimensional lattice points.
It is desirable to perform axial interpolation.

In the above-mentioned embodiment, the case of measuring the two-dimensional space has been described, but it goes without saying that the present invention is not limited to this. This is because, for example, by performing phase encoding in the z-axis direction, the same effect can be obtained even when the measurement space is expanded to three dimensions.

As a high-speed MRSI method, for example, magnetic resonance in medicine (Magnetic Resonance)
Resonance in Medicine) Vol. 1, 370-386 (1984), etc.
When the EPSM method and the PREP method proposed by Field) are used, the same effect can be obtained by applying the method described in the above-mentioned embodiment.

[0058]

As is apparent from the above description,
According to the inspection method and apparatus using nuclear magnetic resonance according to the present invention, it becomes possible to obtain information on the chemical shift of a living body at high speed and with high image quality.

[Brief description of the drawings]

FIG. 1 is a main part configuration diagram showing an embodiment of an apparatus to which an inspection method using nuclear magnetic resonance according to the present invention is applied.

FIG. 2 is a schematic view showing an embodiment of an apparatus to which an inspection method using nuclear magnetic resonance according to the present invention is applied.

FIG. 3 is a diagram showing an example of a pulse sequence for applying an inspection method using nuclear magnetic resonance according to the present invention.

FIG. 4 is an explanatory diagram showing k-space scanning when a sampling rate is constant.

FIG. 5 is an explanatory diagram showing k-space scanning when the sampling rate is changed.

[Explanation of symbols]

RF: high frequency magnetic field, RF ₁ ... 90 ° pulse, RF ₂ ... 18
0 ° pulse, Gz ... Z-axis gradient magnetic field, Gy ... Y-axis gradient magnetic field, Gx ... X-axis gradient magnetic field, Gs ₁ ... 90
° Slice selection gradient magnetic field, Gs ₁ '... slice phase return gradient magnetic field, Gs ₂ ... 180 ° slice selection gradient magnetic field,
Gr ... Signal reading gradient magnetic field, Grq ... Signal reading gradient magnetic field (for 1/4 cycle), Ge ... Phase encoding gradient magnetic field, Sig ... Magnetic resonance echo train signal, TE ... Echo time, TR ... Repeat time.

Claims (8)

[Claims] 1. When a nuclear magnetic resonance signal is generated, the gradient polarity of a signal reading gradient magnetic field is periodically inverted and applied, and is continuously generated by inverting the gradient polarity of the signal reading gradient magnetic field. In an inspection method using nuclear magnetic resonance for reconstructing a chemical shift image from an echo signal after passing through a bandpass filter, the bandpass filter makes its passband substantially proportional to the applied strength of the gradient magnetic field for signal readout. An inspection method using nuclear magnetic resonance characterized in that 2. The set band Δf (t) of the bandpass filter at time t is γ when the intensity of the readout gradient magnetic field at time t is Gr (t) and the imaging field of view is Lx.
The inspection method using nuclear magnetic resonance according to claim 1, wherein the inspection method is changed according to / (2π) × | Gr (t) | × Lx. 3. The band pass filter is composed of a plurality of band pass filters having different pass bands, one of which is selected according to the applied strength of the signal reading gradient magnetic field. An inspection method using the described nuclear magnetic resonance. 4. The nuclear magnetic resonance according to any one of claims 1 to 3, wherein when the nuclear magnetic resonance signal is detected, the sampling rate is changed to perform digitization. Inspection methods. 5. An examination using nuclear magnetic resonance according to claim 1, wherein digitization is performed at a constant sampling rate when detecting the nuclear magnetic resonance signal. Method. 6. When a nuclear magnetic resonance signal is generated, the gradient polarity of a gradient magnetic field for signal reading is periodically inverted and applied, and continuously generated by reversing the gradient polarity of the gradient magnetic field for signal reading. In an inspection apparatus using nuclear magnetic resonance for reconstructing a chemical shift image from an echo signal after passing through a bandpass filter, the bandpass filter is such that its passband is approximately proportional to the applied intensity of the signal readout gradient magnetic field. The inspection apparatus using nuclear magnetic resonance is characterized in that it is configured so as to change. 7. The setting band Δf (t) of the bandpass filter at time t is γ when the intensity of the readout gradient magnetic field at time t is Gr (t) and the imaging field of view is Lx.
7. The inspection apparatus using nuclear magnetic resonance according to claim 6, wherein the inspection apparatus is configured to change in accordance with / (2π) × | Gr (t) | × Lx. 8. The bandpass filter is composed of a plurality of bandpass filters having different passbands, one of which is selected according to the applied intensity of the signal readout gradient magnetic field. An inspection apparatus using the described nuclear magnetic resonance.