JP2564428B2 - Nuclear magnetic resonance equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures nuclear magnetic resonance (hereinafter referred to as "NMR") signals from hydrogen, phosphorus, etc. in an object, and visualizes nuclear density distribution and relaxation time distribution. The present invention relates to a nuclear magnetic resonance apparatus.

[0002]

2. Description of the Related Art Conventionally, in a nuclear magnetic resonance apparatus (hereinafter referred to as an MR apparatus), various head coils or abdominal coils surrounding a region of interest of a subject (for example, a person), influence of movement of the heart, etc. Examination and imaging of the subject have been performed using a surface coil that is difficult to receive.

The surface coil has higher sensitivity than the head coil and the abdominal coil, but the field of view is limited, and when the wide area such as the spine is inspected, the surface coil is moved, There has been a problem that it takes time to capture images several times.

On the other hand, a plurality of surface coils are appropriately overlapped and arranged so that each surface coil does not mutually couple with an adjacent surface coil, and the NMR signals received by the surface coils are synthesized. Therefore, there is a method of substantially widening the field of view. The principle of this method is described in Japanese Patent Publication No. 2-500175, Japanese Patent Publication No. 2-3432, or Magnetic Resonance in Medicine (mag).
network resonance in medicin
e) 16 volumes 192 to 225 (1990).

[0005]

In the above-mentioned conventional technique, the signal-noise ratio (S / N) is not improved when the probe outputs are simply added and detected. Therefore, FIG. 2 is used to increase the S / N. Moreover, as in the conventional example, it is necessary to independently detect a plurality of probe outputs. Therefore, a plurality of signal processing systems are required, and there is a problem that the device is complicated, large in size, and expensive. Further, when the probe output is time-sequentially divided and processed by one signal processing system, although these problems can be solved, the imaging time becomes long and a practical problem occurs. The present invention proposes a signal addition system with improved S / N and provides a practical MR device using the same.

[0006]

The basic feature for achieving the above object is to detect an RF signal of a living body from a high frequency probe having a plurality of output terminals in an MR apparatus.
In the device, the plurality of output high-frequency signals are amplified by a plurality of amplifiers corresponding to the respective outputs, and then each of them is subjected to frequency conversion.
Lowering the signal frequency by a plurality of frequency converters for conversion,
Next, means for filtering each of the low-frequency signals by a plurality of band-pass filters and then adding the plurality of signals is provided.

[0007]

The S / N is improved because the low-frequency signals are amplified by a plurality of amplifiers corresponding to the respective outputs, the low-frequency signals are filtered by a plurality of band-pass filters, and then the signals are added.

[0008]

The present invention will be described in detail below with reference to the embodiment shown in FIG. FIG. 1 shows a circuit for simultaneously detecting and adding output signals 1-1 to 1-3 from three high frequency probes. Output signals of coils 15-1 to 15-3 1-
1-1 to 3 have a first signal frequency, which exactly corresponds to the Larmor frequency of biological nuclei. The Larmor frequency is determined by the magnetic field strength of the MR device and the nucleus of interest in the living body.

This will be described with reference to FIG. Taking a vertical magnetic field type MR device as an example, the magnetic field strength is the gradient G ₀ of the illustrated static magnetic field (in the z direction) and a z-direction magnetic field having an intensity distribution in the direction x orthogonal to the static magnetic field. It is an added value of the magnetic field strength G (x). Larmor frequency f at this time
Is

Is a [number 1] f = γ (G (x) + G 0) / 2π ... ( number 1). γ is the gyromagnetic ratio peculiar to the nucleus, and is 267 (MHz / T) for protons. G (x) has a constant inclination with the center of the field of view being 0, and this inclination a is, for example, 0.1 (mT / m) to 10 (mT / m) (or 0.
It is about 01 G / cm to 1 G / cm). If the visual field in the x direction is L (m), the band Δf of the detected MR signal is

## EQU00002 ## .DELTA.f = .gamma.aL / 2.pi. ( Equation 2) .

Therefore, at a static magnetic field strength of 0.2 (T), for example, a relatively weak gradient magnetic field, for example, a magnetic field strength of 0.5.
When detecting protons using (mT / m), the imaging field of view is +0.3 (m) to -0.3 (m), 0.6 (m) in total.
Then, the frequency of the MR signal is 12.75 (kHz) centered on 8.5 (MHz).

As shown in FIG. 3, the field of view of the coil 15
When the image is equally divided by the three surface coils -1 to 15-3, the center frequencies of the signals 1-1 to 1-3 from each coil are 8.49575 (MHz) from (Equation 1 ),
8.50000 (MHz), 8.50425 (MHz)
Is. And each frequency band is 4.25 than (the number 2) (k
Hz). That is, when the signal detection spaces of the plurality of probes are different, the corresponding gradient magnetic field strengths are different, and the frequency of the high-frequency signal to be detected and the first frequency are slightly different in each coil.

Although the vertical magnetic field method has been described above, it goes without saying that the same can be applied to the frequency of the detection signal in the case of the horizontal magnetic field method.

Returning to FIG. 1, the output signals 1-1 to 1-3 are amplified by the amplifiers 2-1 to 2-3, respectively. The gain of the amplifier is typically about 20 (dB) to 50 (dB). The output signals 7-1 to 7-3 of the amplifier are converted into signals 8-1 to 8-3 having the second signal frequency by the first frequency converters 3-1 to 3-3, respectively. This second signal frequency has a close relationship with the band of the band pass filter in the subsequent stage, and in the present invention, this frequency is set to a sufficiently low frequency so that the filter band described later becomes a practical band.

As an example, the output 12- of the high frequency generator 14
The frequencies of 1, 12-2 and 12-3 are 7.495 respectively.
75 (MHz), 7.50000 (MHz), 7.50
425 (MHz) and all the outputs 8-1 from the frequency converter <br/> unit 3-1 3-3 8-3 by inputting a reference signal of the from the frequency converter 3-1 3-3 1
(MHz). About 16 (MHz) generated at this time
Since the frequency of the harmonics of 1 differs greatly from 1 (MHz), it can be easily removed by using a low-pass filter (not shown) as well known, if necessary. Output 8
Frequency band from -1 8-3, as obtained from equal to the bandwidth of the signal 1-1 to 1-3 (number 2) 4.25 (k
Hz). Next, the outputs 8-1 to 8-3 are filtered by the bandpass filters 4-1 to 4-3, respectively. It is desirable that the band of the band pass filter be set to be approximately the same as or slightly wider than the signal band. The center frequency of the bandpass filter is the same as the signal frequency.

[0015] In the present embodiment be the same center frequency of the plurality of band pass filters for the conversion to the second frequency to each other having the same center frequency with each other signal group different signal frequencies from each other at a first frequency converter It is sufficient to make a plurality of filters having the same characteristics, which is preferable in manufacturing the device. However, the present invention is not necessarily limited to this. The Q value of the bandpass filter is generally represented by f / Δf. In this embodiment,

## EQU00003 ## Q = 1 (MHz) /4.25 (kHz) = 235 (Equation 3) , which can be easily created by a generally known technique such as the bandpass filter circuit shown in FIG. it can. In this embodiment, as described above, the first frequency is set to the sufficiently low second frequency.
Q given by (Equation 3) because the frequency is converted to
The value is easy to realize.

Returning to FIG. 2, the description will be continued. Second filtered by band pass filter 4-1 to 4-3
The signals 9-1 to 9-3 having the frequency of 1 to 3 are transmitted by the second frequency converters 5-1 to 5-3 to the signal group 1 having the third signal frequency.
It is converted again from 0-1 to 10-3. Here, the third signal frequencies are different from each other as necessary.
That is, the outputs 13-1, 13-2 of the high frequency generator 14,
13-3 frequencies of 27.49575 (MH
z), 27.50000 (MHz), 27.5042
5 (MHz) and frequency converters 5-1 to 5-
By inputting as the reference signal of No. 3, the outputs 10-1 to 10-3 of the frequency converters 5-1 to 5-3 are 26.49575 (MHz) and 26.50000 (M, respectively).
Hz) becomes 26.50425 (MHz). Here, the outputs 10-1 to 10-3 are the first signal frequency and the second signal frequency.
It is desirable that they are sufficiently different to eliminate interference with the signal frequency of. However, it goes without saying that they may be the same if the mutual interference between the amplifiers can be made sufficiently small. In the present embodiment, the case where the frequency is set to twice the first signal frequency or more is shown. The difference in frequency between the reference signals of the frequency converters 5-1 to 5-3 is made to coincide with the difference in frequency between the output signals 1-1 to 1-3 described above. This is also equal to the difference between the center values of the Larmor frequencies detected by the coils 15-1 to 15-3 shown in FIG. This setting is suitable for MR image reproduction as described later. Returning to FIG. 2 again, the signal groups 10-1 to 10-3 of the third signal frequency
Are added by the adder 6. Resulting signal 11
Has a frequency band of 12.75 (kHz) and a center frequency of 2
It becomes a signal of 6.5 (MHz). The noise per frequency of the added signal is sufficiently small, and there is no increase in noise due to signal addition. This is the added signal group 10-1 to 10-3.
However, this is because the noise in each unnecessary region has already been removed by the bandpass filter groups 4-1 to 4-3.

That is, for example, even if the addition is performed in an analog manner, the S / N ratio of the added signals is good. Therefore, although not shown in the figure, the signal after addition is one A /
The D converter can perform analog / digital conversion, and the digital signal can be processed by various digital signals.
In this case, the number of A / D converters can be significantly reduced as compared with the conventional example, and the device can be downsized and the cost can be reduced.

Taking a vertical magnetic field method as an example, a two-dimensional Fourier transform method on the xy plane is considered as an MR image reconstruction method with the static magnetic field direction as z. The frequency encode direction is the x direction and the phase encode direction is the y direction.
In this case, the signal 11 is subjected to the first Fourier transform by a signal processing device (not shown) in the subsequent stage. As a result,
A signal corresponding to each frequency gives position information in the x direction.
In this embodiment, the added signal 11 is the signal 10-
Since 1 to 10-3 are shifted from each other in the frequency domain, they are given as different position information at the time of the Fourier transform. In particular, the difference between the frequency center values of the signals 10-1 to 10-3 before addition exactly matches the difference between the frequency of the output signals 1-1 to 1-3. Therefore, coil 15-
1-15-3 is equal to the difference between the detected Larmor frequency center values, which accurately represents the relative position of the coil signal detection. Therefore, the value obtained by Fourier transforming the addition signal 11 accurately gives the relative position information of the plurality of coils 15-1 to 15-3. The position information in the phase encode direction is obtained by adding the value obtained by the first Fourier transform to the second value in the phase direction.
Is given by the Fourier transform of. As a result, an MR image in the xy direction is obtained. The reconstructed image has a wide field of view and a high S / N, since the signal 12 to be signal-processed is sufficiently denoised and contains signal components in a plurality of spatial regions.

As is clear from the above description, according to the present invention,
Detected nuclear gyromagnetic ratio γ and gradient of instrument gradient
a, the output signal 12 of the high frequency generator 14 depending on the visual field L
-1 to 12-3 and 13-1 to 13-3 frequencies and
The characteristics of the band pass filters 4-1 to 4-3 are determined.
You. That is, the adjacent first frequencyconversionReference signal between instruments
The difference between the frequencies of signals (eg 12-1 and 12-2)df
₁ Is

## EQU00004 ## df ₁ = Δf / n = γaL / 2πn ( Equation 4) Here, the imaging field of view is frequency-enhanced with n coils.
It was assumed that the code direction was divided into n equal parts. The bandwidth Δf ′ of the band pass filters 4-1 to 4-3 is

[Number 5 is a Δf '= Δf / n = γaL / 2πn = df 1 ... ( number 5). That is, the frequency difference df ₁ of the reference signals (for example, 12-1 and 12-2) between the adjacent first frequency converters .
And the bandwidth Δf ′ of the band pass filters 4-1 to 4-3.
Is desirable to be equal. As will be described later, when 10% to 20% of the field of view of each coil are overlapped with each other when the coils are adjacent to each other,

## EQU6 ## It is desirable that 0.8 × Δf'≤df ₁ ≤0.95 × Δf ' ( Equation 6) . Furthermore, the first frequency of each sequence
The frequency difference df2 between the reference signal of the converter and the reference signal of the second frequency converter (for example, 12-1 and 13-1) is equal between the series. That is, 12-1 and 13-1, 12-
The frequency differences between 2 and 13-2 and 12-3 and 13-3 are all equal.

As a more general case, n coils 1
When the fields of view from 5-1 to 15-n are different, the above description is as follows. That is, the reference signal of the i-th first frequency converter , the frequency f _1i of 12-i,

[Equation 7] and _{f 1i = A + f i ...} ( 7). Where A is an arbitrary frequency and f _i is the center signal frequency of the i-th coil. The bandwidth Δf ′ _i of the bandpass filter 4- _i is

[ Formula 8] Δf ′ _i = γa L _i / 2π ( Formula 8) L _i is the field size in the frequency encoding direction of the i-th i-th coil.

Reference signal of the second frequency converter , 13-i
Frequency f _2i of

[Equation 9] f _2i = B- f _i is the ... (number 9), where B is an arbitrary frequency. Furthermore, in order for the Q values of the band pass filters 4-1 to 4-n to fall within a practical range, for example, 50 to 300,

[ Expression 10] Since 50 Δf ′ _i ≦ | A | ≦ 300 Δf ′ _i ( Expression 10) is required,

Equation 11] _{50γa L i / 2π ≦ | A} | ≦ 300γa L i / 2π ... ( number 11) is a value to be taken for A. In the above example, A = -1
(MHz) and Δf ′ _i = 4.25 (kHz).
When (Equation 10) is calculated, 50 × 4.25 (kHz) ≦ | −1 (MHz) | ≦ 30
0 × 4.25 (kHz) 0.2125 (MHz) ≦ 1 (MHz) ≦ 1.275
(MHz), and it is confirmed that A is a possible value.

Next, a block diagram of the structure of the MR device, FIG.
A method of controlling the high frequency generator and the band pass filter will be described using. The subject 16 is placed in the static magnetic field created by the magnet 19 operated by the static magnetic field generator 23. The gradient magnetic field coil 18 generates a gradient magnetic field and the excitation high frequency pulse generator 21 generates a gradient magnetic field and a high frequency magnetic field, respectively, which act on the subject. The coil unit 15 including a plurality of coils receives a high frequency magnetic field signal (MR signal) from a subject.

This signal is processed and added by the high-frequency signal processing / adding unit 20 shown by 1 to 14 in FIG. 1, and then image processing and signal correction are performed by the signal processing unit 22 and the MR image (MRI, MRS, MRIS, etc.) is displayed on the display unit 25. ) Is displayed.

Static magnetic field generator 23, gradient magnetic field generator 24,
The excitation high frequency pulse generator 21, the high frequency signal processor / adder 20, the signal processor 22, and the display 25 are controlled by the controller 26. In particular, the control unit 26 mutually optimizes and controls the gradient magnetic field strength and the parameters at the time of high frequency signal processing. That is, since the gradient magnetic field strength is generally determined by factors other than the filter of the present invention, for example, the imaging sequence, the imaging speed, and the visual field, the parameters of the high-frequency signal processing unit described in the present invention according to the selected gradient magnetic field strength and visual field, For example, the bandwidth and center frequency of a bandpass filter,
The signal frequency of the high frequency generator can be set arbitrarily. In this case, a variable band filter may be used as the filter, or a plurality of filters may be provided for each channel and switched appropriately.

As the coil used in this embodiment, for example, a phased array coil can be used. It is also applicable to saddle type coils, birdcage coils, slotted tube resonator coils, etc.

In the above description, the vertical magnetic field method is taken as an example,
Although described, it is clear that the present invention can be applied to the horizontal magnetic field method. Further, although the magnetic field strength of 0.2T has been described, the present invention can be applied to other magnetic field strengths. The gradient magnetic field strength can be applied to strengths other than those described in the examples. Furthermore, the configuration diagram of the MR device is an example, and the present invention can be applied to other configurations.

[0027]

According to the present invention, in an MR device for detecting a high frequency signal from a subject by a high frequency probe having a plurality of output terminals, a plurality of amplifiers corresponding to the respective output high frequency signals. After amplification with
Since the signal frequency is lowered by a plurality of frequency converters for frequency conversion , each of the low frequency signals is filtered by a plurality of band pass filters, and then the plurality of signals are added, a high S / N images are obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a device configuration of an embodiment of the present invention.

FIG. 2 is a block diagram showing a device configuration of a conventional example.

FIG. 3 is an operation explanatory diagram of the embodiment of the present invention.

FIG. 4 is a circuit diagram of a bandpass filter.

FIG. 5 is a block diagram showing a device configuration of an embodiment of the present invention.

[Explanation of Codes] 1-1 to 1-3 ... Output signal from high-frequency probe, 2-
1-2-3 ... Amplifier, 3-1-3-3 ... First frequency change
Exchanger, 4-1 to 4-3 ... band-pass filter, 5-1 to 5
-3 ... 2nd frequency converter , 6 ... Adder , 7-1 to 7-
3 ... Output signal of amplifier , 8-1 to 8-3 ... Second signal frequency
Signals of wave number , 9-1 to 9-3 ... Signals of second frequency , 1
0-1 to 10-3 ... Signal of third signal frequency , 11 ... Addition
Arithmetic signal , 12-1 , 12-2 , 12-3 ... High frequency generator
Output (reference signal of the first frequency converter) , 13-1,
13-2, 13-3 ... Output of high frequency generator (second frequency
Number converter reference signal) , 14 ... High frequency generator , 15 ...
Iil part , 15-1 to 15 -n ... Coil , 16 ... Subject ,
17 ... Excitation high-frequency coil , 18 ... Gradient magnetic field coil , 19
... magnet , 20 ... high-frequency signal processing adder , 21 ... high frequency excitation
Wave pulse generator , 22 ... Signal processor display , 23 ... Magnetostatic
Field generation unit , 24 ... Gradient magnetic field generation unit , 25 ... Display unit , 26
… Control unit.

Claims (8)

(57) [Claims] 1. A nuclear magnetic resonance apparatus for detecting a high-frequency signal of a living body having a plurality of first frequencies from a high-frequency probe having a plurality of output terminals, wherein each of the output terminals is provided.
Are connected to increase the high frequency signals of the plurality of first frequencies.
Amplifiers and the amplifiers connected to each of the amplifiers
Frequency conversion of each output signal of the instrument
A first frequency converter for reducing the signal frequency to
A first frequency converter connected to each of the frequency converters.
Bandpass filtering each output signal of the instrument
Filters and respective output signals of the band pass filter
And a means for adding the . 2. Connected to each of the bandpass filters.
Further comprising second frequency conversion means,
It is special to convert each frequency of 2 to the third frequency.
The nuclear magnetic resonance apparatus according to claim 1, which is used as a characteristic. 3. The static magnetic field and the gradient of the gradient magnetic field strength have a predetermined value.
A gradient magnetic field having a value is applied, and the frequency encoding direction
Field of view detected by being divided by multiple coils
In the sounding device, connected to each of the plurality of coils
First frequency detected by each of the plurality of coils
An amplifier for amplifying a high frequency signal having a number and its amplifier
The signal of each output signal of the amplifier connected to each
A first frequency converter for reducing the frequency to a second frequency,
The first frequency converter is connected to each of the first frequency converters.
Filter each output signal of wavenumber converter
The bandpass filter and each of the bandpass filter
And a means for adding output signals
Magnetic resonance device. 4. Connected to each of the bandpass filters.
Further comprising second frequency conversion means,
To convert each of the two frequencies into a third frequency
The nuclear magnetic resonance apparatus according to claim 3, which is characterized in that. 5. The gyromagnetic ratio of nuclei to be detected is γ, where
A constant value in the frequency encoding direction of the a, i-th coil
When the visual field size is L _i ,
The bandwidth Δf ′ _i of the i-th band-pass filter
Is approximately Δf ′ _i = γaL _i / 2π
The nuclear magnetic resonance apparatus according to claim 3. 6. A reference applied to the first frequency converter.
The signal frequency of the i-th reference signal of the signals
f _1i , the center signal of the i-th coil of the plurality of coils
Signal frequency or the i-th
The center signal frequency of the coil is f _i , and the band pass filter is
The bandwidth of the i-th band-pass filter of the
When f ′ _i , 50Δf ′ _i ≦ | A | ≦ 300Δf ′
using A satisfying the _i the relationship, the relationship of f _1i = A + f _i
The method according to claim 1 or claim 3, characterized in that
Nuclear magnetic resonance apparatus. 7. The gyromagnetic ratio of nuclei to be detected is γ, where
A constant value in the frequency encoding direction of the a, i-th coil
7. The relation of 50γa L _i / 2π ≦ | A | ≦ 300γa L _i / 2π is satisfied when the size of the visual field is L _i, and A is satisfied.
Nuclear magnetic resonance apparatus. 8. The i-th coil of the plurality of coils
The center signal frequency or the first high frequency probe
The center signal frequency of the i-th coil is f _i , an arbitrary frequency
B of the reference signal applied to the second frequency converter
When out of the frequency of the i-th of said reference signal and f _2i
And satisfying the relationship of f _2i = B−f _i
The nuclear magnetic resonance apparatus according to claim 2 or 4.