JPH04279151A - Nuclear magnetic resonance apparatus - Google Patents

Abstract

PURPOSE:To provide an MRI device capable of photographing an MR image in a wide range by adding a plurality of high frequency MR signal with a high S/N in an MRI apparatus. CONSTITUTION:In an MRI apparatus which detects a high-frequency signal of a living body with a high-frequency probe having a plurality of output terminals, a plurality of output high-frequency signals involved are amplified with a plurality of amplifiers corresponding to the respective outputs and the signal frequency thereof are lowered with a plurality of modulators for mudulating the signal, respectively. Then, low frequency signals are filtered individually with a plurality of band pass filters, and then, the plurality of signals are added up. This enables the production of the MRI apparatus which can photograph an MR image over a wide range with a high S/N ratio.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention measures nuclear magnetic resonance (hereinafter referred to as "NMR") signals from hydrogen, phosphorus, etc. in a sample, and visualizes nuclear density distribution, relaxation time distribution, etc. Related to nuclear magnetic resonance apparatus.

0002

[Prior Art] Conventionally, in a nuclear magnetic resonance apparatus (hereinafter referred to as an MR apparatus), various head coils and abdominal coils surrounding the region of interest of a subject (for example, a person), the influence of the movement of the heart, etc. Testing and imaging of objects have been performed using surface coils that are less sensitive to radiation.

[0003] The above-mentioned surface coil has higher sensitivity than the above-mentioned head coil and above-mentioned abdominal coil, but the field of view is limited, and when examining a wide area such as the spine, it is necessary to move the above-mentioned surface coil. A problem has arisen in that images must be taken several times and it takes time.

In contrast, a plurality of surface coils are arranged with appropriate overlap so that each surface coil does not couple with an adjacent surface coil, and the NMR signals received by each of the surface coils are synthesized. There is a way to substantially widen the field of view. The principle of this method can be found in Japanese Patent Publication No. Hei 2-500175, Japanese Patent Publication No. 2-13432, or Magnetic Resonance in Medicine (magnetic resonance in medicine).
neticresonance in medicine
ine) Vol. 16, pp. 192-225 (1990).

[0005]

[Problems to be Solved by the Invention] In the above-mentioned conventional technology, the signal-to-noise ratio (S/N) does not improve if the probe outputs are simply added together for detection. As in the conventional example, each of the plurality of probe outputs had to be detected independently. As a result, multiple signal processing systems are required, making the device complex, large, and expensive. Furthermore, if the probe output is divided in time series and processed by one signal processing system, these problems can be solved, but the imaging time increases, causing a practical problem. The present invention proposes a signal addition method that improves S/N, and provides a practical MR device using this method.

[0006]

[Means for Solving the Problems] The basic feature of achieving the above object is that the MR device detects high frequency signals of a living body from a high frequency probe having a plurality of output ends.
In the device, the plurality of output high frequency signals are amplified by a plurality of amplifiers corresponding to each output, the signal frequency is lowered by a plurality of modulators that modulate each signal, and then the low frequency signal is lowered by a plurality of bandpass filters. Means is provided for filtering each of the signals and then adding the plurality of signals.

[0007]

[Operation] Each output is amplified by multiple amplifiers, each of the low frequency signals is filtered by multiple band pass filters, and then the signals are added.
/N improves.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail below using the embodiment shown in FIG. FIG. 1 shows a circuit that simultaneously detects and adds output signals 1-1 to 1-3 from three high-frequency probes. Output signal 1- of coils 15-1 to 15-3
1 to 1-3 have a first signal frequency, which exactly corresponds to the Larmor frequency of the biological nucleus. The Larmor frequency is determined by the magnetic field strength of the MR device and the atomic nucleus of interest in the living body.

This will be explained using FIG. 3. Taking a vertical magnetic field type MR device as an example, the magnetic field strength is the strength G0 of the static magnetic field (in the z direction) shown in the figure, and the gradient magnetic field that generates the z-direction magnetic field with an intensity distribution in the direction x orthogonal to the static magnetic field. This is the added value of the intensity G(x). The Larmor frequency f at this time is f=γ(G(x)+G0)/2π
(1). γ is the gyromagnetic ratio specific to the atomic nucleus, and for protons it is 26
7 (MHz/T). For example, G(x) has a constant slope with the center of the visual field set to 0, and this slope a is, for example, 0.1.
(mT/m) ~ 10 (mT/m) (or 0.01G
/cm to 1G/cm). The field of view in the x direction is L(
m), the band Δf of the detected MR signal is Δf=
γaL/2π
(2)
It is.

Therefore, for example, at a static magnetic field strength of 0.2 (T), 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), a total of 0.6 (m).
If so, the frequency of the MR signal will be a frequency band of 12.75 (kHz) centered around 8.5 (MHz).

As shown in FIG. 3, the imaging field is
-1 to 15-3, the center frequency of the signals 1-1 to 1-3 from each coil is 8.49575 (MHz), respectively, from equation (1). 8
．． 50000 (MHz), 8.50425 (MHz). Also, each frequency band is 4.25 (kHz) from equation (2).
z). That is, when the signal detection spaces of a plurality of probes are different, the corresponding gradient magnetic field intensities are different, and the frequency of the detected high-frequency signal, the first frequency, is slightly different for each coil.

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

Returning to FIG. 1, output signals 1-1 to 1-3 are amplified by amplifiers 2-1 to 2-3, respectively. Typically, the gain of the amplifier is about 20 (dB) to 50 (dB). The output signals 7-1 to 7-3 of the amplifiers are modulated into signals 8-1 to 8-3 at second signal frequencies by first frequency modulators 3-1 to 3-3, respectively. This second signal frequency has a close relationship with the band of the subsequent band-pass filter, and in the present invention, this frequency is set to a sufficiently low frequency so that the filter band, which will be 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 each 7.495
75 (MHz), 7.50000 (MHz), 7.50
425 (MHz) and by inputting these as reference signals of frequency modulators 3-1 to 3-3, outputs 8-1 to 8-3 of frequency modulators 3-1 to 3-3 are all 1.
(MHz). Approximately 16 (MHz) generated at this time
The harmonics of 1 (MHz) have a significantly different frequency, so if necessary, as is well known, a low-pass filter (
(not shown). Output 8-
The frequency band from 1 to 8-3 is equal to the band of signals 1-1 to 1-3, and is 4.25 (kHz) as obtained from equation (2).
z). Next, the outputs 8-1 to 8-3 are filtered by 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 the signal band, or slightly wider. Also, the center frequency of the bandpass filter is made the same as the signal frequency.

In this embodiment, since the first frequency modulator modulates a group of signals having different signal frequencies to a second frequency having the same center frequency, the center frequencies of the plurality of band-pass filters are the same. , it is sufficient to create a plurality of filters with the same characteristics, which is preferable in terms of device manufacturing. However, the present invention is not necessarily limited to this. The Q value of a bandpass filter is generally expressed as f/Δf. In this example, Q=1(MHz)/4.25(kHz)=235
(3), and can be easily created using generally known techniques, such as the bandpass filter circuit shown in FIG. 4, for example. In this embodiment, as described above, the first frequency is frequency-converted to the sufficiently low second frequency, so the Q value given by equation (3) is a value that is easy to realize.

[0016] Returning to FIG. 2 again, the explanation will be given. The second filter filtered by bandpass filters 4-1 to 4-3
The signals 9-1 to 9-3 at the frequencies are converted into the signal group 1 at the third signal frequency by the second frequency modulators 5-1 to 5-3.
It is modulated again from 0-1 to 10-3. Here, the third signal frequencies are different from each other as required. That is, the outputs 13-1, 13-2 of the high frequency generator 14,
13-3 frequency to 27.49575 (MHz
), 27.50000 (MHz), 27.50425
(MHz) and these are frequency modulators 5-1 to 5-3.
frequency modulator 5 by inputting it as a reference signal of
-1 to 5-3 outputs 10-1 to 10-3 are 26.49575 (MHz) and 26.50000 (MHZ), respectively.
z) becomes 26.50425 (MHz). Here, it is desirable that the outputs 10-1 to 10-3 are sufficiently different to eliminate interference with the first signal frequency and the second signal frequency. However, it goes without saying that they may be the same as long as the mutual interference between the amplifiers can be made sufficiently small. In this embodiment, a case is shown in which the frequency is set to twice or more the first signal frequency. The difference in frequency between the reference signals of the frequency modulators 5-1 to 5-3 is made to match the difference in frequency between the output signals 1-1 to 1-3 described above. This is also equivalent to the difference in 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 will be 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. The resulting signal 11 has a frequency band of 12.75 (kHz) and a center frequency of 26
．． 5 (MHz) signal. The noise per frequency of the added signal is sufficiently small, and there is no increase in noise due to signal addition. This is because the signal groups 10-1 to 10-3 to be added have already been subjected to noise removal in unnecessary regions, respectively, by the band-pass filter groups 4-1 to 4-3.

That is, for example, even if 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/D
Analog/digital conversion can be performed using a converter, and the digital signal can be subjected to various digital signal processing. In this case, the number of A/D converters can be significantly reduced compared to the conventional example, and the device can be made smaller and cheaper.

Taking the perpendicular magnetic field system as an example, and assuming that the direction of the static magnetic field is z, consider, for example, a two-dimensional Fourier transform method on the xy plane as an MR image reconstruction method. The frequency encoding direction is set in the x direction, and the phase encoding direction is set in the y direction. In this case, the signal 11 is subjected to a first Fourier transform by a subsequent signal processing device (not shown). As a result,
Signals corresponding to each frequency give position information in the x direction. In this embodiment, the added signal 11 is the signal 10- before addition.
Since the numbers 1 to 10-3 are shifted from each other in the frequency domain, they are given as different position information during the Fourier transform. In particular, the mutual difference in frequency center values of the signals 10-1 to 10-3 before addition exactly matches the mutual frequency difference of the output signals 1-1 to 1-3. Therefore, coil 15-1
~15-3 is equal to the difference between the center values of the detected Larmor frequencies, which accurately represents the relative position of the signal detection of the coil. Therefore, the value obtained by Fourier transforming the summed signal 11 provides accurate relative position information of the plurality of coils 15-1 to 15-3. Note that the position information in the phase encoding direction is given by further performing a second Fourier transform on the value obtained by the first Fourier transform in the phase direction. As a result, an MR image in the x-y direction is obtained. Since the signal 12 subjected to signal processing has been sufficiently removed from noise and contains signal components of a plurality of spatial regions, the reconstructed image has a wide field of view and a high S/N.

As is clear from the above explanation, in the present invention, the output signal 12 of the high frequency generator 14 is determined by the gyromagnetic ratio γ of the atomic nucleus to be detected, the gradient a of the gradient magnetic field of the device, and the field of view L.
The frequencies of -1 to 12-3 and 13-1 to 13-3 and the characteristics of bandpass filters 4-1 to 4-3 are determined. That is, the frequency difference df1 between the reference signals (for example, 12-1 and 12-2) between adjacent first frequency modulators is df1=Δf/n=γaL/2πn
(4). Here, it is assumed that the imaging field of view is divided into n equal parts by n coils in the frequency and encoding directions. The bandwidth Δf' of the bandpass filters 4-1 to 4-3 is Δf'=Δf/n=γaL
/2πn=df1
(5). In other words, it is possible to make the frequency difference df1 between the reference signals (for example, 12-1 and 12-2) between adjacent first frequency modulators equal to the bandwidth Δf' of the bandpass filters 4-1 to 4-3. desirable. In addition, as described later, when multiple coils are overlapped with each other by 10% to 20% of the field of view of each coil, 0.8
×Δf'≦df1≦0.95×Δf'
(6) is desirable. Furthermore, the reference signal of the first frequency modulator of each series and the reference signal of the second frequency modulator (for example, 12-1 and 13-1)
The frequency difference df2 between them is equal between each series. That is, the differences in frequency between 12-1 and 13-1, 12-2 and 13-2, and 12-3 and 13-3 are all equal.

As a more general case, n coils 1
If the fields of view from 5-1 to 15-n are different, the above description becomes as follows. That is, the frequency f1i of the i-th first frequency modulator reference signal 12-i is f1
i=A+fi
(7
). Here, A is an arbitrary frequency, and fi is the center signal frequency of the i-th coil. Bandpass filter 4-i
The bandwidth Δf'i of is Δf'i=γaLi/2π
(8). Li is the field of view size of the i-th i-th coil in the frequency encoding direction.

Reference signal of second frequency modulator, 13-i
The frequency f2i of is f2i=B−fi

(9), where B is an arbitrary frequency. Furthermore,
In order for the Q value of the bandpass filters 4-1 to 4-n to fall within a practical range, for example 50 to 300, 50Δf'i≦|A|≦300Δf'i
(10) is required, so 50γaLi/2π≦|A|≦300γaLi/2π
(11) is the possible value of A. In the above example, A=-1 (MHz), Δf'i=4.2
5 (kHz). When formula (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.
We will explain how to control the high frequency generator and bandpass filter using . The subject 16 is placed in a static magnetic field generated by a magnet 19 operated by a static magnetic field generator 23 . Further, the gradient magnetic field coil 18 and the excitation high-frequency coil 17 generate a gradient magnetic field and a high-frequency magnetic field by an excitation high-frequency pulse generator 21, respectively, and act on the subject. A coil section 15 made up of a plurality of coils receives a high frequency magnetic field signal (MR signal) from a subject.

This signal is processed and added in the high frequency signal processing adder 20 consisting of 1 to 14 in FIG.
2, the image is processed and the signal is corrected, and the display unit 25 displays the MR image (M
RI, MRS, MRIS, etc.) are displayed.

Static magnetic field generating section 23, gradient magnetic field generating section 24,
Excitation high frequency pulse generation section 21, high frequency signal processing addition section 2
0, the signal processing section 22 and the display section 25 are controlled by a control section 26. In particular, the control unit 26 mutually optimizes and controls the gradient magnetic field strength and the parameters during high frequency signal processing as described above. That is, since the gradient magnetic field strength is generally determined by factors other than the filter of the present invention, such as the imaging sequence, the imaging speed, and the field of view, the parameters of the high-frequency signal processing unit described in the present invention, For example, it is possible to arbitrarily set the bandwidth and center frequency of the bandpass filter, the signal frequency of the high frequency generator, etc. 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 as appropriate.

The coil used in this embodiment may be, for example, a phased array coil. It can also be applied to saddle-shaped coils, birdcage coils, slotted tube resonator coils, etc.

[0026] In the above explanation, the perpendicular magnetic field method is taken as an example.
Although described above, it is clear that the present invention is also applicable to the horizontal magnetic field method. Further, although the description has been made using a magnetic field strength of 0.2 T, the present invention can be applied to other magnetic field strengths. It is also possible to apply gradient magnetic field strengths other than those described in the examples. Further, the configuration diagram of the MR apparatus is one example, and the present invention can be applied to other configurations.

[0027]

According to the present invention, in an MR device that detects high frequency signals from a subject using a high frequency probe having a plurality of output terminals, the plurality of output high frequency signals are connected to a plurality of amplifiers corresponding to each output. After amplification, the signal frequency is lowered using multiple modulators that modulate each low-frequency signal, and then each of the low-frequency signals is filtered using multiple band-pass filters, and then the multiple signals are added, allowing a wide field of view. An image with high S/N can be obtained.

[0028]

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an apparatus according to an embodiment of the present invention.

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

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

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

FIG. 5 is a block diagram showing the configuration of an apparatus according to an embodiment of the present invention.

Claims (8)

[Claims] Claim 1: A nuclear magnetic resonance apparatus for detecting biological high frequency signals having a first frequency from a high frequency probe having a plurality of output ends, each of which outputs at least the plurality of output high frequency signals having the first frequency. After amplification by a plurality of amplifiers corresponding to A nuclear magnetic resonance apparatus having means for adding the plurality of signals. Claim 2: The nuclear magnetic resonance apparatus according to claim 1,
A nuclear magnetic resonance apparatus comprising a second frequency modulation means after the filtering and modulates each of the plurality of modulation signals of the second frequency to a third frequency. 3. A nuclear magnetic resonance apparatus having a gradient magnetic field having at least a gradient a of gradient magnetic field strength, and in which a field of view in the frequency encoding direction is divided and detected by a plurality of coils,
When detecting an atomic nucleus with a gyromagnetic ratio γ, a first plurality of coils modulates each output high-frequency signal of at least a first frequency after amplifying the output high-frequency signal of the plurality of coils with a plurality of amplifiers corresponding to each output. A nuclear magnetic resonance apparatus comprising means for lowering the signal frequency to a second frequency by a modulator, then filtering each of the low frequency signals with a plurality of bandpass filters, and then summing the plurality of signals. 4. The nuclear magnetic resonance apparatus according to claim 3,
A nuclear magnetic resonance apparatus comprising a second frequency modulation means after the filtering and modulates each of the plurality of modulation signals of the second frequency to a third frequency. 5. The bandwidth Δf'i of the i-th bandpass filter among the bandpass filters according to claim 3 is approximately:
Δf'i=γaLi/2π, where γ is the gyromagnetic ratio of the detected nucleus, ,L
A nuclear magnetic resonance apparatus characterized in that i is a field size in the frequency encoding direction of the i-th coil. Claim 6: The apparatus according to claim 1 or 3,
The signal frequency f1i of the i-th reference signal among the reference signals given to the first frequency modulator is set to the center signal frequency f of the i-th coil that is a component of the high-frequency probe.
As i, f1i=A+fi, A is the following relationship, 50Δf'i≦|A|≦300Δf'i where Δf'i
A nuclear magnetic resonance apparatus characterized in that satisfies the bandwidth of the i-th bandpass filter among the filters. 7. In the apparatus according to claim 6, A is the following relationship: 50γaLi/2π≦|A|≦300γaLi/2π where γ is the gyromagnetic ratio of the nucleus to be detected, a is the gradient of the gradient magnetic field strength, A nuclear magnetic resonance apparatus characterized in that Li satisfies the field of view size in the frequency encoding direction of the i-th coil. 8. The apparatus according to claim 2 or 4, wherein the frequency f2i of the i-th reference signal among the reference signals of the second frequency modulator is f2i=B−fi, where B is arbitrary. A nuclear magnetic resonance apparatus characterized by a frequency of .