JPS59202050A - Nuclear magnetic resonance video apparatus - Google Patents

Abstract

PURPOSE:To compensate for deviation between a resonance frequency and a reference frequency without using nuclide differing from an object to be measured and without controlling a static magnetic field by feedback controlling of a reference signal generator based on deviation between the resonance frequency and a reference frequency with the application of the static magnetic field. CONSTITUTION:A nuclear magnetic resonance signal generated in an object 2 to be inspected with the application of an electrostatic magnetic field from an electrostatic magnetic field generator 1 is received with a receiver coil 6 and applied to a phase detector 8 via a low noise amplifier 7 to be detected in the phase by a reference signal from a reference signal generator 3. Then, deviation between the frequency of the resonance signal thus detected in the phase and the frequency of the reference signal is detected with a frequency analyzer 9. A reference signal generator 3 by the detection signal is feedback controlled to compensate the deviation between the reference signal and the resonance signal thereby forming a nuclear magnetic resonance image at a high accuracy and a high efficiency without using nuclide differing from an object to be measured and without controlling an electrostatic magnetic field.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to nuclear magnetic resonance (NMR).
Diagnostic NMR-CT (CT: comp.
In order to measure the density and relaxation time of symmetrical atomic nuclei with high precision and efficiency, especially in order to measure the density and relaxation time of symmetrical atomic nuclei with high precision and efficiency, the resonant frequency and excitation signal of the target atomic nuclide corresponding to the static magnetic field strength are used. NMR with high precision matching with the reference signal frequency of
This relates to video equipment.

[Technical background of the invention]

A diagnostic NMR-CT device is a device that obtains image information using NMR phenomena, and in particular, the obtained image information is used for diagnosis.

In general, in NMR devices that perform measurements using NMR phenomena, the resonance frequency of the nuclide to be measured corresponds to the uniform static magnetic field strength applied to the specimen, and the frequency of the excitation signal, that is, the reference signal used to obtain the excitation signal. It is necessary to match the frequency for highly accurate measurement.

In the NMR apparatus used in conventional chemical analysis, the object to be measured is small and it is not possible to obtain a two-dimensional image such as a cross section, so the required uniform field area is also small. Therefore, in this case, in order to maintain the relationship between the static magnetic field strength and the reference signal frequency in the so-called Bohr relationship (details will be described later) over a long period of time, it is necessary to
Fluctuations in the static magnetic field are detected using the resonance signal obtained from a nearby nuclide with a resonance frequency different from that of the nuclide to be measured (this nuclide may be mixed with the sample) as a reference resonance signal. A so-called "NMR lock method" is used in which feedback is applied to the static magnetic field generator.

On the other hand, a diagnostic NMR-CT apparatus obtains a two-dimensional image, and since the subject to be measured is an indivisible living body, the required uniform magnetic field area is large. When the above-mentioned NMR locking method is applied to this diagnostic NMR-CT device, the required homogeneous magnetic field area is large, so special consideration must be given to the placement position of the reference nuclide. NMR-CT for position selection and diagnosis
This creates many problems in terms of device operability. Furthermore, since the required uniform magnetic field area is large, it is not always possible to detect errors uniformly and accurately depending on the reference nuclide described above. Therefore, it is preferable to use a method that does not use a variety of such standards.

Furthermore, in the magnetic field locking method described above, it is not easy to perform stable control because the strong magnetic field must be varied by variably controlling the static field applied to the subject. In particular, when a wall-core superconducting magnet is used in a static magnetic field generator, a magnetic field is generated in the superconducting coil in a permanent mode, that is, in a state where a so-called permanent current is passed through the coil that generates the magnetic field in an electromotive conduction state. In this state, the magnitude of this current cannot be controlled externally, and the magnetic field lock method described above must be applied.In this case, the electric current σ1t flowing through the superconducting coil gradually decreases, so the static Measures against magnetic field fluctuations are especially required.

[Purpose of the invention]

The purpose of the present invention is to compensate for the deviation between the resonant frequency and the reference frequency for generating the excitation signal without using a different nuclide from the measurement target and without controlling the static magnetic field.
An object of the present invention is to provide an imaging device that makes it possible to obtain NMR images with high accuracy and efficiency. It is characterized by frequency-analyzing the resonance signal obtained from the excited object to determine the deviation between the resonance frequency and the reference signal frequency, and feeding the result back to the reference signal generator as an error signal.

EXAMPLE Hereinafter, an example of the present invention will be explained in detail based on its basic principle.

Theoretical analysis of NMR phenomena has been established by Bloch et al., and measurement methods can be roughly divided into continuous wave methods and pulse methods. Since the pulse method is common in diagnostic NMR-CT apparatuses, etc., the pulse method will be explained here.

For the target atomic nucleus, the static magnetic field (strength) Ho is p B
Angular frequency ω determined by the following equation derived from the relationship of ohr
When a rotating magnetic field of 0 is applied at the same time as a static magnetic field Ho, it resonates with the rotating magnetic field and exhibits a phenomenon called precession of Larm or r.

ω0=γHO (1) (where ω0: resonant resonance wave number + Ho: m magnetic field strength; γ: nuclear gyromagnetic ratio determined by the nuclide) This phenomenon is called an NMR phenomenon.

FIG. 1 shows the configuration of a simple embodiment in which the present invention is applied to a diagnostic NMR-CT apparatus. Note that FIG. 1 shows the configuration directly necessary for the present invention, and a diagnostic NM that can be used in the present invention.
This figure shows a configuration that is generally common to the R-C'T equipment, and the configuration that is not shown and that is necessary for a diagnostic NMR-CT device is not directly related to the present invention, and is compatible with various diagnostic NMR-CT devices.
It may differ depending on the t-CT device.

In Fig. 1, 1 is a static magnetic field generator that generates a uniform static magnetic field Ho for the imaging target region, 2 is the object to be examined (shown schematically), and 3 is the resonance angular frequency of the symmetric nuclide in the static magnetic field Ho. When compatible with ω0, a variable frequency reference signal generator (hereinafter referred to as "SSG"), 4 generates an excitation nocellus H1 consisting of a pulsed high frequency (angular frequency ωr) signal based on the reference signal generated by SSG 7. an excitation pulse generator 5 that generates and amplifies the power; 5 a transmitter coil that applies the excitation pulse H1 generated by the excitation pulse generator 4 to the subject 2 as a rotating magnetic field for NMR excitation; A receiver coil receives the NMft signal, 7 is a low-noise amplifier that amplifies the NMR signal received by the receiver coil 6, and 8 phase-detects the amplified NMR signal with reference No. 46 having a reference angular frequency ωr generated by 5SG3. A phase detection device 9 frequency-analyzes the NMR signal detected by the phase detection device 8 to determine the deviation Δω of the resonance angular frequency ω0 from the reference angular frequency ωr.
A frequency analyzer 10 returns the frequency deviation Δω detected by the frequency analyzer 9 to the SSG 3, that is, controls the frequency of the SSG 30 according to the frequency deviation Δω to correct the frequency deviation Δω, and the SSG 3, Erection/Motherus generation area 4. This is a system control device that controls the operation of the amplification device 7, phase detection device 8, and frequency analysis device 9.

In this case, transmitter coil 5 and receiver coil 6
However, in some cases, a so-called single-coil type configuration is adopted in which these are integrated and a common coil is used to apply the p-excitation pulse H and receive the NMR signal.

In such a configuration, the pulsed high-frequency rotating magnetic field determined by the above equation (1), that is, the excitation pulse H1i)
When the magnetic moment μ generated in the target atomic nucleus by applying υ to the Ransmitsuta coil 5 returns to an equilibrium state, the free visiting conduction decrease g (FID: free 1
duction decay ~hereinafter 1'-FID
When the phase detection device 8 detects the phase of the signal (referred to as J) using the reference signal of the reference angular frequency ωr generated by the SSG 3, it is possible to convert the high frequency NMR signal (FID signal in this case) to a frequency that is easy to process. (Hereinafter, considering such a state will be referred to as ``viewing in a rotating coordinate system''). Here, in general, the angle θ at which the magnetization M rotates as shown in Figure 2 is ωO due to the resonance circumference.
In the rotating coordinate system, it is expressed by the following equation.

θ=γH,t (2) (However, t: HtO application time) As a detection system, the sensitivity is the lowest when θ=π/2 (rad). In this implementation flu, exactly θ=π
/2, but in the following, NMR-CT
Let θ be close to the π force to the extent that the sensitivity of the device is satisfied. Under such conditions, the FID signal V□0 detected at the reference angular frequency ωr by the phase detection device 8 is expressed by the following equation.

72,. fart. . (, +y+Z). −(+/T2 + j
((7JO-(77・)゛)=..(1,y+z)e
"" j(lo-(IJ・)t=-(3) (However,
”O(X+y+Z): density of the target nuclide at (X+y+Z), T2, time constant due to the spin-spin relaxation time of the target nuclide and the inhomogeneity of the magnetic field) In this equation (3), the frequency deviation Δω is expressed as Δω=1ω0
-ωrl, it can be seen that the value of the frequency deviation Δω can be found by frequency-analyzing ■ and □D using the frequency analyzer 9.

Here, as in the conventional NMR locking method, the frequency deviation Δω
Let us consider the case where the current is fed back to the static magnetic field generator 1 as a current. According to the above equation (1), the error ΔHO in the static magnetic field strength HO is ΔHo−Δω/γ (4). Therefore, the current source i to be fed back can be found by the following equation.

1=fcΔHo)=f(Δω/r) −= (5
) (However, f(I(O): Static magnetic field strength H of the normally conducting magnet
(relationship between o and current i) In the conventional tow-lock method, since a sample nuclide different from the original target nuclide is used, it is possible to always detect an error signal, and the feedback circuit is always used to detect the error signal. Can be controlled.

In the calculation using the above equation (5), the static magnetic field generator 1
Errors may be caused by the shape of the magnet, the way the coil is wound, etc. Therefore, when detecting the individual frequency difference Δω and feeding it back to the static magnetic field generator 1 as a current based on the above equation (5), continuous feedback operation is required to avoid a decrease in accuracy due to the above error factors. , the presence of the sample nuclide mentioned above is important. However, considering the operability of the NMR-CT apparatus, the presence of the sample nuclide causes undesirable problems in terms of maintenance of the nuclide, positional relationship with the subject 2, and the like.

On the other hand, in this embodiment, in order to ignore the error factor based on the above equation (5), S
Return directly to SG 3. This is the first % characteristic of the present invention. In this way, since the process according to Equation 51 described above is avoided and no error factors are mixed in, it is possible to obtain sufficient accuracy without constantly detecting and feeding back the error signal. In this embodiment, the sample nuclide described above is not required, and the target for obtaining the error signal is the diagnostic site itself of the subject 2. This is the second feature of the present invention. It is sufficient to detect the frequency individual difference Δω in the state where magnetostatic 'JA) (O is applied and to correct the SSG 3 accordingly.
Since it is not necessary to detect the sample nuclide and the sample nuclide mentioned above is not necessary, the sequence time as an NMR-e T apparatus is not affected.

That is, in the configuration shown in FIG.
At t10, information on the frequency deviation Δω obtained by the frequency analyzer 9 is given to the SSG 3, and as a result, the NMR-CT apparatus is fixed at the angular frequency ω0 determined by the above equation (3). To determine whether the frequency difference Δω is positive or negative, the detection method in the phase detection device 8 is, for example, a so-called (quadrature) two-phase detection method (QD: quadrature detection method).
This can be easily done by using ecNon).

Here, SSG 3 will be explained in detail. In this implementation, information on the frequency deviation Δω is used as an error signal in SSG 3
given to. Since this signal is the result of frequency analysis (which may be considered as fast Fourier transform), it is advantageous to treat it as a digital signal when considering the efficiency of the NMR-CT apparatus.

Considering these points, the so-called P
A configuration using a frequency synthesizer using LL (phase 1ocked 1oop) is suitable.

FIG. 3 shows the basic configuration of the SSG 3 using a PLL frequency synthesizer.

In FIG. 3, 31 is a highly stable oscillator that generates a PLL reference signal of frequency fc, 32 is a phase comparator, 33 is a low-pass filter as a loop filter, and 34 is a voltage that oscillates at a frequency f that corresponds to the input voltage Vi. A controlled oscillator (hereinafter referred to as 1-VCOJ), 35 is the frequency f. This is a programmable frequency divider that frequency-divides the input signal of 1/n at a frequency division ratio of 1/n according to the input information Δω and inputs the result to the phase comparator 32.

Briefly explaining the operation, the outputs of the stable oscillator 3 and the programmable frequency divider 35 are input to the phase comparator 32, and this closing is performed so that the frequencies f; and j'o/n of these two always match. The circuit works. Therefore, the frequency f of the output of this circuit, that is, the output of the VCO 34. and P
The LL standard reference signal frequency table 1 relationship is as shown in the following equation.

fi = fo/n -'-fo ''' n-fi - (6) That is, programmable frequency divider 350 frequency division ratio 1/n,!:
Multiple output frequency f depending on PLL reference signal frequency fi. can be set freely. Also, the output frequency f. Although the frequency stability depends on the circuit constants of each part, it can be increased to almost the same level as the stability of the highly stable oscillator 31 that generates the PLL reference signal, and by using a constant temperature crystal oscillator or the like as the oscillator 31. For example, it can be fully used as an NMR-CT device. Therefore, the frequency division setting value n of the programmable frequency divider 35 is changed by the digital error signal corresponding to the frequency deviation Δω which is the result of frequency analysis, and a frequency corresponding to the fluctuation of the static magnetic field strength Ho is outputted.

The configuration shown in Figure 3 is the most basic one, and although there are variations of the configuration depending on frequency variable steps and frequency stability requirements, configurations using PLL synthesizers are basically the same. It is. In this case, the frequency variable step of SSG 3 requires a step comparable to the resolution of frequency analysis.

Furthermore, considering the efficiency of signal transmission, it is considered appropriate that the output waveform be a sine wave.

Next, how to obtain the frequency deviation Δω will be explained.

FIG. 4 shows a timing sequence for determining the frequency deviation Δω.

As shown in Fig. 4, if a 90° pulse (high frequency excitation pulse such that θ = π/2) is applied as the excitation pulse H1 to the subject 2 in the static magnetic field Ho, immediately FID 1 or FID 2 FID signals like this can be observed. F.I.D.
l is an example of the FID signal in the case of ωr'6ω0, and F
ID2 is the FID signal in the case of ωr-ω0. The frequency spectra of these FID signals D and FID2 are as shown in FIG. The overlap torque position matches the origin.

Thus, the principle of this embodiment described above can be easily understood.

When detecting FID signals, errors due to saturation of the detection system amplifier due to 900 pulses and S/N due to the average addition method
In order to improve the signal-to-noise ratio (signal-to-noise ratio) and to avoid the effects of signal attenuation due to magnetic field inhomogeneity, many NMR devices measure signals called NMR echoes. The echo signal can also be used in this actual model. The timing sequence in this case is shown in FIG.

The object 2 in the static magnetic field Ho is placed at 90° as shown in Fig. 6.
After applying the A pulse, apply the 180°/(' pulse (high frequency excitation pulse such that θ = π), and perform frequency analysis on the echo signal obtained after that in the same way as for the FID signal. Accordingly, the frequency deviation Δω can be determined. In this case, by averaging the first echo and the second echo, the S/N can be improved.

In addition, in FIGS. 4 and 6, if a series of sequences starting from time t8 is used as the first sequence, and a sequence starting from time tb is used as the second sequence, it is possible to solve the problem when a sufficient S/N cannot be obtained with the first sequence. ps is calculated by sequentially repeating and averaging the sequences after the second sequence.
/N is improved, and the accuracy of frequency deviation Δω verification and frequency control is improved.

Note that whether to detect the FID signal or the echo signal depends on the S/N of each method and π/2 of θ in the above equation (2), as in the case of general pulse detection. If the deviation from Δθ is Δθ, the error should be determined in consideration of the fact that when a 180° pulse is used, the error will be 31θ, and this is not particularly limited in the present invention.

Next, the static magnetic field generator 1 will be explained in detail.

Of course, an air-core superconducting magnet may be used as the static magnetic field generator 1, but it is also effective to use an air-core superconducting magnet.

When using an air-core superconducting magnet, as mentioned earlier, while operating in permanent mode (persistent current state), if you want to control the current, you must cancel the permanent mode and adjust the current. It is not desirable to adjust the flow rate using an error signal, since the characteristics of the air-core superconducting magnet cannot be fully utilized. Another method for controlling the static magnetic field strength no when an air-core superconducting magnet is used in the static magnetic field generator 1 is to wind a compensation coil around the air-core superconducting magnet and adjust the current value of the coil. It is also possible to make it . However, in this case, a compensating coil and a constant current source for driving the coil are separately required, and the configuration becomes complicated.

On the other hand, if this embodiment is used, the above-mentioned external device is not required, and the corrective damping sequence can be exactly the same as when using an air-core normal-conducting magnet. It is possible to further improve the accuracy of imaging and, in turn, the accuracy of diagnosis, without complicating the configuration of the device. It will look like this: '5000 G
Considering a superconducting magnet that can generate an auss magnetic field, we assume that the change in the magnetic field in the permanent mode is 0. lppm/h, and further M
If the operating time of the CT apparatus is 8 h/day, the fluctuation of the magnetic field will be 4 mGauss/day. Assuming that the nuclide to be measured is a proton, the frequency fluctuation is approximately 17.0
Hz/day, frequency analysis resolution 20.0
If it is Hz, then in the method of canceling the permanent mode of the superconducting magnet and making adjustments, the permanent mode must be canceled every day to adjust the current value. When the method of this embodiment is applied to this problem, the half width of the tuning section in the signal amplification section is
1.9kHz (Q=150), and the allowable variable range is approximately 7
If it is 1 kHz, (71X10'/17-) approximately 41
After 76 days, it is not necessary to cancel the permanent mode. In this case the excitation pulse H.

It is thought that the above-mentioned period during which the permanent mode does not need to be released will be shortened, considering the spread of the frequency sic trams due to the nollus width and the effects of evaporation of the coolant. however,
Even if these effects are considered, if the method of this embodiment is applied to a superconducting magnet, one of the characteristics of a superconducting magnet [during permanent mode operation, the static magnetic field strength is stable and there is no need to compensate for the current value] It is clear that this advantage can be utilized for light.

As mentioned above, the conventional NMI seven-lock system can be replaced with NMR-C.
If it is applicable to the T device, it is necessary to provide a sample nuclide other than the measurement target at a position different from the subject, and operability is limited. Further, in a method in which the signal from the measurement target (subject) is returned to the static magnetic field generator as an error signal by some method, the operability is improved, but feedback cannot always be applied, so adjustment is involved. An error occurs due to the A difference factor. Furthermore, when a 6 superconducting magnet is used in the static tS lift generator, in contrast, according to this embodiment, the NMR signal from the temporary specimen (measurement target) is frequency-analyzed at a convenient timing, such as immediately before imaging. By directly feeding back the result to the SSG as an error signal, the frequency of the SSG can be made to closely follow fluctuations in the static magnetic field strength, etc. Therefore, it is possible to reduce the causes of errors without impairing the operability of the NMR-CT apparatus, and it is possible to improve the imaging efficiency and, in turn, the diagnostic stacking. Further, the control sequence of the conventional NMR-CT apparatus can be fully utilized and can be used without major changes.

Furthermore, when a superconducting magnet is used in an NR4R-CT device, there is no need to cancel the permanent mode because the SSG compensates for static magnetic field fluctuations, and there is no need to take the trouble to provide an external device such as a compensation coil.

It should be noted that the present invention is not limited to the embodiments described above and shown in the drawings, but can be implemented with various modifications within the scope of the invention.

For example, the components of the frequency analyzer 9, system control equipment 10, etc. may be implemented using an electronic instrument for image reconstruction, and may also be realized by software processing. It is good to superimpose, 7#
It is also possible to limit the region in the magnetic field and compensate for frequency deviations in the stable and stable regions.

However, the timing for performing the correction control is not limited to before the original NMR image is taken, but may be performed after the image is taken or during the image taking as necessary.

[Effects of the Invention] According to the present invention, it is possible to compensate for the deviation between the resonance frequency and the reference frequency due to fluctuations in the static magnetic field, etc., without using a nuclide different from that of the measurement target, and without controlling the static magnetic field. In addition, it is possible to provide an image capture device that can obtain NMR images with high efficiency.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, FIG. 2 is a diagram showing the movement of the magnetization vector due to the excitation pulse H1 in a rotating coordinate system for explaining the embodiment, and FIG.
The figure is a block diagram showing the configuration when the PLL synthesizer method is adopted as the reference signal generation device in the same implementation, and Figure 4 is a sequence of excitation-Δω detection sequences (FID signal Fig. 5 is a timing chart showing the sequence of frequency spectra in the same column, and Fig. 6 is a timing chart showing the sequence of excitation-Δω detection in the same column (in the case of using an echo signal). FIG. 1... Static magnetic field generator, 3... Reference signal generator (
SSG), 4... Excitation pulse generator, 5... Transmitter coil, 6... Receiver coil, 7... Low noise amplifier, 8... Phase detector, 9... Frequency analyzer , 10... system control device.

Claims (2)

[Claims] (1) Generate a nuclear magnetic resonance phenomenon in the specimen, detect the induced nuclear magnetic resonance signal, and obtain image information that reflects at least one of the spin density and relaxation time constant of a specific atomic nucleus in the specimen. A nuclear magnetic resonance imaging apparatus for obtaining a magnetic resonance imaging apparatus includes a static magnetic field generator that generates a uniform static magnetic field over substantially the entire imaging area to be applied to a subject, a reference signal having a frequency corresponding to the target atomic nuclide, and a reference signal generator capable of adjusting and controlling the generated frequency; and an excitation signal applying device that forms a high-frequency excitation signal for nuclear magnetic resonance excitation based on the reference signal output of the reference signal generator and applies it to a subject. , a receiving and detecting device that receives and detects nuclear magnetic resonance signals induced in the subject; a first control means that operates the static magnetic field generator and energizes the excitation signal applying device; and the receiving and detecting device. The frequency between the reference frequency and the resonance frequency when the nuclear magnetic resonance signal detected by the device is frequency-analyzed and the excitation signal is applied to the subject in the uniform static magnetic field by the first control means. Nuclear magnetic resonance characterized by comprising a frequency analysis means for determining the deviation, and a second control means for controlling the reference signal generator according to the frequency deviation obtained by this means and correcting the frequency deviation. Video equipment. (2) The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the static magnetic field generator is constructed using an air-core superconducting magnet.