JPH09206286A - Nuclear magnetic resonance imaging equipment - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To enable measurement and observation of the effect of an eddy current at a position symmetrical with respect to the center of a gradient magnetic field. SOLUTION: Two eddy current measurement samples 4 are arranged at positions symmetrical with respect to the center of the gradient magnetic field in the direction of the gradient magnetic field to be measured,
4 ′ is placed, an eddy current measurement pulse sequence is executed to generate an NMR signal, and the NMR signal is converted into a Fourier transformer 1.
In step 2, Fourier transform is performed to obtain frequency information, and the data processing mechanism 13 extracts the frequency information at the symmetrical position from the center of the gradient magnetic field of the two samples 4, 4 ', and displays it on the display 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear magnetic resonance imaging apparatus for performing imaging or spectroscopy measurement by utilizing the NMR (nuclear magnetic resonance) phenomenon, and more particularly, a nuclear magnetic resonance imaging apparatus having a function of measuring an eddy current generated by a gradient magnetic field. The present invention relates to a magnetic resonance imaging apparatus.

[0002]

2. Description of the Related Art In a nuclear magnetic resonance imaging apparatus (MRI apparatus), in order to observe a tomographic image of a subject, the subject is placed in a static magnetic field created by a superconducting magnet or the like, and in a plane perpendicular to the static magnetic field direction. A high-frequency magnetic field rotating at is applied to obtain an NMR signal according to the specific nuclear density distribution of the subject. Further, in order to add the spatial position information to the NMR signal and select the imaging direction and the tomographic plane of the subject, the magnetic field strength is inclined in each of the three orthogonal X, Y and Z axes superimposed on the static magnetic field. The generated gradient magnetic fields Gx, Gy, and Gz are generated in a pulse shape. The gradient magnetic field is generated by applying a current from the gradient magnetic field power supply to the gradient magnetic field coil. Since each of the gradient magnetic fields Gx, Gy, and Gz has a pulse shape, a constant temperature bath of a superconducting magnet for generating a static magnetic field (for example, a shield material that constitutes the constant temperature bath, a support member that supports the superconducting magnet, a frame, etc.) Eddy currents are induced in the conductor structure arranged around the gradient coil. Due to this eddy current, the waveform of the magnetic field actually generated becomes distorted with respect to the ideal gradient magnetic field even if the current waveforms flowing through the Gx, Gy, and Gz coils are pulse waveforms.
For example, an exponential rise and decay of the gradient field is observed during and after applying a rectangular current pulse to the gradient coil.

Therefore, in order to properly operate the MRI apparatus, it is necessary to compensate the distortion of the gradient magnetic field due to the influence of such eddy current. Therefore, in this type of MRI apparatus, a current waveform shaping circuit including an adjustable exponential function type attenuation component (usually a capacitor or a resistance) is provided in the gradient magnetic field power source, and thereby, each of Gx, Gy, and Gz coils is provided. An ideal gradient magnetic field is obtained by setting the current waveform to be flowed in consideration of the effect of eddy current. The effect of eddy current must be measured for eddy current compensation adjustment by adjusting the current waveform by the current waveform shaping circuit.

Therefore, conventionally, in order to measure the distortion of the gradient magnetic field due to the influence of the eddy current, a small NMR operation sample is placed at a certain distance from the point of zero gradient in the gradient magnetic field coil, and at least one of the samples is placed. Applying a gradient pulse along an axis of one dimension, and within a predetermined time after applying the gradient pulse, irradiating the sample with an RF excitation pulse to generate an NMR signal and applying the gradient pulse; The phase of the NMR signal is measured as a function of the total time of, the measured phase is related to the impulse response of the gradient pulse, and the impulse response is used to observe the eddy current due to the gradient magnetic field ( JP-A-62-18
9057).

However, in the shapes of the superconducting (static magnetic field) magnet and the gradient magnetic field coil, or in the actual manufacture of both, an error that is mechanically asymmetrical is unavoidable, so that the position is always symmetrical with respect to the center of the magnetic field system. Since the magnetic field due to the same eddy current is not always generated,
Using the adjustable exponential decay component of the current waveform shaping circuit installed in the gradient magnetic field power supply, measuring the effect of eddy currents generated at equal distances from the center of the gradient magnetic field and observing the balance of each, It is necessary to shape the current applied to the gradient coil. Therefore, conventionally, in the direction of the gradient magnetic field system to be adjusted, a small sample is placed at a position several cm away from the center, and the above measurement is performed. Puts the sample at the same distance in the opposite direction from the center, and again performs the same measurement to make eddy current compensation adjustments so that both are in a well-balanced state.

[0006]

However, the measurement / observation of the influence of the eddy current and the eddy current compensation adjustment are repeatedly performed while the sample is alternately replaced from the center of the gradient magnetic field to a symmetrical position as in the conventional case. Therefore, it takes time and effort to measure or compensate the eddy current, and since it is not possible to measure and observe the effect of the eddy current at the same distance from the center of the gradient magnetic field at the same time, it is very difficult to adjust the left and right balance. There was a problem.

The present invention has been made in view of the above circumstances, and simultaneously measures and observes the influence of the eddy current at a position symmetrical with respect to the center of the gradient magnetic field without replacing the eddy current measuring sample with the right and left alternately. It is an object of the present invention to provide a nuclear magnetic resonance imaging apparatus capable of performing well-balanced eddy current compensation adjustments at both positions all at once.

[0008]

In order to achieve the above object, in the nuclear magnetic resonance imaging apparatus of the present invention, an eddy current measuring sample is placed at symmetrical positions equidistant from the magnetic field center in a predetermined gradient magnetic field direction. The pulse sequence for eddy current measurement is executed every time the waveform shaping degree is adjusted by the waveform shaping circuit for eddy current compensation of the gradient magnetic field generating means, and the NMR signal from the sample received thereby is subjected to frequency analysis. The feature is that the frequency analysis result is displayed.

According to this structure, the adjustment pulse sequence is executed every time the waveform shaping degree is adjusted by the adjustment of the adjustment parts (normal resistance and capacitor) of the waveform shaping circuit of the gradient magnetic field generating means, and each time the adjustment pulse sequence is executed. , NMR signals from both eddy current measurement samples arranged at symmetrical positions equidistant from the magnetic field center are received, and the received NMR signals are
The frequency is analyzed by the frequency analysis means. By this frequency analysis, the effect of the eddy current due to the gradient magnetic field appears as a frequency shift with respect to the resonance frequency.

Therefore, by displaying the frequency analysis result, it is possible to simultaneously measure and observe the influence of the eddy current generated at the same distance from the center of the magnetic field, and the waveform shaping degree at which the frequency deviation becomes the minimum value at the time of measuring the eddy current is obtained. The effect of eddy current can be compensated by setting the adjustment component of the waveform shaping circuit for eddy current compensation of the gradient magnetic field generation means, that is, the distortion of the gradient magnetic field due to the eddy current can be corrected, and the compensation can be performed in a short time. ,and,
Can be done accurately. In the frequency analysis by the frequency analysis means, the digitized NMR signal is subjected to Fourier transform to take out frequency information and the peak value of the frequency information is detected, or the peak value of the Fourier transformed frequency information is detected. Instead, the signal strength contrast may be used.

Thus, by extracting the frequency information of the two eddy current measurement samples and displaying the information, it is possible to grasp the influence of the eddy current at the same distance from the magnetic field center. In order to understand the effect of eddy currents at the same distance from the center of the magnetic field, N of two eddy current measurement samples should be used.
It is essential to distinguish and display MR signals,
For this purpose, the two eddy current measurement samples are made different in size, or one having water and the other having different resonance frequencies such as oil is used, and the peak value of the frequency information of the NMR signal or the NMR signal is used. The frequency information of the signal may have a strength difference.

Further, in displaying, when the peak value of the frequency information is detected, it is displayed as it is or in order to more clearly identify and display the NMR signals of the two eddy current measurement samples, two eddy current measurement samples are displayed. Color-coded display may be performed according to the difference in the signal intensity of the peak value of the eddy current measurement sample, and if the peak value of the frequency information is not detected, it may be displayed as the signal intensity contrast. Good.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION A preferred embodiment of the present invention will be described below with reference to the drawings. The MRI apparatus according to the embodiment of the present invention is configured as shown in FIG. In the MRI apparatus shown in FIG. 1, reference numeral 1 is a superconducting magnet that generates a uniform static magnetic field, and an NMR
A gradient magnetic field coil 2 for generating a gradient magnetic field to be superimposed on a uniform static magnetic field for adding spatial position information to a signal is installed. This gradient magnetic field coil 2 is composed of three sets of coils, and each of them generates three gradient magnetic fields Gx, Gy, Gz whose magnetic field strengths are respectively inclined in the three axial directions of X, Y, Z.

The gradient magnetic field power source 7 supplies a current (current pattern) of a predetermined waveform to each coil of the gradient magnetic field coil 2. The gradient magnetic field waveform generator 5 generates a current pattern of a pulse waveform for Gx, Gy, and Gz necessary for creating Gs, Gr, and Gp pulses of a desired current waveform (current pattern), and the current waveform shaping circuit 6 Send to. The current waveform shaping circuit 6 shapes the current pattern of the gradient magnetic field waveform generator 5 and sends it to the gradient magnetic field power supply 7 in order to cancel (compensate) the influence of the eddy current. The gradient magnetic field power supply 7 amplifies the pulse-shaped current of which the waveform has been shaped to Gx, Gy, G
Each of the three coils of z is supplied. The gradient magnetic field coil 2, the gradient magnetic field waveform generator 5, the current waveform shaping circuit 6, and the gradient magnetic field power supply 7 constitute the gradient magnetic field generating means in the present invention.

Inside the gradient magnetic field coil 2 for generating a static magnetic field and a gradient magnetic field, a subject (usually a human body) is exposed to a subject during imaging.
During eddy current measurement, eddy current measurement samples (hereinafter referred to as "samples") 4 and 4'are irradiated with an RF excitation pulse to perform NMR.
A high-frequency magnetic field transmission / reception coil (hereinafter, referred to as RF) for causing a phenomenon and receiving an NMR signal generated in the subject or the samples 4 and 4'according to the specific nuclear density distribution.
3) is arranged. The RF coil 3 is connected to an oscillator 8 and is supplied with a high frequency pulse generated from the oscillator 8 to irradiate an RF excitation pulse.

A receiver 10 for receiving an NMR signal and an A / D converter 11 for converting the received NMR signal into digital data are connected to the RF coil 3 as a receiving system. The digital data is Fourier transformed by the Fourier transformer 12 and sent to the data processing mechanism 13. The data processing mechanism 13 detects the peak value of the data after the Fourier transform, extracts the frequency information at the same distance from the magnetic field centers of the two samples, and the frequency information is sent to the display 14 and displayed. The Fourier transformer 12 and the data processing mechanism 13 correspond to the frequency analysis means in this invention.

Reference numeral 9 indicates various types of pulse sequences for imaging for collecting data for reconstructing a tomographic image of a subject and pulse sequences for measuring eddy current for collecting data for eddy current measurement. Is a timing controller that sends the command of (1) to the transmission system, the reception system and the gradient magnetic field generation system. That is, a command is sent to the gradient magnetic field waveform generator 5, the oscillator 8 and the like to control the generation timing of the RF pulse and the gradient magnetic field pulse and the waveforms of the RF pulse and the gradient magnetic field pulse.

The two samples 4 and 4'used at the time of eddy current measurement are substances having specific atomic nuclei which cause an NMR phenomenon such as water and oil, and inside the high frequency magnetic field transmission / reception coil 3 at the time of eddy current measurement, The magnetic field gradient center (generally, the geometrical center of the gradient magnetic field coil 2) is placed at equal left and right distances (for example, 80 mm to the right and 80 mm to the left) so as not to impair the sensitivity of the high frequency magnetic field transmission / reception coil 3. It is desirable that the two samples 4 and 4'be as small as possible in order to obtain an NMR signal from a more accurate position. The samples 4 and 4'may have the same size, or may have different sizes such that one of them is larger and the other is smaller. Different substances may be used.

The difference between these left and right samples 4, 4'appears in the frequency data processing (frequency analysis of NMR signals) by the data processing mechanism 13. The eddy current generated in the peripheral conductor structure arranged around the gradient magnetic field coil such as the constant temperature bath of the superconducting magnet 1 is generated at the moment when the gradient magnetic field changes. Since this eddy current generates a magnetic field in a direction that disturbs the change of the magnetic field, when a pulse-shaped current waveform is applied to the gradient magnetic field coil 2, the current waveform becomes exponentially blunt at the rising and falling portions of the current waveform. . The rounding of the current waveform is forcibly shaped by the current waveform shaping circuit 6. As is well known, the current waveform shaping circuit 6 is composed of a potentiometer, a variable resistor, and a variable capacitor, and the shape of the current waveform output to the gradient magnetic field power supply 7 can be shaped by changing the value of this adjusting component. .

With this shaped current waveform, a gradient magnetic field is generated according to the shaped current waveform. In this way, the current waveform shaping circuit 6 is adjusted to shape the current waveform output from the gradient magnetic field waveform generator 5 so as to cancel the influence of the eddy current. Does not appear evenly at positions equidistant from the center of the magnetic field to the left and right, and depending on the shape of the superconducting magnet 1 and the gradient magnetic field coil 2, the eddy current is strongly influenced on either the left or the right. Therefore, when correcting the eddy current, it is necessary to shape the current waveform by the current waveform shaping circuit 6 while observing the left-right balance.

In the present invention, two samples are used when measuring the eddy current, the influence of the eddy current due to the gradient magnetic field is reflected in the NMR signals obtained from both samples, and the obtained NMR signals are subjected to frequency analysis. This makes it possible to simultaneously measure and observe the effect of eddy currents at equal distances from the center of the magnetic field, and eddy current compensation adjustment at well-balanced two points can be performed. Next, the measurement operation of the influence of the eddy current and the eddy current compensation adjustment by the current waveform shaping circuit 6 will be described. Before measuring the eddy current, 2
The samples 4, 4'are arranged at symmetrical positions equidistant to the center of the magnetic field in the gradient magnetic field direction to be measured. In FIG. 1, it is arranged in the X direction in order to measure Gx.

When measuring the eddy current, the timing controller 9 sends the timing signal shown in FIG. 2A to the gradient magnetic field waveform generator 5 and the oscillator 8. The gradient magnetic field waveform generator 5 and the oscillator 8 which have received the timing signal generate the current waveform and the high frequency pulse at the timings shown in FIG.
The pulse sequence for eddy current measurement in FIG. 2 is repeatedly executed by the current waveform shaping circuit 6 every time the degree of shaping of the current waveform is changed by a predetermined step amount. The current waveform output from the gradient magnetic field waveform generator 5 is supplied to the gradient magnetic field coil 2 from the gradient magnetic field power supply 7 through the current waveform shaping circuit 6. As a result, the gradient magnetic field coil 2 generates a gradient magnetic field according to the current waveform.

When a current according to the current waveform shown in FIG. 2 (a) is supplied, the gradient magnetic field is blunted by the effect of the eddy current at the rising and falling portions. FIG.
At the timing as shown in (b), the high frequency pulse is generated from the oscillator 8 immediately after the fall of the current. The high frequency pulse output by the oscillator 8 is generated by the RF coil 3
Creates a high frequency magnetic field. The sample causes an NMR phenomenon by the high frequency magnetic field, and emits high frequency energy from the moment when the high frequency magnetic field is stopped. The RF coil 3 serves as an antenna and the receiver 10 receives this high frequency energy as N.
It is received as an MR signal. This makes it possible to obtain NMR signals immediately after the generation of the gradient magnetic field of the two samples 4 and 4 ′ until the influence of the eddy current disappears. This NMR signal contains the effect of eddy current as a frequency shift.

The NMR signal obtained by the receiver 10 is converted into digital data by the A / D converter 11 at a sampling interval as shown in FIG. 2 (c), and this data is converted by the Fourier transform unit 12 by the Fourier transform. Convert to frequency data. This frequency data (frequency information)
Has a band depending on the size of the samples 4 and 4 ′, the data processing mechanism 13 detects the maximum value of the frequency data of the samples 4 and 4 ′, and the two samples 4 and
Frequency information at the same distance from the center of the magnetic field 4'is extracted. The frequency information obtained by the data processing mechanism 13 is displayed on the display unit 14 to grasp the state of the eddy current equidistant from the center of the magnetic field gradient due to the gradient magnetic field. The distortion of the gradient magnetic field due to the eddy current is corrected by adjusting the current waveform shaping circuit 6 to the degree of waveform shaping that minimizes the frequency shift based on this display data, that is, the eddy current compensation adjustment at the point of symmetry is balanced. You can do it all at once.

FIG. 3 shows the results of Fourier transform of the NMR signals obtained from the sample 4 ′ and the sample 4 of different sizes placed at a distance of X mm to the left and right from the center of the magnetic field gradient. As shown in FIG. 3, the NMR signal obtained from each sample after the Fourier transform is frequency data that is deviated from the central frequency f at the center of the magnetic field gradient by Δf.
This is because the influence of the eddy current increases in proportion to the distance from the magnetic field gradient center, as shown in FIG.
The cause is that the resonance frequencies of the samples placed in mm and -Xmm shift. In the absence of eddy currents, the resonance frequency of the sample comes to the position of f shown in FIG. This means that A
As the A / D sampling in which the / D converter 11 takes in the NMR signal progresses, the frequency spectrum of the NMR signal by the two samples approaches the center frequency. If the display 14 shows how the frequency shift approaches the center frequency, the effect of the eddy current can be measured and observed.

A method for simultaneously displaying the influence of eddy currents that can be placed equidistant from the center of the magnetic field gradient on a display is as follows:
In some embodiments, the data processing mechanism 13 processes the frequency data of the NMR signal to be displayed. When the sample as shown in FIG. 3 is used, the data processing mechanism 13 takes out the peak value of the frequency data of the sample 4 and the sample 4'to accurately grasp the positions of + Xmm and -Xmm, and displays the peak value on the display 14 Send to. To detect the peak value of the frequency data, the peak value can be obtained by taking the derivative of the frequency as a function of the frequency.

Further, in order to distinguish the peak value by the left and right samples, two samples having different sizes as shown in FIG. 3 are used to make a difference in the signal intensity of the peak value of each frequency data. The effect of eddy current in + Xmm and -Xmm is distinguished and displayed by the method of expressing the difference by color. As a method of distinguishing the left and right NMR signals,
In addition to the above-described embodiment, water and oil may be combined as a sample, and the difference due to the resonance frequency may be processed by the data processing mechanism 13 and displayed. It is also possible to make the two left and right samples the same size, display the Fourier-transformed data as it is without searching for the peak of the frequency data in the data processing mechanism 13, and express it as the contrast of the signal intensity. In any case, the sample
It is desirable to use the smallest possible size.

Further, a means for automatically changing the waveform shaping degree of the current waveform shaping circuit of the gradient magnetic field generating means and executing a pulse sequence for eddy current measurement for each change is provided, and a waveform shaping degree with a minimum frequency deviation is provided. If a means for searching the adjustment value is provided, the eddy current compensation adjustment can be automatically performed.

[0029]

As described above, according to the nuclear magnetic resonance imaging apparatus of the present invention, it is possible to simultaneously measure and observe the effects of eddy currents at two points which are symmetrical in the direction of the gradient magnetic field to be measured. It becomes possible to perform eddy current compensation adjustment at points in a good balance at once. Therefore, the eddy current compensation adjustment time can be shortened and the work can be facilitated.

[Brief description of drawings]

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

FIG. 2 is a time chart showing a pulse sequence for eddy current measurement.

FIG. 3 is a diagram showing a relationship between an eddy current measurement sample and an NMR signal.

FIG. 4 is a diagram showing a relationship between a gradient magnetic field gradient and a gradient magnetic field gradient due to an eddy current.

[Explanation of symbols]

1: Superconducting magnet 2: Gradient magnetic field coil 3: High frequency magnetic field transmission / reception coil (RF coil) 4, 4 ': Eddy current measurement sample 5: Gradient magnetic field waveform generator 6: Current waveform shaping circuit 7: Gradient magnetic field power supply 8: Oscillator 9: Timing controller 10: Receiver 11: A /
D converter 12: Fourier converter 13: Data processing mechanism 14: Display

Claims (1)

[Claims] 1. A means for generating a static magnetic field, a gradient magnetic field generating means having a gradient magnetic field coil for generating a gradient magnetic field having a magnetic field strength inclined in a predetermined direction by superimposing the gradient magnetic field on the static magnetic field. A core having an RF irradiating means for irradiating an RF excitation pulse and a receiving means for receiving an NMR signal, wherein the gradient magnetic field generating means has a waveform shaping circuit for eddy current compensation for shaping a current waveform applied to the gradient magnetic field coil. A magnetic resonance imaging apparatus, wherein an eddy current measurement sample placed at a symmetrical position equidistant from a magnetic field center in a predetermined gradient magnetic field direction, and an eddy current measurement every time the waveform shaping degree is adjusted by the waveform shaping circuit Means for executing a pulse sequence for measurement, a means for frequency-analyzing an NMR signal from the sample received by the execution of the pulse sequence for eddy current measurement, and a frequency analysis result are displayed. And a magnetic resonance imaging apparatus.