JPH10127603A - Nuclear magnetic resonance examination system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively remove a low frequency noise component which varies timewisely. SOLUTION: A signal received by an RF coil 12 is detected by a phase detection circuit 42 and sample/digital converted at an A/D converter 43 to provide series of data and these data are stored in a memory 44. A correction circuit 45 finds low frequency noise component data by evaluating several pieces at the head of series of data on respective lines as low frequency noise components and by subtracting these found data from the respective data on these lines, correction is performed for removing the influence of low frequency noise components.

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 inspection apparatus for performing imaging and spectroscopy measurement using a nuclear magnetic resonance phenomenon (MR phenomenon), and particularly to a nuclear magnetic resonance apparatus having a configuration for removing low-frequency noise. It relates to an inspection device.

[0002]

2. Description of the Related Art When performing imaging with a nuclear magnetic resonance inspection apparatus, an object is irradiated with an RF pulse to excite it, thereby generating an NMR signal. Then, the NMR signal generated by the subject is received by an RF coil, phase-detected, sampled at a predetermined rate by an A / D converter, and converted into digital data. By the way, in such a receiving system, it is inevitable that a ground level fluctuates due to a temporal change due to a temperature or the like, and that an AC power supply goes around the receiving system. Due to such a DC offset and a low frequency drift, a DC component and a low frequency drift component are mixed in the collected data, which adversely affects the reconstructed image.

For this reason, conventionally, a pre-scan is performed prior to a normal scan, only low-frequency noise components are collected, and low-frequency noise component data is converted from raw data obtained by a normal scan performed thereafter. 2. Description of the Related Art A nuclear magnetic resonance inspection apparatus is known which removes an adverse effect on a reconstructed image by drawing.

[0004]

However, in the above-mentioned conventional nuclear magnetic resonance inspection apparatus, it is inevitable that a certain time interval is generated between the pre-scan and the normal scan. There is a fundamental problem that if a component fluctuates, it cannot be removed. In addition, noise due to temperature fluctuation, AC power supply noise, and the like correspond to this, and cannot be effectively removed.

[0005] In view of the above, it is an object of the present invention to provide a nuclear magnetic resonance inspection apparatus improved so as to effectively remove the adverse effects of low frequency noise inevitable in a receiving system.

[0006]

In order to achieve the above object, a nuclear magnetic resonance inspection apparatus according to the present invention comprises: means for generating a static magnetic field; and means for generating a gradient magnetic field so as to be superimposed on the static magnetic field. , RF transmitting means, RF receiving means, and means for evaluating low-frequency noise components for each line by evaluating several data at the head of data of each line obtained by sampling a received signal. Means for subtracting and correcting the low-frequency noise component from each data of the detected line.

In the nuclear magnetic resonance inspection apparatus, scanning is performed for each line, and a series of data is obtained for each line. The vicinity of the center of this series of data corresponds to the peak of the NMR signal and has a large signal intensity, but the end portion hardly contains a signal component. Therefore,
If you extract several data at the beginning of the sampling pulse for signal sampling or at the leading end by increasing the sampling pulse for signal sampling to the leading end from the original number of data points, it will be a low-frequency noise component. It turns out that. However, since there is a possibility that spike noise or the like may be included, it is necessary to evaluate several pieces of the extracted data to determine that the data is not spike noise or the like. This evaluation is performed, for example, when the several data have similar values. Then, when the low-frequency noise component is evaluated, an average value of several data is taken, or representative data such as the top data is selected, and data as the low-frequency noise component is determined. obtain. Since the data of the low-frequency noise component is subtracted from all the data of the line, the low-frequency noise can be removed. Since the data collection time of each line (the time from the start to the end of the generation of the sampling pulse) is extremely short, even if the low-frequency noise component fluctuates with time, the fluctuation cycle is the data collection time of each line. As long as the noise is not short, the low frequency noise can be effectively removed. As a result, it is possible to effectively remove not only AC power supply noise but also most other low frequency noise components actually occurring.

[0008]

Next, embodiments of the present invention will be described in detail with reference to the drawings. A nuclear magnetic resonance inspection apparatus according to the present invention is configured as shown in FIG. In FIG. 1, the magnet assembly 11 includes a main magnet for generating a static magnetic field, and a gradient coil for generating gradient magnetic fields Gx, Gy, Gz superimposed on the static magnetic field. A subject 61 placed on an examination table 62 is inserted into the space to which the static magnetic field and the gradient magnetic field are applied. The subject 61 is irradiated with an RF pulse and the NM generated by the subject 61 is emitted.
An RF coil 12 for receiving the R signal is attached.

A magnetic field control circuit 21 is provided as a circuit for supplying a gradient magnetic field current to the gradient coil of the magnet assembly 11. A waveform signal from the waveform generation circuit 53 is sent to the magnetic field control circuit 21. In the waveform generating circuit 53, information on each pulse waveform of the gradient magnetic fields Gx, Gy, Gz is set in advance from the computer 51. At the timing instructed by the sequence controller 52, the gradient magnetic field Gx,
Waveform signals for each of Gy and Gz are generated and sent to the magnetic field control circuit 21, whereby pulsed gradient magnetic fields Gx, Gy and Gz having a predetermined waveform are generated.

R generated by the RF oscillation circuit 31
The F signal is sent to the amplitude modulation circuit 32, which becomes a carrier signal, and is amplitude-modulated according to the RF waveform signal sent from the waveform generation circuit 53. The RF signal after the amplitude modulation is amplified through the RF power amplifier 33 and then applied to the RF coil 12. The oscillation frequency of the RF oscillation circuit 31 is controlled by the computer 51 and the subject 61
Is matched to the resonance frequency of the body tissue. Information on the waveform of the modulation signal is given from the computer 51 to the waveform generation circuit 53 in advance. Waveform generation circuit 53
The timing of the RF oscillation circuit 31 is determined by the sequence controller 52.

NMR received by RF coil 12
The signal is sent to a phase detection circuit 42 via a preamplifier 41 and is subjected to phase detection. An RF signal from the RF oscillation circuit 31 is sent as a reference signal for the phase detection. The signal obtained by the phase detection is sampled at a predetermined sampling timing by the A / D converter 43 controlled by the sequence controller 52, and is converted into digital data. The data obtained from the A / D converter 43 is taken into the computer 51 and stored in the memory 44, and the low frequency noise component is removed by the correction circuit 45.
The image data of each pixel is reproduced by receiving an image reconstruction process such as a dimensional Fourier transform.

The computer 51 includes a display device 54, a keyboard 55, a mouse 56, and a recording device 5.
7 is connected. The display device 54 displays the reconstructed MR image and the like. Keyboard 5
5. Input and setting of an imaging sequence, imaging parameters, and the like are performed by the mouse 56 and the like. The recording device 57 is composed of a magneto-optical disk device or the like, and records obtained data such as images.

As a pulse sequence for imaging, for example, a pulse sequence based on a field echo method as shown in FIG. 2 is performed. A gradient magnetic field pulse for selecting a slice (here, Gz
(Pulse) 72 to selectively excite one location in the Z direction. Thereafter, a phase-encoding gradient magnetic field pulse (Gy pulse) 73 is added, and the reading (and frequency encoding) gradient magnetic field pulse (Gx pulse) 7 is inverted.
4 to generate a resonance signal 75. Then, a number of sampling pulses 76 are generated so as to include a period in which the resonance signal is generated, and A / A
The D converter 43 converts the signal into a sample and digital data. Thus, as shown in FIG.
In the repetition period, data for one line is collected.

This pulse sequence is repeated by the number corresponding to the matrix of the image to be reconstructed while changing the magnitude of the Gy pulse, and the data of all the lines necessary to reconstruct the image of the matrix is reproduced. To collect. Although the number of sampling pulses in one repetition period (that is, the number of data in one line) corresponds to the reconstructed image matrix, several pulses may be added to the head of the matrix. In any case, DC data is detected by evaluating several digital data obtained by sampling with the first few pulses 77.

More specifically, referring to FIG. 3, when an imaging sequence is started, each line is scanned by a pulse sequence as shown in FIG. 2, and a series of data is collected for each line, Several data at the head 77 of a series of data on the line are evaluated to confirm that they are low frequency noise components. The vicinity of the center of the series of data in each line corresponds to the peak of the NMR signal and has a large signal intensity, but the end portion hardly contains a signal component. Therefore,
If a few pieces of data in the leading portion 77 are extracted, it is a low-frequency noise component. However, since there is a possibility that spike noise or the like is also included, the extracted few pieces of data are evaluated and spikes are evaluated. It is necessary to determine that it is not noise or the like.

This evaluation is performed, for example, by determining whether the difference between the data is within a predetermined range. If it is not within the predetermined range, it cannot be evaluated as a low-frequency noise component, so that line is scanned and data collection is performed again. When the difference between the first few data is within a predetermined range and is evaluated as a low-frequency noise component, an average value of the several data is taken, or a representative data such as the first data is taken. To obtain data as a low-frequency noise component.

Then, the low frequency noise component data is subtracted from each of the series of data on the line, and the DC component is corrected. After saving the corrected data in the memory 44, the next line is scanned. By repeating this, data of each line from which the low-frequency noise component has been removed is collected, and when the raw data of all the lines is completed, image reconstruction processing such as two-dimensional Fourier transform is performed. As a result, the ground noise (D
It is possible to effectively remove a low-frequency noise component due to a temporal change of (C offset) and a low-frequency noise component due to leakage from an AC power supply, and to reconstruct an image in which these effects are reduced.

Of course, the present invention can be variously modified without departing from the gist thereof. For example, in the above description, the low frequency noise component is removed by using the correction circuit 45. However, the computer 51 may be configured to perform this by software. As the imaging pulse sequence, other pulse sequences such as a spin echo method can be used in addition to the field echo method shown in FIG.

[0019]

As described above, in the nuclear magnetic resonance inspection apparatus of the present invention, low frequency noise is detected for each line, and the low frequency noise is subtracted from the data of the line.
The low-frequency noise component is corrected, so that the low-frequency noise component can be effectively removed in response to the temporal fluctuation of the low-frequency noise component. Deterioration of the reconstructed image due to adverse effects such as noise can be prevented.

[Brief description of the drawings]

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

FIG. 2 is a time chart showing a pulse sequence used in the embodiment.

FIG. 3 is a flowchart showing an operation in the embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 Magnet assembly 12 RF coil 21 Magnetic field control circuit 31 RF oscillation circuit 32 Amplitude modulation circuit 33 RF power amplifier 41 Preamplifier 42 Phase detection circuit 43 A / D converter 44 Memory 45 Correction circuit 51 Computer 52 Sequence controller 53 Waveform generation circuit 54 display device 55 keyboard 56 mouse 57 recording device 61 subject 62 examination table 71 RF excitation pulse 72 gradient magnetic field pulse for slice selection 73 gradient magnetic field pulse for phase encoding 74 gradient magnetic field pulse for reading 75 resonance signal 76 sampling pulse 77 sampling pulse (The part for DC data detection)

Claims (1)

[Claims] 1. A means for generating a static magnetic field, a means for generating a gradient magnetic field to be superimposed on the static magnetic field, an RF transmitting means, an RF receiving means, and data of each line obtained by sampling a received signal. And a means for evaluating low-frequency noise components on a line-by-line basis by evaluating several data at the beginning of the line, and a means for subtracting and correcting the detected low-frequency noise components from each data of the detected lines. A nuclear magnetic resonance inspection apparatus characterized by the above-mentioned.