JPH10113339A - Magnetic resonance imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To separate received signals of respective receiving passages from each other by the different frequency axes, add and process the signals before input to a signal processing system, and simplify the constitution of a receiving system by providing plural receiving passages in the receiving system, and arranging signal genarators to generate different frequencies in the respective receiving passages. SOLUTION: Received signals inputted to respective coils 301 to 304 are inputted to mixers 601 to 604 after being amplified by preamplifiers 401 to 404. A difference between frequencies fs of respective signals and reference frequencies f1 to f2 from respective oscillators 501 to 504, is found by the respective mixers 601 to 604, and they are separated into a frequency component of f1 to fS-f4 , to fs . Next, separated received signals are added together by an adder 900 through LPFs 701 to 704, and are inputted to a signal processing system 6 after being digitally converted by an A/D converter 800. Here, the signals separated by the frequency component are reconstructed as an image. According to this, the A/D converter 800 can make shift with only one, and the device constitution is simplified, and cost can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging (hereinafter, referred to as MRI) apparatus for imaging a desired portion of a subject by utilizing a nuclear magnetic resonance (hereinafter, referred to as NMR) phenomenon. It is. For more information, MR
The present invention relates to a signal detection and reception system of the I device.

2. Description of the Related Art An MRI apparatus measures a nuclear spin density distribution, a relaxation time distribution, a chemical shift, and the like at an inspection site in a subject by utilizing an NMR phenomenon, and obtains the measurement data.
The image of the section of the subject is displayed from the data. The outline of the principle is described below.

A nuclear spin of a subject placed in a uniform static magnetic field precesses around a direction of the static magnetic field at a Larmor frequency corresponding to the magnetic field strength. When an object is irradiated with an electromagnetic wave having a frequency equal to the Larmor frequency from the outside, nuclear spins in the object are excited, and high energy
Transition to the state. When this irradiation is stopped, the nuclear spin returns to a low energy state with a relaxation time constant corresponding to the bonding state. At this time, an NMR signal is emitted from the subject to the outside. This NMR signal is detected by a high frequency receiving coil tuned to its resonance frequency. At this time, a gradient magnetic field of three axes of X, Y and Z is applied to the static magnetic field space in order to add positional information to this signal. As a result, position information in the space is detected as a signal of frequency information, and data processing is performed by a computer to form an image.

In recent years, a multi-coil having a plurality of receiving coils has been used to improve the sensitivity of a receiving system. A receiving system using a multi-coil includes a plurality of coils 301 to 304 for receiving NMR signals as shown in FIG.
And preamplifiers 401 to 404 for amplifying the received signal;
An oscillator 500 for generating a reference frequency, and mixers 601 to 604 for differentiating between the signal frequency of each coil and the reference frequency
And an LPF 701 for filtering the output from the mixer
To 704, and A / D converters 801 to 804 for digitally converting the filtered signal. The signals received by the coils 301 to 304 are converted into frequency components of f ₀ -fc by a mixer, are digitally converted by A / D converters 801 to 804 via LPFs 701 to 704, and output to the signal processing system 6.

Since the conventional receiving system can generate only one reference frequency (usually called local transmission or local frequency), the A / D converters 801 to 804 have coils. 301 to 304 were required. An A / D converter used in an MRI apparatus generally uses a high-speed A / D converter of 16 bits or more because the dynamic range of a received signal is large and the signal band is wide. However, such high performance A / D
Since the converter is expensive, mounting the same number of coils increases the cost of the product.

An object of the present invention is to provide an MRI apparatus provided with a receiving system having a simple configuration without lowering the sensitivity.

[0005]

To achieve the above object, the present invention provides a static magnetic field system for forming a uniform magnetic field, a transmission system for applying an electromagnetic wave to a subject placed in the magnetic field, A gradient magnetic field system for applying a gradient magnetic field to a specimen, a reception system for detecting a nuclear magnetic resonance signal generated from the subject by applying the electromagnetic wave and the gradient magnetic field, and a signal processing system for imaging a signal from the reception system , The reception system has a plurality of reception paths, and includes a signal generator for generating a different frequency in each reception path.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 3 is a block diagram showing the overall configuration of the MRI apparatus according to the present invention. This apparatus obtains a tomographic image of the subject 7 using the NMR phenomenon, and includes a static magnetic field generating magnet 4, a central processing unit 1, a sequence controller 2,
It comprises a transmission system 3, a gradient magnetic field generation system 21, a reception system 5, and a signal processing system 6. The static magnetic field generating magnet 4 generates a uniform static magnetic field around the subject 7, and is provided with a permanent magnet type, a normal conduction type, or a superconducting type magnetic field generating means. The sequence controller 2 operates under the control of the central processing unit 1, and sends various commands necessary for data acquisition of tomographic images of the subject 7 to the transmission system 3 and the gradient magnetic field generation system 2.
1 and to the receiving system 5.

The transmitting system 3 comprises a high-frequency oscillator 8, a modulator 9, a high-frequency amplifier 10, and a high-frequency coil 11 on the transmitting side. The high-frequency signal output from the high-frequency oscillator 8 is transmitted to the sequence controller 2 by a command. The amplitude is modulated by the modulator 9 with the value of the amplitude output by the above, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 10 and then supplied to the high-frequency transmission coil 11 arranged close to the subject 7. The electromagnetic wave is applied to the subject 7.

The gradient magnetic field generating system 21 comprises a gradient magnetic field coil 13 wound in three directions of X, Y and Z and a gradient magnetic field power supply 12 for driving each coil. X, Y, Z by driving the gradient magnetic field power supply 12 of each coil in accordance with the command from
The gradient magnetic fields Gx, Gy, Gz in the three axial directions are applied to the subject 7. With this method of applying a gradient magnetic field, a slice plane for the subject 7 can be set.

The receiving system 5 includes a high-frequency coil 14 on the receiving side, an amplifier 15, a receiver 16 including an A / D converter and a detector.
The NMR signal of the subject 7 due to the electromagnetic wave radiated from the high-frequency coil 11 of the transmission system is detected by the high-frequency receiving coil 14 and passed through the amplifier 15 and the receiver 16 including the A / D converter and the detector. Converted to digital quantities,
The signal is sent to the signal processing system 6.

The signal processing system 6 includes a central processing unit 1, a recording device such as a magnetic disk 20 and an optical disk 19,
A central processing unit 1 comprising a display 18 such as a CRT performs processing such as Fourier transformation, correction coefficient calculation and image reconstruction, and obtains a signal intensity distribution of an arbitrary cross section or an appropriate operation on a plurality of signals. The distribution is imaged and displayed on the display 18.

In this embodiment, the high-frequency coils 11 and 14 and the gradient coil 13 on the transmitting and receiving sides are arranged in the magnetic field space of the static magnetic field generating magnet 4 arranged in the space around the subject 7. Have been.

FIG. 1 is a block diagram of a receiving system according to the present invention. In this embodiment, four coils are used. However, the number of coils is not particularly limited to four. The configuration of the receiving system of this embodiment is NM
Coils 301 to 304 for receiving the R signal; preamplifiers 401 to 404 for amplifying the received signal;
501 for generating reference frequencies corresponding to.
To 504, mixers 601 to 604 for differentiating the signal frequency from each coil 301 to 304 and the reference frequency, and LPF for filtering the signals from mixers 601 to 604
701 to 704, an adder 900 for adding the filtered signals, and an A / D converter 800 for digitally converting the signal from the adder 900. Each coil 30
When a reception signal is input to each of the preamplifiers 401 to 304,
The signals are amplified at ４０４404 and input to mixers 601１〜604. In the mixer 601-604 by subtracting the reference frequency f ₁ ~f ₄ from the frequency fc and each oscillator 501-504 of each signal, it is separated into frequency components of f ₁ -fc~f ₄ -fc.
The received signals thus separated are LPFs 701 to 704
Via an adder 900, an A / D converter 800
Are converted into digital signals and input to the signal processing system 6. Then, the signal processing system 6 reconstructs an image of the signal separated by the frequency component.

As described above, in this embodiment, each coil 301
Since the reference frequencies to the mixers 601 to 604 are different from those of the coils 301 to 304, the signal frequencies after passing through the LPFs 701 to 704 are f ₁ −fs,
f ₂ −fs, f ₃ −fs, and f ₄ −fs. The signals separated by the frequency components as described above are passed through the adder 900 and can be reconstructed by the operation of the signal processing system 6 to perform A / D conversion.
Only one converter 800 need be provided. For this reason,
Since the use of the A / D converter can be reduced, the cost can be reduced, and the configuration can be simplified without lowering the receiving sensitivity.

[0014]

According to the present invention, the received signals on the respective receiving paths can be separated on different frequency axes, so that the signals can be added and processed before input to the signal processing system. Therefore, the configuration of the receiving system can be simplified, and the sensitivity characteristics are not deteriorated.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a receiving system according to the present invention.

FIG. 2 is a block diagram showing a configuration of a conventional receiving system.

FIG. 3 is a block diagram showing the overall configuration of the MRI apparatus of the present invention.

[Explanation of symbols]

 301-304 Coil 401-404 Preamplifier 500 Oscillator 601-604 Mixer 701-704 LPF 800 A / D converter 900 Adder

Claims (2)

[Claims] 1. A static magnetic field system for forming a uniform magnetic field, a transmission system for applying an electromagnetic wave to a subject placed in the magnetic field,
A gradient magnetic field system for applying a gradient magnetic field to the subject, a receiving system for detecting a nuclear magnetic resonance signal generated from the subject by applying the electromagnetic wave and the gradient magnetic field, and a signal processing for imaging a signal from the receiving system System, the receiving system has a plurality of receiving paths,
A magnetic resonance imaging apparatus comprising: a signal generator that generates a different frequency in each reception path. 2. The reception system has a plurality of reception paths, includes a signal generator that generates at least two or more reference frequencies in each reception path, and performs addition and digital processing on the signals of each reception path. 2. An output to a signal processing system.
A nuclear magnetic resonance imaging apparatus as described in the above.