JPH1147111A - Magnetic resonance computed tomograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reception means which can secure a sufficient S/N value by inhibiting degradation of the S/N value of a large NMR signal. SOLUTION: An NMR signal 21 received by an RF coil is input to a frequency converter 27 via an attenuator 22 and an amplifier 24. The frequency converter 27 mixes the NMR signal from the amplifier 24 with a local reference signal generated from a local clock 25 and amplified by an amplifier 26, thus converting the frequency of the NMR signal to a frequency in an audible zone, and feeding the signal through an anti-aliasing filter 28 and a variable-gain buffer amplifier 30 to an A/D converter 11, where the signal is converted to sampling and digital data. The attenuation value of the attenuator 22 and the gain value of the butter amplifier 30 are adjusted so as to be output as the gain of a large NMR signal is lowered, the amount of noise intrinsic in a receiving system is reduced, and the degradation of the SIN value of an input is minimized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to nuclear magnetic resonance (Nucl).
The present invention relates to an apparatus that performs tomographic imaging of a subject using the ear magnetic resonance (abbreviated as NMR) phenomenon, and in particular, receives an NMR signal detected from the subject in response to irradiation of the subject with an RF (Radio Frequency) pulse. The technology for processing.

[0002]

2. Description of the Related Art In recent years, a magnetic resonance tomography apparatus (hereinafter, referred to as an MRI apparatus) that performs imaging using an NMR phenomenon has been widely used in the field of medical form diagnosis. As is well known, a magnetic resonance tomography apparatus generates a uniform static magnetic field in an imaging region space, and generates three gradient magnetic fields whose magnetic field strengths change in three-dimensional directions orthogonal to the static magnetic field space at a predetermined timing. An RF pulse for nuclear spin excitation is superimposed and transmitted to the subject, and thereby one or more NMR signals returned from the subject are received. A sequence in which a gradient magnetic field or an RF pulse is applied at a predetermined timing in order to obtain a predetermined number of NMR signals is called a pulse sequence. In order to perform one tomography, the above pulse sequence is cyclically repeated, and a large number of NMR signals are continuously detected. The detected NMR signals are sequentially digitized, and further subjected to data processing such as Fourier transform to reconstruct a tomographic image of the subject.

[0003] In a magnetic resonance tomography apparatus, a filter processing for noise removal (for example, FIR filter processing = finite-length impulse response type filter processing) or the like is applied to an NMR signal before performing data processing such as Fourier transform. In addition, signal processing such as enhancement processing may be performed on a specific signal component. By this signal processing, for example, the tone of the finally obtained image can be made more suitable for the condition of the affected part and the purpose of diagnosis.

[0004] In the MRI apparatus, the reception system of the NMR signal includes:
It is configured as shown in FIG. The NMR signal received by the receiving coil (RF coil) 31 has a resonance frequency f
r, which is passed through the preamplifier 32 and mixed with the signal of the frequency f1 by the frequency converter 33 to be converted into the frequency of the audio band of the frequency f0 (= fr−f1), and the anti-aliasing filter 34 and , A buffer amplifier 35, and an A / D converter 36.
After performing sampling and conversion to digital data, the data is sent to a digital signal processing device 37 to perform processing such as Fourier transform. In FIG. 3, the NMR signal is converted to the frequency of the audio band by one down-conversion. However, usually, two stages of frequency converters are provided and a down-conversion is performed twice, that is, a so-called double superheterodyne system is used for high frequency NMR. The signal is converted to a frequency in the audio band.

[0005]

However, in the conventional magnetic resonance tomography apparatus, the setting of the amplification (gain) in the analog signal processing of the NMR signal reaching the A / D converter in the means for receiving the NMR signal is as follows. The first high-frequency stage, that is, a variable gain preamplifier 32 connected to the RF coil 31, or an attenuator interposed between the RF coil 31 and the preamplifier 32 is used.
The buffer amplifier 35 in the audio band immediately before the A / D converter 36 had a fixed gain. In this case, R
There is no problem as long as the NMR signal detected by the F coil 31 has an appropriate amplitude for the entire receiving means system.

However, a comparison between a case where a very large signal is input and a case where a very small signal is input shows that, in the case of a large signal, a large attenuation is applied by the first stage attenuator. Although the noise component is greatly reduced, in the case of a small signal, the attenuator has a small attenuation, and the noise component from the RF coil system does not decrease much. Then, in proportion to the magnitude of the received NMR signal, the S / N of the receiving means system up to a certain signal level
Although the noise component that determines the S / N value after a certain value becomes dominant, the noise component inside the receiving system other than the RF coil system becomes dominant, and the S value and the N value similarly increase, and the S / N value increases.
There is a problem that the N value does not increase.

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a magnetic resonance tomography apparatus capable of securing a sufficient S / N value even when the magnitude of an input resonance signal (NMR signal) increases. With the goal.

[0008]

In order to achieve the above-mentioned object, in the magnetic resonance tomography apparatus of the present invention, N is converted into an audio band frequency in a receiving means.
It is characterized in that the amplifier preceding the A / D converter for amplifying the MR signal is a variable gain amplifier.

Therefore, in the magnetic resonance tomography apparatus of the present invention, the received NMR signal receives two-stage attenuation before reaching the A / D converter. That is, the first stage is an attenuation by the variable gain preamplifier of the high frequency stage connected to the RF coil, and the second stage is an attenuation by the variable gain amplifier before the A / D converter after the frequency conversion. In addition, the attenuation may change the gain of the amplifier, the gain of the amplifier may be fixed, and the signal input to the fixed gain amplifier may be attenuated by providing an attenuator in front of the amplifier. The "variable gain amplifier" in the above includes both of the above (a combination of the former variable gain amplifier and the latter a combination of the attenuator and the fixed gain amplifier).

[Operation] The operation of the present invention is as follows. First, a static magnetic field is generated in the imaging region space by the static magnetic field generating means. A gradient magnetic field is superimposed on the static magnetic field space at a predetermined timing by the gradient magnetic field generation means, and an RF pulse for nuclear spin excitation is transmitted to the subject arranged in the imaging region space by the transmission / reception means. Is done. Based on this, the NMR signal returned from the subject is detected by the transmitting / receiving means. The NMR signal is A
After being digitized by the / D conversion means, it is provided to the signal processing unit. After the signal processing unit performs signal processing on the digital NMR signal, the signal processing unit sends the processed NMR signal to the data processing unit. The data processing means of the magnetic resonance tomography apparatus performs data processing such as Fourier transform based on the sent NMR signal and reconstructs each image according to the signal processing.

In such a series of processes, in the reception system of the resonance signal (NMR signal), the reception gain is adjusted prior to each imaging. At the beginning of this adjustment, the gain of the buffer amplifier of the succeeding stage interposed in the front stage of the A / D converter is fixed at the default value, and the gain of the first stage amplifier (preamplifier) connected to the RF coil or the attenuator value of the attenuator is variable. To bring the level of the input resonance signal to an appropriate value. If a further attenuation is required after a certain appropriate attenuation value, the gain of the subsequent buffer amplifier connected to the A / D converter is lowered and the gain of the first-stage amplifier or the attenuator is reduced again. To make the input signal an appropriate level.

As described above, the gain of the amplifier (preamplifier) connected to the RF coil or the attenuator and A
By adjusting the gain of the amplifier connected to the / D converter or the attenuator, the noise from the coil system and the intrinsic noise other than the coil system in the noise component in the entire receiving system depending on whether the input resonance signal is large or small. Becomes almost constant, and the magnitude of the input signal does not become dominant in either of the noise components.
The value also does not saturate from a certain level due to the input signal value.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the magnetic resonance tomography apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram illustrating the entire configuration of the magnetic resonance tomography apparatus according to the embodiment. The apparatus according to the embodiment includes a main magnet 2 for generating a uniform static magnetic field in the internal space of the gantry section 1.
And three gradient magnetic field coils 3 (3X, 3Y, 3Z) for applying a gradient magnetic field so as to be superimposed on the static magnetic field. Gradient field coil (GradientField Coil) 3
The magnetic field strength changes in three-dimensional directions (X, Y, Z) orthogonal to the uniform static magnetic field Ho by the main magnet 2.
Sets of gradient magnetic field coils 3X, 3 on which pulses of two gradient magnetic fields Gx, Gy, Gz (reading gradient magnetic field, phase encoding gradient magnetic field, slice region selecting gradient magnetic field) are superimposed.
Y, 3Z. An imaging region of the subject is arranged at the imaging center in the internal space (imaging region space) of the gantry unit 1 to which the static magnetic field and the gradient magnetic field are applied, and an RF for nuclear spin excitation is provided near the subject.
A coil (Radio Frequency Coil) 4 is provided.

A gradient magnetic field power supply 5 is connected to the gradient magnetic field coil 3 to supply electric power necessary for generating the respective gradient magnetic fields Gx, Gy, Gz. A waveform signal from the waveform generator 6 is input to the gradient magnetic field power supply 5, and the gradient magnetic field Gx,
Gy, Gz gradient magnetic field waveforms are controlled. The main magnet 2 corresponds to a static magnetic field generating means in the present invention, and the gradient coil 3, the gradient power supply 5 and the waveform generator 6
This corresponds to the gradient magnetic field generating means in the present invention. An RF pulse is supplied to the RF coil 4 via an RF power amplifier 7, and the RF coil 4 transmits the RF pulse to the subject. This RF pulse is applied to the RF signal generator 8
AM (amplitude)
The modulator 9 is AM-modulated according to the waveform sent from the waveform generator 6. The RF power amplifier 7, the RF signal generator 8, the AM (amplitude) modulator 9, and the like correspond to transmission / reception means in the present invention.

After the NMR signal generated by the subject is received and detected by the RF coil 4,
The signal is sent to the / D converter (A / D conversion means) 11. This A
The sampling pulse is input from the sampling pulse generator 12 to the / D converter 11, and the analog NMR signal is digitized at a timing synchronized with the sampling pulse and sent to the next signal processing unit 13. The digital NMR signal is subjected to predetermined signal processing by the signal processing unit 13 and then further processed by the host computer 1.
4

Host computer (data processing means) 1
4 processes the captured data, reconstructs an image, displays a tomographic image on the output display 15, determines the timing of the entire sequence via the sequencer 16, and controls the operation of the signal processing unit 13. Or
In the host computer 14, a three-dimensional image is reconstructed by data processing such as three-dimensional Fourier transform. The host computer 14 determines, for example, the generation timing and oscillation frequency of the RF band oscillation signal for the RF pulse, controls the waveforms and the strengths of the Gx, Gy, and Gz gradient magnetic fields, and also controls the RF pulse. Or define the envelope. Therefore, this host computer 14
In addition to the role of the data processing means, it also partially plays the role of the gradient magnetic field generating means and the transmitting / receiving means. The operation unit 17 is used by an operator to input various data necessary for operating the apparatus and to perform an operation for setting conditions.

Next, the receiving means of the magnetic resonance tomography apparatus of the present invention will be described more specifically with reference to the drawings. FIG. 2 is a block diagram showing the configuration of the receiving means. The resonance signal (NMR signal or FID signal) 21 input to the receiving system from the RF coil is
The signal is appropriately attenuated by the first-stage attenuator 22 at the value set by the attenuation control signal 23, and amplified by the next-stage amplifier 24. Then, thereafter, the frequency converter 27 generates the amplifier 2
After being mixed with the local reference signal f1 amplified in step 6, the frequency becomes the audio band frequency f0, and is input to the anti-aliasing filter 28 and filtered, and then the variable gain buffer amplifier 30 controls the gain control signal 29.
A / D converter 11
And output as digital data.

FIGS. 4, 5 and 6 show changes in the S / N value at each stage with respect to the input signal level of the receiving system. FIG. 4 shows a case of a medium signal level, and the S / N value of the input is 3
In this case, the output is a value slightly smaller than 3. This is because the intrinsic noise (noise B) of the receiving system is added as a noise component, so that the S / N value is deteriorated. FIG. 5 shows a case of a small signal level. When the S / N value of the input is 2, the output becomes a value slightly smaller than 2. This is because the intrinsic noise (noise B) of the receiving system is added as a noise component as in the case of the medium signal level.

FIG. 6 shows a case where the signal level is large. The signal having an input S / N value of 4 deteriorates to about 2 at the output. This is because the original noise component A becomes small, and even if the S / N value does not deteriorate in the first stage attenuator and the frequency converter stage, the overall noise rises due to the superposition of the internal noise B of the receiving system. . Therefore, in the case of a large signal, the gain is reduced, and the amount of the intrinsic noise B is reduced.
The gain of the attenuator 22 in front of the amplifier (preamplifier) to which the NMR signal is input and the gain of the buffer amplifier 30 in front of the A / D converter 11 so that the deterioration of the input S / N value is minimized. To adjust. As a result, even if the received NMR signal becomes large, it is possible to suppress the deterioration of the S / N value and to secure a sufficient S / N value.

[0020]

According to the magnetic resonance tomography apparatus of the present invention, even in the case of an input signal having a high S / N ratio and a large input resonance signal (NMR signal), deterioration of the S / N value in the receiving means system. Can be minimized, and a sufficient S / N value can be secured, so that good diagnostic image data can be obtained regardless of the level of the input signal.

[Brief description of the drawings]

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

FIG. 2 is a block diagram illustrating a configuration of a receiving unit.

FIG. 3 is a block diagram illustrating a configuration of a receiving unit according to a conventional example.

FIG. 4 is a diagram showing a change in S / N value at each stage of a receiving system having a medium input signal level.

FIG. 5 shows S in each stage of a receiving system having a small input signal level.
It is a figure showing a change of / N value.

FIG. 6 shows S in each stage of a receiving system having a large input signal level.
It is a figure showing a change of / N value.

[Explanation of symbols]

2 ... Main magnet 3 ... Gradient magnetic field coil 4 ... RF coil 5 ... Gradient magnetic field power supply 8 ... RF signal generator 11 ... A / D
Converter 13 ... Signal processing unit 14 ... Host computer 21 ... Input resonance signal (NMR signal) 22 ... Attenuator 24 ... Amplifier (preamplifier) 27 ... Frequency converter 28 ... Anti-aliasing filter 30 ... Variable gain buffer amplifier

Claims (2)

[Claims] 1. A static magnetic field generating means for generating a uniform static magnetic field in an imaging region space, and a gradient magnetic field generating means for generating three gradient magnetic fields whose magnetic field strengths change in three-dimensional directions orthogonal to each other in the static magnetic field space. Transmitting / receiving means for transmitting an RF pulse for nuclear spin excitation to the subject and receiving an NMR signal from the subject; A / D converting means for digitizing the received NMR signal; N
In a magnetic resonance tomography apparatus having data processing means for performing an arithmetic process on an MR signal to reconstruct a tomographic image of a subject, the means for receiving the NMR signal includes a first amplifier for amplifying the NMR signal; A frequency converter for converting an output signal of the first amplifier into a frequency in an audio band;
A magnetic resonance tomography apparatus, comprising: a second amplifier for amplifying an output of the frequency converter, wherein the first and second amplifiers are variable gain amplifiers. 2. The magnetic resonance tomography apparatus according to claim 1, wherein gains of the first and second amplifiers are switched according to a magnitude of a received NMR signal.