JPH10165392A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance apparatus reduced in an image degradation caused by a poor phase accuracy in a detection circuit of a NMR signal. SOLUTION: A standard high frequency signal from a high frequency oscillator 8 is separated into components with a 90 deg. phase error by an orthogonal phase-distributor 24 and subjected to the phase error correction by the phase- distributor 24 through a phase correction circuit 25 which has delayed line patterns 27a-27c with different lengths, prior to an input to detection mixers 23a and 23b, in a receiving system which reconstitutes an image by a CPU 1 through A/D conversion in ADCs 17a and 17b for a NMR signal which was detected by a receiving coil 14 is amplified by an amplifier 15 and subsequently detected by a premixer 22 and the detection mixers 23a and 23b which performs the orthogonal detection. The phase error is determined by the CPU 1 from an output of the ADCs 17a and 17b by directly inputting the standard high frequency signal thereinto, and the CPU 1 controls the phase correction circuit 25.

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 apparatus (hereinafter, referred to as "MRI apparatus").
Nuclear magnetic resonance signals (hereinafter referred to as N
(Referred to as an MR signal).

[0002]

2. Description of the Related Art A conventional example will be described with reference to FIGS. FIG. 3 shows a configuration example of an MRI apparatus, and FIG. 4 shows a configuration example of a conventional receiving system.

In FIG. 3, MRI apparatuses are roughly divided into a central processing unit (hereinafter abbreviated as CPU) 1, a sequencer 2, a transmission system 3, a magnet 4 for generating a static magnetic field, and a reception system 5.
And a signal processing system 6.

The CPU 1 controls the sequencer 2, the transmission system 3, the reception system 5, and the signal processing system 6 according to a predetermined program. Sequencer 2 has C
It operates based on a control command from the PU 1 and sends various commands necessary for data acquisition of an image of a tomographic plane of the subject 7 to the transmission system 3, the gradient magnetic field generation system 21, and the reception system 5. I have.

The transmission system 3 includes a high-frequency oscillator 8 and a modulator 9
And an irradiation coil 11 as a high-frequency coil. The amplitude of the reference high-frequency pulse from the high-frequency oscillator 8 is modulated by the modulator 9 in accordance with a command from the sequencer 2, and the high-frequency pulse subjected to the amplitude modulation is passed through the high-frequency amplifier 10. The pulsed electromagnetic wave is applied to the irradiation coil 11 so as to irradiate the subject 7 with a predetermined pulsed electromagnetic wave.

The static magnetic field generating magnet 4 is for generating a uniform static magnetic field in a predetermined direction around the subject 7. The irradiation coil 1 is provided inside the static magnetic field generating magnet 4.
1, a gradient coil 13 and a receiving coil 14 are arranged. The gradient magnetic field coil 13 includes a gradient magnetic field generation system 21.
And receives a current from the gradient magnetic field power supply 12 to generate a gradient magnetic field under the control of the sequencer 2.

The receiving system 5 includes a receiving coil 14 as a high-frequency coil, an amplifier 15 connected to the receiving coil 14,
A detection circuit 16 and an analog / digital converter (hereinafter, referred to as “analog / digital converter”)
When the receiving coil 14 detects an NMR signal from the subject 7, the NMR signal is converted into a digital value via an amplifier 15, a detection circuit 16 and an ADC 17, and the sequencer 2 Is converted into the collected data sampled by the ADC 17 at the timing according to the command, and sent to the CPU 1.

The signal processing system 6 includes an external storage device such as a magnetic disk 20 and an optical disk 19, and a display 18 such as a CRT.
When input to the PU1, the CPU 1 executes processing such as signal processing and image reconstruction, and displays an image of a desired tomographic plane of the subject 7 on the display 18 as a result of the processing.
The data is stored in the magnetic disk 20 or the like of the external storage device.

FIG. 4 shows details of a receiving system of a conventional MRI apparatus. In FIG. 4, the receiving coil 14 detects the NMR signal emitted from the subject 7. The NMR signal detected by the reception coil 14 is amplified by the amplifier 15 and then input to the pre-stage mixer 22 of the detection circuit 16. The NMR signal used as image information is frequency-converted by the pre-stage mixer 22. The output from the previous-stage mixer 22 includes a high-frequency component that is an intermediate frequency and a low-frequency component that is required for image reconstruction.
a and 23b, and only low frequency components are extracted. The NMR signal composed of the low frequency component is output from the ADC17.
a, 17b, which are digitized and input to the CPU 1, where the CPU 1 reconstructs an image from this NMR signal,
An image of the tomographic plane of the subject 7 is obtained.

In the above, the detection mixers 23a, 23b
, A reference high-frequency signal from the high-frequency oscillator 8 is input, but the reference high-frequency signal input to both detection mixers for quadrature detection is given a 90 ° phase difference by the quadrature phase divider 24. I have. These reference high-frequency signals may include a phase error when the phase is converted by the quadrature phase divider 24, and the phase correction circuit 2 corrects the phase error.
After passing through 5a and 25b, they are input to the detection mixers 23a and 23b. LC is provided in the phase correction circuits 25a and 25b.
A circuit is used, and only one phase correction circuit has a variable capacitor inserted in the LC circuit, and the variable capacitor is manually adjusted to correct the phase error.

[0011]

However, in the conventional detection circuit, the correction of the phase error of the quadrature phase divider 24 must be manually adjusted by the variable capacitor of the LC circuit.
Depending on the skill of the worker who performs the adjustment, there is work variation,
There was a problem that the correction accuracy was significantly different.

Accordingly, it is an object of the present invention to provide an MRI apparatus which addresses such a problem, automates an adjustment operation with a simple configuration, and reduces image deterioration caused by poor phase accuracy. .

[0013]

In order to achieve the above object, an MRI apparatus according to the present invention comprises a receiving coil for detecting an NMR signal from a subject, and a high-frequency NMR signal detected by the receiving coil, which has an intermediate frequency. A first detection mixer for converting the NMR signal of the intermediate frequency, a second detection mixer for performing quadrature detection on the NMR signal of the intermediate frequency to convert the NMR signal to an NMR signal of an audio frequency, and the first detection mixer and the first detection mixer. A reference high-frequency oscillation circuit for transmitting a reference high-frequency signal to a second detection mixer; a quadrature phase distribution circuit for separating the reference high-frequency signal into two reference high-frequency signals having a phase difference of 90 °; An audio / digital conversion circuit for converting an NMR signal into a digital signal; and a central unit for performing image reconstruction and various controls based on the digital signal. The second detection mixer mixes the NMR signal of the intermediate frequency with each of the two reference high-frequency signals having a phase difference of 90 °, so that the second detection mixer has an phase difference of 90 °. A delay line pattern having a plurality of different lengths between said quadrature phase distribution circuit and said second detection mixer in a magnetic resonance imaging apparatus configured to convert the signal into an NMR signal of a digital frequency; And a phase correction circuit having a selection means for selecting an arbitrary delay line pattern, wherein one of the delay line patterns is selected to correct the phase error of the reference high-frequency signal. (Claim 1).

Further, the MRI apparatus according to the present invention includes signal switching means for switching the output of the quadrature phase distribution circuit to an audio / digital conversion circuit, and switches the signal switching means to directly convert the reference high-frequency signal into a digital signal. The phase information of the reference high-frequency signal is extracted from the digital information, and the delay line pattern of the phase correction circuit is selected based on the phase information (claim 2).

In the MRI apparatus configured as described above, the central processing unit captures the phase error of the quadrature phase distribution circuit as the phase information of the reference high-frequency signal. The central processing unit determines which length of the delay line pattern to pass through. As a result, the phase error of the quadrature phase distribution circuit is corrected, and the deterioration of the tomographic image due to poor phase accuracy is greatly improved.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 3 is a block diagram of M to which the present invention is applied.
This is an example of the RI apparatus, which has been described in the section of the related art, and has the same configuration as the conventional MRI apparatus. This MR
The part to which the present invention is applied in the I device is the receiving system 5.

FIG. 1 shows details of a receiving system 5 which is a main part of the present invention.
Shown in In FIG. 1, a receiving coil 14 detects an NMR signal emitted from the subject 7. Since the NMR signal detected by the receiving coil 14 is a weak high-frequency signal, it is amplified by the high-frequency amplifier 15 and then input to the pre-stage mixer 22 of the detection circuit 16. With this pre-stage mixer 22,
An NMR signal used as image information is frequency-converted. The output from the pre-stage mixer 22 includes a high-frequency component as an intermediate frequency and a low-frequency component required for image reconstruction. This output is output to the detection mixers 23a,
3b, where it is detected to extract only the low frequency components.

Since quadrature detection is performed in the detection mixers 23a and 23b, a reference high-frequency signal having a phase difference of 90 ° is input. The reference high-frequency signal generated by the high-frequency oscillator 8 is used as the reference high-frequency signal. The reference high frequency signal emitted from the high frequency oscillator 8 is
4 and output as two reference high-frequency signals having a phase difference of 90 ° and input to the detection mixers 23a and 23b.
Here, the two types of reference high-frequency signals having a phase difference of 90 °, which are the outputs of the quadrature phase divider 24, may include a phase error. Therefore, in this embodiment, one of the detection mixers 23a
Only the reference high-frequency signal having a phase difference of 90 ° is input to the detection mixer 23a after the phase is corrected by the phase correction circuit 25. The phase correction circuit 25 includes a plurality of delay line patterns 27a to 27c having different lengths, and a relay switch 28 for selecting the delay line patterns.
a, 28b, and the relay switches 28a, 28b are arranged at both ends of the delay line patterns 27a to 27c. An arbitrary delay line pattern 27 can be selected by switching the relay switches 28a and 28b in accordance with a control signal from the CPU 1.

Next, the delay line pattern length and the phase difference will be described. Assuming that the frequency of the reference high-frequency signal generated by the high-frequency oscillator 8 is 10 MHz, a difference in delay line pattern length of 5.56 cm corresponds to a phase difference of 1 °. On the other hand, since the phase error of the quadrature phase divider 24 is about 3 ° at the maximum, if it is desired to adjust the phase error with an accuracy within 1 °, a delay line pattern 27b having a reference length and 5. 56cm,
11.11 cm, 16.67 cm long delay line pattern 2
7a, 5.56 cm, 11.11 cm, 1
A delay line pattern 27c shorter by 6.67 cm is prepared. Any one of these delay line patterns 27a to 27c is controlled by a relay switch 2 according to a control signal from the CPU 1.
8a and 28b can be selected. With this configuration, the component having a phase difference of 90 ° is ±
Correction within 3 mm is possible.

The above description shows one embodiment of the present invention.
Delay line patterns of various lengths depending on the phase error
27 can be prepared.

The reference high-frequency signal separated by the quadrature phase splitter 24 is supplied to the detection mixer 23 when the NMR signal is measured.
are input to the ADCs 17a and 17b when the phase error is corrected by switching the relays 26a and 26b.
b. The ADCs 17a and 17b are, for example, 1M
The sampling is performed at the timing of Hz, and here, the timing is the ADC1 to which the signal of the 0 ° component is input.
7b is started so that the output becomes zero.

The sampling in the ADCs 17a and 17b will be described with reference to FIG. FIG. 2 shows an example of sampling at 1 MHz. FIG. 2 (A) shows a sample at an arbitrary timing, and FIG. 2 (B) shows a sample at a timing when the output becomes zero. . In this embodiment, since the reference high-frequency signal to be digitized is a signal of only 10 MHz, it is 100 ns.
Are collected at the same phase every time. That is, the digitized reference high-frequency signal is regarded as DC. Therefore, the ADC 17 of the signal sampled at 1 MHz is used.
The magnitudes of the outputs a and 17b are values corresponding to the phase at the time of sampling. Therefore, the phase difference can be determined by looking at the outputs of the ADCs 17a and 17b at the time of sampling. In this method, the determination can be made with an accuracy within ± 2%. Further, as the means for determining the phase difference, it is possible to make the determination by looking at the degree of distortion of the reconstructed image.

On the other hand, the reference high-frequency signal having a phase difference of 90 ° input to the ADC 17a is sampled 50 ns later than the start timing at which the reference high-frequency signal having a phase difference of 0 ° input to the ADC 17b is sampled. To be configured. By doing so, if the phase difference between the two signals is exactly 90 °, the ADC 17a to which the reference high-frequency signal having the phase difference of 90 ° is input.
Will be zero.

Therefore, the CPU 1 operates the relays 28a and 28a in the phase correction circuit 25 so that the output of the reference high-frequency signal having a phase difference of 90 ° at the ADC 17a becomes zero.
b to the delay line patterns 27a to 27c.
By performing the switching, accurate phase adjustment can be automatically performed.

In this embodiment, the performance of the ADC 17 will not be described in detail. However, it is assumed that the ADC 17 is provided with a sample hold circuit or the like as necessary, and can collect a signal of 10 MHz discretely.

As described above, the signal composed of the low-frequency components detected by the detection mixers 23a and 23b is
The data is digitized by 7a and 17b and input to the CPU 1. The CPU 1 reconstructs an image based on the signal composed of the low frequency component, and can obtain an image of the tomographic plane of the subject 7.

[0027]

As described above, the MRI according to the present invention
According to the device, since the phase error of the reference high-frequency signal input to the detection circuit can be accurately and automatically corrected, image deterioration due to poor phase accuracy in the detection circuit can be significantly reduced. it can.

[Brief description of the drawings]

FIG. 1 is a diagram showing details of a receiving system which is a main part of the present invention.

FIG. 2 shows an example of sampling at 1 MHz.

FIG. 3 is a configuration example of an MRI apparatus.

FIG. 4 is a configuration example of a receiving system of a conventional MRI apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 CPU 2 Sequencer 3 Transmission system 4 Static magnetic field generating magnet 5 Receiving system 6 Signal processing system 7 Subject 8 High frequency oscillator 9 Modulator 10 Amplifier 11 Irradiation coil 12 Gradient magnetic field power supply 13 Gradient magnetic field coil 14 Receiving coil 15 Amplifier 16 Detection circuit 17, 17a, 17b ADC 18 Display 19 Optical disk 20 Magnetic disk 21 Gradient magnetic field generation system 22 Pre-stage mixer 23a, 23b Detection mixer 24 Quadrature phase distributor 25, 25a, 25b Phase correction circuit 26a, 26b Relay 27a-27c Delay Line pattern 28a, 28b Relay switch

Claims (2)

[Claims] A receiving coil for detecting a high-frequency nuclear magnetic resonance signal (hereinafter referred to as an NMR signal) from a subject;
A first detection mixer for converting the high-frequency NMR signal detected by the receiving coil into an intermediate-frequency NMR signal; and a first detection mixer for performing quadrature detection on the intermediate-frequency NMR signal and converting it to an audio-frequency NMR signal. Two detection mixers,
A reference high-frequency oscillation circuit for transmitting a reference high-frequency signal to the first detection mixer and the second detection mixer, and a quadrature phase distribution circuit for separating the reference high-frequency signal into two reference high-frequency signals having a phase difference of 90 ° An audio / digital conversion circuit for converting the NMR signal of the audio frequency into a digital signal, and a central processing unit for performing image reconstruction based on the digital signal and performing various controls, The second detection mixer mixes the intermediate frequency NMR signal with each of the two reference high frequency signals having a phase difference of 90 °, thereby obtaining an NMR signal of an audio frequency having a phase difference of 90 °. In a magnetic resonance imaging apparatus configured to convert a signal into a plurality of signals, a plurality of lengths are provided between the quadrature phase distribution circuit and the second detection mixer. A phase correction circuit having a different delay line pattern and a selecting means for selecting an arbitrary delay line pattern, wherein one of the delay line patterns is selected and a phase error of the reference high-frequency signal is obtained. A magnetic resonance imaging apparatus characterized in that: 2. The magnetic resonance imaging apparatus according to claim 1, further comprising signal switching means for switching an output of said quadrature phase distribution circuit to an audio / digital conversion circuit, and switching said signal switching means. Magnetic resonance imaging, wherein a reference high-frequency signal is directly digitized, phase information of the reference high-frequency signal is extracted from the digital information, and a delay line pattern of the phase correction circuit is selected based on the phase information. apparatus.